Superfinishing

Superfinishing, also known as micromachining, microfinishing, and short-stroke honing, is a metalworking process that improves surface finish and workpiece geometry. This is achieved by removing just the thin amorphous surface layer left by the last process with an abrasive stone or tape; this layer is usually about 1 μm in magnitude. Superfinishing, unlike polishing which produces a mirror finish, creates a cross-hatch pattern on the workpiece.

The superfinishing process was developed by the Chrysler Corporation in 1934.

After a metal piece is ground to an initial finish, it is superfinished with a finer grit solid abrasive. The abrasive is oscillated or rotated while the workpiece is rotated in the opposite direction; these motions are what causes the cross-hatching. The geometry of the abrasive depends on the geometry of the workpiece surface; a stone (rectangular shape) is for cylindrical surfaces and cups and wheels are used for flat and spherical surfaces. A lubricant is used to minimize heat production, which can alter the metallurgical properties, and to carry away the swarf; kerosene is a common lubricant.

The abrasive cuts the surface of the workpiece in three phases. The first phase is when the abrasive first contacts the workpiece surface: dull grains of the abrasive fracture and fall away leaving a new sharp cutting surface. In the second phase the abrasive "self dresses" while most of the stock is being removed. Finally, the abrasive grains become dull as they work which improves the surface geometry.

The average rotational speed of abrasive wheel and/or workpiece is 1 to 15 surface m/min, with 6 to 14 m/min preferred; this is much slower compared to grinding speeds around 1800 to 3500 m/min. The pressure applied to the abrasive is very light, usually between 0.02 to 0.07 MPa (3 to 10 psi), but can be as high as 2.06 MPa (299 psi). Honing is usually 3.4 to 6.9 MPa (490 to 1,000 psi) and grinding is between 13.7 to 137.3 MPa (1,990 to 19,910 psi). When a stone is used it is oscillated at 200 to 1000 cycles with an amplitude of 1 to 5 mm (0.039 to 0.197 in).

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