More on grinding and polishing
In the previous article, the subject of crankshaft grinding was discussed. In addition to the perils of grinding incorrectly in terms of ‘grinding cracks’, there is evidence to suggest that the direction of travel of the workpiece relative to the grinding wheel is also significant.
When cylindrical grinding takes place, the grinding wheel, which is usually much larger than the journal or crankpin being ground, rotates at a much higher speed than the workpiece. The ratio of surface (peripheral) speeds between the grinding wheel and workpiece is very large, so one might expect that the direction of rotation of the crankshaft during grinding would be unimportant, but there is widespread agreement that the direction of rotation of crankshaft travel should be in the same direction as that of the grinding wheel. This means that, at the point of material removal, the surfaces are travelling in opposite directions (if this is confusing, think of gears in mesh – they rotate in opposite senses but in the mesh area the teeth are both travelling in the same linear direction and at the same pitch line velocity).
The act of grinding and material removal tends to shear and lift material from the surface. When the grinding takes place as described above, the action of the grinding wheel on the ground surface acts to ‘flatten’ any material that has been pulled up from the surface (imagine a splinter of material still attached to a piece of wood).
In choosing the direction of crankshaft travel, we need to refer to the direction of rotation of the crankshaft in the engines, as we want the ‘splinters’ still attached to the crankshaft to be flattened by the rotation of the crankshaft rather than be torn out. During grinding, the crankshaft should rotate in the same direction as it does in the engine.
Any polishing operation should also aim to achieve the same effect. The end result should be that any ‘splinters’ should be as flat as possible to the crankshaft surface. Polishing is often carried out to achieve a level of surface finish that is not readily achieved by grinding.
There are several methods of crankshaft polishing. The simplest is manual ‘tape polishing’, where a special-purpose polishing tape is held with manually applied pressure against the crankshaft while the crankshaft rotates. The tape is very strong and has very fine particles of abrasive (commonly industrial diamond) embedded or bonded onto its surface. The polishing action takes place over the ‘wrap’ angle formed by the tape, and the maximum polishing pressure is nominally at the halfway point of the wrap angle.
A better method than manual polishing is continuous tape polishing, where a slowly moving continuous tape ‘belt’ polishes the crankshaft. It has many advantages over the manual – for example, the fact that it is not a manual process removes the ‘artisan’ aspect of polishing; and the pressure and duration can be easily controlled. The process is also much more repeatable than manual tape polishing, so the machine operator can be put to good use on other skilled tasks.
An alternative manual method is to produce a specially sized grinding lap into which polishing tape is fixed or a polishing compound is used; this arrangement is sometimes referred to as a ‘nutcracker’. This results in a more even pressure during polishing and the pressure is more easily controlled, either manually or by pre-loading the nutcracker using a spring of known load.
Written by Wayne Ward