machining shop theory

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Mechanical Engineering 203 Machine Shop Practice

SECTION 921, MAY 1-4, 2000, 8:30-4:30

SECTION 922, MAY 8-12, 2000, 8:30-4:30

FIRST LECTURE: RH 102, 8:30 AM

INSTRUCTOR: E.A. Croft

822-6614, [email protected]

YOU ARE EXPECTED TO READ THESE NOTES BEFORE COMING TO THE

FIRST CLASS.

PLEASE NOTE THE SAFETY REQUIREMENTS ESPECIALLY IN TERMS OFAPPROPRIATE CLOTHING WORN IN THE MACHINE SHOP. SPECIFICALLY:

• SHOES MUST BE WORN. NO OPEN TOED SHOES

• LONG HAIR MUST BE TIED BACK OR COVERED

• NO LOOSE CLOTHING OR JEWELRY

YOU MUST SIGN AND HAND IN THE FORM ON PAGE 3 AT THE BEGINNING

OF THE COURSE.

THE QUIZ ON PAGE 20 OF THE NOTES IS DUE ON THE FIRST DAY OF CLASS.



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The University of British Columbia

Department of Mechanical Engineering

STUDENT MACHINE SHOP REGULATIONS

IN THE INTEREST OF SAFETY AND EFFICIENCY, STUDENTS USING THE MECHANICALENGINEERING MACHINE SHOP MUST OBSERVE THE FOLLOWING RULES. FAILURE TO DOSO MAY LEAD TO A SUSPENSION OF MACHINE SHOP PRIVILEGES.

1. GENERAL REQUIREMENTSThe student must present proof of current accident insurance to the Machine Shop Supervisor. This insurance coveragecan be arranged through the Mech Office at a cost of $7. Permission will then be given to work in the Student Area of theMachine Shop on the West Side of the room.

2. TOOLSONLY THE TOOLS IN THE STUDENT WORKSHOP AREA ARE AVAILABLE FOR STUDENT USE. A basic tool set is

available in the Student Area. UNDER NO CIRCUMSTANCES MAY THESE TOOLS BE TAKEN FROM THE AREA. Iftools are required for use elsewhere in the Department they may be signed-out from Stores at the following times:

Mon. Wed. Fri. 8:30 - 10:00 a.m. 3:00 - 4:00 p.m.Tues. and Thurs. Stores Closed.

TOOLS FROM ANY OTHERWORK AREA OR WORKBENCH IN THE MACHINE SHOP MUST NOT BE USED. Most ofthese tools are the personal property of technicians.

3. MATERIALSPermission must be obtained from the Machine Shop Supervisor before removing from stock any material required for aproject. All material must be booked against a designated project.

4. SAFETYThe following regulations apply when working in the Machine Shop, and must be adhered to at all times:

a) Safety Glasses (available for purchase at Mech. Stores at a cost of $3) MUST be worn;b) Shoes must be worn - no open toed shoes;c) Long hair must be tied back or covered;d) No loose clothing.

Do not attempt to operate any piece of equipment with which you are not totally familiar.

Do not disturb technicians when they are working on machines. See the Machine Shop Supervisor if you have questions.

Safety electrical cut-outs are located on both sides of the Machine Shop. FAMILIARIZE YOURSELF WITH THEIRLOCATION. A lock-out system is in place (see Machine Shop notice board).

First Aid kits are available - one beside the north exit door; one in the lunch room. Familiarize yourself with the location ofthese first aid kits.

Machines and work areas must be cleaned before leaving, and when a job is finished (IHSR #8.50; 8.54). Tools andequipment MUST be returned to their designated location.

5. NO FOOD OR BEVERAGES are to be brought into, or consumed in the Machine Shop.



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The University of British Columbia

Department of Mechanical Engineering

The following procedures must be observed when using the Department of Mechanical

Engineering machine shop facilities:

- Power tools must not be used unless there is at least one other person in the machine shop;

- A technician must be consulted before using any tool with which you are not familiar;

- Machines and benches must be cleaned after use and tools returned to their designated

location;

- Any observed damage or malfunction of equipment must be reported to the Machine Shop

Supervisor.

I am fully aware of operating and safety procedures that must be observed when using machine andhand tools.

I have read and am aware of the machine shop regulations, as posted.

Name: (print) Signed:

Faculty Advisor: Date:



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Mechanical Engineering 203 Machine Shop PracticeI.Yellowley, May 1998

revised E. Croft, April 2000

Introduction

These notes are intended as a very brief introduction to the course. Attached to the notes

you will find a set of notes on Machine Shop Safety. YOU ARE EXPECTED TO READ

THESE NOTES AND COMPLETE THE PROBLEM SET ON THE LAST PAGE. THE

PROBLEM SET IS DUE ON THE FIRST DAY OF CLASS.

NOTE: the machine shop environment houses many potential hazards, you must

follow the safety rules and you should not attempt to operate or adjust any equipment

without having received instruction and understood the correct procedures. Failure tocomply with the safety rules will result in immediate termination and a failing grade.

CAREFULLY REVIEW THE FOLLOWING SHOP DRAWINGS FOR A TAP

WRENCH HANDLE. YOU WILL MAKE THIS ITEM DURING THE COURSE.



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I. Basic Geometry of Common Tools

The basic geometry of all tools is defined with respect to the relative work/tool velocity,

the first thing one does in examining the geometry of a cutting tool is to orient the tool

with respect to this velocity. Figure (1) shows the basic geometry of the cutting

operation. Figure (2) shows a simple turning tool with primary and secondary cutting

edges separated by a nose radius. The important planes as well as the directions for

measurement of rake angle are identified within Figure (2)

Figure (1) Basic Geometry of the cutting operation.

Rake Angles are measured between the rake face and a plane perpendicular to the cutting

velocity. Two orthogonal values are used to characterise the rake face. US practice uses

back rake and side rake, (related to shank). The rest of the world uses the more sensible

directions shown, (related to the edge). Clearance angles are measured between clearance

planes and the cutting velocity, (normally +5 to +10 degrees).



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Figure (2) Basic Geometry of a single point cutting tool.



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Figure (3) Typical cemented carbide tipped tools for lathe operations.

Figure (4) How surfaces are generated.

Figure (5) Typical Turning Operations and Tools.



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II. Basic Geometry and Terminology of Cutting Processes

While tool geometry is fairly standard, process terminology is somewhat different for

each operation. This section will consider the most common processes of turning :end

milling.

a) Turning

The turning process usually refers to the process of cutting a rotating workpiece with a

single point stationary tool, (there are exceptions however these are relatively minor).

The machine tool used for turning is called a lathe, those which are designed for large

pieces, held in the horizontal plane are termed vertical turning lathes.

A set of typical plan geometries and associated applications are shown in Figure (5), the

basic variables are shown in Figure (6), finally the detailed geometry of a representative

operation is shown in Figure (7).

Figure (6) Basic Plan View of Turning

Cutting Velocity perpendicular to page.

V={(RPM)*π*(Dia)/60}} m/s

NOTE: However, V is usually specified as m/min OR

ft/min!!!

Feed Direction

Feed Velocity Given by product of feed per revolution and

RPM of the workpiece. .

Vfeed=s*(RPM), either in mm/min or inch/min

The usual units for feed per rev are mm or inches.



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Figure (7) Detailed Geometry of the Turning Process

The detailed tool geometry is of great importance in a production environment. The

combination of approach angle and trail angle determine the range of applications which

a particular tool is capable of. While a decreasing point angle gives greater flexibility,this is at the expense of tool strength. The most common point angles are between 90

degrees and 60 degrees, (square and triangular throw away insert tooling). The tool nose

radius and the trail angle influence surface finish, (in the absence of a nose radius then the

approach angle also has an influence). A typical example of a machined surface finish

profile is shown in Figure (8). The approach angle and the nose radius are also used to

"thin" the chip allowing higher feedrates to be achieved before tool breakage occurs. (In

theory a change in approach angle from 0 degrees to 45 degrees would allow an increase

in feedrate of approximately 40% in those cases where the depth is considerably greater

than the nose radius, in practice other factors such as " work push off " and stability,

("chatter"), mitigate against the use of high approach angles for multipass roughing).



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Figure (8), Surface Produced during Machining



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b)Milling

There are a very large variety of milling cutter types and associated operations, Figure

(9) shows a group of typical operations and cutters. The nomenclature of milling cutters

is, to say the least, inconsistent if not downright confusing. You should not worry to

much about mastering the terminology but understand the major applications of face

mills, end mills and slot drills , (a slot drill has at least one tooth which cuts to centre

and hence is capable of an axial plunge cut ... do not try this with an end mil!!!). The

basic geometry of the milling process is more complex than most others. The cutting

edge must enter and leave the work, and of course the chip thickness in cut is variable.

The actual path traced out by each tooth with respect to the workpiece is trochoidal.

Luckily the peripheral velocity, (the cutting speed), is much larger than the centreline

velocity of the cutter, (the feeding speed). Given that the cutting speed is 2 orders of

magnitude higher than the feed velocity, one may assume that each individual path is of

a circular shape and is translated by the feed per tooth from the path traveled by the

previous tooth. The detailed geometry of the cut is shown in Figure (9) where it is seen

that the instantaneous chip thickness, (equivalent to the feed/tooth in turning), is

sinusoidal. Most HSS (High Speed Steel) milling cutters and almost all slab mills or

end mills utilise a helix angle to attempt to minimise the dynamic force components; in

terms of cut geometry, the helix corresponds to an angle of obliquity and also serves to

direct the chip away from the surface cut by the face of the tool.

The milling process has many more variables than turning, the major decisions to be

made now are the selection of axial depth of cut, feed per tooth, cutting velocity, radial

width of cut and the mode of milling. There are two modes of milling as shown in

Figure (11), namely Up Cut, (Conventional), and Down Cut, (Climb). Figure (11)

shows the force components acting upon a single tooth, it should be evident that, for a

single tooth in contact, there is a obvious possibility of a force reversal in the feed

direction for down milling and in the normal direction for up milling. The force reversal

in the feed direction for down milling may cause trouble if there is backlash in the

power screw driving the machine table, (the table will be moved bodily by the amount of

the backlash). The normal result of the table motion mentioned above is tool breakage,

and perhaps component damage, as well as danger to the operator. Down milling is

only, (commonly), used on machine tools with backlash elimination, (power screws), or

on servo driven mills which utilize preloaded ball screws. The majority of milling on

conventional machine tools is then carried out using an upcut or conventional action,

there are some advantages to down milling, principally that it produces a better surface

finish, it does however create more tool push off force, (normal to surface), so accuracy

is generally compromised relative to up milling. Finally there can be impact problems

with down milling, and some materials make up milling almost impossible, (soft

austenitic stainless steels or nickel base alloys tend to produce chips which adhere to the

tooth on exit and cause destruction of the edge on reentry).



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Figure (9) Representation of Chip thickness as the Normal Distance between Two

Circular Paths

Figure (10), Typical Milling Operations



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Figure (11) Milling Operations and Force Directions

III. The Selection of Machining Conditions

a) Turning

The normal method of selecting turning conditions in a production environment is as

follows:

a) Select the highest allowable depth of cut, (chatter and tool geometry are the

limiting constraints).

b) Select the maximum feedrate, (tool forces, torque, surface finish and tool breakage

are the limiting constraints).

c) Select a cutting speed which leads to a reasonable tool life, (check power

constraint). Normally a handbook value is used to start, this is adjusted as parts are

produced and the actual values become apparent. One should also realise that

velocities which are too low will result in the formation of built up edge which

causes both a poor, (dull), surface finish and accelerated tool wear/breakage.

For the sake of convenience, a couple of tables for turning are included, (one for carbon

steel, (25O BHN-Brinell Hardness Number), the other for brass). NOTE these tables take

account of the rather flexible nature of the components and the variation in size of lathe,

you may be using. Production rates using carbide tooling, rigid workpieces and heavy

duty machine tools would be up to 10 times higher.



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b)Milling

The selection process here is very similar to that in turning, however one must be aware

that when using small diameter milling cutters, (as will be the case here), then the

avoidance of tool shank breakage is a major issue.

a) Select a reasonable cutter diameter for the radial width to be cut. Usually in

production one prefers to have a cutter diameter which is comparable to the intended

radial width since it provides a much lower cost process, in low volume/job shop

environments this is not critical.

b) Maximise depth of cut, again chatter is a major constraint however, in this case,

even before selecting feed per tooth some reasonable data must be consulted to avoid

breakage.

c) Maximise feed, (breakage is the major constraint, however finish and, accuracy

may also need to be considered).

d) Select a reasonable starting value of spindle speed, the major constraint is spindle

power.



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IV. Tables of Recommended Machining Conditions

Introduction

Approximate values of the machining conditions are given for the processes already

introduced, (turning and milling), a very brief table is also included for standard twist

drills, (see Figure (9), for the basic geometry of these tools). Given the machine tools

available, all data are given in Imperial units!

NOTE MACHINING TABLES WHICH FOLLOW ARE APPROXIMATE YOU

SHOULD START MACHINING CONSERVATIVELY, (CERTAINLY IN THE

CASE OF FEEDRATES), THEN EXPERIMENT TO SEE WHETHER THE

VALUES GIVEN HERE ARE REASONABLE

1.)Machining Conditions, Turning of Carbon Steel, HSS Tools

Cutting Speed

(ft/min)

Feed per Rev.

(inch)

Depth of Cut

(inch)

Roughing 85 0.006 to 0.014 0.05 to 0.125

Finishing 1100 0.002 to 0.006 0.015 to 0.05

NOTE: Reduce cutting speed by a factor of 3 for threading.

Interrupted cuts will also require lower speeds, (a factor of 2 is a reasonable first

estimate)

2.)Machining Conditions, Turning of Brass, HSS Tools

Cutting Speed

(ft/min)

Feed per Rev.

(inch)

Depth of Cut

(inch)

Roughing 1 00 0.006 to 0.017 0.05 to 0.125

Finishing 120 0.002 to 0.006 0.01 to 0.05

NOTE: Reduce Cutting Speed by a factor of 3 for threading Interrupted cuts will

also require lower speeds, (a factor of 2 is a reasonable first estimate)



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3.)Machining Conditions, Low Carbon Steel, Milling with HSS End Mills

Cutter Diameter, (inch) (1/4) (1/2) (1)

Feed per Tooth, (inch) 0.001 to

0.004

0.002 to

0.006

0.003 to

0.008

Axial Depth of Cut, (inch), (MAX) 0.150 0.375 0.500

Rotational Speed, (RPM) 1200 750 400

Feeding Speed, (inch/min)=

RPM*(number-of-teeth)*(feed-per-tooth)

?? ?? ??

4.)Machining Conditions, Brass, Milling with HSS End Mills

Cutter Diameter, (inch) (1/4) (1/2) (1)

Feed per Tooth, (inch) 0.002 to

0.006

0.003 to

0.007

0.003 to

0.012Axial Depth of Cut, (inch), (MAX) 0.150 0.375 0.500

Rotational Speed, (RPM) 1300 850 500

Feeding Speed, (inch/min)=

RPM*(number-of-teeth)*(feed-per-tooth)

?? ?? ??

5.)Machining Conditions, Drilling with Standard Chisel Point Drills

Peripheral Cutting Speed, carbon steel, 80ft/min, (all diameters).

Peripheral Cutting Speed, brass, 100ft/min, all diameters

Feedrates for both work materials as follows:

Diameter, (inch) Feed/Rev, (inch)

0.5>Dia>0.375 0.006 to 0.010

0.375>Dia>0.188 0.0025 to 0.006

0.188>Dia>0.031 0.001 to 0.0025

Figure (9) Basic Drill Geometry



V. Quiz

1) On what machine tool is the turning process done?

2) A steel shaft, with a diameter of 1/2 inch is being turned at 90 ft/min. What is the

cutting velocity? If the feed per revolution is 0.010 inch, what is the depth of cut?

3) What is the difference between up-milling and down-milling. Which process isnormally used. Under what conditions is down-milling used?

4) Why does a higher approach angle allow higher feed-rates?

5) What distinguishes a slot drill from a end mill? What is this feature used for?

6) How much should one reduce cutting speed for threading?

7) You are milling a steel block, using a 1/2 inch HSS End Mill with 4 teeth. The millingmachine has the following available Rotational Speeds: 400, 600, 800, 1000, 1200.What rotational speed should you use? What feed speed should you use (a) Whenyou initiate cutting. (b) after cutting for some time.

machining shop theory

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