Work Holding Methods
Work holding is one of the most important factors when carrying out a lapping operation. The flatness of a part can be adversely affected by incorrect work holding.
A nest type work holder is the most common method of locating work for lapping. This takes the form of a metal or plastic disc slightly smaller in diameter than that of the inside of the conditioning ring. The disc can have one or more holes cut in to suit the shape of the part being lapped but, where possible, these holes should be towards the outside of the disc leaving the centre clear. Small feet are then fixed to the disc face to prevent it running directly on the lap surface. The parts are then located in the disc within the control ring. It is necessary to apply a pressure to the parts being lapped, but this must not be of such a weight that it would distort the parts. A felt pad is often used between the parts and the weight to compensate for variations in height between parts, but no more than say 2mm should be allowed for.
In recent years, a more common method of applying weight has also served to hold work in position during lapping. This method consists of facing the weight with a Self Adhesive Non Skid Disc (these are available in sets of 3). When placing the faced weight on top of the work within the conditioning ring, no nest work holder is required as the part or parts are held in position using the non-skid facing. This method is faster and cleaner than using felt pads, but only 1mm part thickness variations can be compensated for.
On complicated parts, special work holding fixtures may be required. When producing such fixtures, the following points should be borne in mind:
- The weight of the fixture should be balanced when holding the parts.
- Ideally, three or more parts should be held in the fixture.
- Often height setting blocks are helpful when assembling parts into the fixture.
- When holding round or similar shapes where the end face is to be lapped, use a 'V' type location rather than a hole in a block.
- The fixture should be a loose fit within the conditioning ring so it is free floating on the Lapping Plate.