The Basics - Fundamentals of Mass Finishing
By A. F. Kenton
President Nova Finishing Systems Inc.
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 heavy thick coatings of paint, plastics, or epoxy. Another finish normally used on metal parts are treatments or thin coatings that are 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, 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 usually 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 creates work action or applies 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 or gravity force on the parts and mass within the work chamber in a equal x and y and some z directional method of operation. The newest equipment is consider high energy systems of both barrel and what are called disc finishing systems. These systems can generate up to 30 g’s or 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
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 predicable in how it operates or performs a designed function of deburring, burnishing, or polishing. Most system 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 processes.
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 beats up on the weakest or at least has some major influence in its 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 and this 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. Also, 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 very effectively as an abrasive media
Now, probably the most important thing you want to remember about mass finishing media is the heavier and larger the media used, the faster it will process the parts that have to be worked. Also 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 3/4 full, so that the parts and media can slide, causing the work action and processing. The weight rule works good for any part, media, process, or equipment.
What also effects the weight of a preformed shape are what 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 effect the over all 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. As a good back up for the weight rule is the speed or rate of break down of the bond. That rule is normally 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. But 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, for 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
abrasive grain size possible, which is probably .060 in size and often goes by the name fast cut. This selection is also normally unrealistic because you also need a shape small enough to get into all the holes, angles, and slots without getting stuck and yet work all of the areas that have to be worked. The media size and shape does not or should 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 do not or 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 still can 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 replaced sooner than half life if 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 gets impregnated onto 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.
As mentioned, beside 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 finalize part. That is, parts are made out of different materials with different hardness factors and they may require different finishing requirements. So not only do you have to deburr the part, it is 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 of 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 normally recommended except when the 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 faster choice for non-ferrous metals would be a fine cut ceramic or a hybrid light weight ceramic media which is suppose 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 again the longer time cycles are not normally cost effective, except when used in high energy equipment systems.
I have not mentioned anything about burnishing media up to now. That is because, other than size and shape there are no major differences or variations for this kind or type of media, but size still determines weight and that is a factor for selection. Non abrasive shapes work parts the same way that abrasives do; however, because there are no abrasives there is no or little material removal. Any material removed is due to metal fatigue caused by flexing. An exception to this is a sharp spiral cut cylinder that is only made by one company and is designed to actually remove material as long as the spiral ribs are still in tact. Fine inorganic materials can be added to steel to do deburring, but it is not recommended because of cost factors and all steel media is heat treated to create a case hardening that is very thin. Besides steel and stainless steel metal shaped media, porcelain is also used to accomplish the same task. The big difference between these two compositions is again the weight factor. In fact, that is the main popularity of steel media. Because it is heavy, about 300 lbs. per cubic foot versus about 100 lbs. for porcelain and most other ceramic abrasives, steel works relatively fast to produce a bright shinny surface appearance; however, shine does not necessarily translate into smoothness. In addition to steel and porcelain, aluminum shaped materials, brass, zinc, and other metals are available in cut wire products and balls.
Before we talk about the subject of organic materials, I want to mention a few things about the physical shape of media, because shapes are a factor in the processing of parts. Most shapes can fall into two categories. I have classified them as either bulldozers or steamrollers. Maybe rollers or scrapers would be better terminology. In either case you have shapes that have either a lot of diameter or straight edge exposed in contact with the parts being worked. That means that the main function of the media shapes is either to roll or crush and the other to scrape. The shape, in mass, also effects the way the parts move within the equipment. That is, rounded shapes tend to move more and allow parts to seek greater depths than straight edge shapes. Geometric shapes tend to have a build up of resistance and force that removes material and somewhat supports parts higher up in the work mass. Both shapes work, provided the media can get into the work areas, but for smoothness I suggest rounded shapes and for a lot of material removal I suggest geometric shapes.
One of the biggest problems with either shape is the media getting stuck in the part to be worked. One of the more common suggestions is to select a shape that is larger than the holes in the part. If you have to go smaller, try not to select a media that will get stuck in the hole or is close to the hole diameters when you double or triple up the media in a bunch. Round or diameter media seems to get stuck more than the geometric shapes. Before selecting a media, just get a couple of sample pieces, bunch them up and just trying to force them into possible problem areas of the part or parts is one of the best ways to check out this lodging problem.
Why lodging occurs at all is interesting story; however, the main reason is that the shape of the media is such that the center of gravity is usually right in the physical center of the shape. That means that the actual movement of the media is very stable and tends not to want to move at all, which is contrary to the purpose of mass finishing. That stability factor is usually over come by the energy forces or action of the equipment, which sets this media into motion. However, if the media should find itself restricted, it usually just rattles around to a very small degree until it can’t move any more.
Now, with all of this information about media shapes, there is one except to all of the above. There is one shape, called either the V shaped cylinder, cylinder wedge, or tri-cylinder that looks and is made different from almost all the other shapes. It is interesting because of its unusual appearance and behavior characteristics. This shape looks like a piece of pie or triangle in one direction and a cylinder in the other direction and it has its center of gravity on the out side edge. This latter statement means that the media shape is very unstable and very mobile. It exhibits the characteristics of both a roller ( it has an over all round shape) and a scraper ( two flats forming a very sharp wedge); therefore, it is usually the best general purpose shaped media available for all applications.
Up to now, all of the media that we have talked about is run in what is called wet processes. That is, these shapes are run with water and some chemical compound. All mass finishing systems are all built with drain systems and provision for liquid input. Because parts are made with cutting oils and pick up oils, greases, and dirt either by design for protection or through accident, chemicals are normally necessary to aid in the processing of the parts. Common practice is to use a water based biodegradable product in a diluted strength which can either be premixed or proportioned into the system. The pH of the product is important, but not the only factor. Inhibitors for protection and wetting agents are also desirable. The pH of waters is listed as 6.7 pH. Any number above water is considered basic, or caustic after 11, or acidic under the pH of water. Most chemical additives are interchangeable with either ferrous or non-ferrous parts, but most people use basic chemicals for ferrous parts and acidic products for non-ferrous and burnishing.
At one time chemical compounds that produced a lot of suds were considered desirable for cleaning of the parts. However, it was determined that the suds actually slowed down the mechanical action of media in mass causing longer time cycles. This same slow down of the media in mass can also be accomplished by just using too much water in the process, but it can also be accomplished by accident when drains become clogged or restricted due to debris. Even though chemicals are used to assist cleaning and brightening metals, which is a removal process, an inhibitor in the product is usually desirable to protect parts against oxidation. Lastly, there are now some stronger chemical additives called accelerators that are used to help speed up the deburring process. Because the chemical does most of the work instead of the media, it is usually recommended the media contain no abrasive; thereby, there is cost savings of the media which does not have the same wear rates as abrasives.
Dry organic media is the last category of media used in mass finishing systems and as the name implies, this media is run dry. Anything that can be processed wet can be done with dry organic materials. The only problem is the longer cycle time due to the weight factor. The main advantage over wet processing of this media is in the deburring or polishing of small or flat parts. Both types of parts mentioned have a tendency to either stick together due to water adhesion and they also adhere to the sides of the equipment being used. This characteristic causes uneven surface finishing. The other advantage of dry processing is the elimination of water pollution controls or restrictions, but there is a trade off. Instead of water and waste disposal problems, there is some concern for dust particles that need either a cover or proper air ventilation.
Dry organic media comes in the form of small random shaped particles of either granules or sawdust fiber, which can be used by itself, or it is is used in a two part form. The finer material is normally used with larger wood shapes in a 5:1 mix of shapes to particles. The reason for this two part mix is because there is hardly any weight to the organic materials. They weight only between 20 to 35 pounds per cubic foot; therefore, bulk or the wood shapes are desirable for adding weight to the process to improve cycle times. Other non-abrasive heavier media can also be added for bulk and produce good results.
When any dry organic materials are combined or mixed with polishing rouge, they are excellent in producing almost hand buffed looking polished parts. When combined with inorganic materials, they are very effective as abrasives. Because of the weight factor, the deburring and polishing qualities of these blended mixes still take a longer time to produce the same results as wet process media, but usually the surface finish is of superior finer and cleaner quality.
Within the last 5 years, a new form of dry organic materials has been developed that looks something like plastic media. There is now a patented composite process that takes inorganic and dry organic materials and makes them into shapes, which are used in a dry processing. These shapes are made in such a way that they can actually have more inorganic abrasives than the dry organic material, but they are still used dry. Also because of the increased weight of this preform, it is now competitive to wet processing media in time cycles and without the problems associated with water. According to the manufacturer, this new material will out last all other abrasive media by 5 to 20%. The advantages offer by this dry media and applications normally warrant the extra high cost of this new product.
As you can tell, there are three main factors that control deburring or burnish of parts in a mass finishing system, they are: the equipment, the media, and either the liquid or additive to the operation. Surface finishing on parts can be repeated over and over again if these elements are constant. Any variation of one of these elements will change the results or time cycle. Basically once a machine system is selected you are locked into some limitations of that machine system. Therefore, that means that media selection is probably the most important variable in the processing of parts and effects the costs of operation and the surface finish the most. Hopefully, now with some of these basic fundamentals down, you can better achieve the processing results you are looking for; however, if you need help or further information, you can contact A.F. Kenton at Nova Finishing Systems Inc. or call 1-800-444-4159.
NOTE: This author has written a book which completely classifies all methods of deburring and/or surface finishing into 5 energy classes of equipment and then rates them with a numbering system based upon how they perform and what they are capable of achieving. The book is called. “Understanding Deburring and Mass Finishing Systems”.
NOTE: Although zirconia is listed here, the more common fast cut media is made with aluminum oxide and has very similar characteristics and normally costs less.
• Nova Finishing Systems Inc., manufactures small, heavy-duty bowl finishers that stack up to most of the big equipment on the market, but cost much less. Nova series vibratory equipment also comes with the same warranties of the larger machines. Form more information on this equipment line, contact:
NOVA Finishing Systems
559 Crook Street
Hampton, TN 37658
980 429-5773 Tel, 704 665-5658 Fax