Machining powder metal is like machining pure abrasion. So opines Charles Gerlach, president of Gerlach Machine. Located in St. Henry, Ohio, a small town landlocked by corn fields and quaint farm houses, Mr. Gerlach's shop has specialized in machining powder metal parts since the 1980s.

The soil in those nearby cornfields is loosely similar to powder metal, in that it is a mixture of nutrients and organic material that come together to provide the base for what farmers hope will be a bumper crop. Along those lines, powder metal parts are a mixture of tiny metal and alloying elements (think talcum powder tiny) that, after being compacted together and heated, form a near-net-shape component.

Unfortunately (for the powder metal molders, anyway), secondary machining operations are typically required to bring the part to its final form. At this stage, the powder metal part may have an apparent hardness rating that isn't very daunting in terms of machineability (30 Rc, for example). However, those individual powders retain their individual hardnesses--which could be 50 Rc or higher--yen after the part is molded. A tool cutting into such material will be slicing through very small, very hard particles--in essence, it's pure abrasion.

The result is excessive and often unpredictable tool wear as cutting edges break down. Success or failure in machining this material rests primarily on the choice and application of the cutting tools.

Why Powder Metal?

The number of powder metal components finding their way into the latest automobile designs continues to rise. In fact, such components are now used in more high-profile engine and transmission applications. Though powder metal parts have become the darling of the auto industry, they can still be a demon for the shops that must machine the finishing touches.

Gerlach Machine didn't set out with the intention of specializing in powder metal machining. The shop's Midwestern location had a lot to do with it. A number of local powder metal molders approached the company 25 years ago to machine the features that couldn't be created in the molding process. Many of these early parts required only boring operations to bring IDs to tolerance or tapping operations, because threads can't be formed in the mold.

In many cases, the ODs of the powder metal part the shop machines are molded to net shape. This is why these materials are especially attractive for gear components, as the traditional hobbing process to form gear teeth is not necessary. The molded teeth can then be induction-hardened for wear resistance, while the core remains soft to reduce the chance of fracture during operation.

It is because of molding process limitations that other part features, such as threads, require secondary machining. While holes that are parallel to the axis of mold compression can be molded, cross holes (those perpendicular to the compression axis) and grooves around the periphery of the part can't be molded. There are also limits to how thin part walls can be molded. In addition, machining may be necessary to reach the tighter tolerances and better surface finishes that are required of the latest automotive components.

Tooling Considerations

Most powder metal parts require turning and tapping operations rather than milling, which is why Mr. Gerlach's shop has more CNC lathes than mills. Because powder metal parts are near net shape, heavy roughing cuts are typically not required. Appropriate tooling, therefore, is that which is geared toward semifinishing and finishing duties. For turning operations, Mr. Gerlach primarily uses cermet inserts from Valenite (Madison Heights, Michigan). These inserts have essentially no edge prep in order to provide an extremely sharp cutting edge and free cutting action, which Mr. Gerlach has found effective in turning powder metal parts.

In certain instances, powder metal parts will be hardened via heat treating or induction hardening prior to machining work. Parts that are induction-hardened may require a finish machining pass because the quality of the finished surface was altered. For these hardened parts, the shop is more likely to use cubic boron nitride (CBN) inserts that are typically used for most hard turning applications. Again, those CBN inserts have minimal edge prep, so that their cutting edge is as sharp as possible.

As for threading, the shop has settled on titanium nitride (TIN) coated taps, and it has machined threads without coolant for the past 20 years. It also tries to mill and turn dry, with some machines fitted with air blast units used more for chip control than to keep tool cutting edges cool. Machining without coolant means that chips remain dry and are less likely to adhere to the part after machining. But more importantly, dry machining reduces the possibility of rust forming on the parts after machining.

Rust is an inherent problem with powder metal parts because of the material's porosity and high iron content. For that reason, not only does the shop typically machine dry, but also, most parts are submerged in an oil bath very soon after machining to prevent oxidation. Mr. Gerlach notes that in some cases, rust seemed to appear almost immediately after machining. This might be an indication that rust somehow developed in the molding process and was captured inside the molded part. The shop may break a pre-machined part in half to see if rust is found in the part's core to determine if a problem may exist in the molding process.