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Superfinishing for super quality.

Superfinishing is a process that improves product quality, consistently produces metal finishes to one micro-inch, and allows fabricators to work with exotic materials that traditionally have been considered difficult to process.

The basic process has been around for more than 50 years. However, recent advances in superfinishing and market demands for tighter tolerances have spawned a significant increase in awareness and acceptance of superfinishing as the ultimate finishing process for progressive applications and traditionally difficult-to-machine materials. These include flame and plasma coatings; ceramics, such as chrome oxide, aluminas, and aluminum oxide; and more standard materials, such as hard and soft steel, stainless steel, aluminum, Monel, tungsten carbide, and hard chrome.

Superfinishing defined

Superfinishing is a gentle process that induces a vibratory "rubbing action" to accomplish micro-fine finishing. In essence, superfinishing involves high-speed oscillation of an abrasive stone at rates up to 2800 cpm that contacts the workpiece as it rotates in turning equipment (usually a lathe).

Superfinishing is most commonly used to achieve micro-finished surfaces, high-tolerance geometries, extended product wear life, and superior cosmetic appearance. A growing trend is to have turned workpieces go directly from a lathe to superfinishing, where the final surface operation is performed--bypassing the usual grinding operation and thus saving both time and money, while improving product quality.

In comparison, traditional turning and grinding processes can be considered "ploughing operations" that generate metal chips, leaving in their wake minuscule craters marked by turned-up edges of "fuzz." While this surface may appear relatively smooth, magnification shows a recurrence of peaks and valleys. Superfinishing provides finishes in the 2 rms to 1 rms range and below. Machine turning produces an average surface of 60 rms to 30 rms. Even fine grit abrasive wheels generally produce finishes in the 4 rms to 6 rms range.

Operational characteristics of superfinishing are low peripheral work speed, moderate unit pressure, low-to-no heat generation, and wide workpiece contact area. The end results are improved surface finishes, as low as one micro-inch; consistently improved parts geometries with tighter tolerances; and removal of the amorphous surface layer created by traditional turning and grinding processes. Also, superfinished pieces have virtually 100% bearing surfaces, which result in significantly longer part life. For example, superfinished rollers in the bearing industry have a 30% life extension over similar parts ground to final finish.

Revitalized technology

Introduced in the automotive industry in the early 1930s, superfinishing did not gain widespread recognition because the need for advanced finished parts in the larger American engines was less critical. However, mass production of smaller engines in the 1970s triggered a new set of surface finishing requirements for the more highly stressed engine components. As superfinishing gained popularity in automotive applications, it became recognized as a viable technology and spread to various other industries involved with metalworking and finishing applications.

As a new technology, superfinishing had definite and focused applications, which fostered the development of equipment dedicated to specific functions. An example is finishing ball bearings. Today, nearly 100% of all precision bearings are superfinished by equipment designed and dedicated to finishing bearings.

As awareness of the technology spread, the benefits of superfinishing became applicable to a growing number of metalworking applications. This set the stage for the development of more versatile and affordable superfinishing equipment to accommodate the growing number of actual and potential applications.

Superfinishing today

New developments in superfinishing equipment coupled with growing awareness of the benefits of the process are fostering increased acceptance of superfinishing for traditional applications, as well as supporting new applications in industries ranging from appliance manufacturing to medical devices to wire drawing.

Superfinishing is now being used in new applications involving thermal spray coatings to enhance product performance where corrosion and wear resistance is necessary, or to rebuild the surfaces of worn parts, saving costly part replacement. Component repair and preventive maintenance applications include steel and paper mill rolls, power plant and shipboard turbine journals, and such oil-patch applications as bearing journals, piston-seal diameters, compressor rods, and hydraulic and ball valves used with drilling equipment.

Superfinishing systems have recently been introduced that offer advanced capabilities at lower prices than previously available equipment. One such device is a lathe-mountable attachment that achieves micro-finished surfaces to one micro-inch, high tolerance geometries, and is capable of accommodating irregularly shaped workpieces. With traditional superfinishing equipment, the process could only be performed on cylindrical or flat workpieces. Designed for versatility, the attachment can be modified to accommodate such irregularly shaped workpieces as crank and cam shafts, which require increasingly tighter tolerances.

For smaller machine shops and tool and die businesses, a one stone attachment may be readily affixed to a lathe or cylindrical grinder. In larger, high volume shops, multi-station operations allow more consistent roughness at each station and require less compromise in stone selection. In addition, the greater number of heads at work in a multi-station operation will generally reduce cycle time. For example, each part may spend two seconds at each of ten heads. While the time required to finish a part may still be twenty seconds, the machine produces a finished part every two seconds because the heads are working simultaneously.

Attachments are more routinely mounted on engine lathes because of their better adaptability. Cylindrical grinders may also be converted to dedicated superfinishing machines by removing the machine spindle and permanently installing the attachment. The cylindrical grinder most often offers the flexibility of infinite feeds and speeds and automated operation.

Used in combination with lathes, superfinishing attachments open the way to parts finishing immediately following turning, thus circumventing grinding. This new handling sequence has special payback value to machine shops where advanced work finishes are a growing requirement.

In addition to lathe-mountable superfinishing devices, centerless throughfeed systems for high volume applications have recently been introduced at about half the cost of traditional systems with comparable features. These systems were developed to meet the growing demand for affordable, compact superfinishing machines that offer the versatility to handle parts ranging in size from 0.1" to 20 ft in length and 0.06" to 3" in diameter.

These developments in superfinishing equipment make it possible for metalworking operations to cost effectively improve existing production as well as expand their services to include superfinishing, thus strengthening the company's competitive position and potential for increased business.

Superfinishing abrasives

Companies such as the Norton Co offer a wide variety of stones to meet specific application needs. Choosing the correct abrasive depends on the material type, desired stock removal, and required surface finish. Abrasives typically used in superfinishing include:

* White aluminum oxide, typically used for single-station superfinishing machines on ferrous materials or in the first station(s) of multi-head superfinishing machines on ferrous materials.

* Levigated alumina, which has an average particle size of 800 to 1000 grit. It was very popular domestically before the finer grit white aluminum oxides became available. Levigated alumina was primarily used on single-station machines for finishing steel parts. Levigated alumina provides an intermediate superfinishing action. Though levigated alumina doesn't cut as well as white aluminum oxide, it polishes better. It also cuts better than green silicon carbide abrasive, but doesn't polish as well.

* Green silicon carbide, which is used in single-station operations on nonferrous materials, in all stations of a multi-head operation on nonferrous materials, and in the last stations on ferrous materials. In bearing applications, green silicon carbide is generally used as the finishing stone, placed in the last one or two stations of multiple-station machines. Green silicon carbide is generally used on all stations when the application involves M50 aircraft bearing steel.

* NVCA, a special green silicon carbide compound blend, was developed by Norton Co for polishing or finishing--with minimal cutting action. NVCA is used in the last station on multi-station machines. NVCA is replacing carbon, graphite resin, and green chrome stones in many applications.

Superfinishing in action

A 30-year old full-service commercial coating facility wanted to maintain its competitive edge by providing customers with as many services as possible, and superfinishing was a natural extension of its machining, grit blasting, coating, and grinding capabilities. Superfinishing was identified as the wave of the future and a lathe-mountable unit was acquired.

As a full-service facility, the company works with a broad base of customers with applications ranging from pulleys and capstans for the wire industry, to shaft and sleeve repair for heavy industry, to materials research and development projects at technical institutions exploring future processes.

An application where superfinishing significantly increased the company's competitive posture was finishing components for the wire industry. The company is a major supplier of pulleys and capstans used to draw wire through dies with high tension at speeds up to 20,000 ft/min. To combat this wear-intensive application, the company applies various ceramic coatings to the grooves of the pulleys. Critical to this operation is superfinishing the ceramic coating consistently.

The company has found that superfinishing reduces the drag on the wire, thus significantly reducing the chances of wire being scratched by the ceramic coating. This results in an improved product and cost saving for the customer since the pulleys do not wear as fast, reducing both coating and maintenance costs while improving performance.

In another application, a bearing manufacturer supplying the aircraft industry used hand polishing on bearing races to yield the required surface finish. The process achieved a measure of finish refinement needed, but the associated inconsistency of pressure control produced race contours that were too often altered beyond the required specifications. To correct this situation, the company installed a superfinishing machine with a two-stone oscillator that precisely roughs and finishes the roller paths of cylindrical roller bearings. Such superfinishing machine operations can take a plain cylindrical ground race and generate a precisely located crown in the track or produce a perfectly straight path within 0.000 02" on straight races. Surface finishes generated are one rms or less.

Another example of the benefits of superfinishing comes from the automotive industry. A company finishing the faces of hydraulic lifters needed a work cycle of 6.5 seconds per unit for the operation. By adding a CNC superfinishing machine to its existing production line, the company was able to meet the time parameter and improve the quality and consistency of the faces.

While superfinishing has been around for more than 50 years, only recently has it been recognized as a viable technology that can improve product quality, while cost effectively expanding a metalworking company's capabilities and competitive position.

The future looks even more promising. Increasingly tighter tolerances and the growing use of materials that traditionally have been difficult to machine, such as thermal sprays, will support further development and use of superfinishing. Advances in superfinishing technology and equipment will contribute to superfinishing's status as a revitalized metalworking technology.

For more information on superfinishing from companies listed in this article, circle the appropriate number:
Norton 305
Supfina Machine Co 306

Ultra Precision

Technologies 307

Superfinishing basics

Superfinishing is a gentle process that involves high-speed oscillation of an abrasive stone that contacts the workpiece as it rotates in turning equipment, usually a lathe. Shown is a typical configuration for a cylindrical workpiece. The three forces in place are the downward pressure of the superfinishing stone, the back-and-forth oscillation of the superfinishing equipment, and the rotation of the turning equipment.

Lathe-mounted attachment

Devices such as the SFG 7108 from Supfina Machine Co, Warwick, RI, can be easily retrofitted to existing lathes and grinders through the use of an adaptor plate. The device eliminates waviness, chatter, and feed marks on cylindrical parts of various sizes and materials, without a large investment.

Dedicated superfinishing

A mid-sized centerless throughfeed superfinishing system from Ultra Precision Technologies, Cranston, RI, provides advanced capabilities in a turnkey system. The UPT-5600 performs cylindrical superfinishing operations cost-effectively, achieving micro-finished surfaces, improved geometries, extended product wear life, and superior cosmetic appearance. Typical applications include hydraulic valve spools, shock absorber rods, bearing rollers, gage pins, and piston wrist pins.

Capable of finishing cylindrical pieces down to 1 Ra and achieving a precise roundness down to 0.000 005", the UPT-5600 provides the versatility to handle parts ranging in size from 0.1" to 20 ft in length and 0.06" to 3" in diameter. Special centerless transport rolls are also available to handle noncylindrical workpieces such as tapered components.

Designed for flexible working arrangements, the unit features six separate stone guides that can be individually adjusted and equipped with finishing stones of varying roughnesses for precise finishing of special applications. Materials typically handled by the machine include flame and plasma coatings; ceramics, including chrome oxide; aluminas and aluminum oxides; as well as such standard materials as hard and soft steel, stainless steel, aluminum, Monel, tungsten carbide, and hard chrome.
COPYRIGHT 1992 Nelson Publishing
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Copyright 1992 Gale, Cengage Learning. All rights reserved.

Article Details
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Title Annotation:includes related article; application of superfinishing in metalworking industry
Author:Stovicek, Donald R.
Publication:Tooling & Production
Article Type:Cover Story
Date:Dec 1, 1992
Previous Article:A fine cutting method for tough materials.
Next Article:Casting process gaining ground.

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