Presentation on theme: "Ch. 26 – Abrasive Machining and Finishing Operations"— Presentation transcript:
1 Ch. 26 – Abrasive Machining and Finishing Operations Brenton Elisberg, Jacob Hunner, Michael Snider, Michael Anderson
2 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.
3 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.
4 Abrasives and Bonded Abrasives Commonly used abrasives in abrasive machining are:Conventional AbrasivesAluminum OxideSilicon CarbideSuperabrasivesCubic boron nitrideDiamond
5 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.
6 Abrasive Types Abrasives commonly found in nature include: Emery CorundumQuartzGarnetDiamond
7 Abrasive Types Synthetically created abrasives include: Aluminum oxide (1893)Seeded gel (1987)Silicon carbide (1891)Cubic-boron nitride (1970’s)Synthetic diamond (1955)
8 Abrasive Grain SizeAbrasives 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.
10 Grinding WheelsLarge 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.
11 Bonding AbrasivesBonding materials act as supporting posts or braces between grains.Bonding abrasives are marked with letters and numbers indicating:Type of abrasiveGrain sizeGradeStructureBond type
12 Bond TypesVitrified: 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.
13 Bond TypesReinforced 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.
14 The Grinding ProcessGrinding 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.
16 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.
17 Grinding ForcesForces in grinding are usually smaller than those in machining operations because of the smaller dimensions involved.Low grinding forces are recommended for dimensional accuracy.
18 Problems with Grinding Wear FlatAfter 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.
19 Problems with Grinding SparksSparks produced from grinding are actually glowing hot chips.TemperingExcessive heat, often times from friction, can soften the work-piece.BurningExcessive heat may burn the surface being ground. Characterized as a bluish color on ground steel surfaces.
20 Problems with Grinding Heat CheckingHigh temps in grinding may cause cracks in the work-piece, usually perpendicular to the grinding surface.
21 Grain FractureAbrasive 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.
22 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.
23 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.
24 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.
25 Grinding Operations and Machines Surface GrindingCylindrical GrindingInternal GrindingCenterless GrindingCreep-feed GrindingHeavy Stock Removal by GrindingGrinding fluids
26 Grinding Operations and Machines Surface Grinding - grinding of flat surfacesCylindrical Grinding – axially ground
27 Grinding Operations and Machines Internal Grinding - grinding the inside diameter of a partCreep-feed Grinding – large rates of grinding for a close to finished piece
29 Grinding Operations and Machines Heavy Stock Removal - economical process to remove large amount of materialGrinding FluidsPrevent workpiece temperature riseImproves surface finish and dimensional accuracyReduces wheel wear, loading, and power consumption
30 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.
31 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.
33 Finishing OperationsCoated Abrasives – have a more pointed and open structure than grinding wheelsBelt Grinding – high rate of material removal with good surface finish
34 Finishing OperationsWire Brushing - produces a fine or controlled textureHoning – improves surface after boring, drilling, or internal grinding
35 Finishing OperationsSuperfinishing – very light pressure in a different path to the pieceLapping – abrasive or slurry wears the piece’s ridges down softly
36 Finishing OperationsChemical-mechanical Polishing – slurry of abrasive particles and a controlled chemical corrosiveElectropolishing – an unidirectional pattern by removing metal from the surface
37 Deburring Operations Manual Deburring Mechanical Deburring Vibratory and Barrel FinishingShot BlastingAbrasive-Flow MachiningThermal Energy DeburringRobotic Deburring
38 Deburring OperationsVibratory and Barrel Finishing – abrasive pellets are tumbled or vibrated to deburrAbrasive-flow Machining – a putty of abrasive grains is forced through a piece
39 Deburring OperationsThermal Energy Deburring – natural gas and oxygen are ignited to melt the burrRobotic Deburring – uses a force-feedback program to control the rate and path of deburring
40 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.
42 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 hardnessToo brittle or too flexibleComplex shapesSpecial finish and dimensional tolerance requirementsTemperature rise and residual stresses
43 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.
44 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.
45 Chemical MachiningChemical 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.
46 Chemical MachiningChemical 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.
47 Chemical MachiningSurface Roughness and Tolerance table
48 Chemical Machining Photochemical blanking/machining Applications Modification of chemical milling.Can be used on metals as thin as mm.ApplicationsFine screens.Printed circuit boards.Electric-motor laminations.Flat springs.Masks for color televisions.
49 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.
50 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.
51 Electrochemical Machining Electrochemical machining cont.The cavity produced is the female mating image of the tool shape.Process capabilitiesGenerally used to machine complex cavities and shapes in high strength materials.Design considerationsNot suited for producing sharp square corners or flat bottoms.No irregular cavities.
53 Electrochemical Machining 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.
54 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.
55 Electrochemical Grinding Electrochemical honingCombines 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 EGCAvoid sharp inside radii.
56 Electrical Discharge Machining (EDM) Principle of operationBased on the erosion of metal by spark dischargeComponents of operationShaped toolElectrodeWorkpieceConnected to a DC power supplyDielectricNonconductive fluid
57 Electrical Discharge Machining (EDM) 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 workpieceCapacitor dischargekHzThis process can be used on any electrically conductive material
58 Electrical Discharge Machining (EDM) Volume of material removed per discharge10^-10 to 10^-8 in^3Material removal can be predictedMRR = 4*10^4 I*Tw^-1.23MRR is mm^3/minI is current in amperesTw is melting point (C)Mechanical energy is not a factorThe hardness, strength, and toughness do not necessarily influence the removal rate
59 Electrical Discharge Machining (EDM) Movement in the X&Y axis is controlled by CNC systemsOvercut (in the Z axis) is the gap between the electrode and the workpieceControlled by servomechanismsCritical to maintain a constant gap
60 Electrical Discharge Machining (EDM) Electrical Discharge Machining (EDM)Dielectric fluidsAct as a dielectricProvide a cooling mediumProvide a flushing mediumCommon fluidsMineral oilsDistilled/Deionized waterKeroseneOther clear low viscosity fluids are available which are easier to clean but more expensive
61 Electrical Discharge Machining (EDM) ElectrodesGraphiteBrassCopper-tungsten alloysFormed by casting, powder metallurgy, or CNC machiningOn right, human hair with a inch hole drilled through
62 Electrical Discharge Machining (EDM) Electrode wearImportant factor in maintaining the gap between the electrode and the workpieceWear ratio is defined as the amount of material removed to the volume of electrode wear3:1 to 100:1 is typicalNo-wear EDM is defined as the EDM process with reversed polarity using copper electrodes
63 Electrical Discharge Machining (EDM) Process capabilitiesUsed in the forming of dies for forging, extrusion, die casting, and injection moldingTypically intricate shapes
64 Electrical Discharge Machining (EDM) Material removal rates affect finish qualityHigh removal rates produce very rough surface finish with poor surface integrityFinishing cuts are often made at low removal rates so surface finish can be improvedDesign considerationsDesign so that electrodes can be simple/economical to produceDeep slots and narrow openings should be avoidedConventional techniques should be used to remove the bulk of material
65 Wire EDM Similar to contour cutting with a bandsaw Typically used to cut thicker materialUp to 12” thickAlso used to make punches, tools and dies from hard materials
66 Wire EDM Wire Usually made of brass, copper, or tungsten Range in diameter from – inchesTypically used at 60% of tensile strengthUsed once since it is relatively inexpensiveTravels at a constant velocity ranging from in/minCutting speed is measured in cross sectional area per unit time (varies with material)18,000 mm^2/hour28 in^2/hour
67 Wire EDMMultiaxis EDMComputer controls for controlling the cutting path of the wire and its angle with respect to the workpiece planeMultiheads for cutting multiple partsFeatures to prevent and correct wire breakageProgramming to optimize the operation
68 Electrical Discharge Grinding Similar to the standard grinderGrinding wheel is made of graphite or brass and contains no abrasivesMaterial is removed by spark discharge between the workpiece and rotating wheelTypically used to sharpen carbide tools and diesCan also be used on fragile parts such as surgical needles, thin-wall tubes, and honeycomb structuresProcess can be combined with electrochemical discharge grindingMaterial removal rate is similar to that of EDMMRR = KI where K is the workpiece material factor in mm^3/A-min
69 Laser Beam Machining The source of the energy is the laser Light Amplification by Stimulated Emission of RadiationThe focus of optical energy on the surface of the workpiece melts and evaporates portions of the workpiece in a controlled mannerWorks on both metallic and non-metallic materialsImportant considerations include the reflectivity and thermal conductivity of the materialThe lower these quantities the more efficient the process
70 Laser Beam MachiningThe cutting depth can be calculated using the formula t = CP/vd wheret is the depthC is a constant for the processP is the power inputv is the cutting speedd is the laser spot diameterThe surface produced is usually rough and has a heat affected zone (discussed in section 30.9)
71 Laser Beam MachiningLasers may be used in conjunction with a gas such as oxygen, nitrogen, or argon to aid in energy absorptionCommonly referred to as laser beam torchesThe gas helps blow away molten and vaporized materialProcess capabilities also include welding, localized heat treating, and markingVery flexible processFiber optic beam deliverySimple fixturesLow setup times
72 Laser Beam Machining Design considerations Sharp corners should be avoidedDeep cuts will produce tapered wallsReflectivity is an important considerationDull and unpolished surfaces are preferableAny adverse effects on the properties of the machined materials caused by the high local temperatures and heat affected zones should be investigated
73 Electron Beam Machining Energy source is high velocity electrons which strike the workpieceVoltages range from kVElectron speeds range from 50-80% the speed of lightRequire a vacuum
74 Electron Beam Machining Plasma arc cuttingIonized gas is used to rapidly cut ferrous and nonferrous sheets and platesTemperatures range from ,000 FThe process is fast, the kerf width is small, and the surface finish is goodParts as thick as 6” can be cutMuch faster than the EDM and LBM processDesign considerationsParts must fit in vacuum chamberParts that only require EBM machining on a small portion should be manufactured as a number of smaller components
75 Water Jet Machining Also known as hydrodynamic machining The water jet acts as a saw and cuts a narrow groove in the materialPressures range from 60ksi to 200ksi
76 Water Jet Machining Process capabilities Can be used on any material up to 1” thickCuts can be started at any location without predrilled holesNo heat producedNo flex to the material being cutSuitable for flexible materialsLittle wetting of the workpieceLittle to no burr producedEnvironmentally safe
77 Water Jet Machining Very similar to water jet machining Water contains abrasive materialSilicon carbideAluminum oxideHigher cutting speed than that of conventional water jet machiningUp to 25 ft/min for reinforce plasticsMinimum hole diameter thus far is approximately 0.12 inchesMaximum hole depth is approximately 1 inch
78 Abrasive Jet Machining Uses high velocity dry air, nitrogen, or carbon dioxide containing abrasive particlesSupply pressure is on the order of 125psiThe abrasive jet velocity can be as high as 100 ft/secAbrasive size is approximately micro-inches
79 Economics of Advanced Machining Processes Advanced machining processes each have unique applicationsThe 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 operationsChemical machining has the added cost of reagents, maskants, and disposalTable 27.1 lists material removal rates for all advanced machining processes