The Technique of Lapping
Aspects of Production Lapping
As today’s flat lapping machines are capable of high material removal rates and therefore short machining times, the idle times are becoming more and more significant in the cost-effectiveness of the process. These trends lead to the development of loading, turning, and unloading devices, which are usually designed as an integral part of the flat lapping machines, fixed to the bed, and capable of adjustment to the required height.
As high material removal rates depend mainly on high loading, pneumatic cylinders (Figure 59) are often used to apply the lapping pressure on production machines. However, the pressure must be appropriate to the surface, the thickness, and the type of workpieces; a certain amount of experience is needed in applying it. A high-performance cooling system should be provided to dissipate the heat generated by the high material removal rates (also see Figure 74, on the left side of the machine).
Assuming allowances between 0.05 and 0.1 mm and 50% of the surface area are being lapped, lapping times usually lie in a range of 10 to 20 minutes per side. It is unfortunate that lapping is still seen only as a means of obtaining the final levels of accuracy, flatness, and finish. Pre-ground parts are usually supplied to the lapping machine with allowances between 0.02 and 0.05 mm, but it is not yet widely realized that in the same time, production machines such as that shown in Figure 59 can remove from 0.2 to 1 mm of material, depending on the surface area and material composition.
A. W. Stahli, lapping machine manufacturers, achieve economically lapping rates on 80% of all parts sub-contracted for machining, without regrinding. The material and the surface area to be machined are more influential on the lapping process than on grinding because lapping particles cannot be relied upon to machine an indefinitely large surface without breaking. It is thus understandable that a steel ring, e.g. diameter 200/180×20 mm, can be lapped in a shorter time than a disc of the same diameter.
Idle times assume most importance when large numbers of small parts have to be lapped on both sides. Manual turning of 1000 and more workpieces per load can take from 20 to 30 minutes, whereas the same work can be done with a special device in 2 to 3 minutes. Depending on the size of the machine and the type of loading-turning-unloading device, automatic handling is feasible for parts down to 0.2 mm thickness.