Superfinishing
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Superfinishing, also known as micromachining, microfinishing, and short-stroke honing, is a
metalworking Metalworking is the process of shaping and reshaping metals to create useful objects, parts, assemblies, and large scale structures. As a term it covers a wide and diverse range of processes, skills, and tools for producing objects on every scale ...
process that improves
surface finish Surface finish, also known as surface texture or surface topography, is the nature of a surface as defined by the three characteristics of lay, surface roughness, and waviness.. It comprises the small, local deviations of a surface from the p ...
and workpiece geometry. This is achieved by removing just the thin
amorphous In condensed matter physics and materials science, an amorphous solid (or non-crystalline solid, glassy solid) is a solid that lacks the long-range order that is characteristic of a crystal. Etymology The term comes from the Greek language ...
surface layer left by the last process with an
abrasive An abrasive is a material, often a mineral, that is used to shape or finish a workpiece through rubbing which leads to part of the workpiece being worn away by friction. While finishing a material often means polishing it to gain a smooth, reflec ...
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 Stellantis North America (officially FCA US and formerly Chrysler ()) is one of the " Big Three" automobile manufacturers in the United States, headquartered in Auburn Hills, Michigan. It is the American subsidiary of the multinational automoti ...
in 1934.


Process

After a
metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typi ...
piece is ground to an initial finish, it is superfinished with a finer grit solid
abrasive An abrasive is a material, often a mineral, that is used to shape or finish a workpiece through rubbing which leads to part of the workpiece being worn away by friction. While finishing a material often means polishing it to gain a smooth, reflec ...
. 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 Swarf, also known as chips or by other process-specific names (such as turnings, filings, or shavings), are pieces of metal, wood, or plastic that are the debris or waste resulting from machining, woodworking, or similar subtractive (material- ...
;
kerosene Kerosene, paraffin, or lamp oil is a combustible hydrocarbon liquid which is derived from petroleum. It is widely used as a fuel in aviation as well as households. Its name derives from el, κηρός (''keros'') meaning " wax", and was reg ...
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 , but can be as high as . Honing is usually and grinding is between . When a stone is used it is oscillated at 200 to 1000 cycles with an amplitude of . Superfinishing can give a surface finish of 0.01 μm.


Types

There are three types superfinishing: Through-feed, plunge, and wheels. ;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 pin In internal combustion engines, the gudgeon pin (UK, wrist pin or piston pin US) connects the piston to the connecting rod, and provides a bearing for the connecting rod to pivot upon as the piston moves.Nunney, Malcolm James (2007) "The Reciproc ...
s,
shock absorber A shock absorber or damper is a mechanical or hydraulic device designed to absorb and damp shock impulses. It does this by converting the kinetic energy of the shock into another form of energy (typically heat) which is then dissipated. Mos ...
rods,
shaft Shaft may refer to: Rotating machine elements * Shaft (mechanical engineering), a rotating machine element used to transmit power * Line shaft, a power transmission system * Drive shaft, a shaft for transferring torque * Axle, a shaft around wh ...
s, and
needle Needle may refer to: Crafting * Crochet needle, a tool for making loops in thread or yarn * Knitting needle, a tool for knitting, not as sharp as a sewing needle * Sewing needle, a long slender tool with a pointed tip * Trussing needle, a long sl ...
s. ;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 Silicon carbide (SiC), also known as carborundum (), is a hard chemical compound containing silicon and carbon. A semiconductor, it occurs in nature as the extremely rare mineral moissanite, but has been mass-produced as a powder and crystal ...
,
cubic boron nitride Boron nitride is a thermally and chemically resistant refractory compound of boron and nitrogen with the chemical formula BN. It exists in various crystalline forms that are isoelectronic to a similarly structured carbon lattice. The hexagon ...
(CBN) and
diamond Diamond is a solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Another solid form of carbon known as graphite is the chemically stable form of carbon at room temperature and pressure, ...
. 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
ceramic A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelai ...
s and M50 tool steel. Note that
graphite Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on la ...
may be mixed with other abrasives to add lubricity and to enhance the appearance of the finish. Abrasive grains must be very fine to be used with superfinishing; usually 5–8 μm.


Advantages and disadvantages

Advantages of superfinishing include: increasing part life, decreasing
wear Wear is the damaging, gradual removal or deformation of material at solid surfaces. Causes of wear can be mechanical (e.g., erosion) or chemical (e.g., corrosion). The study of wear and related processes is referred to as tribology. Wear in ...
, closer tolerances, higher load bearing surfaces, better sealing capabilities, and elimination of a break in period. The main disadvantage is that superfinishing requires grinding or a hard turning operation beforehand, which increases cost. Superfinishing has a lower cutting efficiency because of smaller chips and lower
material removal rate Material removal rate (MRR) is the amount of material removed per time unit (usually per minute) when performing machining operations such as using a lathe or milling machine Milling is the process of machining using rotary cutters to remove m ...
. Superfinishing stones are softer and wear more quickly, however they do not need to be dressed.


Applications

Common applications include:
steering rack A rack and pinion is a type of linear actuator that comprises a circular gear (the ''pinion'') engaging a linear gear (the ''rack''). Together, they convert rotational motion into linear motion. Rotating the pinion causes the rack to be driven i ...
components,
transmission Transmission may refer to: Medicine, science and technology * Power transmission ** Electric power transmission ** Propulsion transmission, technology allowing controlled application of power *** Automatic transmission *** Manual transmission ** ...
components,
fuel injector Fuel injection is the introduction of fuel in an internal combustion engine, most commonly automotive engines, by the means of an injector. This article focuses on fuel injection in reciprocating piston and Wankel rotary engines. All com ...
components,
camshaft A camshaft is a shaft that contains a row of pointed cams, in order to convert rotational motion to reciprocating motion. Camshafts are used in piston engines (to operate the intake and exhaust valves), mechanically controlled ignition systems ...
lobes,
hydraulic Hydraulics (from Greek: Υδραυλική) is a technology and applied science using engineering, chemistry, and other sciences involving the mechanical properties and use of liquids. At a very basic level, hydraulics is the liquid counter ...
cylinder rods, bearing races,
needle roller A needle roller bearing is a special type of roller bearing which uses long, thin cylindrical rollers resembling needles. Ordinary roller bearings' rollers are only slightly longer than their diameter, but needle bearings typically have rollers ...
s, 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.


See also

*
Abrasive machining Abrasive machining is a machining process where material is removed from a workpiece using a multitude of small abrasive particles. Common examples include grinding, honing, and polishing. Abrasive processes are usually expensive, but capable of t ...
* Honing *
Lapping 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 often follows other subtractive processes with more aggressive material removal as a first ste ...


References


Notes


Bibliography

*. *{{citation , first1 = Robert H. , last1 = Todd , first2 = Dell K. , last2 = Allen , first3 = Leo , last3 = Alting , year = 1994 , title = Manufacturing Processes Reference Guide , publisher = Industrial Press Inc. , url = https://books.google.com/books?id=6x1smAf_PAcC , isbn = 0-8311-3049-0. Grinding and lapping