Grinding and Finishing Processes Questions and Answers

Answer: B

A grinding ratio of 200 means the grinding wheel wears 0.005 times the volume of the material removed.

The grinding ratio (G-ratio) is the volume of work material removed divided by the volume of wheel worn away. A G-ratio of 200 means 200 units of material are removed for each unit of wheel wear, so the wheel wear is 1/200 = 0.005 of the material removed.

The first option inverts the ratio. The grinding ratio is about volumes removed and worn, not about the shape (aspect ratio) of the abrasive grains or the volume share of abrasive in the wheel.

Answer: D

The best surface finish comes from A80L5V.

In the standard marking, A is the abrasive (aluminium oxide), the number is the grain size, L the grade (hardness), 5 the structure and V the vitrified bond. The four wheels differ only in grain size. A higher grain number means a finer mesh and smaller abrasive grains, which cut shallower scratches and give a smoother surface.

Grain sizes 36, 54 and 60 are coarser than 80 and suit faster stock removal rather than fine finishing, so 80 gives the best finish of the four.

Answer: A

The artificial abrasive is aluminium oxide.

Aluminium oxide is made in an electric furnace by fusing bauxite, and it is the abrasive in most general-purpose grinding wheels (marked A or WA). Silicon carbide is the other common manufactured abrasive.

Emery and corundum are natural abrasives mined from the ground; emery is a natural mixture of corundum and iron oxide. Diamond, in the sense the question uses, is the natural mineral; although synthetic diamond is also made, it is classed with diamond rather than offered here as the manufactured abrasive.

Answer: A

Dressing means restoring the cutting action of the wheel.

In use, a grinding wheel becomes glazed, with its abrasive grains dulled, or loaded, with metal particles clogging the pores. Dressing removes that surface layer so that fresh, sharp grains are exposed and the wheel cuts freely again.

Shaping the wheel and making it run concentric with its axis are truing, a separate operation (see the question on truing later in this paper), although one tool, the diamond dresser, often does both. Changing the wheel is simply replacement, not dressing.

Answer: B

The operation is truing.

Truing shapes the wheel so that its face runs concentric with the spindle axis and has the required profile. It is done with a diamond tool that cuts away the high spots, and it is needed after mounting a new wheel or when the wheel runs out of round.

Dressing restores the cutting action of a glazed or loaded wheel by exposing fresh grains. Balancing corrects uneven weight distribution so that the wheel does not vibrate. Glazing is a fault, the dulled shiny face that dressing removes, not an operation.

Answer: B

The correct statement is: grade is soft, grit size is not fine and abrasive is green silicon carbide.

Grinding wheel markings follow a fixed order: abrasive, grit size, grade, structure, bond. Here GC is green silicon carbide; 20 is a coarse grit; A is the softest end of the grade scale, which runs from A (soft) to Z (hard); 7 is the structure; and E is the bond, the letter for shellac.

A and WA are aluminium oxide abrasives, not the green silicon carbide marked GC. The bond letter for metal is M, not E. Black silicon carbide is marked C, and grade A is soft, not hard.

Answer: D

The answer is All of the above.

Each of these removes metal with hard abrasive grains rather than a defined cutting edge. Grinding uses a rotating bonded abrasive wheel. Honing uses bonded abrasive stones that rotate and reciprocate in a bore to correct its geometry and finish. Lapping uses loose fine abrasive in a paste between the work and a lap to produce very flat or very fine surfaces.

Since all three are abrasive processes, picking only one of them is incomplete.

Answer: D

The answer is All of the above.

Aluminium oxide is the most common manufactured abrasive, used in grinding wheels for steels. Cubic boron nitride is a superabrasive, second only to diamond in hardness, used for grinding hardened steels. Sand, mainly quartz (silica), is a natural abrasive used in sandpaper and sand blasting.

Since each one is used as an abrasive, no single option is the complete answer.