The first mass finishing media product for tumble finishing was crushed limestone, which was shortly followed by red granite, a much longer-lasting product. Soon these natural stone products were available as screen-sieved natural stone. These low-tech media worked quite well, as did the ground soybean flour compounds and natural abrasives such as pumice used in early mass finishing systems. In fact, this set-up can still be found in use today, more than 80 years after its initial development.
Jump to the 1960s—truly a period of advancement for mass finishing media and equipment. Developments included centrifugal barrel and centrifugal disk finishing machines, and the wider array of machine types expanded possibilities for media selection. This article will outline the types of media available today and factors in their selection.
General Media Types
The "media population" available for mass finishing operations today has many categories. These include preformed ceramic and resin-bonded media, steel media and synthetic random-shape media.
Preformed ceramic media includes porcelain, light-cutting and fast-cutting materials. Today, it is the workhorse of mass finishing systems, and is selected because of its availability in a variety of desirable shapes and sizes, low cost and low wear rate.
All preformed ceramics are manufactured by mixing clay-like materials and water with abrasives forming this mud into shapes, then drying and firing these shapes to vitrify the binder. Variables include the binder used, firing temperature, the amount, size and type of abrasive grains they contain, and their uniformity of firing.
Porcelain media with no abrasive is long-wearing, can develop extremely good surface finishes and has very little cut. Used primarily for light deburring and for brightening metal surfaces, these products have relatively high densities and good wear resistance in the presence of abrasive compounds commonly employed in barrel finishing. Porcelain media are available with very finely divided aluminum oxide, which imparts no cut in itself but which improves the attrition rate of the media.
Light-cutting ceramic media is used primarily for rapid, light deburring of steel and die-cast parts. It is long wearing and extremely efficient. Users can select fast-cutting ceramic media with a wide range of abrasiveness and media depreciation rates. Extremely fast-cutting materials are used in applications that require short finishing cycle times. Their relatively high wear generally makes them less efficient than slower-cutting grades. Poor shape retention, high media depreciation rates, and higher soil generation also characterize these grades. Higher soil generation, however, sometimes minimizes
Preformed ceramic media is available in myriad sizes and shapes. These include angle- or straight-cut triangles, cylinders, diamonds, stars, arrowheads, spheres and cones. About 80% of preformed ceramic media use involves angle-cut cylinders and angle-cut triangles.
Preformed resin-bonded media, as the name implies, use plastic or resin as the binder material. They are low-cost and available with a wider range of abrasive types and sizes than preformed ceramics. The most popular grades are those using quartz as an abrasive, but aluminum oxide, silicon carbide and other abrasives are also used.
Resin-bonded media can develop excellent preplate finishes on any metal. Applications include preplate finishing on zinc die castings, cutting steel parts prior to plating, and deburring when ceramic media would either impinge the surface or roll over the burrs because of its weight and hardness.
Low-cost polyester resins are the most commonly employed binder material, and the various shapes are cast. The most popular shapes for are cones and triangles, though pyramids, stars, tetrahedrons and other shapes are also available.
Steel media consist of case-hardened, stress-relieved steel preformed shapes available in a variety of sizes and configurations. Balls, balls with flat spots, oval balls (footballs), diagonally cut wire similar to angle-cut cylinders, ball cones and cones (both of which are different than the standard concept of cones), and pins are the most commonly used.
Steel media weighs approximately 300 lb/ft3 and is expensive for initial installation but, because it has a minimal attrition rate and because of its extreme cleanliness, steel media is being more widely used for light deburring applications and for cleaning. Compounds are available that keep steel burnishing media clean and bright for years so that it can rapidly clean, brighten, and debur metal, plastic or ceramic components.
Synthetic random-shaped media include the popular fused aluminum oxide, available in a number of grades. More loosely bound, coarse-grained materials are characterized by fast cutting and high depreciation rates. Fine-grained fused aluminum oxide is generally employed for burnishing. Where some light cutting is required, fine-grained aluminum oxide can develop a better luster on stainless steels and other hard surfaces than can be achieved with steel burnishing media.
Regardless of type, the primary functions of mass finishing media are cutting, promoting luster, separating parts and scrubbing surfaces.
Cutting. Media that cuts can remove burrs and smooth surfaces. Cutting grades of media may be considered abrasive metering devices, releasing a given quantity and size of abrasive per unit time. As a carrier of abrasive grain, the large media pieces effectively increase the impact force of the abrasive on the metal part to be cut, thereby improving the efficiency of the abrasive. Cutting media develop dull, matte surfaces.
Promoting Luster. Some media grades are generally non-abrasive or have a very low degree of abrasiveness, designed to promote luster on the surface of metal parts. They deburr by peening rather than actually removing the burr. Media selection, therefore, will control the degree of surface luster imparted to finished components, making the parts bright and shiny, developing a matte surface with a random scratch pattern, or anything in between.
Parts Separation. An important media function is to separate parts during deburring, cutting, surface improvement or burnishing operations. The volume ratio of media to parts is normally used to control the amount of part-on-part contact which will occur in a vibratory or barrel finishing operation. Considerable part-on-part contact occurs at low ratios, while part-on-part contact is limited at higher ratios.
Scrubbing Surfaces. When it comes to surface scrubbing, media has the unique ability to physically assist compounds in their cleaning function. Both abrasive and non-abrasive media are effective in providing scrubbing action, and they work on organic soils as well as on mill scale and other inorganic residues.
The following characteristics make different types of media unique in their capabilities.
Cut rate or surface roughness reduction per unit time refers to the ability of media to remove metal by scraping the part surface to remove high spots and gradually smooth the surface.
Minimum surface roughness capability depends on a number of variables, including the compound used. All medias are characterized by their ability to develop a minimum surface roughness as measured using the root mean square (rms) or other methods. Normally, fast-cutting media will have a higher minimum and slow-cutting media will have a lower minimum.
Media wear rate is generally a function of the material's ability to remove metal. However, little media wear occurs from abrasion by parts; wear is primarily a function of media abrading against itself. This is necessary in vibratory and barrel finishing because it helps keep the media clean, free-cutting and capable of performing the function for which it was selected. Generally speaking, faster-cutting media have higher wear rates.
Inherent hardness of various media types, for example ceramic versus resin-bonded, is a characteristic that frequently drives media selection. High-hardness media types such as ceramics cannot be used where their hardness would cause damage to components being finished. For this reason, high-quality surface finishing on relatively soft metals is normally done with resin-bonded media, which has a lower hardness.
Available shapes and sizes, while not a physical media characteristic are critical in certain applications because of access, lodging, separation, or for other reasons. If a given type of media is not available in the optimum shape or size, it may not be selected for that application.
Sensitivity to compounds. Certain media types show sensitivity to certain compounds regarding cut rate and minimum surface roughness capabilities. Burnishing compounds on certain types of media reduce cut rate and surface roughness minimums to much lower levels. Some media types will give a bright, lustrous appearance on metal surfaces when used with the proper compound. Some ceramic media types, for example, have been designed as "cut and color" media, where a change in compound type alters performance. They will cut while a deburring compound is used, and then "color" or develop luster during burnishing.
Soil Generation. In general, fast-cutting media wears at a much greater rate than slow-cutting media. In addition, soils generated by the media itself can be troublesome in subsequent finishing if cleaning during finishing is not adequate. Some grades of fused aluminum oxide develop tremendous amounts of a dark, smutty soil capable of depositing on metal surfaces. Plastic media can show this effect if the wrong compound is used. Steel and porcelain ceramic media provide the cleanest processes.
Chipping and Fracture
Sensitivity. Automated finishing machinery may use a media-handling system that drops media a considerable distance. This can result in chipping or fracturing of highly vitrified ceramic media without sufficient toughness. Media fragments can cause lodging problems, and the shape change can also cause abnormal media wear.
Bulk density is often overlooked in mass finishing, but is important as the media wears from its original size and shape and a full media size distribution is developed. Wear can increase bulk density by 20-25%, and can increase consumption rate.
Media consolidation due to increased bulk density must be differentiated from wear. Higher bulk density media types impart greater pressures on parts. For example, hardened steel burnishing media at 300 lb/ft3 imparts tremendous forces on metal parts and is capable of peening surfaces very rapidly, while non-abrasive ceramic at 100 lb/ft3 or less will require a much longer time to accomplish the same results.
Shape retention. Certain media show excellent retention of their shape and cutting edges. In general, slower-cutting products are better in this respect than faster-cutting materials.
When access to slots or small part areas is required, shape retention can be critical to success. A rotary barrel has the ability to develop edge radii more efficiently than vibratory equipment. Therefore, media in a rotary barrel will tend to become round and lose shape more readily than in vibratory equipment.
Noise. Large, hard media is very loud in any size vibratory equipment. Agencies overseeing worker health and safety have rules governing allowable noise exposure levels, sometimes limiting the size of media selected.
Cost is normally a prime concern to many users, as it should be. But product cost can be misleading. Media types such as hardened steel are very expensive in dollars per cubic foot, but because their attrition rate is essentially zero, cost per part produced can be extremely low. Cost analysis, therefore, should never be attempted without good media wear rate data and until after productivity or cycle times have been established.
Making a Choice
So, how do you pick a media type, size or shape? Selection is driven by what type of finish you require and what your parts are made from.
If parts are non-ferrous metal and the surface will be either plated or requires a very smooth finish, a resin-bonded material is the first choice. If parts require deburring and surface improvement, ceramics are a good bet. Media size and shape are determined by the size of the part. The general rule is small parts=small media, large parts=large media, but keep in mind separation of the parts and media after the process.
Shapes are determined by part geometry. Triangles and angular media work well for edges and corners. Cylinder shapes do well for general finishing or parts with through holes. Keep in mind that all medias wear and expose new abrasive. As the shape wears, the geometry changes and the media becomes smaller. This can cause problems such as lodging, but can be avoided by careful planning.