Steel Shot: Hit or Myth | By Duane Howard Today there are many manufacturers of steel shot blasting machines. Each one may have their own method of operation. Each manufacturer may have their own theory of how and why these machines work the way they do. Hopefully, the following may help you decide what is correct and/or best for your machine(s). Most rebuilders are aware that steel shot is available in various size ranges from S-70 to very large S-780 shot. The most recommended and widely used shot size today is S-170. This is also generally the smallest size used. It cleans quickly and well, and is used on both steel and aluminum parts in the same machine. Because it is on the fine side, it is usually not too rough for alu- minum parts (see Figure 1, page 12). The next size larger, S-230, is also used by automotive electrical parts rebuilders. Although it will last a little longer, it will cause a rougher surface on aluminum parts. The next larger size, S-280, will last longer yet. However, it will leave too rough a surface on many aluminum parts. If a rebuilder has a machine dedicated to steel parts only, this shot size may be a consideration. Steel shot is available in different Rockwell hardness ranges as well. Remember, hardness is “resistance to deformation.” It is probably the wisest choice to use a shot of a medium or low hardness (low if available), because the longer a particle of shot stays in use, the shinier it gets. We all know that if a product is hard it tends to be brittle. The same goes for steel shot. If the shot is too hard it will break down too quickly, causing more than normal dust in your machine. If a shot is too hard it will not clean well because the shot will never get bright and shiny. Also, the harder the shot, the quicker it will wear out your machine and machine parts (blades, wheel well, baskets, walls, etc). So what do you do? You simply use a medium or low hardness shot. This does several things. It lasts longer—it will tend to “shine” (if your dust collector is maintained). Once the shot is broken in and starts to “shine” your parts will “shine” and look brighter and cleaner. Plus, you will get the maximum life possible from your machine! It is important to buy your steel shot (what we call “real steel”) from a rep- utable manufacturer or distributor. Do not use chilled iron shot or grit. Do not use decarburized iron shot or grit. Those products are cheaper, but break down quicker and can wear your machine out four times faster! They also will not clean your parts well. “Real steel” shot is not expensive considering its life. (There is a product called steel grit. However, it is not used or recommended in up blast machines as owned by most automotive electrical rebuilders.) What happens to the abrasive shot, as you blast, is unique in its own way. For instance, you may start your cleaning process with S-170 or S-230, medium hardness. As it is used, it work-hardens and eventually fragments (example: one piece into two pieces). These two frag- ments will have jagged edges. After continued use these fragmented pieces will round off again into smaller balls or ovals and will fracture again (two pieces into four). See Figure 2. This process repeats continuously and now you have the best kind of “shot” in your machine. It is called an “operating mix.” (Your dust collector must be maintained prop- erly). See Figures 3a, b and c on page 20. The amp meter of your machine is almost like the speedometer of a vehicle. It should be checked several times with each load. Why? Because small amounts of new steel shot added is the best way to maintain your operat- ing mix. If your operator waits until your amp meter is too low, he has to add a large amount of shot to bring it back to the proper reading. Those next loads may come out a bit too rough because he has upset the “operating mix.” So much for uniform finishes. New shot is not shiny. It may clean, but it does not brighten the parts because the new shot itself is not “clean.” It must be broken in to remove the oxidation. Some points to think about in summary: *Use steel shot with medium or lower hardness. «Keep dust collector clean and maintained. «Make small additions of shot frequently—keep the amp meter where it should be. Check the plate on the motor for proper operating amperage. ¢ It is the combination of the larger, the medium size and the finer shot particles (the operating mix) which produce the fastest, cleanest and brightest parts, day after day, with a minimum of machine wear. * Poor choice of steel shot will result in longer cleaning and lack of uniform finish. «An easy way to determine the proper size shot for the job is to understand: a) the cycle time to accomplish amount of work needed; b) cost— start to finish; c) finish required on the end product. Duane Howard, formerly of ARE Industries, Wichita, KS. Also contributing, Ron Fine, Fine Industrial Supply, Baldwin, NY.