Internal Burrs Quadruple Finish Time

 Q. I have a deburring problem on the inside of a steel ring. The ring is cut from three-inch steel tubing of 3/8-inch wall thickness; each piece is ½-inch long. Four 0.375-inch holes have been drilled radially around the ring. There is a light burr along the chamfered ID and OD edges, and heavier burrs are around the drilled holes in the ID. My only mass finishing equipment is a 3-ft3 vibratory bowl. The media I use for most parts are 5/8-inch, 45-degree-angle cut triangles of a medium cutting composition; I also have some seldom-used 3/8- × 1-inch angle cut, carbide cylinders. The machine will hold about 200 parts; the edges are deburred in the first 30 minutes, but it takes at least two hours to debur the drilled holes. The carbide media reduces the time cycle to 1.5 hours, but the media wear rate and higher price does not offset the time savings nor make up for the time it takes to change media in the machine. What can you recommend to reduce the time cycle without using carbide media? S.C.

 

A. The first thing that comes to mind is to brush debur the inside of the ring with a brush large enough to debur the entire ID in one pass. I’m not a wire-brushing expert, but I have seen mop-like abrasive brushes with long strands that may remove the OD and ID burrs as well as the interior burrs with a single in-and-out pass. It’s worth investigating. This may get those drilling burrs down to where they can be vibratory finished as quickly as the rest of the part.
The angle cut triangle is an excellent shape for this job. It will do ID and OD edges quickly, and the points will work around the drilled holes as well as any media shape, particularly in this specific part configuration. That leaves media composition as a variable. You already demonstrated that a very aggressive composition is one answer, albeit an expensive one. There is a different composition I can recommend, and that is a very heavy (130–140 lb/ft3) composition with the most cutting ability available in that density range.

The higher density media will apply considerably greater pressure on the work area. Vibratory finishing with cutting media is similar to a filing action. Think of working a burr with a hand file: the more pressure you apply, the faster you will remove the burr. In this case, the media will be about 65 percent heavier, and it will do the job more quickly. In addition, most high density media has longer life, and that translates to less residue, reducing disposal requirements. Often, lower compound flow rates can be used with this media while still getting clean parts.

While taking a good look at this application, you also may consider equipment changes. First, let us look at just getting a larger vibrator. Your 3-ft3 machine produces 100 parts per hour under present circumstances. Moving up to a larger machine will give proportionately more production at less than proportional cost increases. You also will find that a larger capacity machine does not take up much more floor space.

You also can investigate centrifugal disc (CD) finishing. This is a process using the same media and compounds used in vibratory or tumble finishing, but the action is different and several times faster than vibratory finishing. The CD process is contained in a stationary round chamber with a rotating bottom. It is filled with parts and media in a ratio similar to vibratory and tumbling processes. As the bottom is rotated, the mass is forced against the wall of the chamber where it has nowhere to go except up the wall until it falls down in a whirlpool action. The action is similar to a food blender.

The CD process uses the sliding action of media against parts to accomplish deburring. It does this with considerable pressure, and typical time cycles are one-third or less than those of a vibratory finisher. The CD process is similar to tumbling barrels in that burrs have to be accessible to a straight line of action; vibratory finishing excels in reaching semi-interior areas, such as inside a bell-shaped part. The burrs described in this application, however, will be accessible to the CD process. Here are some caveats about CD processes and equipment: Initial cost is very high; time cycles are not always reduced as much as advertised—insist on a production size demonstration; media wear is perhaps out of proportion to the work done; compound flow rates are higher than with vibratory finishing; separation of parts from media and returning the media bowl may offset much of the process time advantage; maintenance and downtime can be costly. Be sure you have optimized the vibratory finishing process that you are using for a comparison in your purchasing analysis.

Another process to consider is blast finishing. It is by-far the most efficient method if the resulting finish is acceptable. To blast these parts efficiently you would use a tumble/blasting wheel type machine and small, probably about S-170 or S-230 steel shot. The time cycle for a whole load of parts will be about five minutes. Tumble-blast machines are made in a wide range of sizes, so whatever the production requirement, these batch machines can be very efficient. If the blast finish is acceptable, and if you do not have a tumble/blaster in your shop, consider sending the work to a qualified metal finishig job shop with that equipment.

Other deburring processes are available, however the processes described above will be the most cost effective.