INDUSTRY NEWS
Article on Shot Missed the Mark LETTERBOX
Te article published in the Janu-
ary issue entitled “Choosing Blast Media” (p. 34) took several positions I
believe were incomplete and in some cases inaccurate. Te article indicates choosing the
correct abrasive size and grade is the result of “trial and error,” which does not have to be the case. Rather, by evaluating such criteria as the work piece to be cleaned and specific equip- ment parameters, such as the number and position of wheels, motor rpm, line speed, and desired surface profile for surface preparation or cycle time, the correct size and grade can be cho- sen the first time.
In another statement, the article
implies you should charge a blast machine with 100% virgin S390 to run a trial. Tis should never be done. Rather, an operating mix consisting of the original S390 and the next two smaller sizes should be blended for such a trial. The article states that low carbon
shot is “interchangeable with other steel shot.” This is not the case. Low carbon shot, due to its micro- structure, is by nature softer than high carbon shot and therefore does not demonstrate as high a level of energy transfer. Lower energy transfer translates to slower and less thorough cleaning of the work piece. Typically, low carbon users must use one size larger than high carbon us- ers, which means less coverage and slower cleaning rates. Te article implies that size is the most important consideration when selecting a blast material. Tis overlooks the role hardness plays in energy transfer. Te SAE specifica- tion indicates a wide range of 40-51 Rockwell C for shot. Again, a harder shot of, say, 47 Rockwell C will transfer more energy than one at 44, thereby cleaning faster and result- ing in shorter cycle times. Also, grit comes in four hardness ranges that must be considered when making a grit recommendation.
Bob Schmidt
Vice President of Sales W Abrasives Bedford, Va.
16 | MODERN CASTING March 2011
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