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The Basic Fundamentals Of Mass Finishing

June - 2008
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By: A.F. Kenton, President of Nova Finishing Systems

All man made items or objects can be considered parts and are mostly made from metals, castings, or molded parts. Raw materials for these parts can be ferrous or non-ferrous metals, plastics (which can include epoxies), and any inorganic or organic, capable of being formed. Basically, that means that almost everything and anything that man uses can be worked via machine or hand. Before such items or industrial parts can be used or exchange hands to its end use or user, they must be made to fit, form, and function safely. The word safely is open to interpretation; however, it primarily means that it should not hurt, cut, or damage surrounding objects or people.

The word ‘safely', or that end product condition of the item or part in question, is normally achieved by a finishing process or just the term "FINISHING". Now, even though we are talking about an end item or part, there is still a question about what a ‘finish' is. That is, how a part looks or is finished depends to a large extent as to how it is to be used. That means that the word finishing can take on other meanings than just safety. The word can also refer to the coating applied to the part or its surface modifications for aesthetic or treatment purposes to prolong or protect its life.

Most parts or objects exposed to out of doors environments are usually finished with a heavy, thick coating which is desirable for protection, but a lot of poor workmanship can be covered up with thick coatings of paint, plastics, or epoxy. Another finish, normally used on metal parts, is a treatment or thin coating that is considered plating or surface treatment. Because this finish is relatively thin, this end process will not - or will only slightly - change the current surface features of the item in question. Basically, that means that what you see before you treat the part is what you will get after you finish the part.

To accomplish surface modification of a permanent uniform nature requires the mechanical working of the part or object in question, either by hand or machine. That means material removal and/or the blending of previously worked areas, generally with abrasives in a mechanical type operation. This finishing task is usually best accomplished using mass finishing systems, because it requires the least amount of time and care by an operator and produces a uniform finished product. Even though this is a finishing process and the end product may be the final appearance of the part, it still may require some kind of protective coating or alternative finish. Blast finishing is an option in some cases; however, blasting with inorganic materials can leave a surface finish extremely rough, which is good for the adhesion of heavy coatings but not for tight tolerances, smoothness, or appearance sake.

Mass finishing systems have evolved over the years into three types of energy systems that generate mechanical forces, which create work action or apply pressure to a mix of both abrasives and parts within this equipment. The first system, developed from ancient times, is considered a barrel system from which we derive the terminology ‘tumbling'. This equipment is the slowest method because it exerts only 1 g (or gravity force) to the mass of parts and abrasives within the barrel and primarily moves the mass in one direction. The next system developed is considered vibratory. This equipment uses an open work chamber that is energized by an eccentric or out of balance spinning weight. This equipment can generate up to 8 g's on the parts and mass within the work chamber, in an equal x and y and some z directional method of operation. The newest equipment is considered high energy systems of both barrel, and what are called disc finishing systems. These systems can generate up to 30 g's (gravity forces) to the parts within them in the x, y, and primarily z work action.

The energy forces of the mechanical motion of the type of equipment [1] described above is only part of the work action taking place within the equipment. The energy transfer from the equipment to the part is best accomplished with a solid abrasive medium, which is called media, and which can be controlled or is predictable in how it operates or performs a designed function of deburring, burnishing, or polishing. Most systems use a form of abrasive media, with liquid systems for deburring, and hard non-abrasive shapes and materials for burnishing. Polishing is best accomplished in mass finishing equipment using a dry organic process.

Mass finishing media supplies come in many sizes, shapes, and compositions. They all deburr or modify surface features of metals and plastics to some extent. Choosing the right media makes a world of difference in time and costs. How efficient the media is in achieving the end results you are looking for is also critical to this selection process. To begin with, all media products can be considered abrasives, even burnishing media. That is, they all have the capability to remove some material off the item or part in question, be it surface dirt or heavy metal. All media will work or do something to a part in a mass finishing system. It is a relative thing, a kind of guilty by association. In mass, the strongest beat up on the weakest, or at least have some major influence in the behavior or final appearance of the parts.

Because we are talking mostly about machined parts or man made objects, the design configuration of these parts are made in such a way that they are not too compatible with nature. What I mean by that is that naturally occurring media or random abrasive supplies are normally not good enough to work most parts because of their irregular shapes. In short, because of the irregularities of random sizes and shapes of abrasive materials and their mass behavior, the media usually gets stuck within the configuration of the part, thereby neutralizing the surface modification process, which can result in a non-uniform finish. If the part is relatively simple and can be worked with this random media, it is normally the least expensive media and way to process parts. There are now some extremely hard, man made abrasive products made in this random form, which are very effective for both deburring and burnishing; therefore, this media should be considered when and where possible.

In addition to random, abrasive products classified into specific size ranges are man made abrasive shapes of uniform specific size. Generally speaking, all mass finishing media, like parts, are man made into preformed shapes from 1 of 4 basic composition materials or bonding agents. The most abrasive materials are made with ceramic and plastic materials. These bonding agents are used to bind uniform small grains of abrasives together into a shape. Burnishing media is made from non-abrasive porcelain ceramic and either molded or cut steel, stainless steel, aluminum, brass or other metals. Lastly, there is a category of organic materials that are used dry without water systems, and are used primarily to polish; but when blended with inorganic materials they can be used as effectively as an abrasive media.

Now, the most important thing you want to remember about mass finishing media is that the heavier and larger the media used, the faster it will process the parts that have to be worked. That means that the more media you can get into a machine system, the faster it will work on the parts because of the weight factor. Where this is not true is in the old barrel tumbling systems. The older barrel systems need an air gap. Proper fill of media in a barrel should be between 2/3rds to 3/4ths full, so that the parts and media can slide, causing the work action and processing. The weight rule works well for any part, media, process, or equipment.
The weight of a preformed shape is also affected by the minerals it is composed of. The most common material used in abrasive preformed media is aluminum oxide. Given the same physical size and shape of the more common compositions, silica or sand is about the lightest mineral preform shape; then aluminum oxides, silicon carbide, and zirconia. Now, given that information about mineral weights, the actual materials that make up the bond can also affect the overall weight of the preform. If the supplier of the media does not know or understand the significance of the bond, go by the weight rule for the aggressiveness of the media. A good back up for the weight rule is the speed or rate of break down of the bond. That rule being ‘the faster the media wears or breaks down, the more aggressive the media', because new, fresh abrasive is exposed to do the cutting. However, besides weight and speed, surface finish is also an important factor to be considered before a media is selected. A heavy coarse media is not normally suitable for plating or surface treatments. Then again, that type of media may be required in a 2 or 3 step operation prior to a plated surface treatment.

Going by the rules above, to achieve the greatest amount of material removal or the fastest deburring media, you normally want to use the largest preformed ceramic shape, available with the coarsest zirconia[2] abrasive grain size possible - which is probably .060 in size and often goes by the name ‘fast cut'. This selection is often unrealistic, because you need a shape small enough to get into all the holes, angles, and slots without getting stuck, while also working all of the areas that need to be worked. The media size and shape does not necessarily have to be smaller than the hole or work area, but it should be able to poke a small portion of the shape into that same work area. Normally, inside dimension should not be worked because of tight tolerances. On the other hand, if a media shape is too large it will not work corners or recesses, leaving a slight shadow appearance or texture in these areas on the finished parts, even after a secondary treatment.

When a media shape wears down to about half its original size, it basically becomes ineffective for the part, or parts, it was selected for deburring. This is what I call its ‘half life'. It can still be used on smaller parts, but because of its size and/or mass, it loses its effectiveness or efficiency. On tight tolerance parts where lodging is a problem, this media shape may have to be replaced a lot sooner than its half life. Media may also have to be replaced sooner than half life if the deburring process is not properly maintained. That is, wet systems are designed to flow or function, thereby removing oils and debris. If the media becomes glazed, it is basically ineffective as an abrasive media and may have to be replaced way before it reaches half life. Glazing occurs when oils, metals, or other debris get imbedded into the surface of the media, due mostly to poor liquid flow or chemical additives in the process. Basically, glazing is a coating on the media that creates a barrier so the media cannot break down.

Besides the abrasive grain size, the next controlling factor for selecting a media is the bond or glue that holds the matrix together like cement. In fact, the manufacturing process of preformed shapes is almost exactly like making cement, except the finished shaped must be cured in an oven and baked to create the proper hardness. Each manufacturer of preformed abrasive shapes makes at least 5 to 7 standard grades, compositions, or formulations of the same size or shape. Another rule to consider here is that the smoothness of the part's surface finish can only be the same as the size variation of the largest particle grain size that makes up the media. That is the reason for these different formulations. They are necessary for regulating the cut and the surface finish of the final part. Parts made out of different materials with different hardness factors may have different finishing requirements. Therefore, not only do you have to deburr the part, it is also necessary that you create the right surface smoothness.

Now, I have told you what the fastest deburring media is, but there are a lot of exceptions to this rule. First of all, ceramic media is almost always used on ferrous metals because its hardness and rigid form makes it very aggressive. Plastic bonded media is almost always used on non-ferrous metals because it is more flexible and gives on metals, making it a more gentle media. For deburring plastics, ceramics are generally recommended, except when appearance is a concern. In some cases, ceramics can be used on non-ferrous materials; however, the media will normally leave the metal rougher than what its surface finish was prior to processing. A better and faster choice for non-ferrous metals would be a fine-cut ceramic, or a hybrid, light weight ceramic media, which is supposed to be an all purpose media for both ferrous and non-ferrous materials. Light weight ceramic is about the same weight as plastic and works well in some applications, but it still takes longer than normal on ferrous parts, and leaves non-ferrous parts somewhat rougher than plastic media. Some plastics can be used on ferrous materials, but the longer time cycles are not normally cost effective - except when used in high energy equipment systems.

Please see the July issue of Finishing Talk for Part II of "The Basic Fundamentals of Mass Finishing"

You can reach the author at :

Nova Finishing
PO Box 185
Hatboro, PA 19040
Phone: 215-942-4474 * 800-444-4159
Fax: 215-953-1342