1. Bonded Abrasives: Part 2
MACH 122: Abrasive Machining
Bonded Abrasives: Part 2

2. OBJECTIVES Identify the structure of grinding wheels.
Explain the varying impacts of structure selection.
Identify various bonding materials used in bonded abrasives.
Discuss and Identify factors involved in wheel selection.

3. Fourth Symbol: Structure (A 60-J8V)
Selection & Identification of Grinding Wheels
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Fourth Symbol: Structure (A 60-J8V)
Structure, the spacing of the abrasive grains in the wheel is indicated by numbers.
Structure provides chip clearance so chips may be thrown from the wheel by centrifugal force or washed out by the grinding fluid.
1 is a dense structure. 8 is a more medium structure is an open structure.

4. Fourth Symbol: Structure (A 60-J8V)
Selection & Identification of Grinding Wheels
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Fourth Symbol: Structure (A 60-J8V)
If chips are not thrown clear, the wheel will becomes loaded with work-piece particles.
The wheel at the left is loaded with bits of metal embedded in its grinding face. It is inappropriate to do off-hand grinding of soft metals like aluminum on a pedestal grinder. The same wheel, right, has been dressed to remove the loading.

5. Fifth Symbol: Bond (A 60-J8V)
Selection & Identification of Grinding Wheels
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Fifth Symbol: Bond (A 60-J8V)
Bond is identified by letter according to the following: V B R E M
Vitrified
Resinoid
Rubber
Shellac
Metal
Vitrified bonds are most common, used mostly for precision grinding.

6. Fifth Symbol: Bond (A 60-J8V)
Selection & Identification of Grinding Wheels
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Fifth Symbol: Bond (A 60-J8V)
Resinoid bond wheels are typically used in rough- grinding operations.
Shellac finds limited use in finish grinding the wear surfaces of cam shafts.
Metal-bonded diamond wheels are also used for grinding hard nonmetallics such as ceramics and stone.

7. Other Factors in Wheel Selection
Selection & Identification of Grinding Wheels
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Other Factors in Wheel Selection
Grinding is probably the most challenging part of machine tool operation because so many variables must be integrated to get first-class results.
Four factors relate to work-piece materials, which are constantly being changed in the shop environment.
Five other factors in wheel selection are relatively fixed.

8. Variable Factors - Composition
Selection & Identification of Grinding Wheels
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Variable Factors - Composition
Composition of the Workpiece
For most steels & steel alloys, aluminum oxide grinding wheels are the usual choice.
With high-temperature alloys, cubic boron nitride can be far preferable.
For grinding cemented carbides, diamond is preferred.
For cast irons, silicon carbide is the abrasive of choice.
Aluminum oxide is best for Ferritic & martinsitic stainless.

9. Variable Factors - Material Hardness
Selection & Identification of Grinding Wheels
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Variable Factors - Material Hardness
A factor in the choice of both grit size and grade.
For soft, ductile materials, the grit is coarser and the grade can be harder.
For hard materials, finer grit &softer grades are used.
In the machine shop, coarse abrasive is considered to be in the range of 36 to 60, and a fine abrasive in the range of 80 to 100.
Grades F, G, and H are considered soft.
J, K, and L are considered relatively hard.

10. Fixed Factors - Area of Grinding
Selection & Identification of Grinding Wheels
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Fixed Factors - Area of Grinding
On any given design of grinding machine, the area of grinding contact remains essentially constant.
The rule is finer grit sizes and harder wheels for small areas of contact, and coarser grit sizes and softer wheels for larger areas of contact.

11. Fixed Factors - Wheel Speed
Selection & Identification of Grinding Wheels
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Fixed Factors - Wheel Speed
Wheel speed is usually fixed by the nature of the machine.
Wheel diameter affects the sfm.

12. Fixed Factors - Use of Grinding Fluid
Selection & Identification of Grinding Wheels
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Fixed Factors - Use of Grinding Fluid
The use of grinding fluid is a relatively minor factor in grinding wheel selection and typically results in only one step of grade change.
With grinding fluids as opposed to dry grinding, a wheel one grade harder is typically chosen.
In any shop that has been operational for some time, there should be information on which wheels have worked best on specific jobs.

13. REVIEW Define structure as it pertains to bonded abrasives.
How is structure identified?
Which wheel has a more open structure?
A 60 H 12 V
A 120 K 4V
What symbol is used to identify a resinoid bond?

14. REVIEW A 80J 10V Rough grinding mild steel.
Select the correct wheel for the following operations.
Rough grinding mild steel.
A200 G6 V
A200 J 6V
A80 G10 V
A 80J 10V
A 80J 10V

15. REVIEW A 200 G6 V Finishing grinding hardened tool steel.
Select the correct wheel for the following operations.
Finishing grinding hardened tool steel.
A200 G6 V
A200 J 6V
A80 G10 V
A 80J 10V
A 200 G6 V

16. QUESTIONS