Page 26 - MetalForming March 2013

The Quest for Q5
a number of process variables.” Metalformers typically use two types of garnet for abrasive-waterjet cutting: crushed and alluvial. For Q5 cuts, Schwartz recommends using crushed garnet. “It generally has sharper edges that cut better and faster, while alluvial garnet exhibits smoother edges,” he says. Opta Minerals’ Bill Meyerholtz explains the difference between
crushed and alluvial garnet:
“More angular grains of garnet have sharper edges. This allows for cleaner cuts and better control. Usually, crushed garnet has sharper edges than alluvial garnet because alluvial garnet has had the sharp edges rounded a bit by the processes that eroded the crys- tals out of the host rock and transport- ed the garnet grains into beach sand or gravel deposits.
“Using the proper mesh size also is
important,” Meyerholtz continues. “Mesh sizes identify a particle-size range or sometimes an average grain size present in a particular product. A smaller mesh number indicates larger grain sizes. The finer the average par- ticle size the smoother the edge quali- ty of the cut and the more precise the control. However, cutting speed will be slower, since larger particles cut faster. Most shops try to achieve the best combination of cutting speed and edge quality. A #80-mesh garnet pro- vides a nice combination of speed and control, while a #100 or #120 mesh will cut more precisely but at a lower speed. Likewise, a #50 mesh will cut more quickly but will leave a rougher edge and wider kerf.”
Meyerholtz reminds metalformers to be mindful of production costs when selecting garnet.
“Look for the best combination of attributes that will minimize overall production costs, including the cost of cutting and of additional workpiece fin- ishing. If you cut quickly but the work- piece requires excessive edge finishing, overall costs will rise. Likewise, if you cut with cheap garnet and sacrifice time and productivity during cutting, you may not achieve the lowest overall pro- duction cost. Cut control, cut quality and productivity are best served by using good-quality, clean garnet.
“Almandite garnet offers the best combination of (high) specific gravity and hardness,” Meyerholtz continues, “to efficiently deliver cutting energy to the surface of the workpiece. The gar- net should be clean—very low content of other minerals and free of clay and other slimes. The combination of grain sizes should be consistent, with no oversize particles to plug nozzles or feed systems. And, the product should be dry and flow freely so that it feeds through the system smoothly.”
“Using the proper amount of garnet also proves critical to making quality cuts,” adds Pavlov. To determine the correct amount of garnet for a partic- ular cut, consult your manufacturers’ cutting parameters and perform a test cut.
24 MetalForming/March 2013
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