The Technique of Lapping
Hand lapping is now only rarely used. This method is expensive and requires a high degree of skill, and as the skilled specialists retire, companies find it difficult to obtain younger men who possess the same necessary ability. It is still suitable for one-off items, fitting work and small production series. In cases where hand lapping is indispensable, the same conditions as for mechanical lapping must be maintained. In other words, good hand lapping plates (Figure 13) and suitable lapping agents must be available.
Hand lapping is not an easy process. It must be realized, for example, that holding the workpieces with the fingers or with unsuitable workholders can lead to undesirable results such as uneven warming of the part, distortion, or imperfect flatness due to uneven loading of the lapping surface. Even an intermittent, stop-start action can be a source of error. ideally, the part should be lapped with a figure-of-eight movement so as to ensure that all areas on the surface travel approximately the same distance. This type of movement requires large lapping plates. Oval movements are simpler, and are effective if the workpiece is turned several times. One problem is maintaining the plate flatness. Lapping plates should be re-ground, or better still machine-lapped, from time to time. This can be performed by the plate manufacturer. It is common practice to give the plates a slightly convex form in order to counter the usual heavier wear in the middle.
The most important requirement in hand lapping is that the lapping-oil film should be saturated evenly with grit particles. If this lapping film contains too much abraded material, it becomes black and resinous in most cases. A rule of thumb is that when lapping with the intention of removing material,
the movement should be light and even; for polishing, more pressure can be applied. More frequent cleaning of the lapping surface and inspection of the process produce the best results.
A bench lapping machine with a working plate diameter of 300 mm was developed as an interesting intermediate solution (Figure 14) This machine allows both manual and
machine lapping. The difference is that the working plate (lapping
plate) rotates, while the workpiece is moved to-and-fro across the surface. The process is considerably simplified, and results of high to very high quality, comparable to those given by larger machines, can be obtained.