1. Manufacturing Engineering Technology in SI Units, 6th Edition Chapter 26: Abrasive Machining and Finishing Operations
Presentation slide for courses, classes, lectures et al.
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2. 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 Operations
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3. Introduction
Some manufacturing processes cannot produce the required dimensional accuracy or surface finish
An abrasive is a small, hard particle having sharp edges and an irregular shape
Abrasives is removing of small amounts of material from a surface through a cutting process that produces tiny chips
Grinding wheels can sharpen knives and tools
Sandpaper or emery cloth can smoothen surfaces and remove sharp corners
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4. Introduction
Abrasives are hard and used in finishing processes for heat-treated metals and alloys
Applications include:
Finishing of ceramics and glasses
Cutting off lengths of bars, structural shapes, masonry and concrete
Removing unwanted weld beads and spatter
Cleaning surfaces with jets of air or water containing abrasive particles.
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5. Introduction
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6. Abrasives and Bonded Abrasives
Abrasives that are commonly used:
Conventional abrasives
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 fracture into smaller pieces
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7. Abrasives and Bonded Abrasives
Abrasive Types
Commonly found in nature are emery, corundum (alumina), quartz, garnet and diamond
Abrasives that have been made synthetically:
Aluminum oxide
Seeded gel
Silicon carbide
Cubic boron nitride
Diamond
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8. Abrasives and Bonded Abrasives
Abrasive Grain Size
Abrasives are very small when compared to the size of cutting tools and inserts
Abrasives have sharp edges and allow removal of small quantities of material
Very fine surface finish and dimensional accuracy can be obtained using abrasives as tools
Size of an abrasive grain is identified by a grit number
Smaller the grain size, larger the grit number
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9. Abrasives and Bonded Abrasives
Abrasive–workpiece-material Compatibility
Affinity of an abrasive grain to the workpiece material is important
The less the reactivity of the two materials, the less wear and dulling of the grains occur during grinding
Recommendations when selecting abrasives:
Aluminum oxide: Carbon steels, ferrous alloys, and alloy steels.
Silicon carbide: Nonferrous metals, cast irons
Cubic boron nitride: Steels and cast irons
Diamond: Ceramics, cemented carbides, hardened steels
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10. Abrasives and Bonded Abrasives: Grinding Wheels
Each abrasive grain removes only a very small amount of material at a time
High rates of material removal can be achieved when large number of these grains act together
Done by using bonded abrasives
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11. Abrasives and Bonded Abrasives: Grinding Wheels
In bonded abrasives, porosity is essential to provide clearance for produced chips and cooling
Porosity can be observed by the surface of grinding wheel
For non-porosity, it is fully dense and solid
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12. Abrasives and Bonded Abrasives: Grinding Wheels
Due to their high cost, only a small volume wheels consists of superabrasives
Bonded abrasives are indicated by the type of abrasive, grain size, grade, structure, and bond type
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13. Abrasives and Bonded Abrasives: Grinding Wheels
Cost of grinding wheels depends on the type and size of the wheel
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14. Abrasives and Bonded Abrasives: Bond Types
Common types of bonds:
Vitrified:
Consist of feldspar and clays
Strong, stiff, porous, and resistant to oils acids, and water
Resinoid:
Bonding materials are thermosetting resins
Resinoid wheels are more flexible than vitrified wheels
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15. Abrasives and Bonded Abrasives: Bond Types
Reinforced Wheels:
Consist of layers of fiberglass mats of various mesh sizes
Thermoplastic:
Used in grinding wheels
With sol-gel abrasives bonded with thermoplastics
Rubber:
Using powder-metallurgy techniques
Lower in cost and are used for small production quantities
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16. Abrasives and Bonded Abrasives: Wheel Grade and Structure
The grade of a bonded abrasive is a measure of its bond strength
The grade is also referred to as the hardness of a bonded abrasive
A hard wheel has a stronger bond and/or a larger amount of bonding material between the grains than a soft wheel
Structure of a bonded abrasive is a measure of its porosity
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17. The Grinding Process
Grinding is a chip-removal process that uses an individual abrasive grain as the cutting tool
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18. The Grinding Process Grinding Forces
Knowledge of grinding forces is essential for:
Estimating power requirements
Designing grinding machines and work-holding fixtures and devices.
Determining the deflections that the workpiece
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19. The Grinding Process
In surface grinding, undeformed chip length and thickness is approximated by equation
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20. The Grinding Process Grinding Forces
A knowledge of grinding forces is essential for:
Estimating power requirements
Designing grinding machines and work-holding fixtures and devices
Determining the workpiece and grinding machine deflections
Grain force is proportional to the process variables:
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21. The Grinding Process Specific Energy
Energy dissipated in producing a grinding chip consists of energy required from:
Chip formation
Plowing
Friction
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22. The Grinding Process EXAMPLE 26.1 Forces in Surface Grinding
A 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.
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23. The Grinding Process Solution Forces in Surface Grinding
The material-removal rate (MRR) is
The power consumed is
Since power is
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24. The Grinding Process Solution Forces in Surface Grinding
Thrust force can be estimated by noting from experimental data in the technical literature that it is about 30% higher than the cutting force
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25. The Grinding Process Temperature
Temperature rise in grinding is important as it can:
Adversely affect the surface properties
Cause residual stresses on the workpiece
Cause distortions due to thermal expansion and contraction of the workpiece surface
Surface-temperature rise in grinding is
Temperature increases with increasing d, D, and V
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26. The Grinding Process Sparks
Sparks produced are chips that glow due to exothermic (heat-producing) reaction of the hot chips with oxygen in the atmosphere
When heat generated due to exothermic reaction is high, chips can melt
Tempering
Excessive temperature rise in grinding can cause tempering and softening of the workpiece surface
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27. The Grinding Process Burning
Excessive temperature during grinding may burn the workpiece surface
A burn is characterized by a bluish color on ground steel surfaces
Heat Checking
High temperatures in grinding may cause the workpiece surface to develop cracks
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28. The Grinding Process: Grinding-wheel Wear
Attritious 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 continuously during grinding
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29. The Grinding Process: Grinding Ratio
Grinding ratio is defined as
Higher the force, greater the tendency for the grains to fracture
Higher the wheel wear, lower the grinding ratio
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30. The Grinding Process: Grinding Ratio
EXAMPLE 26.2
Action of a Grinding Wheel
A 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
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31. The Grinding Process: Dressing, Truing, and Shaping of Grinding Wheels
Dressing is the process of:
Conditioning
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 ground on the workpiece
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32. The Grinding Process: Dressing, Truing, and Shaping of Grinding Wheels
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33. The Grinding Process: Grindability of Materials and Wheel Selection
Grindability of materials is a general indicator of how easy it is to grind a material
Include quality of the surface produced, surface finish, surface integrity, wheel wear, cycle time, and overall economics of the operation
Wheel selection involves shape of the wheel, shape of the part to be produced and characteristics of the workpiece material
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34. The Grinding Process: Grinding Operations and Machines
Selection of a grinding process and machine depends on the workpiece shape and features, size, ease of fixturing, and production rate required
Relative movement of the wheel may be along the surface of the workpiece or move radially into the workpiece
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35. The Grinding Process: Grinding Operations and Machines
Surface Grinding
Surface grinding involve the grinding of flat surfaces
Workpiece is secured on a magnetic chuck attached to the worktable of the grinder
Traverse grinding is where the table reciprocates longitudinally and is fed laterally after each stroke.
In plunge grinding, it involves the wheel moving radically into the workpiece
Vertical spindles and rotary tables allow a number of pieces to be ground in one setup
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36. The Grinding Process: Grinding Operations and Machines
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37. The Grinding Process: Grinding Operations and Machines
Cylindrical Grinding
The external cylindrical surfaces and shoulders of workpieces are ground
Rotating cylindrical workpiece reciprocates laterally along its axis to cover the width to be ground
Capable of grinding rolls with large diameter
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38. The Grinding Process: Grinding Operations and Machines
Cylindrical Grinding
Can also produce shapes in which the wheel is dressed to the workpiece form to be ground
Non-cylindrical parts can be ground on rotating workpieces
Workpiece spindle speed is synchronized between the workpiece and the wheel axis
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39. The Grinding Process: Grinding Operations and Machines
Cylindrical Grinding
Thread grinding is done on cylindrical grinders using specially dressed wheels matching the shape of the threads
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40. The Grinding Process: Grinding Operations and Machines
EXAMPLE 26.3
Cycle Patterns in Cylindrical Grinding
Cycle patterns for a CNC precision grinder
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41. The Grinding Process: Grinding Operations and Machines
Internal Grinding
A small wheel is used to grind the inside diameter of the part
Internal profiles is ground with profile-dressed wheels that move radially into the workpiece
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42. The Grinding Process: Grinding Operations and Machines
Centerless Grinding
A high-production process for continuously grinding cylindrical surfaces
Workpiece is supported by a blade
Parts with variable diameters can be ground by centerless grinding called infeed or plunge grinding
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43. The Grinding Process: Grinding Operations and Machines
Centerless Grinding
In internal centerless grinding, the workpiece is supported between three rolls and is ground internally
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44. The Grinding Process: Grinding Operations and Machines
Creep-feed Grinding
Grinding can also be used for large-scale metal- removal operations to compete with milling, broaching and planing
In creep-feed grinding, the wheel depth of cut, d, is small and the workpiece speed is low
To keep workpiece temperatures low and improve surface finish, the wheels are softer grade resin bonded and have an open structure
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45. The Grinding Process: Grinding Operations and Machines
Heavy Stock Removal by Grinding
Grinding can also be used for heavy stock removal by increasing process parameters
The dimensional tolerances are on the same order as those obtained by most machining processes
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46. 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
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47. The Grinding Process: Grinding Operations and Machines
Other Grinding Operations
Universal tool and cutter grinders are used for grinding single-point or multipoint tools
Tool-post grinders are self-contained units and attached to the tool post of a lathe
Swing-frame grinders are used in foundries for grinding large castings
Portable grinders are driven pneumatically or electrically
Bench and pedestal grinders are used for the routine offhand grinding of tools and small parts
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48. The Grinding Process: Grinding Operations and Machines
Grinding Fluids
Importance of using a fluid:
Reduces temperature rise in the workpiece
Improves part surface finish and dimensional accuracy
Improves the efficiency of the operation
Grinding fluids are water-based emulsions for grinding and oils for thread grinding
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49. The Grinding Process: Grinding Chatter
Chatter adversely affects surface finish and wheel performance
Chatter marks on ground surfaces can be identified from:
Bearings and spindles of the grinding machine
Non-uniformities in the grinding wheel
Uneven wheel wear
Poor dressing techniques
Using grinding wheels that are not balanced properly
External sources
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50. The Grinding Process: Grinding Chatter
Ways to reduce the tendency for chatter in grinding:
Using soft-grade wheels
Dressing the wheel frequently
Changing dressing techniques
Reducing the material-removal rate
Supporting the workpiece rigidly
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51. The Grinding Process: Grinding Chatter
Safety in Grinding Operations
Grinding wheels are brittle and when rotate at high speeds, they can fracture and cause serious injury
There must be care in handling, storage and usage of grinding wheels
Damage to a grinding wheel can reduce its bursting speed severely
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52. Design Considerations for Grinding
Specific attention should be given to:
Parts should be designed so that they can be mounted securely
Interrupted surfaces should be avoided as they can cause vibrations and chatter
Parts for cylindrical grinding should be balanced
Short pieces should be avoided as they may be difficult to grind
Design kept simple to avoid frequent form dressing of the wheel
Holes should be avoided
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53. 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, aluminum oxide or silicon carbide
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54. Ultrasonic Machining Rotary Ultrasonic Machining
Abrasive slurry is replaced by metal-bonded diamond abrasives either impregnated or electroplated on the tool surface
Tool is vibrated ultrasonically and rotated at the same time
It is being pressed against the workpiece surface at a constant pressure
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55. Ultrasonic Machining Design Considerations for Ultrasonic Machining
Basic design guidelines:
Avoid sharp profiles, corners, and radii
Holes produced will have some taper
Bottom of the parts should have a backup plate
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56. Finishing Operations Coated Abrasives
Coated abrasives are made of aluminum oxide, silicon carbide and zirconia alumina
Coated abrasives have more open structure than the abrasives on grinding wheels
They are used to finish flat or curved surfaces of metallic and nonmetallic parts, metallographic specimens, and in woodworking
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57. Finishing Operations Belt Grinding
Used as belts for high-rate material removal with good surface finish
Replace conventional grinding operations
Microreplication perform more consistently than conventional coated abrasives and the temperatures involved are lower
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58. Finishing Operations EXAMPLE 26.5
Belt Grinding of Turbine Nozzle Vanes
Turbine nozzle vanes shown
The vanes were mounted on a fixture and ground dry at a belt surface speed of 1,800 m/min
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59. Finishing Operations Wire Brushing Also called power brushing
The workpiece is held against a circular wire brush that rotates
Wire brushing is used to produce a fine or controlled surface texture
Honing
Used to improve the surface finish of holes
Tool has a reciprocating axial motion and produces a crosshatched pattern on the surface of the hole
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60. Finishing Operations Superfinishing
Light pressure is applied and the motion of the honing stone has a short stroke
Motion of the stone is controlled so that the grains do not travel along the same path on the surface
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61. Finishing Operations Lapping
Used for finishing flat, cylindrical, or curved surfaces
The lap is soft and porous and is made of cast iron, copper, leather or cloth
The abrasive particles are embedded in the lap or carried in a slurry
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62. Finishing Operations Polishing
A process that produces a smooth, lustrous surface finish
Softening and smearing of surface layers by frictional heating and fine scale abrasive removal from the workpiece surface
Produce shiny appearance of polished surfaces
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63. Finishing Operations Chemical–mechanical Polishing
Uses a suspension of abrasive particles in a water- based solution with a controlled corrosion
Removes material from the workpiece through combined abrasion and corrosion effects
Major application of this process is the polishing of silicon wafers
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64. Finishing Operations Electropolishing
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:
Magnetic-float
Magnetic-field-assisted
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65. 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:
Jam and misalignment of parts,
Safety hazard to personnel
Reduce the fatigue life of components
Sheet metal have lower bend ability
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66. Deburring Operations Deburring operations include:
Manual deburring with files and scrapers
Mechanical deburring by machining pieces
Wire brushing
Using abrasive belts
Ultrasonic machining
Electropolishing
Electrochemical machining
Magnetic–abrasive finishing
Vibratory finishing
Shot blasting or abrasive blasting
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67. Deburring Operations Vibratory and Barrel Finishing
Used to remove burrs from large numbers of relatively small workpieces
Container is vibrated or tumbled by various mechanical means
Impact of individual abrasives and metal particles removes the burrs and sharp edges from the parts
Can be a dry or a wet process
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68. Deburring Operations Shot Blasting Also called grit blasting
Involves abrasive particles propelling by a high-velocity jet of air, or by a rotating wheel, onto the surface of the workpiece
Surface damage can result if the process parameters are not controlled
Microabrasive blasting consists of small-scale polishing and etching on bench-type units
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69. Deburring Operations Abrasive-flow Machining
Involves the use of abrasive grains that are mixed in a putty-like matrix
Movement of the abrasive matrix under pressure erodes away both burrs and sharp corners and polishes the part
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70. Deburring Operations Thermal Energy Deburring
Consist of placing the part in a chamber and injected with a mixture of natural gas and oxygen
Drawbacks to the process:
Larger burrs tend to form
Thin and slender parts may distort
Does not polish or buff the workpiece surfaces
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71. Deburring Operations Robotic Deburring
Deburring and flash removal by programmable robots
Using a force-feedback system for controlling the path and rate of burr removal
Eliminates tedious and expensive manual labor
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72. Economics of Abrasive Machining and Finishing Operations
Abrasive machining and finishing operations are necessary to achieve high enough dimensional accuracy or good-quality surface finishes
Abrasive processes are used both as a finishing process and as a large-scale material-removal operation
As the surface-finish requirement increases, more operations are needed and cost increases further
Total cost of abrasive operations depends on part size, shape, surface finish, dimensional accuracy, machinery, tooling and labor involved
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73. Economics of Abrasive Machining and Finishing Operations
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