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54 Cards in this Set
- Front
- Back
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What is abrasive machining? |
Material removal by action of hard, abrasive particles usually in the form of a bonded wheel. |
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What is abrasive machining used for? |
It is generally used as a finishing operation after the part geometry has been established by conventional machining. |
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What is the most important abrasive process? |
Grinding. |
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Why are abrasive processes important? |
They can be used on all types of materials. - Some can produce extremely fine surface finished, up to 0.025 μm - Can hold dimensions to extremely close tolerances |
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What is grinding? |
A material removal process in which abrasive particles are contained in a bonded grinding wheel that operates at very high surface speeds
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Describe grinding wheels |
They are usually disc-shaped and precisely balanced for high rotational speeds. |
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What are the five grinding wheel parameters? |
- Abrasive material - Grain size - Bonding material - Wheel grade - Wheel structure |
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What are the four properties of abrasive materials? |
- High hardness - Wear resistance - Toughness - Friability |
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What is friability? |
The capacity to fracture when cutting edge dulls, so a new sharp edge is exposed. |
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What is the most common abrasive? What is it used for? |
Al2O3 - Used to grind steel and other ferrous high-strength alloys |
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What is silicon carbide (SiC) used for? What are its properties? |
Harder than Al2O3 but not as tough. It is used on aluminium, brass, stainless steel, some cast irons, and certain ceramics. |
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What are the two new abrasive materials? |
Cubic boron nitride (cBN) Diamond |
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What are the properties of cBN and what is it used for? |
Very hard and expensive. It is suitable for steels, and for hard materials such as hardened tool steels and aerospace alloys. |
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What are the properties of diamond? |
Even harder than cBN and also very expensive. - Not suitable for steels - Used on hard, abrasive materials such as ceramics, cemented carbides, and glass. |
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How does grain size affect grinding? |
Small grit sizes produce better finished, while larger grit sizes permit larger material removal rates. Harder work materials require smaller grain sizes, while softer materials require larger ones. |
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How is grit size measured? |
Using a screen mesh procedure. Smaller grit sizes are indicated by larger numbers in the screen mesh procedure, and vice versa. |
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What properties are needed of the bonding materials? |
- They must be able to withstand centrifugal forces and high temperatures. - They must resist shattering during shock loading of the wheel - They must be able to hold abrasive grains rigidly in place for cutting yet allow worn grains to be dislodged to expose new sharp ones. |
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What is the wheel structure? |
The relative spacing of abrasive grains in wheel. |
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How are volumetric proportions of grains, bond material, and pores expressed? |
Psubg+Psubb+Psubp=1.0 |
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What does "open structure" mean? When is it recommended? |
It means that Psubp is relatively large and that Psubg is relatively small.
It is recommended when clearance for chips must be provided. |
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What does "dense structure" mean? When is it recommended? |
It means Psubp is relatively small, and Psubg is larger. Recommended to obtain better surface finish and dimensional control. |
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What does the wheel grade indicate? |
It indicates the bond strength in retaining abrasive grits during cutting. |
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What does the wheel grade depend on? |
Pm the amount of bonding material in the wheel structure (Psubb) |
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How is wheel grade measured? |
It is measured on a scale ranging between soft and hard. |
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Describe soft wheels. What are they used for? |
They lose grains readily - used for low material removal rates and hard work materials. |
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Describe hard wheels. What are they used for? |
They retain grains - used for high stock removal rates and soft work materials. |
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What is the standard grinding wheel marking system? |
It is Abrasive type-grit size-grade-structure-bond material. - Example A-46-H-6-V |
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How is the best surface finish achieved? |
- Small grain sizes - Higher wheel speeds - Denser wheel structure = more grits per wheel area |
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What are the three types of grain action? |
- Cutting - Plowing - Rubbing |
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What does cutting consist of? |
When grits project far enough into the surface to form a chip, and material is removed. |
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What is plowing? |
When the grits project into work, but not far enough to cut. instead, the surface is deformed and energy is consumed. No material is removed. |
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What is rubbing? |
When grits contact the surface but only rubbing friction occurs. Consumes energy and no material is removed. |
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What are the characteristics of grinding? |
High temperatures, and high friction. Most of the energy remains in the ground surface, resulting in high work surface temperatures. |
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What are the possible damaging effects of grinding? |
- Surface burns and cracks - Metallurgical damage immediately beneath the surface - Softening of the work surface if heat treated - Residual stresses in the work surface. |
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How can grinding temperatures be reduced? |
- Decreasing the infeed (depth of cut) - Reducing the wheel speed - Reducing the number of active grits per square inch of the grinding wheel C - Increase work speed vsubw - Use a grinding fluid |
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What are the three causes of wheel wear? |
- Grain fracture - Attritious wear - Bond fracture |
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What is grain fracture? |
When a portion of the grain breaks off, but the rest remains bonded in the wheel. - Edges of the fractured area become new cutting edges. - Tendency to fracture is called friability. |
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What is attritious wear? |
Dulling of individual grains, resulting in flat spots and rounded edges. - Analogous to tool wear in cutting tools - Caused by similar mechanisms including friction, diffusion, and chemical reactions. |
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What is bond fracture? |
When individual grains are pulled out of the bonding materials. - Depends on the wheel grade, among other factors - Usually occurs because grain has become dull due to attritious wear, and resulting cutting force becomes excessive. |
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What is the grinding ratio? |
It indicates the slope of the wheel wear curve. GR=Vsubw/Vsubg, where Vsubw is the volume of work material removed and Vsubg is the corresponding volume of grinding wheel worn. |
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How is dressing accomplished? |
By rotating disc, abrasive sticks, or another grinding wheel against the wheel being dressed as it rotates. |
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Why are wheels dressed? |
- To break off dulled grits to expose new sharp grains. - To remove chips clogged in wheel |
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When is dressing required? |
When the wheel is in the third region of the curve. |
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How is truing accomplished? |
By use of a diamond pointed tool fed slowly and precisely across wheel as it rotates. |
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What depth is used when truing? |
0.025 mm or less. |
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What are the advantages of truing? |
Sharpening the wheel and restoring its cylindrical shape. It also insures straightness across the outside perimeter. |
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To optimize surface finish, we select ___________. |
- Small grit size and dense wheel structure - High wheel speeds v and lower work speeds vsubw - Smaller depths of cut (d) and larger wheel diameters (D) also help. |
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To maximize material removal rate, we select _______________. |
- Larger grit size - More open wheel structure - Vitrified bond. |
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For steel and most cast irons, we use __________________ as the abrasive. |
Aluminium oxide. |
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For most nonferrous metals, we use ___________ as the abrasive. |
Silicon carbide. |
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For hardened tool steels, and certain aerospace alloys, we use _____________________ as the abrasive. |
Cubic boron nitride. |
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For hard abrasive materials (e.g. ceramics, cemented carbides, and glass), we use _______________________ as the abrasive. |
Diamond. |
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For soft metals, we use ____________ grit size and _________ grade wheel. |
Larger-Harder |
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For hard metals, we use ______ grit size and _______ grade wheel. |
Small-Softer |
