ch. 26 – abrasive machining and finishing operations brenton elisberg, jacob hunner, michael...

Ch. 26 – Abrasive Machining and Finishing Operations

Brenton Elisberg, Jacob Hunner, Michael Snider, Michael Anderson

Abrasive Machining and Finishing Operations

• There are many situations where the processes of manufacturing we’ve learned about cannot produce the required dimensional accuracy and/or surface finish.

– Fine finishes on ball/roller bearings, pistons, valves, gears, cams, etc.

– The best methods for producing such accuracy and finishes involve abrasive machining.

Abrasives and Bonded Abrasives

• An abrasive is a small, hard particle having sharp edges and an irregular shape.

• Abrasives are capable of removing small amounts of material through a cutting process that produces tiny chips.

Abrasives and Bonded Abrasives

• Commonly used abrasives in abrasive machining are:– Conventional Abrasives

• Aluminum Oxide• Silicon Carbide

– Superabrasives • Cubic boron nitride • Diamond

Friability

• Characteristic of abrasives.

• Defined as the ability of abrasive grains to fracture into smaller pieces, essential to maintaining sharpness of abrasive during use.

• High friable abrasive grains fragment more under grinding forces, low friable abrasive grains fragment less.

Abrasive Types

• Abrasives commonly found in nature include:– Emery – Corundum– Quartz– Garnet– Diamond

Abrasive Types

• Synthetically created abrasives include:– Aluminum oxide (1893)– Seeded gel (1987)– Silicon carbide (1891)– Cubic-boron nitride

(1970’s)– Synthetic diamond

(1955)

Abrasive Grain Size

• Abrasives are usually much smaller than the cutting tools in manufacturing processes.

• Size of abrasive grain measured by grit number.– Smaller grain size, the larger the grit number.– Ex: with sandpaper 10 is very coarse, 100 is

fine, and 500 is very fine grain.

Grinding Wheels

• Large amounts can be removed when many grains act together. This is done by using bonded abrasives. – This is typically in the form of a grinding wheel.– The abrasive grains in a grinding wheel are

held together by a bonding material.

Bonding Abrasives

• Bonding materials act as supporting posts or braces between grains.

• Bonding abrasives are marked with letters and numbers indicating:– Type of abrasive– Grain size– Grade– Structure– Bond type

Bond Types

• Vitrified: a glass bond, most commonly used bonding material.– However, it is a brittle bond.

• Resinoid: bond consiting of thermosetting resins, bond is an organic compound.– More flexible bond than

vitrified, also more resistant to higher temps.

Bond Types

• Reinforced Wheels: bond consisting of one or more layers of fiberglass.– Prevents breakage rather than improving

strength.

• Rubber: flexible bond type, inexpensive.

• Metal: different metals can be used for strength, ductility, etc.– Most inexpensive bond type.

The Grinding Process

• Grinding is a chip removal process that uses an individual abrasive grain as the cutting tool.

• The differences between grinding and a single point cutting tool is:– The abrasive grains have irregular shapes and are

spaced randomly along the periphery of the wheel.– The average rake angle of the grain is typically -60

degrees. Consequently, grinding chips undergo much larger plastic deformation than they do in other machining processes.

– Not all grains are active on the wheel.– Surface speeds involving grinding are very fast.

(a) (b)

Grinding Forces

• A knowledge of grinding forces is essential for: – Estimating power requirements.– Designing grinding machines and work-

holding fixtures and devices.– Determining the deflections that the work-

piece as well as the grinding machine may undergo. Deflections adversely affect dimensioning.

Grinding Forces

• Forces in grinding are usually smaller than those in machining operations because of the smaller dimensions involved.

• Low grinding forces are recommended for dimensional accuracy.

Problems with Grinding

• Wear Flat– After some use, grains along the periphery of

the wheel develop a wear flat.

• Wear flats rub along the ground surface, creating friction, and making grinding very inefficient.


• Sparks– Sparks produced from

grinding are actually glowing hot chips.

• Tempering– Excessive heat, often times

from friction, can soften the work-piece.

• Burning– Excessive heat may burn

the surface being ground. Characterized as a bluish color on ground steel surfaces.


• Heat Checking– High temps in grinding may cause cracks in

the work-piece, usually perpendicular to the grinding surface.

Grain Fracture

• Abrasive grains are brittle, and their fracture characteristics are important.

• Wear flat creates unwanted high temps.

• Ideally, the grain should fracture at a moderate rate so as to create new sharp cutting edges continuously.

Bond Fracture

• The strength of the abrasive bond is very important!

• If the bond is too strong, dull grains cannot dislodge to make way for new sharp grains. – Hard grade bonds are meant for soft materials.

• If too weak, grains dislodge too easily and the wear of the wheel increases greatly. – Soft grade bonds are meant for hard materials.

Grinding Ratio

• G = (Volume of material removed)/ Volume

of wheel wear)• The higher the ratio, the longer the wheel will

last. • During grinding, the wheel may act “soft” or

hard” regardless of wheel grade. – Ex: pencil acting hard on soft paper and soft on

rough paper.

Dressing, Truing, Shaping

• “Dressing” a wheel is the process of:– Conditioning worn grains by

producing sharp new edges. – Truing, which is producing a

true circle on the wheel that has become out of round.

• Grinding wheels can also be shaped to the form of the piece you are grinding.

• These are important because they affect the grinding forces and surface finish.

Grinding Operations and Machines

• Surface Grinding• Cylindrical Grinding• Internal Grinding• Centerless Grinding• Creep-feed Grinding• Heavy Stock Removal by Grinding• Grinding fluids


• Surface Grinding - grinding of flat surfaces

• Cylindrical Grinding – axially ground


• Internal Grinding - grinding the inside diameter of a part

• Creep-feed Grinding – large rates of grinding for a close to finished piece


• Centerless Grinding – continuously ground cylindrical surfaces


• Heavy Stock Removal - economical process to remove large amount of material

• Grinding Fluids – Prevent workpiece

temperature rise– Improves surface finish

and dimensional accuracy– Reduces wheel wear,

loading, and power consumption

Design Consideration for Grinding

• Part design should include secure mounting into workholding devices.

• Holes and keyways may cause vibration and chatter, reducing dimensional accuracy.

• Cylindrically ground pieces should be balanced. Fillets and radii made as large as possible, or relieved by prior machining.

Design Considerations for Grinding

• Long pieces are given better support in centerless grinding, and only the largest diameter may be ground in through-feed grinding.

• Avoid frequent wheel dressing by keeping the piece simple.

• A relief should be include in small and blind holes needing internal grinding.

Finishing Operations

• Coated abrasives• Belt Grinding• Wire Brushing• Honing• Superfinishing• Lapping• Chemical-Mechanical Polishing• Electroplating


• Coated Abrasives – have a more pointed and open structure than grinding wheels

• Belt Grinding – high rate of material removal with good surface finish


• Wire Brushing - produces a fine or controlled texture

• Honing – improves surface after boring, drilling, or internal grinding


• Superfinishing – very light pressure in a different path to the piece

• Lapping – abrasive or slurry wears the piece’s ridges down softly


• Chemical-mechanical Polishing – slurry of abrasive particles and a controlled chemical corrosive

• Electropolishing – an unidirectional pattern by removing metal from the surface

Deburring Operations

• Manual Deburring

• Mechanical Deburring

• Vibratory and Barrel Finishing

• Shot Blasting

• Abrasive-Flow Machining

• Thermal Energy Deburring

• Robotic Deburring


• Vibratory and Barrel Finishing – abrasive pellets are tumbled or vibrated to deburr

• Abrasive-flow Machining – a putty of abrasive grains is forced through a piece


• Thermal Energy Deburring – natural gas and oxygen are ignited to melt the burr

• Robotic Deburring – uses a force-feedback program to control the rate and path of deburring

Economics of Abrasive Machining and Finishing Operations

• Creep-feed grinding is an economical alternative to other machining operations.

• The use of abrasives and finishing operations achieve a higher dimensional accuracy than the solitary machining process.

• Automation has reduced labor cost and production times.• The greater the surface-finish, the more operations

involved, increases the product cost.• Abrasive processes and finishing processes are

important to include in the design analysis for pieces requiring a surface finish and dimensional accuracy.

Chapter 27 – Advanced Machining Processes

Chapter 27 – Advanced Mechanical Processes

• Advanced Machining Processes can be used when mechanical methods are not satisfactory, economical or possible due to:– High strength or hardness– Too brittle or too flexible– Complex shapes– Special finish and dimensional

tolerance requirements– Temperature rise and residual

stresses

Advanced Mechanical Processes

• These advanced methods began to be introduced in the 1940's.

• Removes material by chemical dissolution, etching, melting, evaporation, and hydrodynamic action.

• These requirements led to chemical, electrical, laser, and high-energy beams as energy sources for removing material from metallic or non-metallic workpieces.

Chemical Machining

• Chemical machining– Uses chemical dissolution to dissolve material from the

workpiece.– Can be used on stones, most metals and some ceramics.– Oldest of the advanced machining processes.

Chemical Machining

• Chemical milling - shallow cavities are produced on plates, sheets, forgings, and extrusions, generally for the overall reduction of weight.– Can be used with depths of metal removal as large as

12 mm.– Masking is used to protect areas that are not meant to

be attacked by the chemical.

Chemical Machining

• Chemical Blanking – similar to the blanking of sheet metals with the exception that the material is removed by chemical dissolution rather than by shearing.– Printed circuit boards.– Decorative panels.– Thin sheet-metal stampings.– Complex or small shapes.

Chemical Machining

• Surface Roughness and Tolerance table

Chemical Machining

• Photochemical blanking/machining– Modification of chemical

milling.– Can be used on metals as

thin as .0025 mm.

• Applications– Fine screens.– Printed circuit boards.– Electric-motor laminations.– Flat springs.– Masks for color televisions.

Chemical Machining

• Chemical machining design considerations– No sharp corners, deep or narrow cavities,

severe tapers, folded seam, or porous workpiece materials.

– Undercuts may develop.– The bulk of the workpiece should be shaped

by other processes prior to chemical machining.

Electrochemical Machining

• Electrochemical machining (ECM)– An electrolyte acts as a current

carrier which washes metal ions away from the workpiece (anode) before they have a chance to plate on the tool (cathode).

– The shaped tool is either solid or tubular.

– Generally made of brass, copper, bronze or stainless steel.

– The electrolyte is a highly conductive inorganic fluid.


• Electrochemical machining cont.– The cavity produced is the female mating image of the

tool shape.

• Process capabilities– Generally used to machine complex cavities and

shapes in high strength materials.

• Design considerations– Not suited for producing sharp square corners or flat

bottoms.– No irregular cavities.


• Pulsed electrochemical machining (PECM)– Refinement of ECM.– The current is pulsed instead of a direct current.– Lower electrolyte flow rate.– Improves fatigue life.– Tolerance obtained 20 to 100 micro-meters.

(a) (b)

Electrochemical Grinding

• Electrochemical grinding (ECG)– Combines ECM with conventional grinding.– Similar to a conventional grinder, except that the wheel is a

rotating cathode with abrasive particles.• The abrasive particles serve as insulators and they remove

electrolytic products from the working area.

– Less then 5% of the metal is removed by the abrasive action of the wheel.

Electrochemical Grinding

• Electrochemical honing– Combines the fine abrasive action of honing

with electrochemical action.– Costs more than conventional honing.– 5 times faster than conventional honing.– The tool lasts up to 10 times longer.

• Design considerations for EGC– Avoid sharp inside radii.

Electrical Discharge Machining (EDM)

• Principle of operation– Based on the erosion of

metal by spark discharge

• Components of operation– Shaped tool

• Electrode– Workpiece

• Connected to a DC power supply

– Dielectric• Nonconductive fluid


• When the potential difference is sufficiently high, the dielectric breaks down and a transient spark discharges through the fluid, removing a very small amount of material from the workpiece

• Capacitor discharge– 200-500 kHz

• This process can be used on any electrically conductive material


• Volume of material removed per discharge– 10^-10 to 10^-8 in^3

• Material removal can be predicted– MRR = 4*10^4 I*Tw^-1.23– MRR is mm^3/min– I is current in amperes– Tw is melting point (C)

• Mechanical energy is not a factor• The hardness, strength, and toughness do not necessarily

influence the removal rate


• Movement in the X&Y axis is controlled by CNC systems

• Overcut (in the Z axis) is the gap between the electrode and the workpiece– Controlled by

servomechanisms– Critical to maintain a

constant gap


• Dielectric fluids– Act as a dielectric– Provide a cooling medium– Provide a flushing medium

• Common fluids– Mineral oils– Distilled/Deionized water– Kerosene– Other clear low viscosity

fluids are available which are easier to clean but more expensive


• Electrodes– Graphite– Brass– Copper-tungsten alloys– Formed by casting,

powder metallurgy, or CNC machining

– On right, human hair with a 0.0012 inch hole drilled through


• Electrode wear– Important factor in maintaining the gap between the

electrode and the workpiece– Wear ratio is defined as the amount of material

removed to the volume of electrode wear• 3:1 to 100:1 is typical

– No-wear EDM is defined as the EDM process with reversed polarity using copper electrodes


• Process capabilities– Used in the forming of

dies for forging, extrusion, die casting, and injection molding

– Typically intricate shapes

http://www.swedm.com/capabilities/57.jpg

http://www.swedm.com/capabilities/57.jpg


• Material removal rates affect finish quality– High removal rates produce very rough surface finish with poor

surface integrity– Finishing cuts are often made at low removal rates so surface

finish can be improved

• Design considerations– Design so that electrodes can be simple/economical to produce– Deep slots and narrow openings should be avoided– Conventional techniques should be used to remove the bulk of

material

Wire EDM

• Similar to contour cutting with a bandsaw

• Typically used to cut thicker material– Up to 12” thick– Also used to make

punches, tools and dies from hard materials

Wire EDM

• Wire– Usually made of brass, copper, or tungsten– Range in diameter from 0.012 – 0.008 inches– Typically used at 60% of tensile strength– Used once since it is relatively inexpensive– Travels at a constant velocity ranging from 6-360

in/min– Cutting speed is measured in cross sectional area per

unit time (varies with material)• 18,000 mm^2/hour• 28 in^2/hour

Wire EDM

• Multiaxis EDM– Computer controls for

controlling the cutting path of the wire and its angle with respect to the workpiece plane

– Multiheads for cutting multiple parts

– Features to prevent and correct wire breakage

– Programming to optimize the operation

Electrical Discharge Grinding

• Similar to the standard grinder• Grinding wheel is made of graphite or brass and contains

no abrasives• Material is removed by spark discharge between the

workpiece and rotating wheel• Typically used to sharpen carbide tools and dies• Can also be used on fragile parts such as surgical

needles, thin-wall tubes, and honeycomb structures• Process can be combined with electrochemical discharge

grinding• Material removal rate is similar to that of EDM

– MRR = KI where K is the workpiece material factor in mm^3/A-min

Laser Beam Machining

• The source of the energy is the laser– Light Amplification by Stimulated Emission of Radiation

• The focus of optical energy on the surface of the workpiece melts and evaporates portions of the workpiece in a controlled manner– Works on both metallic and non-metallic materials

• Important considerations include the reflectivity and thermal conductivity of the material

• The lower these quantities the more efficient the process


• The cutting depth can be calculated using the formula t = CP/vd where– t is the depth– C is a constant for the process– P is the power input– v is the cutting speed– d is the laser spot diameter

• The surface produced is usually rough and has a heat affected zone (discussed in section 30.9)


• Lasers may be used in conjunction with a gas such as oxygen, nitrogen, or argon to aid in energy absorption– Commonly referred to as laser beam torches– The gas helps blow away molten and vaporized material

• Process capabilities also include welding, localized heat treating, and marking

• Very flexible process– Fiber optic beam delivery– Simple fixtures– Low setup times


• Design considerations– Sharp corners should be avoided– Deep cuts will produce tapered walls– Reflectivity is an important consideration

• Dull and unpolished surfaces are preferable

– Any adverse effects on the properties of the machined materials caused by the high local temperatures and heat affected zones should be investigated

Electron Beam Machining

• Energy source is high velocity electrons which strike the workpiece

• Voltages range from 50-200kV

• Electron speeds range from 50-80% the speed of light

• Require a vacuum

Electron Beam Machining

• Plasma arc cutting– Ionized gas is used to rapidly cut ferrous and nonferrous sheets

and plates– Temperatures range from 9400-17,000 F– The process is fast, the kerf width is small, and the surface finish

is good– Parts as thick as 6” can be cut– Much faster than the EDM and LBM process– Design considerations

• Parts must fit in vacuum chamber• Parts that only require EBM machining on a small portion

should be manufactured as a number of smaller components

Water Jet Machining

• Also known as hydrodynamic machining

• The water jet acts as a saw and cuts a narrow groove in the material

• Pressures range from 60ksi to 200ksi

Water Jet Machining

• Process capabilities– Can be used on any material up to 1” thick– Cuts can be started at any location without predrilled

holes– No heat produced– No flex to the material being cut

• Suitable for flexible materials

– Little wetting of the workpiece– Little to no burr produced– Environmentally safe

Water Jet Machining

• Very similar to water jet machining– Water contains abrasive material

• Silicon carbide• Aluminum oxide

– Higher cutting speed than that of conventional water jet machining

• Up to 25 ft/min for reinforce plastics

– Minimum hole diameter thus far is approximately 0.12 inches

– Maximum hole depth is approximately 1 inch

Abrasive Jet Machining

• Uses high velocity dry air, nitrogen, or carbon dioxide containing abrasive particles

• Supply pressure is on the order of 125psi

• The abrasive jet velocity can be as high as 100 ft/sec

• Abrasive size is approximately 400-2000 micro-inches

Economics of Advanced Machining Processes

• Advanced machining processes each have unique applications

• The economic production run for a particular process depends on the costs of tooling, equipment, operating costs, material removal rate required, level of operator skill required, and necessary secondary and finishing operations

• Chemical machining has the added cost of reagents, maskants, and disposal

• Table 27.1 lists material removal rates for all advanced machining processes

References

http://www.electricaldischargemachining.com/http://www.belmont4edm.com/http://www.texasairsonics.com/Cabinet%20Style.html

ch. 26 – abrasive machining and finishing operations brenton elisberg, jacob hunner, michael...

Documents

abrasive bond

grinding process grinding

grinding surface

grinding heat

abrasive machining

abrasive types abrasives

grinding machines

grinding forces forces