Originally Posted by triump  what is lapping and how is it done? Pat |
What is the context of your question? To coin a pun lapping has many facets and one of them is just that; gemstone faceting where a gem is held in a precise position against a flat rotating disc with an abrasive paste or slurry on it. The abrasive wears, or polishes, a flat spot, a facet, on the gemstone which is then repositioned through a precise angle for another facet until eventually you have the sort of thing you see in jewellery showcases. All lapping works the same way; two surfaces moving against each other with an abrasive between.
Lapping is also used to get better contact between mating surfaces such as cpu chips and heatsinks. This can be done either by lapping each piece against a flat surface which can be as simple as sandpaper on a sheet of glass or by lapping the two pieces against each other. In the second case two nicely mating surfaces are produced but they are almost certainly not flat.
Lapping does not have to be done on flat surfaces; tapered valves can be lapped into their seats, spheres can be lapped between two rotating cups.
Special lapping techniques can be used to generate either spherical surfaces or perfectly flat surfaces. Two, initially flat, surfaces lapped together in a circular pattern will eventually become one concave spherical surface and one convex spherical surface. This technique has been used for centuries to make concave or convex lense and mirrors for telescopes.
Conversely, if three surfaces are lapped against each other in pairs a small amount at a time eventually, with experience and care, it is possible to generate three very flat surfaces. This is how optical flats are made; pieces of glass that are flat to within fractions of a wave length of light.
One large commercial application of lapping is that done on silicon wafers that go into chip manufacture. This is done on lapping machines that have a flat rotating disc while the wafers are held in carriers which sort of roam around in a random fashion. The random fashion is needed because this replicates the three plate effect used for optical flats; very flat surfaces are generated on both the plate and the wafer.
Another large, probably the largest, application of lapping is ball bearing lapping. Two flat discs are rotated with many balls between them and eventually all the balls are lapped down to very close to the same size with a very smooth finish.