Superfinishing, also known as micromachining 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; 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.[1]
The superfinishing process was developed by the Chrysler Corporation in 1934.[1]
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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.[1] 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.[2][3]
The abrasive cuts the surface of the workpiece in three phases. The first phase is when the abrasive first contacts the workpiece surface the dull grains of the abrasive fracture and fall away, which produces a sharp new cutting surface. In the second phase the abrasive "self dresses", where a most of the stock is removed. Finally, the abrasive grains dull, which improves the surface geometry.[1]
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 0.20 in).[3]
Superfinishing can give a surface finish of 0.01 μm.[1][3]
There are three types superfinishing: Through-feed, plunge, and wheels.
Common abrasives used for superfinishing include: aluminium oxide, silicon carbide, cubic boron nitride (CBN), and diamond.
Aluminium oxide is used for "roughing" operations. Silicone carbide is harder than aluminium oxide, so it is used for "finishing" operations. CBN and diamond are not as commonly used, but find use with specialized materials, such as ceramics and M50 tool steel. Note that graphite may be mixed with other abrasives to add lubricity and to enhance the appearance of the finish.[1]
Abrasive grains must be very fine to be used with superfinishing; usually 5–8 μm.[3]
Advantages of superfinishing include: increasing part life, decreasing wear, closer tolerances, higher load bearing surfaces, better sealing capabilities, and elimination of a break in period.[1]
The main disadvantage is that superfinishing requires grinding or a hard turning operation beforehand, which increases cost.[4] Superfinishing has a lower cutting efficiency because of smaller chips and lower material removal rate. Superfinishing stones are softer and wear more quickly, however they do not need to be dressed.[5]
Common applications include: steering rack components, transmission components, fuel injector components, camshaft lobes, hydraulic cylinder rods, bearing races, needle rollers, and sharpening stones and wheels.[1]
It has been proven that superfinishing certain parts makes them more durable. For example if the teeth in a gear are superfinished they will last up to four times as long.[6]