Abrasive Control Factors for Mass Finishing Systems
Gravity and weight are the most important elements affecting speed and time.
A growing concern in the surface finishing industry is the aging of those in the business. As the trade matures, more and more is left up to the people performing the surface finishing work. This graying trend is especially noticeable in the mass finishing industry, where subtle processing control factors (rather than say, software programs) are what mark distinctions in quality. And it’s for that reason that certain basic fundamentals of material removal and surface finishing should be passed along to the next generation.
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 biggest 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 affecting 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.
With that in mind, here are some guidelines can be helpful for selecting media supplies for specific applications and working particular parts.
Random, Preformed Shapes
The most typical application of mass finishing equipment is to deburr parts en masse 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 isn’t always easy. One size or shape does not fit all; media choice depends on the surface finishing requirements that are affected by different part configurations, which limit 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—the surface of the metal will have a rougher finish after processing than it did before being worked.
For cost purposes, the cheapest form of any abrasive is usually 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. More commonly, manmade shapes are used in mass finishing systems for their controllable and predictable nature.
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 inches in size. Manmade media is made with uniform-size particles, which predetermine the surface finish of the part 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, overall media size is usually referred to by its physical size in L x W x H and abrasive composition first, followed by its preformed shape.
Ceramic, 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, 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 a very hard abrasive shape.
Ceramic shapes are made with inorganic materials, 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 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 the same surface finishing results on nonferrous parts as ceramic will on steel, but plastics will take a long time to abrade steel parts and therefore are not recommended for that application.
While ceramic media comes in a number of inorganic bonds that determine their rate of decomposition, plastic comes with either a polyester or urea bonding agent variation. Urea formulations are typically 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; so be careful and go by the description of the composition and not the color.
Gravity and Weight
Earlier I mentioned that after equipment, the specific gravity and weight of the abrasive are the most important factors affecting 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 percent media to parts by weight, and up to 80 or 90 percent for burnishing and good looking finishes. Also, remember that typically, the faster the media breaks down or decomposes, the faster it works.
As a guide for deburring, abrasive media averages 100 pounds per cubic foot, but can weigh between 90 to 120 pounds per cubic foot, some fine polishing porcelains can go as high as 150 pounds per cubic foot, and on the opposite side are some hybrid lightweight ceramics that are used on both ferrous and nonferrous materials that come in around 60 pounds per cubic foot. Plastic media normally weighs anywhere from 55 to 80 pounds per cubic foot, and some zirconia abrasive plastic shapes approach 100 pounds per cubic foot range.
Because both ceramic and plastic media are formulated solids, they have a porosity factor that affects moisture content and the weight of the abrasive media. That means with age and depending on the storage environment, moisture content can and will vary. Even though this problem does not significantly affect processing, it does affect comparative test results. When one tries to contrast one media against another, the media is weighed 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 accurately compare apples to apples.
Generally, 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 another—it’s like comparing a bulldozer to a steamroller.
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.
Because these shapes are so mobile en masse, 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, making this shape very good for working edges and inside angles on parts. This resistance also keeps parts from sinking en mass and creates a slightly louder sound while processing.
As mentioned, there is a plus-one hybrid shape that needs special clarification. A cylinder wedge 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, this shape has its center of gravity on the outside edge, meaning it is the least stable and most mobile of all the shapes and therefore less likely to get stuck in part recesses.
Dry Finishing Options
All of the media shapes discussed up to this point, even the random shape materials, are typically used in wet processing systems, because dry inorganic materials do not leave parts clean and may also produce surface finishes rougher than before processing. Also, when used dry, inorganic materials become easily contaminated and cannot be easily removed or separated from the media. Therefore, water and a chemical compound are used in most deburring processes.
Better dry alternative media include wood, wood sawdust, corncob and shell products. When used alone, these organic materials don’t have much capability to deburr, but they can clean well. Because of their small, random particle size, rarely exceeding 1/8 of an inch, they are light, making their processing times long and almost prohibitive. When inorganic pumice is added to organic media, deburring improves drastically and the combination can work well (albeit slowly) on jewelry detail and flat parts.
Of the organic materials, walnut shell media is the heaviest at 35 to 45 pounds per cubic foot. Next is corncob at about 23 to 33 pounds per cubic foot, and then wood at 21 to 27. 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.
New Dry Technology
In the last five years, a new media bonding resin process has been developed that uses small, loose, random organic and inorganic materials to create a media shape and run only dry. It looks, feels, and behaves like ceramic or plastic media, but is used without water or compound, and produces a cleaner, 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, so it’s 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 breakdown rate is anywhere from 5 to 20 times that of the ceramic or plastic formulations. Therefore, despite its high cost, at around $12.00 per pound, it is overall more cost effective than other media over the long run.
Another advantage of using this dry media is that there is no need to treat wastewater. Chemicals, rinsing, inhibitors, and drying processes are eliminated, and part and equipment maintenance is diminished. Parts come out clean and can go directly into another operation without wait time. Some adaptations may be necessary to convert existing equipment systems to this dry process, such as closing and sealing drains, putting a dust cover over open machines, or providing an adequate air collection system nearby.
Currently, this new resin bonded media is only distributed in the USA by Finishing Associates Inc., a Sinto America group company located in Huntingdon Valley, Pennsylvania.
Thanks to Mike Cantwell, Finishing Associates Inc., Huntingdon Valley, PA.; David Davidson, Pegco, Bartlett, NH; and George Bull, Illinois Electro- Deburring, Schiller Park, IL.
A.F. Kenton is president and owner of Nova Finishing Systems Inc. For more information, call 215-942-4474 or email email@example.com.
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