Diso , Justine F.
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
Process
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).
Superfinishing can give a surface finish of 0.01 μm.
3 types superfinishing
Through-feed
This type of superfinishing is used for cylindrical workpieces. The workpiece is rotated between two
drive rollers, which also move the machine as well. Four to eight progressively finer abrasive stones are
used to superfinish the workpiece. The stones contact the workpiece at a 90° angle and are oscillated
axially. Examples of parts that would be produced by process include tapered rolls, piston pins, shock
absorber rods, shafts, and needles.
Plunge
This type is used to finish irregularly shaped surfaces. The workpiece is rotated while the abrasive
plunges onto the desired surface.
Wheels
Abrasive cups or wheels are used to superfinish flat and spherical surfaces. The wheel and workpiece are
rotated in opposite directions, which creates the cross-hatching. If the two are parallel then the result if
a flat finish, but if the wheel is tilted slightly a convex or concave surface will form.
Abrasives
Common abrasives used for superfinishing include aluminum oxide, silicon carbide, cubic boron nitride
(CBN) and diamond.
Aluminum oxide is used for "roughing" operations. Silicon carbide, which is harder than aluminum oxide,
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.
Applications
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.
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.
