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]

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). [5]

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]

Advantages and disadvantages

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. [3]

The main disadvantage is that superfinishing requires grinding or a hard turning operation beforehand, which increases cost. [6] 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. [2] : 575–594 

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] [7]

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. [8]

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 

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">Grinding machine</span> Machine tool used for grinding

A grinding machine, often shortened to grinder, is a power tool used for grinding. It is a type of machining 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">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.

A grinding dresser or wheel dresser is a tool to dress the surface of a grinding wheel. Grinding dressers are used to return a wheel to its original round shape, to expose fresh grains for renewed cutting action, or to make a different profile on the wheel's edge. Utilizing pre-determined dressing parameters will allow the wheel to be conditioned for optimum grinding performance while truing and restoring the form simultaneously.

<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">Surface finishing</span> Range of processes that alter the surface of an item to achieve a certain property

Surface finishing is a broad range of industrial processes that alter the surface of a manufactured item to achieve a certain property. Finishing processes may be employed to: improve appearance, adhesion or wettability, solderability, corrosion resistance, tarnish resistance, chemical resistance, wear resistance, hardness, modify electrical conductivity, remove burrs and other surface flaws, and control the surface friction. In limited cases some of these techniques can be used to restore original dimensions to salvage or repair an item. An unfinished surface is often called mill finish.

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

Polishing and 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">Diamond tool</span>

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.

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 tighter tolerances and better surface finish than other machining processes

<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

Honing is an abrasive machining process that produces a precision surface on a metal workpiece by scrubbing an abrasive grinding stone or grinding wheel against it along a controlled path. Honing is primarily used to improve the geometric form of a surface, but can also improve the surface finish.

<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.

Mass finishing is a group of manufacturing processes that allow large quantities of parts to be simultaneously finished. The goal of this type of finishing is to burnish, deburr, clean, radius, de-flash, descale, remove rust, polish, brighten, surface harden, prepare parts for further finishing, or break off die cast runners. The two main types of mass finishing are tumble finishing, also known as barrel finishing, and vibratory finishing. Both involve the use of a cyclical action to create grinding contact between surfaces. Sometimes the workpieces are finished against each other; however, usually a finishing medium is used. Mass finishing can be performed dry or wet; wet processes have liquid lubricants, cleaners, or abrasives, while dry processes do not. Cycle times can be as short as 10 minutes for nonferrous workpieces or as long as 2 hours for hardened steel.

Electrochemical grinding is a process that removes electrically conductive material by grinding with a negatively charged abrasive grinding wheel, an electrolyte fluid, and a positively charged workpiece. Materials removed from the workpiece stay in the electrolyte fluid. Electrochemical grinding is similar to electrochemical machining but uses a wheel instead of a tool shaped like the contour of the workpiece.

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

Surface integrity is the surface condition of a workpiece after being modified by a manufacturing process. The term was coined by Michael Field and John F. Kahles in 1964.

<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.

Magnetic field-assisted finishing, sometimes called magnetic abrasive finishing, is a surface finishing technique in which a magnetic field is used to force abrasive particles against the target surface. As such, finishing of conventionally inaccessible surfaces is possible. Magnetic field-assisted finishing (MAF) processes have been developed for a wide variety of applications including the manufacturing of medical components, fluid systems, optics, dies and molds, electronic components, microelectromechanical systems, and mechanical components.

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 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 9 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 4 Murty 1996 , p. 187.
  6. ↑ Schwarz, Jeff; Darmann Abrasive Products (1998-12-15), "For Superfinishing Excellence, Start With The Right Finish", Modern Machine Shop.
  7. ↑ "Microfinishing Solutions". Neuteq.
  8. ↑ Krantz, Timothy L. (1 March 2000). Gear Durability Shown To Be Improved by Superfinishing (Technical report). NASA.

Bibliography

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