Learn about Lapping
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An abrasive finishing process, lapping is used to create extremely flat and smooth surfaces. In some cases, lapping can remove marks left by other finishing processes, such as grinding. Lapping begins with a soft material, such as lead, cast iron, cloth or copper, containing embedded abrasives. This material, known as a lap, is rubbed against the surface to be lapped (or vice versa). In hand lapping, the material is moved in a figure-eight pattern over the lap.
Due to the nature of the lap and abrasive, the embedded abrasives perform the actual finishing, while the lap simply provides a bed for the abrasives. This allows the abrasives to cut away very fine amounts of the material while the lap remains intact. The process removes an extremely fine layer of material, typically gauged in Helium Light Bands (HLB), which are a minute fraction of an inch, and uses abrasives ranging from fine powders to 120 grit.
The process is most useful on hard materials, as abrasives can become stuck in softer work pieces. Because of its limited range, lapping was once used only for specific applications; however, the number of operations and products calling for the fine finish provided by lapping has increased, and now the process has become fairly common. Unfortunately, despite its obvious advantages, it is also a slow, costly finishing method, and should be selected only in situations where the flatness of the work piece is of primary importance.
Silicon carbide, garnet and aluminum oxide are regularly used as abrasives for the lapping process, although their use varies per work piece. Typically, they are held between lap and work piece in grease, oil or another liquid. Both metal and nonmetal materials can be lapped; silicon carbide, for example, is effective on cast iron, while garnet is used to lap certain glass and plastic pieces. Diamond dust, is also a commonly used lapping abrasive. Both hand lapping services and machine lapping equipment are available.
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