Abrasive Control Factors For Mass Finishing Systems
By A. F. Kenton
There is a growing concern in the surface finishing industry of the aging of those in the business. It is not a glamorous trade but absolutely necessary to all industrial and commercial products and unlike CNC equipment, there are subtle processing control factors instead of software programs. Maybe for that reason, fewer and fewer young people are seeking the trade knowledge or careers in the business. That also means that as the trade ages, more and more is left up to the people performing the surface finishing work. It is for that reason that certain basic fundamentals of material removal and surface finishing should be passed along, because the graying trend is especially noticeable in the mass finishing industry.
As a general statement, what you put into a mass finishing system determines what you get out of it. All mass finishing systems are designed to perform uniform edge or surface modification on machined or cast parts. The major difference between the various finishing systems is the application of energy forces, which relates to the speed or time necessary to produce the desired surface modifications
Next to equipment, the specific gravity and weight of the abrasive is the most important element effecting speed or time, as well as actual surface texture and smoothness. Just as all equipment will eventually do the job of material removal, so too will all abrasives. However, there are literally hundreds of media compositions, sizes, and shapes available. As a general rule, the greater the pressure that can be exerted onto a part, either by equipment or the weight of the media, the greater the material removal. Therefore, here are some guidelines can be helpful for selecting media supplies for specific applications and working particular parts.
Random and Preformed Shapes
The most typical application of mass finishing equipment is to deburr parts en mass in the cheapest and fastest way possible without getting stuck or creating other finishing problems. To do this, you want to select the largest abrasive possible that is still small enough to reach all the part areas that have to be worked without getting stuck.
Unfortunately, finding the perfect abrasive size is not always easy. One size or shape does not fit all. Media choice depends upon the surface finishing requirements that are effected by different part configurations which limits access and the raw material of the part which determines the hardness and size of the abrasive. An abrasive that works well on one part may not be able to achieve the desired end result on another. That is because there is a relationship to the abrasive size, its hardness, and the part to be worked. For that reason, nonferrous or soft metals are not normally finished with the coarsest abrasive, because the surface of the metal will have a rougher finish after processing than it did before being worked.
For cost purposes, usually the cheapest form of any abrasive is loose random shaped, naturally occurring mineral compounds classified by screen sizes. Although this material is commonly used with abrasive blast finishing equipment, it is rarely used in mass finishing systems because it varies in size and shape making it more likely to jam and get stuck. Therefore, it is more common to use manmade shapes in mass finishing systems, because it is more controllable and predictable.
Unlike loose random natural abrasives, which can be more than 1 inch in size, the largest abrasive particle in a preformed shape rarely exceeds .060 in size. Manmade media is made with uniform size particles which predetermines the surface finish of the part in being worked. Technically, you can not have a surface finish finer than the largest abrasive particle used in the make up of the media. However, over all media size is determined or usually referred to by its physical size in L x W x H and abrasive composition first followed by its preformed shape.
Ceramic and Plastic Preformed Shapes
Most preformed deburring shaped media used in mass finishing systems is made with either a ceramic or plastic bonding agent to hold the uniform abrasive particles together. Ceramic media is made like cement and extruded and cut to size to make a shape; whereas, plastic media is made like an epoxy and then injected into small molded shapes. Both are then baked to achieve an overall hardness and to create a very hard abrasive shape.
Ceramic shapes are made with inorganic materials and primarily aluminum oxide abrasive grits, but silicon carbide, silicon, zirconia, and porcelain compositions are also available. The binders used to hold the abrasive shape together are formulated to decompose at a predetermined rate to expose new sharp abrasive particles. The harder the bond, the longer the shape lasts and the finer the surface finish of the part being worked. The faster the bond breaks down, the faster the media removes material and the coarser the finish on the end product. Ceramic preformed shaped media is relatively hard and very rigid, similar to a grinding wheel, so it is typically used on hard carbon steel parts or those parts requiring a lot of material removal.
Plastic media is softer and more flexible than ceramic media so it is usually used on nonferrous parts. Even though the abrasives used to make the shapes are the same as ceramics the media behaves differently. Plastic media will produce equal surface finishing results on nonferrous parts as ceramic does on steel, but plastics will take a long time to do anything on steel parts and therefore is not recommended for that application.
While ceramic media comes in a number of inorganic bonds, which determines their rate of decomposition, plastic comes in either a polyester or urea variation of the bonding agent. The urea formulations seem to be a little softer and cheaper than the polyester bonds. Ceramic media compositions are basically shades of gray or brown and plastic is multi colored resin formulations that also comes in many more shapes than ceramic, because they are molded. Generally speaking, the darker the color of ceramic media the faster the material removal rate. However, plastic media has no industrial standards or uniform color code that relates to the speed of cut or decomposition; therefore, be carefully and go by the verbal description of the composition and not the color.
Earlier I mentioned that after equipment, specific gravity and weight of the abrasive was the most important factor effecting the speed of processing parts. So, the more media and parts you can get into a machine system, the faster it works, with a couple of exceptions. A barrel system needs a definite air gap for the media to properly slide during processing. For normal deburring in vibratory and high energy systems, the process requires about 60% media to parts by weight, and up to 80 or 90 % for burnishing and good looking finishes. Also, remember normally the faster the media breaks down or decomposes the faster it works.
As a guide for deburring, abrasive media averages 100 lbs/ cu.ft. but can weight between 90 to 120 lbs. per cu.ft., some fine polishing porcelains can go as high as 150 lbs. per cu.ft., and on the opposite side are some hybrid light weight ceramics that are used on both ferrous and nonferrous materials that come in around 60 lbs. per cu.ft. Plastic media normally weights anywhere from 55 to 80 lbs.per cu.ft. and some zirconia abrasive plastic shapes approach 100 lbs. per cu.ft. range.
Because both ceramic and plastic media are formulated solids, they have a porosity factor that effects moisture content and the weight of the abrasive media. That means with age and depending upon the storage environment, this moisture content can and will vary. Even though this problem does not significantly effect processing, it does effect comparative test results. When one tries to compare one media against another, the media is weighted before and after testing to determine performance and attrition. The weight factor due to moisture is the real wild card that can play havoc with testing and there is almost no way to compare apples and apples accurately.
As a general statement, preformed media comes in two basic shapes or the shapes have two different behavior characteristics, plus one. Both work and there is no conclusive studies to indicate one shape is better than the other. In slang terms, what you have is basically a steamroller versus a bulldozer.
Spherical shapes, such as cylinders, cones, or spheres/balls have a lot of rounded surface features and are therefore very mobile like a steamroller. As they move, they rotate en mass and by themselves. These shapes work extremely well on parts with holes, because the media tends to poke itself slightly into holes and rotate before moving on. They also work flat areas very well without marking it.
Because these shapes are so mobile en mass, they do not hold, support, or restrict parts from reaching the bottom of the work chamber. That means the full weight of the media and mass is used to put pressure on the parts. These shapes, especially the balls, are not necessarily effective for finishing inside corners and angles resulting in a shadow appearance in these areas.
Angular geometric shapes, such as triangles and tri-stars move more slowly than curved shapes because their alignment and edges resist movement creating a pushing effect like a bulldozer. This resistance seems to transmit more energy or pressure to the parts on a more constant basis make this shape very good for working edges and inside angles on parts. This resistance characteristic also does not allow parts to sink en mass and it also creates a slightly louder sound while processing than rollers.
As mentioned, there is a plus one, hybrid shape that needs special clarification. There is one shape called a cylinder wedge that is probably the best general purpose shape media of all applications because it uses both a round and angular configuration. There is also something else interesting about this shape that makes it unique. Unlike all the other shapes that have their center of gravity directly in or near the center of its form, this shape has its center of gravity on its outside edge. That fact means that this media shape is the most unstable and most mobile of all the shapes and it also means that it is less likely to get stuck in most part recesses.
Dry finishing options
All of the media shapes discussed up to this point, even the random shape materials, is typically used in wet processing systems because dry inorganic materials do not leave parts clean and they may also produce surface finishes rougher than before processing. Also, when used dry, inorganic materials become easily contaminated and can not be easily removed or separated from the media. Therefore, water and a chemical compound are used in most deburring processes.
A better dry alternative media is wood, wood sawdust, corncob, and shell products. When used by themselves these organic materials don’t have much capability to deburr, but they can clean well. Because their small random particle size, which rarely exceeds 1/8 of an inch, they are light weight making their processing times to be very long and almost prohibitive. When inorganic pumice is added to this organic media, deburring drastically improves and it works well on jewelry detail and flat parts, but time cycles are still long. Processing times can be improved more by adding larger precut wood shapes or porcelain, but other preformed abrasives are not recommended as an addition because of porosity.
Of the organic materials, walnut shell media is the heaviest at 35 to 45 lbs./cu.ft., next is corncob at about 23 to 33 lbs./cu.ft., and then wood at between 21 to 27 lbs./cu.ft. The low number is the media in its untreated natural condition and the high side represents treated materials. Pumice additives add very little weight to these lower numbers but polishing additives push the upper weight limit.
In the last five years, a new media bonding resin process has been developed that produces these small loose random organic and inorganic materials to create a media shape and are run only dry. It looks, feels, and behaves like ceramic or plastic media, but it is used without water or compound, yet it produces a cleaner and smoother surface finish than traditional media.
The new resin bonded media comes in four formulations or grades and can have more inorganic than organic composition, therefore, it is hard to classify. The key to this media seems to be the resin bond that also gives it the longest life of any media on the market today. The attrition or break down rate is anywhere from 5 to 20 times that of the ceramic or plastic formulations. Therefore, despite its high cost of around $12.00 per pound, it is over all more cost efficient than other media over the long run.
Another advantage of using this media dry is that there is no need to treat wastes. Chemicals, rising, inhibitors, and drying processes are eliminated, and the maintenance to parts and equipment is removed. Parts come out clean and can go directly into another operation without waiting. Some precautions may be necessary to adapt existing equipment systems to this dry process such as closing and sealing drains and putting a dust cover over open machines or get an adequate air collection system nearby.
This new resin bonded media is only distributed in the USA by Finishing Associates Inc., a Sinto America group company located in Huntingdon Valley, PA. which also makes special equipment with air collection systems built in.
Although surface finishing industry personnel are retiring and/or going out of business there is a need for the technology and that technology is still changing. Unfortunately, the decreasing number of people in the trade are not as knowledgeable as they once where. The knowledge of equipment is separate from processing. Machines will only produce mechanical energy forces to the media that is in the work chamber. Processing of parts can be summed up with an earlier statement of, what you put into a machine determines what you get out of the machine. If you don’t know what to put into it, along with parts, you are not going to get the results you are looking for. Hopefully, this information will help you with some of those decisions.
For more information, contact A.F. Kenton, president of Nova Finishing Systems Inc., Huntingdon Valley,PA. 19006 or call 215-942-4474 e-mail email@example.com
Mr Kenton has been president and owner of Nova Finishing Systems Inc. for over 13 years. His company makes small bench top deburring equipment. He is a soon to be published author of a book entitled “ Understanding Deburring and Mass Finishing Systems”, which establishes a classification system for all material removal systems.
Finishing Associates Inc
1610 Republic Rd.
Huntingdon Valley, PA. 19006
P.O Box 248
Bartlett, NH. 03812
Illinois Electro- Deburring
Schiller Park, IL. 60176
• 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