Hard turning of powder metal.

Author:Dailey, James H.
 
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Gearmaker achieves 60% increase in annual production using PCBN.

Hardened steel and cast iron have always been the materials for choice for automotive gears and similar components. In the 1990s new materials, such as powder metal (P/M), have demonstrated production advantages over cast iron and hardened steel. The use of powder metal in the North American market doubled from 1991 through 1996. It is expected to double again by 2001 as more components made of powder metal find their way into automotive manufacturing operations.

Since powder metal parts are pressed and sintered to near-net shape (NNS), the machining and finishing cycle times are dramatically reduced. This results in a significant cost savings in production, making P/M components very attractive for the automaker. However, these rough-pressed near-net parts are difficult to machine and finish due to the sintering process, which forms various oxides and carbide micro-particles. The hardness of these particles can be as high as 70 HRc with a normal range falling between 50-60 HRc. Powder metal is as tough to machine as hardened steel but its composition behaves remarkably different in a machining operation. [ILLUSTRATION FOR FIGURE 1 OMITTED] Therefore, a cutting tool material is needed that can successfully overcome P/M's unique characteristics.

When a large North American gear manufacturer was faced with a 60% annual increase in production as its projected output climbed from 270,000 to 450,000 units per year, it turned to GE Superabrasives America Applications Development Center (AADC) in Worthington, OH, to learn about the benefits of hard turning as a production solution. If the manufacturer continued to use conventional grinding technology, it would have to invest $600,000 in three new dedicated grinders to meet its new production requirement.

The manufacturer required certain specifications for the finished gears. If hard turning were to be an option, superabrasive machining parameters would have to be established for an interrupted cut (four oil holes 90 deg apart, [ILLUSTRATION FOR FIGURE 2 OMITTED]) of the ID gear bore, with a surface finish [less than]25 Ra [micro][inches] (0.63[[micro]meter]). The AADC examined several key process variables necessary for the proper development of this superabrasive machining application. The testing procedures were established using the actual powder metal gears on a standard lathe set up for a boring operation. Machining parameters were evaluated,...

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