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and grit for the North American mar- ket use the same specifi cations, each provided by SAE International: SAE J827, SAE J1993 and SAE J444. The fi rst specifi cation is for shot, the sec- ond relates to grit. And the third is for mixing the two. “The three work in concert,” Rhoda-

berger said. A fi nal specifi cation, SAE J2175 re-

lates to a low carbon shot provided by only one of the three North American producers. This material provides a unique microstructure and is inter- changeable with other steel shot.

Taking a Shot Shot is a rounded media used in

the majority of metalcasting facilities to remove sand and fl ash. The materi- als used in the industry are almost all of a standard hardness level (40 to 51 Rockwell C), as dictated by the SAE specifi cation. So, the only variance among the available materials is size (Table 1). This is the most important consideration when deciding what type of shot to use in your facility, according to Rhodaberger, and in general, the size of the shot should correspond to the size of the casting. “The size preference can be dictated

by the type of casting or the type of equipment being used,” he said. “Large steel castings with large blast areas and blast rooms might need a large media. The complexity of the casting [is an- other consideration]. But for the most part, size dictates complexity.” Nick D’Alessandro, a salesperson for

shot blast wear parts producer Astech Inc., Vassar, Mich., agreed that size is the most important consideration. For example, a 2-lb. die casting might require S170 shot (the third to smallest size available), and smaller materials might be selected when a fi ner surface fi nish is desired. D’Alessandro said the tricky part is balancing your part size and complexity with the shot size that gives you the best cycle time. “Cycle times are money. If you

have long times, you are spending too much,” he said. “With smaller shot, you have to throw more at the work piece,” meaning the cycle times are longer. So where do you begin when select-

ing the right shot size? Unfortunately, most experts in the market agree it is not an exact science. Rather, fi nding the right shot size is the result of trial and error. The number of trials you

MODERN CASTING / January 2011

have to conduct, though, can be lim- ited by how close you come with your initial guess. This should be based on standard industry norms, published by a number of shot providers. “Say you start

“You want to use the smallest [material] that still does the work.” —Bill Rhodaberger, Ervin Industries

with an S390,” D’Alessandro said. “You charge the blast machine with 390, run the wheel, run the cycle times, tumble for eight minutes, pull out [the work piece] and see how it’s doing.” If the castings are not coming out clean enough, you’ll have to use a larger shot material, if they are show- ing excessive wear, you may be able to use smaller shot and achieve the appropriate results. The velocity of the shot when it is

blasted at the work piece also plays a role in size selection, as it is directly related to the mass of the blast media, according to Pete Bortnichak, a regional manager for shot blast producer Whee- labrator Group, LaGrange, Ga. “If you’re using a wheel that is throw-

ing at 3,600 rpm, your shot velocities are going to vary [depending on size],” he said. “The amount of fi re power you can get out of [each piece] matters.” Once you have determined the

right size of material for the casting you’re blasting, the next step is main- taining the mix of materials that are actually in the shot blast machine at any given time. When shot strikes a work piece, it breaks down, leaving

Table 2. Work Mix Guidelines for Shot or Grit Size

Original Control Screen Opening (in.)

S660 S550 S460 S390 S330 S280 S230 S170 G12 G14 G16 G18 G25 G40 G50

0.0661 0.0555 0.0469 0.0394 0.0331 0.0278 0.0234 0.0165 0.0661 0.0555 0.0469 0.0394 0.0278 0.0165 0.0117

what Rhodaberger classifi ed as three different classes of material—the origi- nal shot size, inter- mediates and fi nes. “We start with

the original size and develop what is called a work mix,” he said. “It

is the combination of the three sizes that does the work.” With the right work mix (Table 2),

the large pieces of media remove sand and fl ash, and the smaller pieces pol- ish the piece. One of the advantages of the larger pieces of shot, according to Rhodaberger, is that they bounce around the piece after they strike a sur- face, doing more work on the rebound. In order to maintain the proper work mix, new shot should be charged in the machine at the same rate fi nes are removed. According to Rhodaberger, shot blast machines are designed to remove media using a curtain of air when the particles become too small.

True Grit Grit is an angular material that dif-

fers from shot in that it etches into the surface of a casting when it strikes it. The materials are commonly used in non-metalcasting industries to provide a surface receptive to coatings. Increas- ingly, they are also being used by met- alcasters to prepare their castings for coatings, according to Rhodaberger. But that’s not the most common application of the materials in the industry.

Original Control Range Target Take-Out Screen Opening (in.)

30-45% 30-45% 30-45% 30-45% 35-50% 35-50% 35-50% 40-55% 30-45% 30-45% 30-45% 30-45% 35-50% 40-55% 45-60%

0.0165 0.0165 0.0139 0.0139 0.0117 0.0117 0.0098 0.007

0.0165 0.0165 0.0139 0.0139 0.0117 0.007

0.0049 35

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