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Manufacturing Engineering Technology in SI Units, Manufacturing Engineering Technology in SI Units,
66thth Edition Edition Chapter 26: Chapter 26:
Abrasive Machining and Finishing Abrasive Machining and Finishing Operations Operations
Chapter Outline Introduction Abrasives and Bonded Abrasives The Grinding Process Grinding Operations and Machines Design Considerations for Grinding Ultrasonic Machining Finishing Operations Deburring Operations Economics of Abrasive Machining and Finishing Opera
Abrasives and Bonded Abrasives Abrasives that are commonly used:1. Conventional abrasives2. Superabrasives Above abrasives are harder than conventional cutting-
tool materials Cubic boron nitride and diamond are the two hardest
materials known; referred to as superabrasives Friability defined as the ability of abrasive grains to
Abrasives and Bonded AbrasivesAbrasive Types Commonly found in nature are emery, corundum
(alumina), quartz, garnet and diamond Abrasives that have been made synthetically:1. Aluminum oxide2. Seeded gel3. Silicon carbide4. Cubic boron nitride5. Diamond
The Grinding ProcessGrinding Forces Knowledge of grinding forces is essential for:1. Estimating power requirements2. Designing grinding machines and work-holding fixtures
and devices.3. Determining the deflections that the workpiece
The Grinding ProcessGrinding Forces A knowledge of grinding forces is essential for:1. Estimating power requirements2. Designing grinding machines and work-holding fixtures
and devices3. Determining the workpiece and grinding machine
deflections Grain force is proportional to the process variables:
The Grinding ProcessEXAMPLE 26.1 Forces in Surface GrindingA surface-grinding operation is being performed on low-carbon steel with a wheel of diameter D=250 mm that is rotating at N=4000 rpm and a width of cut of w=25 mm. The depth of cut is d=0.05 mm and the feed rate of the workpiece, is 1.5 m/min. Calculate the cutting force and the thrust force.
The Grinding ProcessSolution Forces in Surface GrindingThrust force can be estimated by noting from experimental data in the technical literature that it is about 30% higher than the cutting force
The Grinding ProcessTemperature Temperature rise in grinding is important as it can:1. Adversely affect the surface properties2. Cause residual stresses on the workpiece3. Cause distortions due to thermal expansion and
contraction of the workpiece surface Surface-temperature rise in grinding is
The Grinding Process:Grinding-wheel WearAttritious Grain Wear Similar to flank wear in cutting tools Cutting edges become dull and develop a wear flat Selection of abrasive is based on the reactivity of the
grain, workpiece hardness and toughness
Grain Fracture The grain should fracture at a moderate rate So that new sharp cutting edges are produced
The Grinding Process:Grinding RatioEXAMPLE 26.2 Action of a Grinding WheelA surface-grinding operation is being carried out with the wheel running at a constant spindle speed. Will the wheel act soft or hard as the wheel wears down over time? Assume that the depth of cut, d, remains constant and the wheel is dressed periodically.
As D becomes smaller, the relative grain force increases, the wheel acts softer
The Grinding Process:Dressing, Truing, and Shaping of Grinding Wheels Dressing is the process of:1. Conditioning2. Truing Dressing is required for dulls wheel or when the wheel
becomes loaded Loading occurs when the porosities on the wheel
surfaces become filled with chips from the workpiece Dressing techniques and their frequency affect grinding
forces and workpiece surface finish Grinding wheels can be shaped to the form to be
The Grinding Process:Grinding Operations and Machines
EXAMPLE 26.4 Grinding versus Hard Turning Dimensional tolerances and surface finish in hard
turning approaching to those obtained with grinding Turning requires much less energy than grinding Cutting fluids not necessary and the machine tools are
less expensive Work-holding devices for large and slender workpieces
for hard turning is a problem Tool wear and its control is also a problem
The Grinding Process:Grinding Operations and Machines
Grinding Fluids Importance of using a fluid:1. Reduces temperature rise in the workpiece2. Improves part surface finish and dimensional accuracy3. Improves the efficiency of the operation Grinding fluids are
water-based emulsions for grinding and oils for thread grinding
Chatter adversely affects surface finish and wheel performance
Chatter marks on ground surfaces can be identified from:
1. Bearings and spindles of the grinding machine2. Non-uniformities in the grinding wheel 3. Uneven wheel wear4. Poor dressing techniques5. Using grinding wheels that are not balanced properly6. External sources
Ways to reduce the tendency for chatter in grinding:1. Using soft-grade wheels2. Dressing the wheel frequently3. Changing dressing techniques4. Reducing the material-removal rate5. Supporting the workpiece rigidly
Ultrasonic Machining Material is removed from a surface by microchipping
and erosion with loose, fine abrasive grains in a water slurry
Best suited for materials that are hard and brittle Form tool is required for each shape to be produced Materials for abrasive grains are boron carbide,
Ultrasonic MachiningDesign Considerations for Ultrasonic Machining Basic design guidelines:1. Avoid sharp profiles, corners, and radii2. Holes produced will have some taper3. Bottom of the parts should have a backup plate
Finishing OperationsEXAMPLE 26.5 Belt Grinding of Turbine Nozzle Vanes Turbine nozzle vanes shown The vanes were mounted on a fixture and ground dry at
Finishing OperationsElectropolishing Mirrorlike finishes can be obtained on metal surfaces No mechanical contact with the workpiece For polishing irregular shapes
Polishing in Magnetic Fields 2 basic polishing methods: 1. Magnetic-float2. Magnetic-field-assisted
Deburring Operations Burrs are thin ridges developed along the edges of a
workpiece from operations Burrs can be detected by simple means or visual
inspection Burrs have several disadvantages: 1. Jam and misalignment of parts, 2. Safety hazard to personnel3. Reduce the fatigue life of components4. Sheet metal have lower bend ability
Deburring OperationsThermal Energy Deburring Consist of placing the part in a chamber and injected
with a mixture of natural gas and oxygen Drawbacks to the process: 1. Larger burrs tend to form2. Thin and slender parts may distort3. Does not polish or buff the workpiece surfaces