Media Selection

Ask an Expert From: Products Finishing, from The Markee Corp.

Posted on: 4/1/2009

Your frequent reference to the tri-cyl shape as being one of the most universal has been true for many jobs, but this is apparently an exception.

Q. Our problem is deburring around the ID of a flat, washer-like part 1.5 inches in diameter with a 5/8-inch ID, about 1/4-inch thick. The burr protrudes slightly into the ID following a surface grinding operaton. We have a through-feed vibrator that can process parts for about 20 min, and that is not long enough to get the job done. Our media is a 7/8-inch tri-cyl shape of medium cutting composition.


I am a fan of your column, and I’m always interested in discussion about media selection. Your frequent reference to this shape as being one of the most universal has been true for many jobs, but this is apparently an exception. Can you recommend a media to get this job done in the 20 min available? Also, I should mention that we looked at part-on-part finishing because you once wrote that this method is often successful with flat washers. The problem with part-on-part was that the final finish was too rough for the requirement. H.N.

 

A. You are right about the tri-cyl shape. It is one of the most universal. You are also right that it is not the choice for this application. This shape is also known by various other names, the “double-cut cylinder,” “cylindrical wedge,” and “WEJ,” (by Markee).


This is the second question this month regarding problems with deburring the ID of a washer-like part. Before answering your question, allow me to describe the other application. Both are similar, yet each has a different media recommendation. For clarity, let me call yours “Part A,” and the other “Part B.” Both parts are being deburred in through-feed vibratory finishers with limited time cycles of 20 min or so.


Part B is 5 1/2 -inch OD, 4 1/2 -inch ID, and is 1/2-inch thick. In addition to the ID, there are three slots spaced equally around the outside diameter. These slots are 1/4 -inch wide, 1/4 -inch deep, and are cut straight from the front to the back of the part, perpendicular to the sides. The problem burrs occur around the ID and around the slots. The present media is a 7/8-inch × 11/4-inch angle cut cylinder made from one of the very fastest cutting compositions. After an aggressive 20 min in this large through-feed machine, the deburring is not satisfactory. The media vendor has twice recommended an increase in the aggressiveness of the composition. One of those times he also recommended an increase in the size of the media from 5/8- to 7/8-inch diameter, still in the angle cut cylinder. The result has been a greatly increased consumption of media, a problematic level of media residue, and still unsatisfactory deburring.


Inside diameters of the two parts are considerably different. The 5/8-inch ID of Part A is small enough that the 7/8-inch tri-cyl just slides right across the face of the washer and cannot enter the ID to do any work. You might say it bridges the gap with hardly a care about the edges, or the burr.


Clearly, Part A needs a media that will either pass right through the ID, such as a small cylinder, or a shape that will merely protrude into the ID enough to do some deburring. After discussing the choices with this reader, my recommendation is a 7/8 × 5/16-inch thick three-pointed star. This is generally called an angle-cut tri-star. It is true that this shape can also slide along the face and bridge the gap across the hole. If you watch the media perform in such an application, however, you will notice that an occasional leg will protrude into the ID and the media will briefly dance around the hole, pushed and prodded by other media that wants to get by. The action happens frequently enough to get some decent deburring done on the ID— something that the try-cyl just cannot do. I am also recommending the heavier cutting composition (approximately 110 lb/ft3) so that more pressure will be applied to the burr when contact is made.


Part B has a much larger hole, and we aren’t worried about media getting lodged. The principle that comes into play in this application is that you need a media that will work back and forth and across the edges. If you watch this long, cylindrical media while it is vibrated in the tub you will notice that the media pieces line up with each other, side by side, hardly a piece turning at any angle to the rest. They are stacked up like a cord of wood, each following the exact pattern of its neighbors, in front, in back, and on all sides. This orderly flow of media tends to align the parts and push them along at about the same pace. There is little, if any, interaction between the cutting surfaces of the media and the burrs either in the ID or in the slots. This is one of many reasons why I seldom recommend angle-cut cylinder media.
My recommendation after discussing the options with the reader, is to use a 7/8-inch WEJ of the high-density, heavy cutting composition mentioned before.


Before leaving this discussion of media, let me describe this WEJ, tri-cyl, shape for any not acquainted with it. Most ceramic media is made by extruding abrasive-laden clay through a die in the shape of a circle, triangle, arrowhead, or star with three or four points. The extrusion is then wire cut to the desired length. (Cone-shaped media is made in a different way.) The angle at which the wire cuts the extrusion determines whether it is straight-cut, or angle cut. The distance between the wires determines the length dimension of the product. The abrasive content, if any, and the firing technique determine the “cut” of the media.


The WEJ is a shape cut from a cylindrical extrusion; it is cut at a 45° angle. It differs from an angle cut cylinder in that each end is cut in the opposite direction. You wind up with a shape that, viewed from either end is a circle, and from either side is a pyramid. Properly cut, each side of the pyramid exactly intercepts the top of the cylinder at exactly the same place. This results in a wedge shaped sharp edge, allowing the media to reach into fillets and corners better than most shapes. One interesting fact about this geometry is that it retains its shape right down to drain sizes. It therefore continues to perform as designed even when it is severely worn down. When you specify the WEJ, or tri-cyl, by whatever nomenclature, insist that the intersection of the sides is at the same point at the top edge of the cylinder. If that is not the case, you will not get the benefits of this uniquely designed media; you will wind up with something only marginally better than an angle-cut cylinder.
 


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