The Principles of Deburring and Polishing
Using Mass Finishing Systems.
By A. F. Kenton
President Nova Finishing Systems Inc
There are a lot of ways to deburr and polish parts; however, normally the fastest, cheapest, and most efficient mechanical way is to use mass finishing systems. Unfortunately, there are a lot of variables that effect the time, cost, and finish of a part. The bottom line is that the part must fit, form, and function. In addition to these problems might be the final appearance of the part. To properly address these problems, there are three main factors that control surface finishing using mass finishing systems. These factors are: the equipment, the media, and the additives.
All things are relative. That is, tools or equipment are over all faster than hand operations. This same idea holds true for mass finishing systems. Equipment is the first factor that effects part finishing and basically there are now three generations of automated equipment now in use. These systems are the old tumbling barrel, then vibratory technology, and high energy systems. Briefly, barrel systems rotate an octangular work chamber in one direct filled with parts and an abrasive, called media. The weight of the parts and abrasives applies pressure to the parts to deburr, burnish, or polish the parts. The amount of this pressure to the part in a barrel system is equal to 1 g or the gravitational force of the entire mass and its sliding action. Therefore the larger the work chamber or machine, the greater the mass and pressure and slide action to process the parts and the shorter the time cycle.
Now the limiting factor to the barrel system is a combination of a weight factor, RPM’s of the barrel and the slide action zone determined by the size of the barrel. If there is no slide action, or the part can not move, then the weight of the mass will not effect the part at all because there is no movement or abrasive contact. Without the pressure of a heavy mass, the element of time will be drastically increased to produce acceptable results. A barrel should be filled with a least 50% and up to about 65% of media and parts to maximize its efficiency; however, the slide action zone or processing occurs only in the top layer or in the slide movement. This surface movement can be related to surface feet per minute. Most sanding belts operate at about 1000 surface feet per minute; whereas, a barrel operates at between 50 and 250 surface feet per minute. The speed of rotation of the barrel controls the surface feet per minute time cycle and finish of the part. At too high of a RPM materials can become air borne, loses efficiency and can damage parts. At too low of a RPM the process becomes inefficient and not cost effective.
An improvement over the barrel processing time is the more recent technology, developed in the 1940’s, of the vibratory finishing systems. Instead of rotating a barrel, a vibratory system uses an off center eccentric weight to create energy forces on an X, Y, and some Z axis’s to a stationary work chamber. Energy is directed into the equipment at a constant moving point in a pulsating manner, or energy wave, and transmitted through the media onto the part(s). The amount of force of the energy transfer equals 8 g’s or 8 times the normal force of gravity. There are two versions of this type of equipment. One is a tub and the other a bowl. Both use the same technology principal but apply energy along different lines or patterns to the work chamber and mass.
The configuration of the tub allows it to process much longer parts than bowls. The bowl design on the other hand looks more like a donut than a bowl. Because of the tubs length, the work chamber is usually deeper or more compact than a bowl design. That means that the weight and the media travel in the tub is more like the barrel or moves primarily in one direction but is energized throughout its entire mass. The media is also more concentrated on top of the part(s) and therefore improves or shortens processing time over that of a bowl. In operation, the media of both machines travels up the outside wall and falls back toward the center of the machine in a slightly elongated fashion. Beside this movement pattern, the bowl also moves the material in a clockwise or counter-clockwise direction from the top. This pattern gives the bowl greater versatility when it comes to both loading and unloading of parts and the separation of the media from the parts and keeps the parts separated more.
The last generation of equipment is called high energy systems. This type of equipment uses the Z forces more than the X and Y motion to work parts. That is, the centrifugal forces used in this type of equipment produces pressures of up to 30 g’s to speed up processing time. This speed and pressure relates to about 1000 surface feet per minute, thereby making it behave similar to a sanding belt. Again, there are two basic types of high energy systems. They are called high energy barrels and centrifugal discs. High energy barrel systems were developed back in the 1950’s, but they did not see much service until further refinements in the technology in the late 80’s. These high energy barrels are different from the old fashion barrel tumblers in that there are usually a set or four balanced barrels spinning in one direction while they are also counter rotating; thereby producing high gravity forces on both the media and parts within the barrels. Some systems are designed to position the barrels in such a way that entire mass moves in a figure 8 within the closed barrel. Because of the high speed spinning forces exerted on both the barrel and disc systems, these system are usually completely enclosed while in operation.
The smaller high energy centrifugal disc machines look similar to the vibratory bowls and do not have to be enclosed while in operation. The fully automated systems with material handling capabilities are enclosed. These machines are designed with stationary work chamber walls similar to a vibratory mill, but the bottom of the chamber has a spinning hub, core, or cone. The speed of rotation moves the mass within to travel at up to 28 times the force of gravity along the cone until it goes up the stationary wall and back down into the center cone area again. Although this machine system is slightly slower in cycle time than the barrel systems, it can normally handle larger parts and it is a lot faster to load and unload the entire mass.
As you can tell from the preceding equipment descriptions, these machine systems only provide a mechanical action to energizes an abrasive media. That is, they create or move the entire mass within them in such a way that they work together to cause abrasion to occur resulting in a surface finish. The technologies used to create these energy forces are almost progressive by a factor of 10. That means that if everything is equal, a part is run in a barrel system for 10 hours can be done in 1 hour in a vibratory system and 6 to 10 minutes in a high energy system. However, each machine system is still in use today because they all have certain advantages and disadvantages to either the part or the process.
Now, the media in the machine is what is actually used to create the desired finish to the part. In other words, what you put into the machine determines what you get out of the machine. The more abrasive the media, the faster or shorter the processing time to deburr and the rougher the surface finish. Using a non-abrasive media normally results in a smooth and shiny part. The heavier or more media you can put into a machine system, the faster it works. The exception to this is the tumbling barrel system that has specific limitations. Another factor for deburring that controls the cycle time and surface finish is the composition or make up of the media.
The most common abrasive products or media used by early tumbling systems were random abrasive products classified into size ranges. These materials are the least expensive abrasives and are still in use today; however, machined parts do not lend themselves to random size media because of irregularities that cause the media to get stuck in the part and may not properly work all surface areas uniformly. To reduce this problem of surface finishing and lodging in parts, man made preformed shapes of different sizes and compositions were introduced probably in the 1930’s. Because of the uniformity of the media, processing and surface finishing of parts this technology has become very consistent and lodging is greatly reduced.
There are 4 basic compositions of media in use today, meaning what these sizes and shapes are made out of. They are: ceramic, plastic, burnishing, which can be either a non-abrasive porcelain ceramic or metal, and organic materials. Deburring media made of ceramic or plastic refers to the glue or bond that holds the shape together. That is, a preformed shape consists of a matrix of fine random abrasive held together with either a ceramic or plastic bonding agent. The larger the size of the random abrasive and the heavier it is, normally the faster it works to effect the part. Another purpose of the bond is to break down or decompose in use to allow the shape to expose new sharp abrasive edges. The faster the bond breaks down, the faster it works and if the media does not breakdown it doesn’t work.
While we are talking about breakdown, we should discuss media selection and size. Media is selected to be shaped and large enough to work all the areas of a part that needs to be worked. As the media breaks down, it loses weight and takes longer and longer to produce the same results than when it was new. At about half its size, which I call half life, it basically becomes inefficient to be used on the part it was selected to work. It can be used on smaller parts, but besides taking longer there is a greater tendency for the media to get stuck in internal dimensions. Actually media may have to be replaced a lot sooner than half life if the we are dealing with precision parts or if the media gets glazed due to poor liquid flow and/or chemical usage.
Media comes in many sizes, shapes, and compositions. Making the right selection is important to the surface finish and overall performance of the process. I classify media into maybe 2+1 categories of shape. The 2 being either a steamroller or a bulldozers, or more current terminology of rollers and pushers. A media shape either rolls or scrapes. The more diameter the media has, the easier it rolls and allows parts to move. The more geometric the shape, the greater the resistance the media has against the part and in mass. Both shapes work. But, if the media can not or does not move properly, it doesn’t work and/or can create additional problems. Generally speaking all media shapes have their center of gravity right in the center of the shape making them very stable. Therefore, when they become restricted within a part, they just rattle around until they get stuck tight. The only shape that does not have its center of gravity in the center is a shape called the V cut wedge or cylinder wedge. It has a very point created by two flats, but is round thereby making it a +1 or hybrid of two shapes. Its center of gravity is located on the outside bottom edge of the shape making it very unstable and mobile; therefore it is a good general purpose media shape for a lot of different parts. However, for generally smoother parts, I prefer the roller. For greater material removal, I prefer the geometric shape.
Burnishing media is non abrasive and is designed to lap or smooth parts, but because it is non-abrasive, it only modifies the present surface finish of the part making it shiny not smooth. Therefore, steel or stainless steel media is preferred over porcelain because it weights approximately 300 pounds per cubic foot of material versus the 100 pounds of almost all ceramic based deburring or burnishing media. The size or bulk of the media is about the only factor in improving processing time, because there is only one composition for each non-abrasive. Then again, a lot of machine systems can not take the heavy weight of steel for proper processing thereby leaving porcelain ceramic as the only alternate choice.
The last category of media is organic. Unlike all of the previous media talked about, this media is almost always used dry; whereas, the other media is run in a wet processing. Up to now, nearly all this media was used mostly like random shape abrasives. That is, they were classified to pass through screen sizes and generally speaking the maximum size of a particle is about 1/8 of an inch. In addition to its light weight of between 20 and 35 pounds per cubic foot, this media takes a long cycle time to either deburr or polish. Wood shapes are commonly mixed with this fine grain material in order to provide additional bulk and weight to the processing. Depending on polishing additives or inorganic abrasives, you are still talking about time cycles in excess of 20 times that of wet processes. However, for bright, smooth, mirror finishes, this process is superior to all the other media processes.
Now, after talking about the above dry processing media, new technology has changed or will change a lot of the concepts concerning dry processing. Within the last 5 years, there is a company producing man made preformed organic media shapes that look something like plastic media but it is used dry. That means that just like ceramic and plastic, there is a new category of media which can now be called dry shape media. I dropped the word organic, because these new shapes can actually have more inorganic materials than organic, but they are still run dry. Supposedly, this new media has an attrition rate of 5 to 20 times longer than that of wet preformed shapes for deburring. This longevity plus problems and maintenance associated with wet processing tends to offset its expensive cost.
We have talked about equipment and media. The last factor that effects mass finishing systems is the additive. In the past, I would have basically said water and compound; however, the dry organic material usage has changed a lot of that. However, at one time, this factor was and is considered the third major factor in effecting part finishing. By putting in too much liquid, it is possible to slow down abrasive action and create a greater buffing action in the equipment. This same result can be obtained by putting in too much chemical and that has a tendency to suds. Some barrel processes use a water level higher than the media; however, most systems operate with no water or suds splashing or visible during operation. In either case, proper liquid flow is essential for clean parts and consistency in the time cycle and finish on the part.
There is no clear cut rule about which chemical additives to use with which materials. However, in general, basic compounds, chemicals above the pH of water at 6.7 are used on ferrous metals while chemicals below that are used on non-ferrous metals. Chemical additives can come in liquid or powder form, but the tendency is to liquids because they are easier to regulate. Another desired ingredient to a liquid additive is a wetting agent to increase the effects of cleaning then a good inhibitor against oxidation. Products that have a lot of lubricity are recommended for burnishing. Some chemicals aid in producing a reaction or coloring effect on parts. In fact, new technology has also produced some chemical additives called accelerators. The latter product normally recommends the use of a non–abrasive media with these strong chemicals that produce a reactive oxidation like coating that increases material removal. This is a very effective process where a lot of material needs to be removed. Also, since non-abrasive media is used, costs or media usage is reduced.
Well, there you have it. Simple right? Just like a computer, there are a lot of ways of achieving the desired end result. However, just in case we may not have covered the specific subject area of your concern, or if you have any questions, you may contact A.F. Kenton, president of Nova Finishing Systems Inc. at 215-800-942-4474
 NOTE: This author has written a yet to be published book on the subject which completely classifies all methods of deburring and/or surface finishing into 5 classes of equipment and 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”.
• 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