The lap is then used to cut a harder material - the work piece. The other form of lapping involves a softer material such as pitch or a ceramic for the lap, which is "charged" with the abrasive. This produces microscopic conchoidal fractures as the abrasive rolls about between the two surfaces and removes material from both.
The first type of lapping (traditionally called grinding), involves rubbing a brittle material such as glass against a surface such as iron or glass itself (also known as the "lap" or grinding tool) with an abrasive between them. Lapping is a machining process in which two surfaces are rubbed together with an abrasive between them, by hand movement or using a machine. Lapping and sanding are subsets of grinding. This is why the terms are usually used separately in shop-floor practice. The term "cutting" is often understood to refer to the macroscopic cutting operations, while "grinding " is often mentally categorized as a "separate" process. Each grain of abrasive functions as a microscopic single-point cutting edge (although of high negative rake angle), and shears a tiny chip that is analogous to what would conventionally be called a "cut" chip (turning, milling, drilling, tapping, etc.). Grinding is a subset of cutting, as grinding is a true metal-cutting process. Compared to "regular" machining, it is usually better suited to taking very shallow cuts, such as reducing a shaft’s diameter by half a thousandth of an inch or 12.7 μm.
It is usually better suited to the machining of very hard materials than is "regular" machining, and until recently it was the only practical way to machine such materials as hardened steels. It can produce very fine finishes and very accurate dimensions yet in mass production contexts it can also rough out large volumes of metal quite rapidly. Grinding practice is a large and diverse area of manufacturing and tool-making.
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