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The Basic Fundamentals Of Mass Finishing Part II

July - 2008
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By: A.F. Kenton, President of Nova Finishing 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 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, but because there are no abrasives, there is little to no 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 intact. Fine, inorganic materials can be added to steel for deburring, but it is not recommended because of cost factors, and because 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, once again, the weight factor. In fact, that is the main popularity of steel media - 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, shiny 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 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 affects 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, while somewhat supporting 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,  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 an 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 overcome 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 exception 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 differently 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 outside 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 overall 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 discussed 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  built with drain systems and provisions for liquid input. Because parts are made with cutting oils and pick up oils, greases, and dirt either by design for protection or by 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 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 granules or sawdust fiber, which can either be used by itself, or 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 that there is hardly any weight to the organic materials. They weigh 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 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 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 outlast all other abrasive media by 5 to 20%. The advantages offered 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, with some of these basic fundamentals down, you can better achieve the processing results you are looking for. If you do, however, need help or further information, you can contact A.F. Kenton at Nova Finishing Systems Inc. or call 1-800-444-4159. 

[1] NOTE: The 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 entitled Understanding Deburring and Mass Finishing Systems.
[2] 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.
For more information on this equipment line, contact:

Nova Finishing
PO Box 185, Hatboro,
PA 19040
Fax 215-953-1342

Click here to read Part I.