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IT’S A MATERIAL WORLD A large portion of a part’s machinability stems from its material. Plain


carbon steels typically machine better than low alloy steel. Aluminum alloys containing copper, zinc and magnesium machine easier than other alumi- num alloys. Copper alloys with lead machine better than other copper al- loys. Annealed irons can be cut at high speeds. Along the same lines, alloys with a higher hardness typically are more diffi cult to machine. “You shouldn’t make castings stronger than necessary to meet specifi ca-


tions,” said Charles Bates, a metalcasting industry veteran who has written a number of research articles on machining. “Some think stronger is better, but it hurts machinability.” For some applications, your metalcaster might suggest a more machin-


able alloy for the application. However, machinability takes a lower priority than a part’s required strength. Heat treatment also may be used to improve the part’s machinability. The machining characteristics of carbon and low alloys steels can be improved through heat treatment by as much as 100-200%, according to the Steel Founders Society of America Steel Castings Handbook. Soft aluminum al- loys, such as grade F aluminum, might produce a built-up edge on the tool face, causing machined surfaces to be rough, according to Fields. T5 or T6 heat treatment can harden the alloy enough to avoid the built-up edge. However, heat treatment will change the microstructure and thus the


properties of the material, which should be considered in determining the fi nal production process for a part. 


down the time needed to machine. T e pads then can be quickly ground off after machining, if needed. Finding the right balance of


machine stock can aid in reducing machine time, as well. Machine stock is the extra amount of material added to areas of the casting that are going to be machined. Machining operations hold tighter tolerances and dimensions than a raw cast surface, so enough ma- chine stock should be in place to give a consistent amount of material to re- move. If not enough machine stock is present, the machine tool may end up running into the raw casting surface. However, a heavy hand on the ma-


chine stock results in longer machin- ing times, more metal needed in the raw part, and more material left on the machine room fl oor. “Determining machining stock is


like a moving target for continuous improvement,” Shah said. “Typically the metalcasting facility will determine


28 | METAL CASTING DESIGN & PURCHASING | Sept/Oct 2012


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