high speed machining and conventional die and mould machining

Reprint from Metalworking World

High speed machining and conventional

die and mould machining

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Historical backgroundThe term High Speed Machining (HSM)commonly refers to end milling at highrotational speeds and high surface feeds.For instance, the routing of pockets inaluminum airframe sections with a veryhigh material removal rate. Over the past60 years, HSM has been applied to a widerange of metallic and non-metallic work-piece materials, including the produc-tion of components with specific surfacetopography requirements and machiningof materials with a hardness of 50 HRCand above. With most steel componentshardened to approximately 32-42 HRC,machining options currently include:

• rough machining and semi-finishingof the material in its soft (annealed)condition• heat treatment to achieve the final re-quired hardness (</= 63 HRC)• machining of electrodes and Electri-cal Discharge Machining (EDM) of spe-cific parts of the dies or molds (speci-fically small radii and deep cavities withlimited accessibility for metal cuttingtools)• finishing and super-finishing of cyl-indrical/flat/cavity surfaces with appro-priate cemented carbide, cermet, solidcarbide, mixed ceramic or polycrystal-line cubic boron nitride (PCBN)

With many components, the productionprocess involves a combination of theseoptions and in the case of dies and mo-lds it also includes time consuming handfinishing. Consequently, production costscan be high and lead times excessive.

Typical for the die and mold industryis to produce one or a few tools of thesame design. The process includes con-stant changes of the design. And be-cause of the need of design changesthere is also a corresponding need ofmeasuring and reverse engineering.

The main criteria is the quality of thedie or mold regarding dimensional,geometrical and surface accuracy. If thequality level after machining is poorand if it can not meet the requirementsthere will be a varying need of manualfinishing work. This work gives a satis-

There are a lot of questions about HSM today and many different,more or less complicated, definitions can be seen frequently. Herethe matter will be discussed in an easy fashion and from a practicalpoint of view.

This article is the first in a series of articles about die andmoldmaking from Sandvik Coromant. In a following article HSMwill be further discussed.

HSM- High Speed Machining

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Processes - the demands on shorterthrough- put times via fewer set-ups andsimplified flows (logistics) can be solvedto a big extent via HSM. A typical targetwithin the die and mold industry is tomake a complete machining of fully har-dened small sized tools in one set-up.Costly and time consuming EDM-pro-cesses can also be reduced or elimina-ted via HSM.

Design & development - one of the maintools in today’s competition is to sell pro-ducts on the value of novelty. The ave-rage product life cycle on cars is today4 years, computers and accessories 1,5years, hand phones 3 months... One ofthe prerequisites of this development offast design changes and rapid productdevelopment time is the HSM technique.

Complex products - there is an increaseof multifunctional surfaces on compo-nents. Such as new design of turbine bla-des giving new and optimised functionsand features. Earlier design allowed poli-shing by hand or with robots (manipu-lators). The turbine blades with the new,more sophisticated design has to befinished via machining and preferablyby HSM.

There are also more and more examplesof thin walled workpieces that has to bemachined (medical equipment, electro-nics, defence products, computer parts).

Production equipment - the strong deve-lopment of cutting materials, holdingtools, machine tools, controls and especi-ally CAD/CAM features and equip-ment has opened possibilities that mustbe met with new production methodsand techniques.

fying surface accuracy, but it always hasa negative impact on the dimensionaland geometrical accuracy.

One of the main targets for the die andmold industry has been, and is, to re-duce or eliminate the need of manualpolishing and thus improve the quality,shorten the production costs and leadtimes.

Main economical and technical factors for the development of HSM Survival - the ever increasing competi-tion on the marketplace is setting newstandards all the time. The demands ontime and cost efficiency is getting higherand higher. This has forced the develop-ment of new processes and productiontechniques to take place. HSM provideshope and solutions...

Materials - the development of new,more difficult to machine materials hasunderlined the necessity to find newmachining solutions. The aerospace in-dustry has its heat resistant and stain-less steel alloys. The automotive indu-stry has different bimetal compositions,Compact Graphite Iron and an ever in-creasing volume of aluminum. The dieand mold industry mainly has to facethe problem to machine high hardenedtool steels. From roughing to finishing.

Quality - the demand on higher compo-nent or product quality is a result of thehard competition. HSM offers, if appliedcorrectly, solutions in this area. Substitu-tion of manual finishing is one example.Especially important on dies or moldsor components with a complex 3Dgeometry.

Chip removal temperature as a result of the cutting speed.

The original definition of HSMSalomons theory, “Machining with highcutting speeds...“ on which he got aGerman patent 1931, assumes that “ata certain cutting speed (5-10 times hig-her than in conventional machining),the chip removal temperature at thecutting edge will start to decrease...“.

Giving the conclusion: ...“seem to givea chance to improve productivity in ma-chining with conventional tools at highcutting speeds...“

Modern research has unfortunately notbeen able to verify this theory to its fullextent. There is a relative decrease of thetemperature at the cutting edge thatstarts at certain cutting speeds for dif-ferent materials. The decrease is smallfor steel and cast iron. And bigger foraluminum and other non-ferrous metals.The definition of HSM must be basedon other factors.

What is today’s definition of HSM?The discussion about high speed machi-ning is to some extent characterised byconfusion. There are many opinions,many myths and many different waysto define HSM. Looking upon a few ofthese definitions HSM is said to be:

• High Cutting Speed (vc) Machining...• High Spindle Speed (n) Machining...• High Feed (vf) Machining...• High Speed and Feed Machining...• High Productive Machining...

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Most dies or moulds have a considera-bly smaller size, than mentioned above,in complete machining (single set-up). Typical operations performed are, roug-hing, semi-finishing, finishing and inmany cases super-finishing. Restmil-ling of corners and radii should alwaysbe done to create constant stock for thefollowing operation and tool. In manycases 3-4 tool types are used.

The common diameter range is from 1 -20 mm. The cutting material is in 80 to90% of the cases solid carbide end millsor ball nose end mills. End mills with bigcorner radii are often used. The solidcarbide tools have reinforced cuttingedges and neutral or negative rakes(mainly for materials above 54 HRC).One typical and important design fea-ture is a thick core for maximum ben-ding stiffness.

It is also favourable to use ball nose endmills with a short cutting edge and con-

hm, are much lower compared with con-ventional machining. The material remo-val rate, Q, is consequently and con-siderably smaller than in conventionalmachining. With the exception when ma-chining in aluminium, other non- ferrousmaterials and in finishing and superfini-shing operations in all types of materials.

Application technologyTo perform HSM applications it is ne-cessary to use rigid and dedicated machi-ne tools and controls with specific designfeatures and options. All productionequipment has to be designed for thespecific process of HSM.

It is also necessary to use an advancedprogramming technique with the mostfavourable tool paths. The method toensure constant stock for each opera-tion and tool is a prerequisite for HSMand a basic criteria for high productivityand process security. Specific cuttingand holding tools is also a must for thistype of machining.

Characteristics of today’s HSM in hardened tool steelWithin the die & mold area the maxi-mum economical workpiece size forroughing to finishing with HSM is appro-ximately 400 X 400 X 150 (l, w, h). Themaximum size is related to the relati-vely low material removal rate in HSM.And of course also to the dynamics andsize of the machine tool.

On following pages the parameters thatinfluence the machining process andhaving connections with HSM will bediscussed. It is important to describeHSM from a practical point of view andalso give as many practical guidelinesfor the application of HSM as possible.

True cutting speedAs cutting speed is dependent on bothspindle speed and the diameter of the

tool, HSM should be defined as “truecutting speed“ above a certain level.The linear dependence between cuttingspeed and feed rate results in “highfeeds with high speeds“. The feed willbecome even higher if a smaller cutterdiameter is chosen, provided that thefeed per tooth and the number of teethis unchanged. To compensate for a smal-

ler diameter the rpm must be increasedto keep the same cutting speed...andthe increased rpm gives a higher vf.

Shallow cutsVery typical and necessary for HSMapplications is that the depths of cut, aeand ap and the average chip thickness,

Vf = fz x n x zn [mm/min]

Q = ap x ae x vf [cm3/min]1000

De = 2 �ap(Dc -ap)

Effective cutting speed (ve)

ve = � x n x De m/min1000

Formula for feed speed.

Formula for material removal rate.

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One example:• End mill with 90 degree corner, dia-

meter 6 mm. Spindle speed at truecutting speed of 250 m/min = 13 262rpm

• Ball nose end mill, nominal diame-ter 6 mm. ap 0,2 mm gives effectivediameter in cut of 2,15 mm. Spindlespeed at true cutting speed of 250m/min = 36 942 rpm

tact length. Another design feature ofimportance is an undercutting capabi-lity, which is necessary when machiningalong steep walls with a small clearance.It is also possible to use smaller sizedcutting tools with indexable inserts. Es-pecially for roughing and semi-finishing.These should have maximum shankstability and bending stiffness. A taperedshank improves the rigidity. And so doesalso shanks made of heavy metal.

The geometry of the die or mold couldpreferably be shallow and not too com-plex. Some geometrical shapes are alsomore suited for high productive HSM.The more possibilities there are to adaptcontouring tool paths in combinationwith downmilling, the better the resultwill be.

One main parameter to observe whenfinishing or super-finishing in hardenedtool steel with HSM is to take shallowcuts. The depth of cut should not exceed0,2/0,2 mm (ae/ap). This is to avoid exces-sive deflection of the holding/cuttingtool and to keep a high tolerance leveland geometrical accuracy on the machi-ned die or mold. An evenly distributedstock for each tool will also guaranteea constant and high productivity. Thecutting speed and feed rate will be onconstant high levels when the ae/ap isconstant. There will be less mechanicalvariations and work load on the cut-ting edge plus an improved tool life.

Cutting dataTypical cutting data for solid carbideend mills with a TiC,N or TiAlN-coatingin hardened steel: 48-58 HRC.

RoughingTrue vc: 100 m/min, ap: 6-8% of thecutter diameter, ae: 35-40% of the cut-ter diameter, fz: 0,05-0,1 mm/z

Semi-finishingTrue vc: 150-200 m/min, ap: 3-4% ofthe cutter diameter, ae: 20-40% of thecutter diameter, fz: 0,05-0,15 mm/z

Finishing and super-finishing True vc: 200-250 m/min, ap: 0,1-0,2 mm,ae: 0,1-0,2 mm, fz: 0,02-0,2 mm/z

• HSM is not simply high cutting speed.It should be regarded as a processwhere the operations are performedwith very specific methods and pro-duction equipment.

• HSM is not necessarily high spindlespeed machining. Many HSM appli-cations are performed with moderatespindle speeds and large sized cutters.

• HSM is performed in finishing inhardened steel with high speeds and

feeds, often with 4-6 times conven-tional cutting data.

• HSM is High Productive Machiningin small-sized components in roug-hing to finishing and in finishing andsuper-finishing in components of allsizes.

• HSM will grow in importance themore net shape the components get.

• HSM is today mainly performed intaper 40 machines.

Material Hardness Conv. vc HSM ve, R HSM ve, FSteel 01.2 150 HB <300 >400 <900

Steel 02.1/2 330 HB <200 >250 <600

Steel 03.11 300 HB <100 >200 <400

Steel 03.11 39 -48 HRc <80 >150 <350

Steel 04 48-58 HRc <40 >100 <250

GCI 08.1 180 HB <300 >500 <3000

Al/Kirksite 60-75 HB <1000 >2000 <5000

Non-ferr 100 HB <300 >1000 <2000

HSM Cutting Data by Experience

Typical workpieces for HSM, forging diefor an automotive component, molds for aplastic bottle and a headphone.

Practical definition of HSM - conclusion

Dry milling with compressed air or oil mist under high pressure is recommended.

The values are of course dependent ofout-stick, overhang, stability in the appli-cation, cutter diameters, material hard-ness etc. They should be looked upononly as typical and realistic values. Inthe discussion about HSM applicationsone can sometimes see that extremelyhigh and unrealistic values for cuttingspeed is referred to. In these cases vchas probably been calculated on thenominal diameter of the cutter. Notthe effective diameter in cut.

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The application of High Speed MachiningIn the article about HSM in the Nov/Dec issue1998, the focus was on the background, characte-ristics and definitions of HSM. In this article thediscussion will continue with the focus on applica-

tion areas and the different demands put onmachine and cutting tools. We will also shed lighton some advantages and disadvantages with HSM.

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Forging dies. Most forging dies are sui-table for HSM due to the shallow geo-metry that many of them have. Shorttools always results in higher producti-vity due to less bending (better stability).Maintenance of forging dies (sinkingof the geometry) is a very demandingoperation as the surface is very hard andoften also has cracks.

Main application areas for HSMMilling of cavities. As have been dis-cussed in the previous article, it is pos-sible to apply HSM-technology (HighSpeed Machining) in qualified, high-alloy tool steels up to 60-63 HRc.

When milling cavities in such hard ma-terials, it is crucial to select adequatecutting and holding tools for each spe-cific operation; roughing, semi-finishingand finishing. To have success, it is alsovery important to use optimised toolpaths, cutting data and cutting strategies.These things will be discussed in detailin future articles.

Die casting dies. This is an area whereHSM can be utilised in a productive wayas most die casting dies are made of de-manding tool steels and have a mo-derate or small size.

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Milling of electrodes in graphite andcopper. An excellent area for HSM.Graphite can be machined in a produc-tive way with TiCN-, or diamond coa-ted solid carbide endmills. The trend isthat the manufacturing of electrodesand employment of EDM is steadilydecreasing while material removal withHSM is increasing.

Injection moulds and blow moulds arealso suitable for HSM, especially be-cause of their (most often) small size.Which makes it economical to performall operations (from roughing to finish-ing) in one set up. Many of these moulds

Modelling and prototyping of dies andmoulds. One of the earliest areas forHSM. Easy to machine materials, suchas non-ferrous, aluminium, kirkzite etcetera. The cutting speeds are often ashigh as 1500-5000 m/min and the feedsare consequently also very high.

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have relatively deep cavities. Whichcalls for a very good planning of appro-ach, retract and overall tool paths.Often long and slender shanks/exten-sions in combination with light cuttingtools are used.

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screws. HSM and axial milling is also agood combination as the impact on thespindle bearings is small and the met-hod also allows longer tools with lessrisk for vibrations.

Productive cutting process in small sized componentsRoughing, semi-finishing and finishingis economical to perform when the totalmaterial removal is relatively low.

Productivity in general finishing andpossibility to achieve extremely goodsurface finish. Often as low as Ra ~ 0,2microns.

are shallow and the engagement timefor the cutting edge is extremely short.It can be said that the feed is faster thanthe time for heat propagation.

Low cutting force gives a small andconsistent tool deflection. This, in com-bination with a constant stock for eachoperation and tool, is one of the prere-quisites for a highly productive andsafe process.

As the depths of cut are typically shal-low in HSM, the radial forces on thetool and spindle are low. This savesspindle bearings, guide-ways & ball

Targets for HSM of dies and mouldsOne of the main targets with HSM is tocut production costs via higher produc-tivity. Mainly in finishing operationsand often in hardened tool steel.

Another target is to increase the overallcompetitiveness through shorter leadand delivery times. The main factors,which enables this are:- production of dies or moulds in (a

few or) a single set-up - improvement of the geometrical accu-

racy of the die or mould via machi-ning, which in turn will decrease themanual labour and try-out time

- increase of the machine tool andworkshop utilisation via processplanning with the help of a CAM-system and workshop oriented pro-gramming

Advantages with HSMCutting tool and workpiece tempera-ture are kept low. Which gives a pro-longed tool life in many cases. In HSMapplications, on the other hand, the cuts

HSM is also very often used in direct production of -• Small batch components• Prototypes and pre-series in

Al, Ti, Cu for the Aerospace industryElectric/Electronic industryMedical industryDefence industry

• Aircraft components, especi-ally frame sections but also engine parts

• Automotive components, GCIand Al

• Cutting and holding tools (through hardened cutter bodies)

Top picture HSM, feed faster than heatpropagation. Lower picture, conventionalmilling, time for heat propagation.

Cutting force (Fc) vs cutting speed (vc) for a constant cutting power of 10 kW.

Cutting speed (vc) Vs specific cutting force (Mpa) in aluminium 7050.

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Fc[N]

2500

2000

1500

1000

500

00 500 1000 1500 2000 2500 3000 Vc[m/min]

kC1N/mm2

800

700

600

500

0 500 1000 1500 2000 2500 3000 Vc[m/min]

The impulse law

Fc = PcVc

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machining, the time consuming manu-al polishing work can be cut down dra-matically. Often with as much as 60-100%!

Reduction of process stepsReduction of production processes ashardening, electrode milling and EDMcan be minimised. Which gives lowerinvestment costs and simplifies thelogistics. Less floor space is also nee-ded with fewer EDM-equipment. HSMcan give a dimensional tolerance of0,02 mm, while the tolerance with EDMis 0,1-0,2 mm.

The durability, tool life, of the harde-ned die or mould can sometimes beincreased when EDM is replaced withmachining. EDM can, if incorrectlyperformed, generate a thin, re-harde-ned layer directly under the melted toplayer. The re-hardened layer can be upto ~20 microns thick and have a hard-ness of up to 1000 Hv. As this layer isconsiderably harder than the matrix itmust be removed. This is often a timeconsuming and difficult polishing work.EDM can also induce vertical fatiguecracks in the melted and resolidified

Machining of very thin walls is possible.As an example the wall thickness canbe 0,2 mm and have a height of 20 mmif utilising the method shown in thefigure. Downmilling tool paths to beused. The contact time, between edgeand work piece, must be extremelyshort to avoid vibrations and deflectionof the wall. The microgeometry of thecutter must be very positive and theedges very sharp.

Geometrical accuracy of dies andmoulds gives easier and quicker assem-bly. No human being, no matter howskilled, can compete with a CAM/CNC-produced surface texture and geome-try. If some more hours are spent on

A) Traditional process. Non-harde-ned (soft) blank (1), roughing (2) andsemi-finishing (3). Hardening to thefinal service condition (4). EDM pro-cess - machining of electrodes andEDM of small radii and corners at bigdepths (5). Finishing of parts of thecavity with good accessability (6).Manual finishing (7).

B) Same process as (A) where the EDM-process has been replaced by finish machining of the entirecavity with HSM (5). Reduction of one process step.

C) The blank is hardened to the final service condition (1), roughing (2), semi-finishing (3) andfinishing (4). HSM most often applied in all operations (especially in small sized tools). Reductionof two process steps. Normal time reduction compared with process (A) by approximately 30-50%.

top layer. These cracks can, duringunfavourable conditions, even lead toa total breakage of a tool section.

Design changes can be made very fastvia CAD/CAM. Especially in caseswhere there is no need of producingnew electrodes.

Some disadvantages with HSM• The higher acceleration and decele-

ration rates, spindle start and stopgive a relatively faster wear of guideways, ball screws and spindle bear-ings. Which often leads to highermaintenance costs…

• Specific process knowledge, pro-gramming equipment and interfacefor fast data transfer needed.

• It can be difficult to find and recruitadvanced staff.

• Considerable length of “trial anderror” period.

• Emergency stop is practically unne-cessary! Human mistakes, hard-, orsoftware errors give big consequen-ces!

• Good work and process planning ne-cessary - “feed the hungry machine...”

= Manual finishing

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Some specific demands on cutting tools made of solid carbide• High precision grinding giving run-

out lower than 3 microns • As short outstick and overhang as

possible, maximum stiff and thickcore for lowest possible deflection

• Short edge and contact length forlowest possible vibration risk, lowcutting forces and deflection

• Oversized and tapered shanks, espe-cially important on small diameters

• Micro grain substrate, TiAlN-coatingfor higher wear resistance/hot hard-ness

• Holes for air blast or coolant• Adapted, strong micro geometry for

HSM of hardened steel • Symmetrical tools, preferably balan-

ced by design

Specific demands on cutters with indexable inserts• Balanced by design• High precision regarding run-out,

both on tip seats and on inserts,maximum 10 microns totally...

• Adapted grades and geometries forHSM in hardened steel

• Good clearance on cutter bodies toavoid rubbing when tool deflection(cutting forces) disappears

• Holes for air blast or coolant• Marking of maximum allowed rpm

directly on cutter bodies. Specificdemands on cutting tools will be fur-ther discussed in coming articles.

Cutting fluid in millingModern cemented carbides, especiallycoated carbides, do not normally requ-ire cutting fluid during machining. GCgrades perform better as regards to toollife and reliability when used in a drymilling environment.

This is even more valid for cermets, cera-mics, cubic boron nitride and diamond.

Today’s high cutting speeds results in avery hot cutting zone. The cutting actiontakes place with the formation of a flowzone, between the tool and the work-

• Safety precautions are necessary:Use machines with safety enclosing -bullet proof covers! Avoid longoverhangs on tools. Do not use“heavy” tools and adapters. Checktools, adapters and screws regularlyfor fatigue cracks. Use only tools withposted maximum spindle speed. Donot use solid tools of HSS!

An example of the consequences ofbreakage at high speed machining is thatof an insert breaking loose from a 40 mmdiameter endmill at a spindle speed of40.000 rpm. The ejected insert, with amass of 0.015 kg, will fly off at a speed of84 m/s, which is an energy level of 53 nM- equivalent to the bullet from a pistol andrequiring armour plated glass.

Some typical demands on the machine tool and the data transfer in HSM (ISO/BT40 or comparable size)• Spindle speed range

</ = 40 000 rpm• Spindle power

> 22 kW• Programmable feed rate

40-60 m/min• Rapid traverse

< 90 m/min• Axis dec./acceleration

> 1 g (faster w. linear motors)

• Block processing speed 1-20 ms

• Increments (linear)5-20 microns

• Or circular interpolation via NURBS (no linear increments)

• Data flow via RS23219,2 Kbit/s (20 ms)

• Data flow via Ethernet250 Kbit/s (1 ms)

• High thermal stability and rigidity in spindle - higher pretension and cooling of spindle bearings

• Air blast/coolant through spindle• Rigid machine frame with high vibration absorbing capacity• Different error compensations - temperature, quadrant, ball screw are

the most important• Advanced look ahead function in the CNC

Surface with (red line) and without (blue line) run-out.

Tool life as a funktion of TIR of chipthickness.

Tool life

rpm

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Run outs influence on surface quality

Run outs influence on tool-life

Rt = fz2

4 x Dc

Exempel: Two edge cutter. Profile depthfz

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Essential savings can be done via dry machining:• Increases in productivity as per above.• Production costs lowered. The cost

of coolant and the disposal of itrepresent 15-20% of the total pro-duction costs! This could be compa-red to that of cutting tools, amount-ing to 4-6% of the production costs.

• Environmental and health aspects.A cleaner and healthier workshopwith bacteria formation and badsmells eliminated.

• No need of maintenance of the coo-lant tanks and system. It is usuallynecessary to make regular stoppagesto clean out machines and coolantequipment.

• Normally a better chip forming takesplace in dry machining.

Cutting fluid in HSMIn conventional machining, when thereis much time for heat propagation, itcan sometimes be necessary to usecoolant to prevent excessive heat frombeing conducted into; the workpiece,cutting and holding tool and eventuallyinto the machine spindle. The effectson the application may be that the tooland the workpiece will extend somew-hat and tolerances can be in danger.

This problem can be solved in differentways. As have been discussed earlier, itis much more favourable for the die ormould accuracy to split roughing andfinishing into separate machine tools.The heat conducted into the workpieceor the spindle in finishing can be neglec-ted. Another solution is to use a cuttingmaterial that does not conduct heat,such as cermet. In this case the mainportion of the heat goes out with thechips, even in conventional machining.

It may sound trivial, but one of themain factors for success in HSM appli-cations is the total evacuation of chipsfrom the cutting zone. Avoiding recut-ting of chips when working in harde-ned steel is absolutely essential for apredictable tool life of the cutting edgesand for a good process security.

tions need to be taken. The temperatu-re in the cutting zone should be eitherabove or below the unsuitable areawhere built-up edge appears.

Achieving the flow-zone at highertemperatures eliminates the problem.No, or very small built-up edge is for-med. In the low cutting speed areawhere the temperature in the cuttingzone is lower, cutting fluid may beapplied with less harmful results forthe tool life.

There are a few exceptions when theuse of cutting fluid could be “defen-ded” to certain extents:

• Machining of heat resistant alloys isgenerally done with low cuttingspeeds. In some operations it is ofimportance to use coolant for lubrica-tion and to cool down the component.Specifically in deep slotting opera-tions.

• Finishing of stainless steel and alu-minium to prevent smearing of smallparticles into the surface texture. Inthis case the coolant has a lubricat-ing effect and to some extent it alsohelps evacuating the tiny particles.

• Machining of thin walled componentsto prevent geometrical distortion.

• When machining in cast iron andnodular cast iron the coolant collectsthe material dust. (The dust can alsobe collected with equipment forvacuum cleaning).

• Flush pallets, components and machi-ne parts free from swarf. (Can alsobe done with traditional methods orbe eliminated via design changes).

• Prevent components and vital machi-ne parts from corrosion.

If milling has to be performed wet,coolant should be applied copiouslyand a cemented carbide grade shouldbe used which is recommended for usein wet as well as in dry conditions. It caneither be a modern grade with a toughsubstrate having multilayer coatings. Ora somewhat harder, micro-grain carbi-de with a thin PVD coated TiN layer.

piece, with temperatures of around1000 degrees C or more.

Any cutting fluid that comes in thevicinity of the engaged cutting edgeswill instantaneously be converted tosteam and have virtually no coolingeffect at all.

The effect of cutting fluid in milling isonly emphasising the temperature varia-tions that take place with the insertsgoing in and out of cut. In dry machi-ning variations do take place but wit-hin the scope of what the grade has beendeveloped for (maximum utilisation).Adding cutting fluid will increase varia-tions by cooling the cutting edge whilebeing out of cut. These variations orthermal shocks lead to cyclic stressesand thermal cracking. This of coursewill result in a premature ending of thetool life. The hotter the machining zoneis, the more unsuitable it is to use cutt-ing fluid. Modern carbide grades, cer-mets, ceramics and CBN are designedto withstand constant, high cuttingspeeds and temperatures.

When using coated milling grades thethickness of the coating layer plays animportant role. A comparison can bemade to the difference in pouring boi-ling water simultaneously into a thick-wall and a thin-wall glass to see whichcracks, and that of inserts with thin andthick coatings, with the application ofcutting fluid in milling.

A thin wall or a thin coating lead to lessthermal tensions and stress. Therefore,the glass with thick walls will crack dueto the large temperature variations be-tween the hot inside and the cold out-side. The same theory goes for an insertwith a thick coating. Tool life differen-ces of up to 40%, and in some specificcases even more, are not unusual, tothe advantage of dry milling.

If machining in sticky materials, suchas low carbon steel and stainless steel,has to take place at speeds where built-up edges are formed, certain precau-

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The second best is to have oil mist underhigh pressure directed to the cutting zone,preferably through the spindle.

Third comes coolant with high pressure(approximately 70 bar or more) and goodflow. Preferably also through the spindle.

If using cemented carbide or solid carbide the difference in tool life between the first

and the last alternative may be as much as 50%.

If using cermet, ceramic or cubic boron nitride coolant should not be an option at all.

The best way to ensure a perfect chip eva-cuation is to use compressed air. It shouldbe well directed to the cutting zone.Absolutely best is if the machine tool hasan option for air through the spindle.

The worst case is ordinary, external coo-lant supply, with low pressure and flow.

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Data transfer and tool balance important for HSMTo perform High Speed Machining (HSM) applica-

tions it is necessary to use dedicated machine tools.It is of equal importance to have computer software andmachine controls with specific design features and optionsto ensure that correct tool paths can be programmed. Inthis article the importance of tool holding and balancedtools will be discussed.

This article is the third in a Series of articles dealingwith die and mould making techniques from SandvikCoromant.

CAD/CAM AND CNC STRUCTURESHSM processes have underlined thenecessity to develop both the CAM-,and CNC-technology radically. HSMis not simply a question of controllingand driving the axes and turning thespindles faster. HSM applications cre-ate a need of much faster data commu-

nication between different units in theprocess chain. There are also specificconditions for the cutting process inHSM applications that conventionalCNCs can not handle.

This type of process structure is cha-racterised by specific configuration of

data for each computer. The communi-cation of data between each computerin this chain has to be adapted and trans-lated. And the communication is alwaysof one way-type. There are often seve-ral types of interfaces without a com-mon standard.

PROBLEM AREASThe main problem is that a conventio-nal control (CNC) does not understandthe advanced geometrical informationfrom the CAD/CAM systems withouta translation and simplification of thegeometry data.

This simplification means that the hig-her level geometry (complex curves)from the CAD/CAM is transformed totool paths via primitive approximationsof the tool paths, based on straight linesbetween points within a certain tole-rance band. Instead of a smooth curveline geometry there will be a linearisedtool path. In order to avoid visible facets,vibration marks and to keep the surfa-ce finish on a high level on the compo-nent the resolution has to be very high.The smaller the tolerance band is (ty-pical values for the distance betweentwo points range from 2 to 20 microns),the bigger the number of NC-blockswill be. This is also true for the speeds- the higher cutting and surface speedthe bigger the number of NC-blocks.

This has today resulted in limitationsof some HSM applications as the blockcycle times have reached levels closeto 1 msec.

Such short block cycle times requires avery huge data transfer capacity. Whichwill create bottlenecks for the entireprocess by overloading factory networksand also demand large CNC-memoryand high computing power.

If one NC-block typically consists of250 bits and if the block cycle timeranges between 1 - 5 msec the CNC has

The typical structure for generating data and perform the cutting and measuring process may looklike the illustration above.

Workpiece geometry. CAD - creation/design of a geometrical 3D model based on advanced mathematical calculations (Bezier curves or NURBS)

Generic tool path. Creation of CAM - files representing toolpaths, methods of approach, tool and cutting data et cete-ra

NC program. Generation of a part program (NC-program)via post-processing of CAM - files to a specific type ofcontrol

Workpiece. Machining of the component, die or mould etc.via commands from a CNC

Measuring data. Registration and feedback of measuring data, CAQ, Computer Aided Quality assurance

14 Metalworking World

to handle between 250 000 to 50 000bits/sec!

NEW NURBS-BASED TECHNOLOGYThe recently developed solution onthe above problems is based on whatcould be called “machine independentNC-programming”.

This integration of CAD→CAM→CNCimply that the programming of the CNCconsiders a generic machine tool thatunderstands all geometrical commandscoming from the NC-programming.The technique is based on that the CNCis automatically adapting the specificaxis and cutter configuration for eachspecific machine tool and set-up.

This includes for instance correctionsof displacements of workpieces (on themachine table) without any changes inthe NC-program. This is possible asthe NC-program is relative to differentdeviations from the real situation.

Tool paths based on straight lines havenon-continuous transitions. For theCNC this means very big jumps in vel-ocity between different directions ofthe machine axes. The only way theCNC can handle this is by slowing downthe speed of the axes in the “change ofdirection situation”, for instance in acorner. This means a severe loss ofproductivity.

A NURBS is built up by three para-meters. These are poles, weights andknots. As NURBS are based on non-linear movements the tool paths willhave continuous transitions and it ispossible to keep much higher accelera-tion, deceleration and interpolationspeeds. The productivity increase canbe as much as 20-50%. The smoothermovement of the mechanics also resultsin better surface finish, dimensionaland geometrical accuracy.

Conventional CNC-technology doesnot know anything about cutting con-ditions. CNCs strictly care only aboutgeometry. Today’s NC-programs con-tains constant values for surface andspindle speed. Within one NC-blockthe CNC can only interpolate one con-stant value. This gives a “step-function”for the changes of feed rate and spind-le speed.

These quick and big alterations arealso creating fluctuating cutting forcesand bending of the cutting tool, which

P0, G0

CADgeometry principle

e.g. NURBS

CADgeometry principle

standard

P1, G1

P2, G2

P3, G3K

P1

Original contourLinearized tool path

Tolerance band

Chord error

P2

P3

Number of NC blocks/sec.

1/SizeTolerance band

Tool Pathvelocity

CAMmachine-independent

machine-specific

Postprocessor

CNC Controll

CAM machine-independent

CNC Control

Pol1

Pol3

Pol2

Vpathmax2

Control polygonP1

P2

P3

P4 P5 P6

P7

Vpathmax1Vpath

Vpath

P2 P3 P4 P5 P6 P7 Time

TimeVpathmax2

Vpathmax1

G1

G3

P1P0

P1P0

M


gives a big negative impact on the cut-ting conditions and the quality of theworkpiece.

These problems can however be sol-ved if NURBS-interpolation is appliedalso for technological commands. Sur-face and spindle speed can be pro-grammed with the help of NURBS,which give a very smooth and favoura-ble change of cutting conditions. Con-stant cutting conditions mean successi-vely changing loads on the cutting toolsand are as important as constant amountof stock to remove for each tool inHSM applications.

NURBS-technology represents a highdensity of NC-data compared to linearprogramming. One NURBS-block re-presents, at a given tolerance, a bignumber of conventional NC-blocks.This means that the problems with thehigh data communication capacity andthe necessity of short block cycle timeare solved to a big extent.

LOOK AHEAD FUNCTIONIn HSM applications the execution timeof a NC-block can sometimes be as lowas 1 ms. This is a much shorter timethan the reaction time of the differentmachine tool functions - mechanical,hydraulic and electronic.

In HSM it is absolutely essential tohave a look ahead function with muchbuilt in geometrical intelligence. If thereis only a conventional look ahead, thatcan read a few blocks in advance, theCNC has to slow down and drive theaxes at such low surface speed so thatall changes in the feed rate can be con-trolled. This makes of course no HSMapplications possible.

P0, G0

P2, G2P1, G1

P3, G3

P4, G4

P11, G11

P13, G13

P(u)

P12, G12

P21, G21

P31, G31

P41, G41

P(v)

P¡ = (P0, P1, P2, P3)G¡ = (G0, G1, G2, G3)K¡ = (K0, K1, K2, K3, K4, K5, K6, K7, K8)

P11, G12, P13…P21, G22, P23…P31, G32, P33…. . .. . .. . .

G11, G12, G13…G21, G22, G23…G31, G32, G33…. . .. . .. . .

K11, K12, K13…K21, K22, K23…K31, K32, K33…. . .. . .. . .

Pu,v =

Gu,v =

Ku,v =

• Dramatic changes of cutting conditions• Waste of machine productivity

NC-Blocks NC-Blocks

F6

F5

F4

F3

F2F1

S4

S3

S2

S1

Pro

gram

med

Sp

ind

le R

evol

utio

n S

NC-Blocks

Pro

gram

med

feed

rate

, FP

rogr

amm

ed

spin

dle

rev

olut

ion,

S

ConventionalProgamming

NURBS-basedProgramming andInterpolation

NC-Blocks

• High tool wear• Limited part quality

ConventionalProgamming

NURBS-basedProgramming andInterpolation

Pro

gram

med

feed

rat

e, F


An advanced look ahead function mustread and check hundreds of blocks inadvance in real time and identify/defi-ne those cases where the surface speedhas to be changed or where other actionsmust be taken.

An advanced look ahead analyses thegeometry during operation and opti-mises the surface speed according tochanges in the curvature. It also controlsthat the tool path is within the allowedtolerance band.

A look ahead function is a basic soft-ware function in all controls used forHSM. The design, the usefulness andversatility can differ much dependingon concept.

CHOICE OF HOLDING TOOLSJust as the CAD/CAM and machinecontrols, are important to get good ma-chining results and an optimized pro-duction, the holding/cutting tools areof equal importance.

One of the main criteria when choo-sing both holding and cutting tools is tohave as small run-out as possible. Thesmaller the run-out is, the more eventhe workload will be on each insert in amilling cutter. (Zero run-out would ofcourse theoretically give the best toollife and the best surface texture andfinish).

In HSM applications the size of run-outis specifically crucial. The TIR (TotalIndicator Readout) should be maxi-mum 10 microns at the cutting edge.A good rule of thumb is: “For each 10 microns in added run out- 50% less tool life!

Balancing adds some steps to the pro-cess and typically involves:• Measuring the unbalance of a tool/

toolholder assembly on a balancemachine.

• Reducing the unbalance by alteringthe tool, machining it to remove massor by moving counterweights in abalancable toolholder.

• Often the procedure has to be repe-ated, involving checking the tool again,refining the previous adjustmentsuntil the balance target is achieved.

Tool balancing leaves several sourcesof process instability untouched. Oneof these is error in the fit between tool-holder and spindle interface. The rea-son is that there is often a measurableplay in this clamp, and there may alsobe a chip or dirt inside the taper. Thetaper will not likely line up the same wayevery time. The presence of any suchcontamination would create unbalanceeven if the tool, toolholder, and spind-le were perfect in every other way.

To balance tools is an additional costto the machining process and it shouldbe analysed in each case if cost reduc-tion gained by balancing is motivated.But, some times there is no alternativeto get the required quality.

However, much can be done by justaiming for good balance through pro-per tool selection and here are somepoints to think of when selecting tools:• Buy quality tools and toolholders.

Look for toolholders that have beenpremachined to remove unbalance.

• Favour tools that are short and aslightweight as possible.

• Regularly inspect tools and toolhol-ders for fatigue cracks and signs ofdistortion.

The tool unbalance that the processcan accept is determined by aspects ofthe process itself. These include thecutting forces in the cut, the balancecondition of the machine, and the extentto which these two affect another. Trialand error is the best way to find theunbalance target. Run the intendedoperation several times to a variety ofdifferent values, for instance from 20gmm and down. After each run, upgra-de to a more balanced tool and repeat.The optimal balance is the point bey-ond which further improvements intool balance fail to improve the accu-racy or surface finish of the workpiece,or the point in which the process caneasily hold the specific workpiece tole-rances.

The key is to stay focused on the pro-cess and not aim for a G value or otherarbitrary balance target. The aim shouldbe to achieve the most effective processas possible. This involves weighing thecosts of the tool balancing and thebenefit it can deliver, and strike theright balance between them.

The upper pass on the component is machined with a machine control without sufficient look aheadfunction and it clearly shows that the corners have been cut, compared with the lower pass machinedwith sufficient look ahead function.


The aluminium workpiece on thepicture illustrates tool balancingaffecting surface finish. The balan-ceable toolholder used to machineboth halves of the surface were set totwo unbalance values, 100 gmm and1.4 g-mm. The more balanced toolproduced the smoother surface. Con-ditions of the two cuts were other-wise identical: 12000 rpm, 5486 mm/min feed rate, 3.5 mm depth- and 19mm width of cut, using a toolholderwith a combined mass of 1.49 kg.

Balancing tools to G-class targets,as defined by ISO 1940-1, may de-mand holding the force from unba-lance far less than the cutting forcethe machine will see anyway. In rea-lity, an endmill run at 20000 rpm maynot need to be balanced to any bet-ter than 20 gmm, and 5 gmm is gene-rally appropriate for much higherspeeds. The diagram refers to unba-lance force relating to tool and adap-ter weight of 1 kg. Field A showsthe approximate cutting force on a10 mm diameter solid endmill.

Angular error

Parallell error

9549 x Gu = m x (gmm)

n

F = u ( n )2 (N)9549

Parallel errorCoromant Capto C5 HSK 50 form A

Unbalance Up to 2.6 gmm up to 9.6 gmmBalance class up to G4.4 up to G16.8TIR up to 4.2 �m up to 16 �m

n = 20 000 rpm, weight of adapter and tool m = 1.2 kg

Influence of system accuracy on unbalance for different tool interface.

Angular errorCoromant Capto C5 HSK 50 form A

Unbalance Up to 0.9 gmm up to 3.3 gmmBalance class up to G1.5 up to G5.6TIR up to 3.5 �m up to 13.4 �m

The balance equations contain:F: force from unbalance (Newton)G: G-class value, which has units of mm/secm: tool mass in kgn: spindle speed in rpmu: unbalance in g mm


At high speed, the centrifugal forcemight be strong enough to make thespindle bore grow slightly. This has anegative effect on some V-flange toolswhich contact the spindle bore only inthe radial plane. Spindle growth cancause the tool to be drawn up into thespindle by the constant pull of the draw-bar. This can lead to a stuck tool ordimensional inaccuracy in the Z-axis.

Tools with contact both in the spindlebore and face, radial and axial contact,simultaneous fit tools are more suitedfor machining at high speeds. When thespindle begins to grow, the face contactprevents the tool from moving up thebore. Tools with hollow shank designare also susceptible to centrifugal forcebut they are designed to grow with thespindle bore at high speeds. The tool/spindle contact in both radial and axialdirection also gives a rigid tool clam-ping enabling aggressive machining.The Coromant Capto coupling is due

SURFACE CONTACT OF SPINDLE INTERFACE AT HIGH SPINDLE SPEED

Spindle speed ISO40 HSK 50A Coromant CaptoC5

0 100% 100% 100%20000 100% 95% 100%25000 37% 91% 99%30000 31% 83% 95%35000 26% 72% 91%40000 26% 67% 84%

to its polygon design superior when itcomes to high torque and productivemachining.

When planning for HSM one shouldstrive to build tools using a holder cut-ter combination that is symmetrical.There are some different tool systemswhich can be used. However, a shrinkfit system where the toolholder is heat-

ed up and the bore expands and thenclamps the tool when cooling down isconsidered to be one of the best andmost reliable for HSM. First because itprovides very low run-out, secondly thecoupling can transmit a high torque,thirdly it is easy to build customizedtools and tool assemblies and fourth, itgives high total stiffness in the assembly.

COMPARISON BETWEEN HOLDERS FOR CLAMPING OF SHAFT TOOLS

Weldon/whistle- Collet chuck Power chuck HydroGrip Shrink fit holder CoroGripnotch holder Din 6499 Hydraulic chuck Hydro-mechanical

chuck

Type of Heavy roughing- Roughing - Heavy roughing - Finishing Heavy roughing- Heavy roughing -operation semi finishing Semi finishing finishing finishing finishing

Transmission +++ ++ ++ + +++ +++torque

Accuracy 0.01 - 0.02 0.01 - 0.03 0.003 - 0.010 0.003 - 0.008 0.003 - 0.006 0.003 - 0.006TIR 4 x D [mm]

Suitable for + + ++ ++ +++ +++high speed

Maintenance None required Cleaning and Cleaning and None required None required None requiredchanging changing sparecollets parts

Possibility to No Yes Yes Yes No Yesuse collets


Taper contact surface

External taperFace contact surface

Face contact surface

Length of taper

Length of taper

Inner taper

Nominal taper angle

Manufactured angle of the taper

Nominal taper

Taper diameter tolerance area

Tolerance for roundness

Nominal taper

Cross section tolerance area for roundness

Taper interface angle

The roundness and concentricity are the mostcrucial factors for toolholders and not thetolerance class (AT).


D & M process planning

When machining dies and moulds, and in any machi-ning for that matter, the process has to be carefully

planned to utilize the most efficient method possible andachieve the best result. In this fourth article from SandvikCoromant regarding die and mould machining, the focuswill be shifted somewhat from the high speed machiningtrend to the more basic planning stage of the machiningprocess. Which of course applies to the HSM process as well.

AN OPEN-MINDED APPROACHThe larger the component, and themore complicated, the more importantthe process planning becomes. It is veryimportant to have an open-mindedapproach in terms of machining met-hods and cutting tools. In many cases itmight be very valuable to have an ex-ternal speaking partner who has expe-riences from many different applica-tion areas and can provide a differentperspective and offer some new ideas.

Being a tooling company we are pre-pared to offer all our expertise in holdingand cutting tools as well as in the cut-ting process in a partnership with theworld-wide Die & Mould industry...

AN OPEN-MINDED APPROACHTO THE CHOICE OF METHODS,TOOL PATHS, MILLING ANDHOLDING TOOLSIn today’s world it is a necessity to becompetitive in order to survive. One ofthe main instruments or tools for this iscomputerised production. For the Die& Mould industry it is a question ofinvesting in advanced production equ-ipment and CAD/CAM systems. Buteven if doing so it is of highest impor-tance to use the CAM-softwares totheir full potential.

In many cases the power of tradition inthe programming work is very strong.The traditional and easiest way to pro-gram tool paths for a cavity is to usethe old copy milling technique, withmany entrances and exits into the ma-terial. This technique is actually linkedto the old types of copy milling machi-nes with their stylus that followed themodel.

This often means that very versatile andpowerful softwares, machine and cut-ting tools are used in a very limited way.

Modern CAD/CAM-systems can beused in much better ways if old thin-

Metalworking World

The question that should be asked is,“Where is the cost per hour highest? Inthe process planning department, at aworkstation, or in the machine tool”?The answer is quite clear, as the machinecost per hour often is at least 2-3 timesthat of a workstation.

After getting familiar with the new wayof thinking/programming the program-ming work will also become more of aroutine and faster. If it still should takesomewhat longer time than program-ming the copy milling tool paths, it willbe made up by far in the following pro-duction. However, experience showsthat in the long run, a more advancedand favourable programming of the toolpaths can be done faster than with con-ventional programming.

THE RIGHT CHOICE OF HIGHLYPRODUCTIVE CUTTING TOOLSFOR ROUGHING TO FINISHING

First of all:• Study the geometry of the die ormould carefully.

• Define minimum radii demands andmaximum cavity depth.

• Estimate roughly the amount of ma-terial to be removed. It is important tounderstand that roughing and semi-finishing of a big sized die or mould isperformed far more efficiently and pro-ductively with conventional methodsand tooling. The finishing is alwaysmore productive with HSM. Also forbig sized dies and moulds. This is dueto the fact that the material removal ratein HSM is much lower than in conven-tional machining. With exception formachining of aluminium and non-fer-rous materials.

• The preparation (milled and parallelsurfaces) and the fixturing of the blankis of great importance. This is alwaysone classic source for vibrations. If per-forming HSM this point is extra impor-tant. When performing HSM or also inconventional machining with high de-mands on geometrical accuracy of thedie or mould, the strategy should alwaysbe to perform roughing, semi-finishing,

king, traditional tooling and produc-tion habits are abandoned.

If instead using new ways of thinkingand approaching an application, therewill be a lot of wins and savings in theend.

If using a programming technique inwhich the main ingredients are to “sliceoff” material with a constant Z-value,using contouring tool paths in combina-tion with down milling the result will be:

• a considerably shorter machining time• better machine and tool utilisation• improved geometrical quality of the

machined die or mould• less manual polishing and try out time

In combination with modern holdingand cutting tools it has been provenmany times that this concept can cutthe total production cost by half.

Initially a new and more detailed pro-gramming work is more difficult andusually takes somewhat longer time.

21

22

tool path when it comes to precision.Different persons use different pressu-res when doing stoning and polishing,resulting most often in too big dimen-sional deviations. It is also difficult tofind and recruit skilled, experiencedlabour in this field. If talking about HSMapplications it is absolutely possible,with an advanced and adapted pro-gramming strategy, dedicated machine

finishing and super-finishing in dedica-ted machines. The reasons for this arequite obvious - it is absolutely impossi-ble to keep a good geometrical accura-cy on a machine tool that is used for alltypes of operations and workloads.

The guide ways, ball screws andspindle bearings will be exposed tobigger stresses and workloads whenroughing for instance. This will ofcourse have a big impact on the sur-face finish and geometrical accuracyof the dies or moulds that are beingfinish machined in that machine tool.It will result in a need of more manualpolishing and longer try out times. Andif remembering that today’s targetshould be to reduce the manual polis-hing, then the strategy to use the samemachine tools for roughing to finishingpoints in totally wrong direction. Thenormal time to manually polish, for in-stance, a tool for a bonnet (big sized car)is roughly 400 hours.

If this time can be reduced by goodmachining it not only reduces the cost,but also enhance the geometrical accu-racy of the tool. A machine tool machi-nes pretty much exactly what it is pro-grammed for and therefore the geome-trical accuracy will be better the morethe die or mould can be machined.However, when there is extensive ma-

Metalworking World

nual finishing the geometrical accuracywill not be as good because of manyfactors such as how much pressure andthe method of polishing a person uses,just to mention two of them.

If adding, totally, some 50 hours onadvanced programming (minor part)and finishing in an accurate machinetool, the polishing can often be reduceddown to 100-150 hours, or sometimeseven less. There will also be other con-siderable benefits by machining to moreaccurate tolerances and surface struc-ture/finish. One is that the improvedgeometrical accuracy gives less try outtimes. Which means shorter lead times.Another is that, for instance, a pres-sing tool will get a longer tool life andthat the competitiveness will increasevia higher component quality. Which isof highest importance in today’s com-petition.

A human being can not compete, nomatter how skilled, with a computerised

23

THE VERSATILITY OF ROUND INSERT CUTTERS If the rough milling of a cavity is donewith a square shoulder cutter much stair-case shaped stock has to be removed insemi-finishing. This of course createsvarying cutting forces and tool deflec-tion. The result is an uneven stock forfinishing, which will influence the geo-metrical accuracy of the die or mould.

are usually first choice for all operations.But, it is definitely possible to competein productivity also by using insertedtools with specific properties. Such asround insert cutters, toroid cutters andballnose end mills. Each case has to beindividually analysed...

To reach maximum productivity it isalso important to adapt the size of themilling cutters and the inserts to a certaindie or mould and to each specific opera-tion. The main target is to create anevenly distributed working allowance(stock) for each tool and in each ope-ration. This means that it is most oftenmore favourable to use different dia-meters on cutters, from bigger to smal-ler, especially in roughing and semi-finishing. Instead of using only one dia-meter throughout each operation. Theambition should always be to come asclose as possible to the final shape ofthe die or mould in each operation.

An evenly distributed stock for eachtool will also guarantee a constant andhigh productivity. The cutting speedand feed rate will be on constant highlevels when the ae/ap is constant. Therewill be less mechanical variations andwork load on the cutting edge. Whichin turn gives less heat generation, fati-gue and an improved tool life.

A constant stock also enables for highercutting speed and feed together with avery secure cutting process. Some semi-finishing operations and practically allfinishing operations can be performedunmanned or partially manned. A con-stant stock is of course also one of thereal basic criterias for HSM.

Another positive effect of a constantstock is that the impact on the machinetool - guide ways, ball screws and spind-le bearings will be less negative. It isalso very important to adapt the sizeand type of milling cutters to the size ofthe machine tool.

tools and holding and cutting tools, toeliminate manual polishing even up to100%. If using the strategy to do roug-hing and finishing in separate machinesit can be a good solution to use fixturingplates. The die or mould can then be lo-cated in an accurate way. If doing 5-sidedmachining it is often necessary to usefixturing plates with clamping from be-neath. Both the plate and the blank mustbe located with cylindrical guide pins.

The machining process should be divi-ded into at least three operation types;roughing, semi-finishing and finishing,some times even super-finishing (mostlyHSM applications). Restmilling opera-tions are of course included in semi-finishing and finishing operations.

Each of these operations should beperformed with dedicated and optimi-sed cutting tool types.

In conventional die & mould making itgenerally means:

Roughing Round insert cutters,end mills w. big cornerradii

Semi-finishing Round insert cutters,toroid cutters, ball noseendmills

Finishing Round insert cutters(where possible), toroidcutters, ball nose end-mills (mainly)

Restmilling Ballnose endmills, end-mills, toroid and roundinsert cutters

In high speed machining applications itmay look the same. Especially for big-ger sized dies or moulds.

In smaller sizes, max 400 X 400 X 100(l,w,h), and in hardened tool steel, ballnose end mills (mainly solid carbide)

Metalworking World

If a square shoulder cutter with triang-ular inserts is used it will have relativelyweak corner cross sections, creating anunpredictable machining behaviour.Triangular or rhombic inserts also cre-ates big radial cutting forces and due tothe number of cutting edges they areless economical alternatives in theseoperations.

On the other hand if round inserts,which allows milling in all materialsand in all directions, are used this willgive smooth transitions between thepasses and also leaves less and moreeven stock for the semi-finishing. Re-sulting in a better die or mould quality.

Among the features of round inserts isthat they create a variable chip thick-ness. This allows for higher feed ratescompared with most other insert shapes.The cutting action of round inserts is alsovery smooth as the entering angle suc-cessively alters from nearly zero (very

Stock to beremoved

“Stair caseshaped” stock

shallow cuts) to 90 degrees. At maxi-mum depth of cut the entering angle is45 degrees and when copying with theperiphery the angle is 90 degrees. Thisalso explains the strength of round in-serts - the work-load is built up succes-sively.

Round inserts should always be regar-ded as first choice for roughing and me-dium roughing operations. In 5-axismachining round inserts fit in very welland have practically no limitations.

With good programming round insertcutters and toroid cutters can replaceball nose end mills to a very big extent.The productivity increase most oftenranges between 5-10 times (comparedwith ball nose end mills). Round insertcutters with small run-outs can in com-bination with ground, positive and lightcutting geometries also be used insemi-finishing and some finishing ope-rations. Ballnose endmills, on the other,hand can never be replaced in closesemi-finishing and finishing of complex3D (shapes) geometries.

In the next article in the Die & Mouldseries “Application technologies” willbe put in focus.

Square shouldercutter, 90°

Much material remaining after roughing

Stock to beremoved

Round insert cutter

Less material remaining after roughing

Combinationof milling directions

Smooth transitions-little stock

Metalworking World24


the feed rate as it is dependent on thespindle speed for a certain cutting speed.

If using the nominal diameter value ofthe tool, when calculating cutting speed,the effective or true cutting speed willbe much lower if the depth of cut isshallow. This is valid for tools such as,round insert cutters (especially in thesmall diameter range), ball nose endmills and end mills with big corner radii.

EFFECTIVE DIAMETER IN CUT This is very much a question aboutoptimising cutting data, grades and geo-metries in relation to the specific typeof material, operation and productivityand security demands.

It is always important to base calcula-tions of effective cutting speed on thetrue or effective diameter in cut. If not,there will be severe miscalculations of

In this fifth article about die and mould makingfrom Sandvik Coromant, application technology

will be in focus. Some basic, but none the less veryimportant parameters, will be discussed. Examplesare down milling, copy milling and the importanceof as little tool deflection as possible.

Application technology

The feed rate will of course also bemuch lower and the productivity seve-rely hampered.

Most important is that the cutting con-ditions for the tool will be well belowits capacity and recommended applica-tion range. This often leads to prema-ture frittering and chipping of the cut-ting edge due to too low cutting speedand heat in the cutting zone.

AVOID EXCESSIVE DEFLECTIONWhen doing finishing or super-finishingwith high cutting speed in hardenedtool steel it is important to choose toolsthat have a coating with high hot hard-ness. Such as TiAlN.

One main parameter to observe whenfinishing or super-finishing in harde-ned tool steel with HSM is to take shal-low cuts. The depth of cut should notexceed 0,2/0,2 mm (ap/ae). This is toavoid excessive deflection of the hol-ding/ cutting tool and to keep a hightolerance level and geometrical accu-racy on the machined die or mould.

Choose very stiff holding and cuttingtools. When using solid carbide it is im-portant to use tools with a maximumcore diameter (big bending stiffness).When using inserted ball nose end mills,for instance, it is favourable to usetools with shanks made of heavy metal(big bending stiffness). Especially ifthe ratio overhang/diameter if large.

1000

800

600

400

0TiAIN TiCN TiN Uncoated

ap/ae � 0,2


DOWN MILLING IS IMPORTANTAnother application parameter of im-portance is the use of down millingtool paths as much as possible. It is,nearly always, more favourable to dodown milling than up milling. When thecutting edge goes into cut in down mil-ling the chip thickness has its maximum

heat is generated as the cutting edge isexposed to a higher friction than indown milling. The radial forces are alsoconsiderably higher in up milling, whichaffects the spindle bearings negatively.

In down milling the cutting edge ismainly exposed to compressive stresses,which are much more favourable forthe properties of cemented or solid car-bide compared with the tensile stressesdeveloped in up milling.

When doing side milling (finishing)with solid carbide, especially in harde-ned materials, up milling is first choice.It is then easier to get a better toleranceon the straightness of the wall and alsoa better 90 degree corner. The mismatchbetween different axial passes will alsobe less, if none.

value. In up milling this is when it hasits minimum value. The tool life isgenerally shorter in up milling than indown-milling due to the fact that thereis considerably more heat generated inup-, than in down milling. When thechip thickness in up milling increasesfrom zero to maximum the excessive

Bending Roughing Finishing

Upmilling - 0.02 mm 0.00 mm

Downmilling 0.06 mm 0.05 mm

Roughing Finishing

Downmilling Upmilling

D U

VƒVƒ

Endmills with a higher helix angle have less radial forces and usually run smoother. Endmills with ahigher helix angle has more axial forces and the risk of being pulled out from the collet is greater.

Solid Carbide Endmills - Finishing/Deflection

Example based on zero degree entering angle.


a risk for vibration, deflection or eventool breakage if the feed speed does notdecelerate fast enough. There is alsoa risk of pulling the cutter from theholder due to the direction of the cut-ting forces.

The most critical area when using ballnose end mills is the centre portion.Here the cutting speed is zero, which isvery disadvantageous for the cuttingprocess. Chip evacuation in the centreis also more critical due to the smallspace at the chisel edge. Avoid usingthe centre portion of a ball nose endmill as much as possible. Tilt the spindleor the workpiece 10 to 15 degrees toget ideal cutting conditions. Sometimesthis also gives the possibility to useshorter (and other type of) tools.

If the spindle speed is limited in themachine, contouring will help to keepup the cutting speed. This type of toolpath also creates less quick changes inwork load and direction. This is of spe-cific importance in HSM applicationsand hardened materials as the cuttingspeed and feed are high and the cuttingedge and process is more vulnerable toany changes that can create differencesin deflection and create vibrations. Andultimately total tool breakdown.

This is mainly due to the direction ofthe cutting forces. With a very sharpcutting edge, the cutting forces tend to“pull” or “suck” the cutter towards thematerial.

Up- milling can be favourable whenhaving old manual milling machineswith large play in the lead screw, becausea “counter pressure” is created whichstabilizes the machining.

The best way to ensure down- millingtool paths in cavity milling is to use con-touring type of tool paths. Contouringwith the periphery of the milling cutter(for instance a ball nose end mill) oftenresults in a higher productivity, due tomore teeth effectively in cut on a largertool diameter.

COPY MILLING AND PLUNGINGCopy milling and plunging operationsalong steep walls should be avoided asmuch as possible! When plunging, thechip thickness is large at a low cuttingspeed. This means a risk of frittering atthe centre, especially when the cutterhits the bottom area. If the control hasno, or a poor, look ahead function thedeceleration will not be fast enoughand there will most likely be damageon the centre.

It is somewhat better for the cutting pro-cess to do up-copying along steep wallsas the chip thickness has its maximumat a more favourable cutting speed.

But, there will be a big contact lengthwhen the cutter hits the wall. This means

Large chip thickness at very low vc . Max chip thickness at recommended vc .

The tool-life will be considerably shor-ter if the tool has many entries andexits in the material. This adds theamount of thermal stresses and fatiguein the cutting edge. It is more favoura-ble for modern cemented carbide tohave an even and high temperature inthe cutting zone than having big fluc-tuations.

Copy milling tool paths are often a mixof up-, and down milling (zig-zag) andgives a lot of engagements and disen-gagements in cut. This is, as mentionedabove, not favourable for any millingcutter, but also harmful for the qualityof the die or mould. Each entrance

For a long tool-life, it is also morefavourable in a milling process to stayin cut continuously and as long as pos-sible. All milling operations have inter-rupted or intermittent character cutsdue to the usage of multi-teeth tools.

means that the tool will deflect andthere will be an elevated mark on thesurface. This is also valid when the toolexits. Then the cutting forces and thebending of the tool will decrease andthere will be a slight undercutting ofmaterial in the exit portion.

These factors also speak for contou-ring and down milling tool paths as thepreferred choice.

SCULPTURED SURFACESIn finishing and super-finishing,especially in HSM applications, thetarget is to reach a good geometri-cal and dimensional accuracy andreduce or even eliminate all manualpolishing.

In many cases it is favourable tochoose the feed per tooth, fz, identi-cal with the radial depth of cut, ae(fz = ae).

This gives the following advantages:

• very smooth surface finish in all directions

• very competitive, short machi-ning time

• very easy to polish, symmetricalsurface texture, self detectingcharacter via peaks and valleys

• increased accuracy and bearing resistance on surfacegives longer tool life on die or mould

• minimum cusp or scallop heightdecides values on fz/ae/R

If you have any questions regar-ding die & mould making, send ane-mail to: [email protected]


29

roughing and finishing steel and wherevibration tendencies are a threat to theresult of the operation.

Coarse pitch is the true problem solverand is the first choice for milling withlong overhang, low powered machinesor other applications where cuttingforces must be minimized.

(C) Extra-close pitch cutters have smallchip pockets and permits very hightable feeds. These cutters are suitablefor machining interrupted cast-ironsurfaces, roughing cast-iron and smalldepth of cut in steel. Also in materialswhere the cutting speed has to be keptlow, for instance in titanium. Extra closepitch is the first choice for cast iron.

The milling cutters can have eithereven or differential pitch. The latterone means unequal spacing of teethround the cutter and is a very effectivemeans of coming to terms with pro-blems of vibrations.

(A) Close pitch means more teeth andmoderate chip pockets and permitshigh metal-removal rate. Normallyused for cast-iron and for medium dutymachining operations in steel. Closepitch is the first choice for general pur-pose milling and is recommended formixed production.

CUTTER PITCHA milling cutter, being a multi-edgetool, can have a variable number ofteeth (z) and there are certain factorsthat help to determine the number forthe type of operation. The material andsize of workpiece, stability, finish andthe power available are the more ma-chine orientated factors while the toolrelated include sufficient feed per tooth,at least two cutting edges engaged incut simultaneously and that the chipcapacity of the tool is ample.

The pitch (u) of a milling cutter is thedistance between a point on the edge tothe same point on the next edge. Millingcutters are classified into coarse, closeor extra-close pitch cutters and mostcutters have these three options.

Knowing the processparameters

Metalworking World

In this article in the series about die and mouldmaking some basic factors of the milling

process will be discussed, as well as some troubleshooting hints. It is important to know basicmilling factors such as cutter pitch, entrance andexit of cut, positioning of the cutter, extended toolsand how these parameters influence the cuttingprocess in order to facilitate the understanding inupcoming articles.

(B) Coarse pitch means fewer teeth onthe cutter periphery and large chippockets. Coarse pitch is often used for

A

C

B

u

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ENTRANCE AND EXIT OF CUTEvery time a cutter goes into cut, theinserts are subjected to a large or smallshock load depending on material, chipcross section and the type of cut. Theinitial contact between the cutting edgeand workpiece may be very unfavoura-ble depending on where the edge ofthe insert has to take the first shock.Because of the wide variety of possibletypes of cut, only the effects of the cut-ter position on the cut will be conside-red here.

Where the centre of the cutter is posi-tioned outside the workpiece (D) anunfavourable contact between the edgeof the insert and the workpiece results.

Where the centre of the cutter is posi-tioned inside the workpiece (E) themost favourable type of cut results.

The most dangerous situation howe-ver, is when the insert goes out of cutleaving the contact with the workpie-ce. The cemented carbide inserts aremade to withstand compressive stres-ses which occur every time an insertgoes into cut (down milling). On theother hand, when an insert leaves theworkpiece when hard in cut (up mil-ling) it will be affected by tensile stres-ses, which are destructive for the insertwhich has low strength against thistype of stress. The result will often endin rapid insert failure.

seats, the inserts sitting in the seatswhich are not being in cut can beground down and allowed to remain inthe cutter as dummy inserts.

POSITIONING AND LENGTH OF CUTThe length of cut is affected by thepositioning of the milling cutter. Tool-life is often related to the length of cutwhich the cutting edge must undergo.A milling cutter which is positioned inthe centre of the workpiece gives ashorter length of cut, while the arc whichis in cut (�) will be longer if the cutteris moved away from the centre line (B)in either direction.

Bearing in mind how the cutting forcesact, a compromise must be reached.The direction of the radial cutting for-ces (A) will vary when the insert edgesgo into and out of cut and play in themachine spindle can give rise to vibra-tion and lead to insert breakage.

By moving the milling cutter off thecentre, B and C, a more constant andfavourable direction of the cutting for-ces will be obtained. With the cutterpositioned close to the centre line thelargest average chip thickness is obtai-ned. With a large facemill it can beadvantageous to move it more off cen-tre. In general, when facemilling, thecutter diameter should be 20-25% lar-ger than the cutting width.

When there is a problem with vibra-tion it is recommended that a millingcutter with as coarse pitch as possibleis used, so that fewer inserts give lessopportunities for vibration to arise.You can also remove every second in-sert in the milling cutter so that thereare fewer inserts in cut. In full slot mil-ling you can take out so many of theinserts that only two remain. However,this means that the cutter being usedmust have an even number of teeth, 4,6, 8, 10 etc. With only two inserts in themilling cutter, the feed per tooth canbe increased and the depth of cut canusually be increased several times. Thesurface finish will also be very good. Asurface finish of Ra 0.24 in hardenedsteel with a hardness of 300 HB hasbeen measured after machining with amilling cutter with an overhang of 500mm. In order to protect the insert

Metalworking World

E D

31

of the die or mould decides where tochange.

Cutting data should also be adapted toeach tool length to keep up maximumproductivity.

When the total tool length, from thegauge line to the lowest point on thecutting edge, exceeds 4-5 times diame-ter at the gauge line, tuned, tapered barsshould be used. Or, if the bending stiff-ness must be radically increased, ex-tensions made of heavy metal shouldbe used.

When using extended tools it is impor-tant to choose biggest possible diame-ter on the extensions and adaptersrelatively to the cutter diameter. Everymillimetre is important for maximumrigidity, stiffness and productivity. It isnot necessary to have more than 1 mmradially in difference between holdingand cutting tool. The easiest way toachieve this is to use oversized cutters.

Modular tools increases the flexibilityand the number of tool combination possibilities.

EXTENDED TOOLS IN ROUGHING OF A CAVITYTo maintain maximum productivitywhen roughing a cavity it is importantto choose a series of extensions for thecutter. It is a very bad compromise to

Metalworking World

start with the longest extension, as theproductivity will be very low.

It is recommendable to change to ex-tended tools at pre-determined posi-tions in the program. The geometry


TROUBLE SHOOTING

The basic action to be taken when there is a problem with vibration is to reduce thecutting forces. This can be done by using the correct tools and cutting data.

Choose milling cutters with coarse and differential pitch.

Use positive insert geometries.

Use as small milling cutter as possible. This is particularly important when milling withtuned adapters.

Small edge rounding (ER). Go from a thick coating to a thin one, if necessary use uncoatedinserts.

Use a large feed per tooth, reduce the rotational speed and maintain the table feed (= larger feed per tooth). Or maintain the rotational speed and increase the table feed (= larger feed per tooth). Do not reduce the feed per tooth!

Reduce the radial and axial cutting depths.

Choose a stable tool holder. Use the largest possible adapter size to achieve the beststability. Use tapered extensions for best rigidity.

With long overhangs, use tuned adapters in combination with coarse and differentialpitched cutters. Position the milling cutter as close to the tuned adapter as possible.

Position the milling cutter off centre of the workpiece, which leads to a more favourabledirection of the cutting forces.

Start with normal feed and cutting speed. If vibrations arises try introducing thesemeasures gradually, as previously described:

a) increase the feed and keep the same rpm

b) decrease the rpm and keep the same feed

c) reduce the axial or/and radial depth of cut

d) try to reposition the cutter


Axially weak workpiece

Establish the direction of the cutting forces and position the materialaccordingly.

Try to improve the clamping generally.

Reduce the cutting forces by reducing the radial and axial cutting depth.

Choose a milling cutter with a coarse pitch and positive design.

Choose positive inserts with small corner radius and small parallel lands.

Where possible, choose an insert grade with a thin coating and sharpcutting edge. If, necessary, choose an uncoated insert grade.

Avoid machining where the workpiece has poor support against cuttingforces.

The first choice is a square shoulder facemill with positive insets.

Choose an insert geometry with sharp cutting edge and a large clearanceangle, which produces low cutting forces.

Try to reduce the axial cutting forces by reducing the axial depth of cut,as well as using positive inserts with a small corner radius, small parallellands and sharp cutting edges.

Always use a coarse and differentially pitched milling cutter.

Balance the cutting forces axially and radially. Use a 45-degree enteringangle, large corner radius or round inserts.

Use inserts with a light cutting geometry.

Try to reduce the overhang, every millimetre counts.

Choose the smallest possible milling cutter diameter in order to obtain themost favourable entering angle. The smaller diameter the milling cutterhas the smaller the radial cutting forces will be.

Choose positive and light cutting geometries.

Try up milling.

Try up milling.

Look at the possibility of adjusting the prestress of the washer to the ball-screw (CNC). Adjust the lock nut or exchange the screw on conventionalmachines.

Cause

Poor clamping of the workpiece

Action

Uneven table feed

Large overhang either on themachine spindle or the tool

Square shoulder milling with aradially weak machine spindle

34

any definite stop at block borders.Which means that the movement givessmooth continuous transitions and thereis only a small chance that a vibrationshould start.

• Another solution is to produce a big-ger corner radius, via circular interpo-lation, than stated in the drawing. Thiscan be favourable sometimes as itallows to use a bigger cutter diameterin roughing to keep up maximum pro-ductivity.

In traditional machining of corners thetool radius is identical with the cornerradius. Which gives maximum contactlength and deflection (often one qua-drant).

The most typical result is vibrations,the bigger the longer the tool, or totaltool overhang is. The wobbling cuttingforces often also creates undercuttingof the corner. There is of course also arisk for frittering of edges or total toolbreak down.

METHODS FOR MACHINING OF CORNERSThe traditional way of machining acorner is to use linear movements (G1)with non-continuous transitions in thecorner. Which means that when thecutter comes to the corner it has to beslowed down because of dynamic limi-tations of the linear axes. And therewill even be a very short stop before themotors can change the feed direction.As the spindle speed is the same, thesituation creates a lot of excessive fric-

Effective machining of corners & cavities

Metalworking World

This is the last article in this series about die andmould making from Sandvik Coromant. In

this article the most efficient way to machine cor-ners are discussed as well as different methods formachining of cavities. Finally the advantages ofmachining in segments is also discussed.

Some solutions on this problem are:

• Use a cutter with a smaller radius toproduce the desired corner radius onthe die or mould. Use circular interpo-lation (G2, G3) to produce the corner.This movement type does not create

• The remaining stock in the cornercan then be machined via restmilling(rest = remaining stock) with a smallercutter radius and circular interpola-tion. The restmilling of corners can alsobe performed by axial milling. It is im-

tion and heat. If for instance alumini-um or other light alloys are machinedthey can get burning marks or evenstart to burn due to this heat. The sur-face finish will deteriorate optically andin some materials even structurally,even beyond the tolerance demands.

Stockto remove

Ø8

R10

R4

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13 degrees. Whilst an 80 mm cuttermanages 3.5 degrees. The amount ofclearance also depends upon the dia-meter of the cutter.

Often used within die & mould makingis when the tool is fed in a spiral sha-ped path in the axial direction of thespindle, while the workpiece is fixed.This is most common when boring andhave several advantages when machi-ning holes with large diameters. Firstof all the large diameter can be machi-ned with one and the same tool, se-condly chip breaking and evacuationis usually not a problem when machi-ning this way, much because of the

portant to use a good programmingtechnique with a smooth approach andexit. It is very important to perform therestmilling of corners before or as asemi-finishing operation - gives evenstock and high productivity in finishing.If the cavity is deep (long overhang)the ap/ae should be kept low to avoiddeflection and vibration (ap/ae appr.0,1-0,2 mm in HSM applications inhardened tool steel).

If consequently using a programmingtechnique based on circular interpola-tion (or NURBS-interpolation), whichgives both continous tool paths andcommands of feed and speed rates, it ispossible to drive the mechanic functionsof a machine tool to much higher speeds,accelerations and decelerations.

This can result in productivity gainsranging between 20-50%!

RAMPING AND CIRCULAR INTERPOLATIONAxial feed capability is an advantagein many operations. Holes, cavities aswell as contours can be efficiently ma-chined. Facemilling cutters with roundinserts are strong and have big clearan-ce to the cutter body.

Metalworking World

Those lend themselves to drill/milloperations of various kinds. Rampingat high feed rates and the ability toreach far into workpieces make roundinsert cutters a good tool for complica-ted forms. For instance, profile millingin five-axis machines and roughing inthree-axis machines.

Ramping is an efficient way to appro-ach the workpiece when machiningpockets and for larger holes circularinterpolation is much more power effi-cient and flexible than using a largeboring tool. Problems with chip controlare often eliminated as well.

When ramping, the operation shouldbe started around the centre, machiningoutwards in the cavity to facilitate chipevacuation and clearance. As millingcutters has limitations in the axial depthof cut and varies depending on the dia-meter, the ramping angle for differentsizes of cutters should be checked.

The ramping angle is dependent uponthe diameter of the cutters used, clea-rance to the cutter body, insert size anddepth of cut. A 32 mm CoroMill 200cutter with 12 mm inserts and a cuttingdepth of 6 mm can ramp at an angle of

smaller diameter of the tool comparedto the diameter of the hole to be ma-chined and third, the risk of vibrationis small.

It is recommended that the diameterof the hole to be machined is twice thediameter of the cutter. Remember tocheck maximum ramping angle for thecutter when using circular interpola-tion as well.

These methods are favourable for weakmachine spindles and when using longoverhangs, since the cutting forces aremainly in the axial direction.

MACHINING IN SEGMENTSWhen machining huge press dies it isoften necessary to index the insertsseveral times. Instead of doing thismanually and interrupting the cuttingprocess, this can be done in an organi-sed way if precautions are taken in theprocess planning and programming.

Based on experience, or other infor-mation, the amount of material, or thesurface to machine, can be split up inportions or segments. The segments,or several segments, can be chosenaccording to natural boundaries or bebased on certain radii sizes in the die ormould. What is important is that eachsegment can be machined with one setof insert edges or solid carbide edges,plus a safety margin, before beingchanged to next tool in that specificfamily of replacement tools.

This technique enables full usage ofthe ATC (Automatic Tool Changer)and replacement tools (sister tools).

The technique can be used for roug-hing to finishing. Today’s touch probesor laser measuring equipment givesvery precise measuring of tool diame-ter and length and a matching (of sur-faces) lower than 10 microns. It alsogives several benefits such as:

• Better machine tool utilisation- lessinterruptions, less manual toolchanging

• Higher productivity-easier to optimisecutting data

• Better cost efficiency-optimisation vsreal machine tool cost per hour

• Higher die or mould geometricalaccuracy-the finishing tools can bechanged before getting excessive wear

METHODS FOR MACHINING OF A CAVITY

A. Pre-drilling of a starting hole. Corners canbe pre-drilled as well. Not recommendablemethod as one extra tool is needed. Whichalso adds more unproductive positioning andtool changing time. The extra tool also blocksone position in the tool magazine. From acutting point of view the variations in cuttingforces and temperature when the cutter breaksthrough the pre-drilled holes in the corners isnegative. The re-cutting of chips also increa-ses when using pre-drilled holes.

B. If using a ball nose end mill, inserted orsolid carbide, it is common to use a peck-dril-ling cycle to reach a full axial depth of cutand then mill the first layer of the cavity. Thisis then repeated until the cavity is finished.The drawback with this start is chip evacua-tion problems in the centre of the end mill.Better than using a peck-drilling cycle is toreach the full axial depth of cut via circularinterpolation in helix. Important also then tohelp evacuate the chips.

C. One of the best methods is to do linearramping in X/Y and Z to reach a full axialdepth of cut. Note that if choosing the rightstarting point, there will be no need of millingaway stock from the ramping part. The ram-ping can start from in to out or from out to independing on the geometry of the die ormould. The main criteria is how to get rid ofthe chips in the best way. Down milling shouldbe practised in a continuous cutting. Whentaking a new radial depth of cut it is impor-tant to approach with a ramping movementor, better, with a smooth circular interpola-tion. In HSM applications this is crucial.

D. If using round insert cutters or end millswith a ramping capacity the most favourablemethod is to take the first axial depth of cutvia circular interpolation in helix and followthe advice given in the previous point.

C-5000:3299911

Printed in SwedenCMSE/Idéreklam/Sjöströms/Sandvikens Tryckeri

Peck-drilling cycle witha short delay betweeneach down-feed toevacuate chips.

Requireddepth ofcut formachiningthe firstlayer.

A

B

C

D

high speed machining and conventional die and mould machining

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