Lapping - Operation

Operation

By way of example, a piece of lead may be used as the lap, charged with emery, and used to cut a piece of hardened steel. The small plate shown in the first picture is a hand lapping plate. That particular plate is made of cast iron. In use, a slurry of emery powder would be spread on the plate and the workpiece simply rubbed against the plate, usually in a "figure-eight" pattern.

The second picture is of a commercially available lapping machine. The lap or lapping plate in this machine is 30 centimetres (12 in) in diameter, about the smallest size available commercially. At the other end of the size spectrum, machines with 8-to-10-foot-diameter (2.4 to 3.0 m) plates are not uncommon, and systems with tables 30 feet (9.1 m) in diameter have been constructed. Referring to the second picture again, the lap is the large circular disk on the top of the machine. On top of the lap are two rings. The workpiece would be placed inside one of these rings. A weight would then be placed on top of the workpiece. The weights can also be seen in the picture along with two fiber spacer disks that are used to even the load.

In operation, the rings stay in one location as the lapping plate rotates beneath them. In this machine, a small slurry pump can be seen at the side, this pump feeds abrasive slurry onto the rotating lapping plate.

When there is a requirement to lap very small specimens (from 3" down to a few millimetres), a lapping jig can be used to hold the material while it is lapped (see Image 3, lapping machine and jig). A jig allows precise control of the orientation of the specimen to the lapping plate and fine adjustment of the load applied to the specimen during the material removal process. Due to the dimensions of such small samples, traditional loads and weights are too heavy as they would destroy delicate materials. The jig sits in a cradle on top of the lapping plate and the dial on the front of the jig indicates the amount of material removed from the specimen.