Superfinishing

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Superfinishing, also known as microfinishing [1] 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 of fragmented or smeared metal left by the last process with an abrasive stone or tape; this layer is usually about 1 μm in magnitude.

Contents

Process

After a metal piece is ground to an initial finish, it is superfinished with a finer grit abrasive stone. The stone is oscillated or rotated while the workpiece is moved in such a way that each bonded grain of abrasive follows a random path with variations in speed, direction and pressure. This multi-motion is a key feature of superfinishing because it prevents the sort of smeared finish that results from built up edge. In this way, superfinishing is similar to lapping, but with a bonded abrasive stone rather than loose or embedded abrasive. [2] 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. [3] 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. [4] [5]

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. [3]

Superfinishing differs from grinding in that the relative speed between the abrasive and workpiece is kept low enough to avoid heating and the pressure is light. Superfinishing differs from long-stroke honing in that a controlled viscosity lubricant is used so that an oil wedge forms that automatically terminates cutting at a predetermined cutting pressure. [2] : 359  Superfinishing is unique in involving rapid changes in the speed, direction, and pressure on each grain of abrasive in the abrasive stone. This "multi-motion" is critical to achieving the finest possible finish because it prevents to re-formation of an amorphous layer of smear metal due to built up edge. [2] : 404 

Superfinishing can give a surface finish of 0.01 μm. [3] [5]

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 pins, shock absorber rods, shafts, and needles. [3]
Plunge
This type is used to finish irregularly shaped surfaces. The workpiece is rotated while the abrasive plunges onto the desired surface. [3]
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. [3]

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. [3]

Abrasive grains must be very fine to be used with superfinishing; usually 5–8 μm. [5]

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. [3] [6]

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. [7]

History

Superfinishing was conceived of by the Chrysler Corporation in 1934 in response to widespread damage sustained by wheel bearings installed in automobiles shipped by rail from Detroit to California. The problem manifested as a buzzing or clicking sound that annoyed buyers of new cars and trucks, but the cause was unclear and so car dealerships in the Western United States eventually resorted to replacing all factory-installed wheel bearings with virgin bearings prior to sale. [2] : 29–40 

Brinelling of the bearing races was eventually identified as the cause of the noise, but a way to prevent this damage proved elusive. Thousands of design and process variations were tried in attempting to prevent brinelling, but none had any effect. A batch of bearings that had been brinelled was reworked by removing the brinell marks by hand with fine sandpaper and these reworked bearings were installed in automobiles that were loaded onto a train and shipped from Detroit to California as an experiment. A tool maker traveled on the same train and then inspected the bearings when they arrived in California. He found the bearings to be damage-free, making this the first method to have any effect on the problem. [2] : 39 

The hand finishing method of removing amorphous "grinding fuzz" from crystalline base metal using sandpaper was then mechanized for low-rate production. Small grinding stones in a flexible rubber holder were driven by a manually operated drill press to remove grinding fuzz from bearing cups. Fully automated superfinishing machines were then developed. [2] : 39 

Random motion between the abrasive stone and workpiece was found to be critical to the superfinishing process. Hand lapping and hand sanding naturally involve irregular variations in direction, speed, and pressure, but mechanized metal finishing processes up to the development of superfinishing had not appreciated the importance of "multi-motion." [2] : 60 

Investigation showed that multi-motion is important to achieving the best possible finish because it avoids smearing caused by built up edge. [2] : 198  Multi-motion avoids smearing by continually unloading the cutting edge of a given abrasive grain so that the built-up edge that has formed is interrupted, and never builds to a size sufficient to smear against the workpiece. [2] : 387 

See also

Related Research Articles

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, reflective surface, the process can also involve roughening as in satin, matte or beaded finishes. In short, the ceramics which are used to cut, grind and polish other softer materials are known as abrasives.

<span class="mw-page-title-main">Machining</span> Material-removal process; manufacturing process

Machining is a manufacturing process where a desired shape or part is created using the controlled removal of material, most often metal, from a larger piece of raw material by cutting. Machining is a form of subtractive manufacturing, which utilizes machine tools, in contrast to additive manufacturing, which uses controlled addition of material.

<span class="mw-page-title-main">Tumble finishing</span> Technique for smoothing and polishing a rough surface on relatively small parts

Tumble finishing, also known as tumbling or rumbling, is a technique for smoothing and polishing a rough surface on relatively small parts. In the field of metalworking, a similar process called barreling, or barrel finishing, works upon the same principles.

<span class="mw-page-title-main">Grinding machine</span> Machine tool used for grinding

A grinding machine, often shortened to grinder, is any of various power tools or machine tools used for grinding. It is a type of material removal using an abrasive wheel as the cutting tool. Each grain of abrasive on the wheel's surface cuts a small chip from the workpiece via shear deformation.

<span class="mw-page-title-main">Shot peening</span> Cold metal working process to produce compressive residual stress

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<span class="mw-page-title-main">Grinding wheel</span> Abrasive cutting tool for grinders

Grinding wheels are wheels that contain abrasive compounds for grinding and abrasive machining operations. Such wheels are also used in grinding machines.

<span class="mw-page-title-main">Lapping</span> Process of removing material from two workpieces

Lapping is a machining process in which two surfaces are rubbed together with an abrasive between them, by hand movement or using a machine.

<span class="mw-page-title-main">End mill</span> Milling cutter designed to cut axially

An end mill is a type of milling cutter, a cutting tool used in industrial milling applications. They can have several end configurations: round (ball), tapered, or straight are a few popular types. They are most commonly used in "milling machines" that move a piece of material against the end mill to remove chips of the material to create a desired size or shape. It is distinguished from the drill bit in its application, geometry, and manufacture. While a drill bit can only cut in the axial direction, most milling bits can cut in the radial direction. Not all mills can cut axially; those designed to cut axially are known as end mills.

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<span class="mw-page-title-main">Polishing (metalworking)</span> Abrasive process for creating smooth finished surfaces

Buffing are finishing processes for smoothing a workpiece's surface using an abrasive and a work wheel or a leather strop. Technically, polishing refers to processes that uses an abrasive that is glued to the work wheel, while buffing uses a loose abrasive applied to the work wheel. Polishing is a more aggressive process, while buffing is less harsh, which leads to a smoother, brighter finish. A common misconception is that a polished surface has a mirror-bright finish, however, most mirror-bright finishes are actually buffed.

<span class="mw-page-title-main">Tool wear</span> Gradual failure of cutting tools due to regular use

In machining, tool wear is the gradual failure of cutting tools due to regular operation. Tools affected include tipped tools, tool bits, and drill bits that are used with machine tools.

<span class="mw-page-title-main">Diamond tool</span> Cutting tool with diamond grains

A diamond tool is a cutting tool with diamond grains fixed on the functional parts of the tool via a bonding material or another method. As diamond is a superhard material, diamond tools have many advantages as compared with tools made with common abrasives such as corundum and silicon carbide.

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<span class="mw-page-title-main">Grinding (abrasive cutting)</span> Machining process using a grinding wheel

Grinding is a type of abrasive machining process which uses a grinding wheel as cutting tool.

<span class="mw-page-title-main">Honing (metalworking)</span> Production of a precise surface on a metal workpiece

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<span class="mw-page-title-main">Burnishing (metal)</span> Deformation of a metal surface due to friction

Burnishing is the plastic deformation of a surface due to sliding contact with another object. It smooths the surface and makes it shinier. Burnishing may occur on any sliding surface if the contact stress locally exceeds the yield strength of the material. The phenomenon can occur both unintentionally as a failure mode, and intentionally as part of a metalworking or manufacturing process. It is a squeezing operation under cold working.

<span class="mw-page-title-main">Race (bearing)</span> Track in a bearing along which the rolling elements ride

The rolling-elements of a rolling-element bearing ride on races. The large race that goes into a bore is called the outer race, and the small race that the shaft rides in is called the inner race.

Surface grinding is done on flat surfaces to produce a smooth finish.

<span class="mw-page-title-main">Flat honing</span> Metalworking grinding process

Flat honing is a metalworking grinding process used to provide high quality flat surfaces. It combines the speed of grinding or honing with the precision of lapping. It has also been known under the terms high speed lapping and high precision grinding.

Grinding wheel wear is an important measured factor of grinding in the manufacturing process of engineered parts and tools. Grinding involves the removal process of material and modifying the surface of a workpiece to some desired finish which might otherwise be unachievable through conventional machining processes. The grinding process itself has been compared to machining operations which employ multipoint cutting tools. The abrasive grains which make up the entire geometry of wheel act as independent small cutting tools. The quality, characteristics, and rate of grinding wheel wear can be affected by contributions of the characteristics of the material of the workpiece, the temperature increase of the workpiece, and the rate of wear of the grinding wheel itself. Moderate wear rate allows for more consistent material size. Maintaining stable grinding forces is preferred rather than high wheel wear rate which can decrease the effectiveness of material removal from the workpiece.

References

Notes

  1. ↑ "What is Microfinishing?". Neuteq.
  2. 1 2 3 4 5 6 7 8 9 Swigert, Arthur (1940). The Story of Superfinishing. Ann Arbor, Michigan: The Ann Arbor Press. OCLC   568009.
  3. 1 2 3 4 5 6 7 8 Darmann Abrasive Products. "The Art of Superfinishing" (PDF). Archived from the original (PDF) on 2016-09-09. Retrieved 2017-02-01.
  4. ↑ Todd, Allen & Alting 1994 , pp. 135–136.
  5. 1 2 3 Murty 1996 , p. 187.
  6. ↑ "Microfinishing Solutions". Neuteq.
  7. ↑ Krantz, Timothy L. (1 March 2000). Gear Durability Shown To Be Improved by Superfinishing (Technical report). NASA.

Bibliography

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