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Tumble Blasting Tubular Parts

The Problem:

Many manufacturers need to clean, peen, or finish parts that start out as pins, rods, tubes, and other thin, lightweight cylinders. These companies have seen all kinds of parts processed in tumble barrel and tumble belt cabinets, and they figure tumbling will work for their parts as well. Wrong!

This problem comes up repeatedly through the years. Recently, a distributor asked us to process knitting needles in a tumble cabinet. We knew better, but just to demonstrate the problems, we loaded the parts, set the timer, and hoped for the best.

Then we photographed the results and returned the parts to the customer.

The parts collected in a bird-nest clump.

These parts are generally lightweight and relatively inexpensive. To process them economically, companies load big batches. After only a few turns of the drum, the parts begin to fall into interwoven patterns and form clumps. These clumps hook to each other, until the entire mess gets big enough to bridge the gap between the barrel and the gun array. Eventually, the barrel carries the ball of parts to the top of the gun array, were it settles, relaxes just enough to avoid the moving barrel wall, and remains until the individual parts are pulled free.

Even when the rods are shorter than the distance from the drum to the gun array, their interwoven mass will grow until it bridges the gap. Another concern when placing slender parts into a tumble blast cabinet is the OD of the part compared to the ID of the media drain holes. If any part can possible fit through a hole, it will manage to do so - and it will usually do so when your back is against the wall trying to meet a production deadline. The result is a lot of scrap parts and an hour or more of down time cutting everything loose.

The Solution:

If you already have a tumble cabinet, and you're dying to process pins or tubes in it, start with very small batches. With each subsequent batch, increase the part count. When they begin to nest, back off the count. Keep good records and, once you get the optimum number, make sure you never exceed that limit.

Take special care to explain the process to new employees. It is very tempting when they look into the gaping maw of a large tumble cabinet to want to fill it with parts and get the job done sooner.

Unless you're dead set on tumble blasting, consider other types of automated blast cabinets - such as an indexing or continuous turntable, or a straight-line conveyor. You can place rods vertically in a simple fixture for blasting, but because the fixture covers some areas, the parts will not receive complete coverage.

Tubes can be placed over a dowel for all-over OD coverage in one pass. Pins and rods can be grasped in a spring-loaded fixture that masks just their ends.

Another option, lay the parts on an existing turntable or belt conveyor for the first pass, then turn them over for all-over coverage.

The cost for durable, high-tech fixturing is not always warranted. Sometimes it makes more sense to design and build your own simple, inexpensive fixtures and replace them as they wear out.

When the need for precise blasting will justify the cost, we can create a dedicated cabinet with parts handling tailored to the application.

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