The Technique of Lapping
Some Figures on Lapping
The lapping process actually consists of a very large number of kneading, rolling and cutting actions. A simplified estimation would assume about 200,000 to 300,000 particles per cm squared, uniformly covering half the surface; a typical particle size would be about 15 microns. The lapping area on a lapping machine with a plate diameter of 750 mm is approximately 4000 cm squared. Assuming the above figures, about 800 million lapping abrasive particles would be present. Rotary speed of 70 rpm results in a peripheral speed of 165 m/min, for an average velocity of 100 m/min at the outer diameter of the plate.
Assuming 100 % rolling action, lapping particles of the grain size 15 my or 47 my circumference roll 21 times per millimeter. At the average speed of 100 m/min, a rolling speed of 2.1 million revolutions per minute is possible. As all particles are not the same size and probably interfere with each other during rolling, we can assume a value of approx. 50% here, i.e. approx. 1 million revolutions per minute. At each revolution, the particle leaves three to four impressions, which results in three to four million kneading impacts per minute over a distance of 100 m. Multiplying the figure of 3 million by the number of particles involved in the lapping process at 50 % particle coverage, i.e. 400 million, results in:
(3×10^6 x 400 x 10^6 = 1.2 x 10^15) 1.2 quadrillion (1,200,000,000,000,000)
kneading impacts per minute. This figure supplies an explanation for the rapidity of material removal with lapping machines. Now, looking at grinding, e.g. tangential grinding, it is obvious that in spite of the higher cutting speeds used, the number of active particles in action per time unit is very much less than in lapping. In addition, whereas in grinding only a portion of the workpiece surface is exposed to the wheel at any one time, in lapping the entire surface is exposed the whole time. In other words, there is no so-called “dead” time as with grinding.