Manufacturing parts using powder metal solves many of the issues that plague more conventional machining and fabrication methods, including parts with complex shapes or features. In powder metal manufacturing, a very fine metal powder is mixed with a binder and then compacted into the desired shape and sintered, evaporating the binder and resulting in the hardened metal part.
The powder metal manufacturing process (PM) results in parts with near-net shape, using approximately 97 percent of the material that goes into the mold and thus making it very efficient. In addition, because the raw material is finely powdered metal, alloys based on desired material properties are readily customizable.
The automotive industry accounts for 70 percent of the powder metal manufacturing market, but other markets and applications are growing. The ability to produce small gears using PM molding processes has provided significant cost savings over traditional gear hobbing techniques.
While the outside of a gear is typically acceptable out of the mold, the inner diameter often requires boring to meet tight automotive tolerances. Powder metallurgy is renowned for its ability to produce extremely complex geometries, but as it is a molding process, there are design limitations. Any feature that would prevent molding, such as reverse tapers, threads, or cross holes, must be machined after the part has been molded.
Powder metal parts exhibit different machining characteristics than their traditional metal counterparts. PM parts are typically made of alloys. When determining the hardness of a PM part, the difference between apparent hardness and particle hardness must be understood. The apparent hardness is the hardness of the overall part, while the particle hardness is the discrete hardness of the individual particles within the material.
While the apparent hardness may appear modest for machining, particle hardness must also be taken into account to prevent cutting tool breakage. The abrasive nature of the PM material structure can cause micro-fatigue at the tool edge, resulting in unpredictable wear and shorter tool life.
Tool material thus plays a critical role in the ability to machine PM parts. The three most commonly used tooling materials are cermet, cubic boron nitride (CBN), and carbide. Cermet, a cemented carbide material with titanium-based hard particles, is the most affordable material option and ideal for low-volume applications that require a variety of different tools. Different grades of cermet are available, depending on whether cutting is interrupted or continuous. Coated cermets, made through physical vapor deposition (PVD), cost more but offer greater wear resistance.
Due to its cost, CBN is typically relegated to higher-volume or extremely tight-precision applications. But in those applications, its hardness and durability can make up the cost with extended tool life and optimal cutting results. Specific grades of CBN are available for cutting extremely abrasive materials such as powder metal. For interrupted cutting and threading, carbide tips coated with titanium nitride (TiN) are used due to their sharp cutting edge.
As powder metal manufacturers continue to find widespread success in a variety of applications outside of the automotive and small gear industry, demand for PM machining services is rising. Many cutting tool manufacturers have responded by creating tools designed specifically for cutting powder metals. These tools combine extreme hardness with a fine-grain structure that allows the tool to hold its cutting edge longer.
Approximately 80 percent of powder metal parts are produced using ferrous materials. To prevent rusting, the use of coolant should be avoided when possible. Since the PM manufacturing process produces a near-net shape, most machining processes will fall under finish machining and large roughing cuts will not be necessary. This means dry machining is often possible, especially when using tools with a sharp cutting edge.
If a coolant must be used, synthetic coolant is preferred. After machining, parts should be submerged in oil or thoroughly coated with a rust preventative. On occasion, rust is introduced during the powder metal molding process, so if a significant amount of rusting is observed during machining and all of the above precautions have been taken, this may be the case.
Another issue to consider when machining powder metal is the fact that PM parts are inherently porous and can cause chatter in machine tools. To offset this issue, resin impregnation can be used to fill the micro-voids that are imparted during the sintering process.
In resin impregnation, parts are lowered into a vacuum chamber, where a liquid resin is injected into the part under pressure. Impregnated parts are cured in an oven, which allows the resin to dry and harden. The result is a very consistent part with a fine surface finish that can be much more easily machined.
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Resin impregnation is especially beneficial when a part is going to be subsequently plated, as this process also prevents the blistering that results from the outgassing of PM parts. When done in batches, resin impregnation can be very cost effective. Many powder metal manufacturers offer this service, but there are also standalone facilities that specialize in vacuum impregnation.
Care should be taken to monitor offsets throughout the machining process. Acclimating to the difference in tool wear when machining PM parts versus traditional metal components takes time. Gauges, or other simple metrology tools, should be employed to frequently check measurable dimensions. Close monitoring of the machining process can prevent costly scrap and downtime. Spot checks should be performed, using precision measurement methods, to verify feature-to-feature dimensions.
Powder metal manufacturing is growing from a niche market to a viable manufacturing process for multiple part types across an array of industries. Part of this growth success is due to manufacturers’ ability to accommodate additional part features and refine tolerances via secondary machining.
While the learning curve can be steep, the growing demand for PM machining can make the time and investment required very worthwhile. Using the prescribed processes and understanding the distinct differences between the structure of powder metal and traditionally manufactured metal components, high-quality and consistent results can be achieved.
Zach Arnold is president of Arnold Machine Inc., a provider of automation systems and manufacturing services based in Tiffin, Ohio. Arnold Machine serves the automotive, appliance, and heavy-equipment industries with custom sheet metal fabrication, precision CNC machining, and engineering services. It builds automation equipment that includes spray machines, conveyor systems, and pick-and-place machinery. For more, visit www.arnoldmachine.com.