The Technique of Lapping
The Working Plate
The usual material for the working plate is gray cast iron, hardened by a special method to the required value. The structure should be fine-grained, and free of blow-holes.
For lap-polishing, especially when using diamond grit, plates of steel, copper, zinc, aluminum or even wood are used. Working plates of pitch, plastic, fabrics or coated films are used for ultra-fine polishing. Today, all the crucial parts of a lapping machine can be designed for optimal mutual efficiency. If minimum wear is required for example, the working plate can be made of sufficient hardness, but in view of the ease with which continuous reconditioning can be performed, soft to medium-hard plates are most often used, and these have the additional advantage of being cheaper than harder plates.
Once a lapping plate acquires an imperfect profile (convex, concave lapping surface), correction takes a long time. Some machines are fitted with a device for rapid shape correction, consisting of a means of slightly distorting the plate during use. Residual errors are then compensated for by moving the conditioning rings to the appropriate position as described earlier. Practice shows that a slightly convex working plate can give good results in a short time in certain cases, for example when lapping parts with large surfaces. The rapid form correction device is useful in such applications as well.
The working plates are usually provided with radial slots to drain the used lapping medium off the surface. Unslotted plates are best small, irregular workpieces, or when lapping carbide metal or ceramics using water-based media with a high grit content. Lapping or polishing with diamond powder requires a surface profile similar to a V-thread, flat thread or taper thread. Plastic, fabric or felt films are bonded on base working plates. They are good carriers in terms of structure in most cases, but require frequent cleaning or replacement.
New multi-metal working plates are a combination of 2-3 different materials. They are available as spiral inserts, in mosaic or round form. The first type has the advantage of continuous material removal as the workpieces pass over the plate, as well as additional grooves. The multi-metal working plates are much more expensive and are suitable for use with diamond lapping agents. The conditioning rings must be matched correspondingly.
Further working plates with fixed grains (lapping powders of all types, mixed with binding agent, baked or fired) open up new possibilities.
In this context, we would like to point out that water-based fluids are also suitable for this purpose. This prepares the way for “clean lapping”. Environmental protection will also play an important part in micro-machining technology in future.