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.
Role of Lubricants in Lapping Paste
Oil and grinding paste have contrary functions, while abrasive increases the rubbing and cutting the oil seeks to reduce it. However oil is used to control the abrasive and cutting action. It also is the base in which the abrasive particles float and move under the action of the mating surfaces. Oil or grease is However used in a certain measured amount.
Some people like to use additional lubricant during lapping process. However it must be remembered that the addition of more oil is reducing the effort and the cutting action of the abrasive particles. The abrasive particles are also washed away if extra lubricant is added.
During lapping if you want fast removal of the metal you must lap till the effort to move the mating parts reduces. The reduction of the force indicates that the abrasives have broken down to harmless paste and are now acting like a lubricant. Therefore you must remove the old lapping paste and put fresh paste.
If sufficient care is not taken during this time then metal to metal contact would take place and the effect of spot welding and scuffing would spoil the finish desired.
Taking Care of lapping Paste
Lapping is an art and a master machinist would work down from a coarse to a very fine grade in a proper sequence to get the desired finish and precision as required. Each grade has carefully controlled similar sizes of abrasive particles suspended in the base. Any alien particle of a coarse abrasive in an extra fine tin can create frustration especially when you are on your final finish.
It is a good practice to keep all the lapping and the grinding paste cans closed when not in use. Also when a fresh charge of lapping paste is taken it should be rubbed thoroughly between the fingers to make out for any contaminants. This is very beneficial when you are doing fine lapping.
Cleanliness in Precision Lapping
Cleanliness is one of the most important factors in precision lapping as is the quality of the lapping paste and the skill of the operator. It is a good practice that the work table is cleaned with clean rags and blown dry with compressed air before the lapping. If a surface plate is being used then it should be washed with clean kerosene and then blown dry with compressed air.
Also when removing used lapping paste from items being lapped, they should be wiped with soft tissue paper and cleaned with compressed air.
- Based on the experience of the author in Merchant Marine