Moving a nearly finished part from the last machining operation to the loading dock can be a laborious process, taking days instead of hours or less, especially if the part sits in a batch waiting its turn for hand deburring or other form of final finishing.
Steel media, applied through either barrel or vibratory equipment, can take huge chunks out of down time while providing polished, clean interiors and exteriors, even with delicate parts. And it simultaneously provides manufacturers with a long-term capital investment carrying a life expectancy of a decade or more.
In 1909 George E. Abbott experimented with methods to burnish a bright finish on small metal parts. At that time only steel balls were available, and since a sphere can neither reach into corners, nor recesses smaller than its diameter, early parts showed clearly delineated non-finished areas.
During the ensuing years engineering sophistication grew, and the range of finishing applications expanded. New shapes, improved metallurgy and manufacturing, extensive research, plus better compounds and finishing equipment have all provided new uses for steel media, reducing costs while improving productivity and quality.
Since steel media lasts a decade or more and can perform in one step, it usually pays for itself in about a year. Steel media produces fewer rejects from lodging, and creates better fitting parts for reduced assembly time and increased wear resistance. When the proper shapes and sizes have been chosen, the uniformity of steel media ensures it will contact all critical areas and will not lodge anywhere. When correctly applied and maintained, it will hold its shape for years.
Steel media is most often made from high-quality, high-carbon, casehardened and stress-relieved steel, as well as 302 high-quality stainless steel. The controlled uniform shapes are essential in complex areas where lodging could be a problem. Proper heat treatment and stress-relief combine to form tough, long-lasting media.
Steel media weighs approximately 300 pounds per cubic foot, therefore it cushions and supports parts in the mass, delivering quick contact and fast peening of edges. Its substantial weight exerts added pressure to a mass of components. As pieces force their way through the media, the pressure and increased resistance are especially effective in reducing finishing times. Cycle times are normally 5-20 minutes. Steel media also is non-consumable. With proper care it is not used up in the finishing process.
Systems are designed to efficiently clean soils and remove chips remaining from machining operations in parts having such diverse features as milled grooves, threaded holes, slots, stampings, die cast and headed parts. Steel media scrubs oxides from zinc, aluminum, brass, copper and other materials and is highly effective at removing nuclear residues from metal tools. It also works with biodegradable compounds and does not contribute pollutants to the waste stream.
One of its most cost effective uses is in deburring operations. During cleaning, many parts obtain a burnished finish. Although steel media is not used to cut down on a heavy buff, it is widely used for peening or hammering sharp edges. As a result of its size control and ability to access all areas, complicated castings are successfully deburred in large quantities often in less than 15 minutes. OSHA requirements have been met using this process.
Another common use for steel media is deflashing thermoset plastics, a process that has been successfully used for years at leading manufacturers of thermoset electrical components. Non-abrasive steel media does not damage surfaces, and time cycles are normally short (7-12 minutes). Complex ceramic insulators with intricate cavities are successfully deflashed using this process.
Sometimes called "ball-burnishing," burnishing produces smooth mirror-like finishes on surfaces ranging from matte to polished. In addition, as steel media presses on a part, the working action imparts compressive stress, and its surface becomes work-hardened. The process often can replace steel shot blasting as a work-hardening step. Parts processed with steel media have longer cycle lives and greater resistance to wear as a result of this compressive stress action.
As an added benefit to its cleaning and burnishing capabilities, steel media enhances plating operations. Prior to plating, parts that may appear smooth often are marked by micro-imperfections, which can distort the plating process. The weight of steel media flattens these minute irregularities and prepares a surface for satisfactory plating.
When plated parts are finished with steel media, a compacting action spreads the surface of the softer plate to fill and pinpoint holes. This process helps eliminate porosity and increases the corrosion resistance created by the plating process. This action is especially critical during the deposition of nickel or other solutions that typically do not fill the depressions, but follow the contours of the metal.
Steel media also is essential in solving finishing problems resulting from the complexity of modern parts manufacture. To achieve this it must be understood that each piece of steel media in the mass is an individual polishing tool, and must be flaw free for optimal performance without damage. The media also must maintain its original dimensions to prevent lodging or other problems associated with size reduction.
Abbott requires all shapes, except pins and diagonals, to be checked for dimensional tolerance to plus or minus 0.010 inch during production, the most stringent standard in the industry. Tight tolerances on all flanges, sloping edges, tapers, curves and points assure contacts in angles, grooves, and figured surfaces that eliminate or reduce the potential for lodging.
Abbott also has developed proprietary techniques to ensure that its shapes receive uniform heat treatment for longer life. Ball surfaces are equidistant from a common center, and hardness penetration is uniform. However, formed shapes such as cones and ball cones, can become brittle in the thinner, flanged sections. Media shapes are case hardened, then tempered for stress relief.
Filigrees and engravings on silverware and other detailed parts presented a new challenge. These designs require a pointed object to achieve an overall finish. Thus, pins were developed in double-pointed sizes to reach into these intricate areas. The resultant ball, cone, and pin mixture is common for finishing a variety of detailed components.
Other specialized shapes were later developed to secure surface contacts on irregularly machined pieces. These shapes include diagonals, (a cylindrical steel section with each end cut off at a slant); ball-cones (which differ from the cone in that one side is a cone, the other side a half-ball); and oval balls (a football-shaped piece that provides wider area contacts than a ball, and somewhat more vigorous tumbling action.)
Steel media now consists of a multitude of interlocking, compact, scientifically designed shapes and sizes to match virtually every product contour, increase pressure and improve finishing action. Unique shapes are also available.
When conducting a value analysis of steel media, its "wear resistance" is a major component that should be considered. Steel media will maintain its size tolerances, thus eliminating wear-related size changes and the inevitable lodging problems sure to follow. The media's metallurgical structure is engineered to prolong its life and eliminate the hidden costs of frequent re-ordering.
When the proper media is chosen, consistent performance and repeatability is assured. If proper maintenance procedures are followed for protecting the steel media between uses, many years will elapse before replacement is necessary. Also, steel media will not wear the lining of barrel equipment, saving costly downtime and relining of the equipment, even when using compound solutions.
Non-abrasive, non-wearing steel media coupled with a good cleaning compound containing a built-in corrosion inhibitor, scrubs surfaces and interiors economically and efficiently. This method removes both organic and inorganic soils from various materials ranging from common carbon steels to exotics such as silver and gold.
Compounds are excellent for cleaning parts in conjunction with ferrous steel media. Although they are acidic, rusting due to improper maintenance can be prevented by using a compound solution containing corrosion inhibitor properties or by using steel media made from stainless steel.
Compounds are developed specifically as inhibitors for dipping ferrous and non-ferrous parts after processing. The ferrous inhibitors can be used directly in the media mass over weekends and shutdown periods to insure extra corrosion protection. Proper compound and inhibitor selection will protect the long life of steel media and improve cleaning productivity.
Foremost among the specialized types of media developed by Abbott is the ABCUT line. ABCUT provides accurate deburring and a bright finish on steel, zinc and aluminum parts, in one clean, economical process. The grooves in the media surface deburr and remove material without changing the part shape, often eliminating the need for hand finishing or other secondary finishing processes. This product closely resembles many miniature files working on the burrs of the part.
Stainless Steel Media
There is stainless steel media designed for specialized finishing operations. Also, more acidic compounds have been developed for use with stainless steel media to dramatically cut finishing times. A descaling cycle is typically cut in half when chemistry with less than 4.0 or higher than 10 ph is used. Other finishing operations are also improved with these chemistries.
Stainless steel's inherent ability to resist corrosion often eliminates the need for compound solutions and rust inhibitors. When compounds are required, lower-cost compounds may be substituted. Corrosion-resistant stainless steel reduces the storage, maintenance and handling costs, and maintenance procedures for overnight and longer-term shut downs are considerably simplified.
Stainless steel media also eliminates iron impregnation of components from chemical interactions during some finishing processes. Uncontaminated part surfaces are especially important to manufacturers of medical equipment, surgical tools, food processing equipment and similar items.
Barrel finishing is achieved using optimized concentration and control of working pressures; efficient application of these pressures directly to the part without dissipation; and uniform contact of finishing media on all part surfaces. Other process parameters affect desired surface finish characteristics and run times, from the mix of media shapes and compound to barrel speed and height-to-width ratio as it affects cascading action.
Open-faced barrel design provides variable cascading, and is also used where acids might risk explosion in a closed barrel. The tilting angle of the barrel determines the rate of cascading action. A more horizontal position produces no cascading with gentle rotation that's suitable for polishing; a steeper angle generates parts and media cascading that approaches the levels of a closed barrel. A closed barrel design permits the full rotation of the parts and media for more aggressive action in longer, unattended cycles.
To obtain the desired results with vibratory equipment, the system must be able to support and drive the density of the steel media. Internal or external separators designed into this equipment will allow ease of operation and integration with in-line automation.
External part-media separators allow parts to vibrate onto a screen deck while the media returns to the vibratory equipment by conveyor. Internal separators have built-in screen decks and a gate, allowing media and parts to flow onto the screen deck where media returns to the equipment and only the parts vibrate out. The media never leaves the equipment. Both types greatly increase the production rate by eliminating handling.
Smaller systems are perfect for jewelry, die castings, small decorative parts, fittings, medical and precision components. Larger systems are suited for production runs of larger parts in higher quantity. Since the geometry of steel media is most important, a specification sheet aids in its measurement.