Grinding Paste

Lapping paste is a mixture of hard abrasive particles in a suitable base. The base can be oil-based like grease or water soluble lubricant. The hard particles used are carborandum, aluminum oxide, silica or silicon carbide, glass, boron carbide, etc.

Lapping pastes are used for controlled removal and are used for the close mating of surfaces and for removing rust and the brightening of a metal surface. They are used for lapping of the exhaust and the inlet valves of engines, seat repairs of globe valves, the overhaul of reciprocating compressor valves, etc.

The lapping paste are graded from extra coarse to extra fine. In extra coarse there are few grits of larger size abrasives, and it is used for rough lapping. In extra fine grinding paste, there are a very large number of grits of very fine particles and it is used for fine lapping. The larger the grit rating of the lapping paste, the finer is the paste.

Correct Methods of Manual Lapping

When using hand lapping there are two methods of lapping used, the first is the figure of eight lapping and the other is circular. In case a guide is available, like the nozzle nut in a fuel injector the circular method can be used. The figure of eight lapping needs a mature hand and best not left to novices. It should be remembered that if you can stop that leakage with a minimum amount of lapping then over lapping should be avoided. Under lapping is always preferred to over lapping and lapping must not be done just for lapping sake but the advice of the manufacturers must be sought.

In a figure of eight method a numeric shape of eight is made on the surface plate by the operator. In a circular method the job piece is rotated circularly in place.

The exposed metal should give a uniform polished look and the quality of the lapping can be made out by the patterns. Any stray scratches on the lapped surface should be avoided as under high pressure they can become channels for leakage.