With automakers setting tighter tolerances for powertrain components, microfinishing use is growing, leading to more automation and new equipment from machine manufacturer Impco Microfinishing.
Large engine blocks can soak up a lot of vibration. A cast-iron crankshaft rotating at 5,000rpm in a 5L V-8 can sound pretty sweet. Up that speed, drop two-thirds of the engine mass, and swap that cast-iron crank for tool steel, and every microscopic flaw is going to be noticeable.
Engine downsizing and turbocharging strategies have allowed automotive original equipment manufacturers (OEMs) to replace V-8s with V-6s and even 4-cylinder engines. That shift has improved fuel economy and simplified vehicle assembly by eliminating the need to support dramatically different engine sizes in each vehicle. On the other hand, keeping those small, boosted engines civilized is a major engineering challenge.
OEM specifications for roundness, smoothness, and dimensional accuracy have gotten more stringent in the past five years, causing headaches for suppliers who now find themselves struggling to meet requirements measured in microns.
Pat Cebelak, on the other hand, doesn’t mind. The president of Lansing, Michigan-based Impco Microfinishing makes machines that remove the last micron or so of material on ground parts such as crankshafts and camshafts. As tolerances get tighter, Impco’s processes become more vital to production.
“The tighter the tolerances, the better things are for us,” Cebelak says.
Crankshafts, camshafts, and similar rotating parts tend to be made from cast iron or tool steel, and manufacturers typically grind them to shape. Impco Global Sales Director Mark Hendel says that the grinding process has become significantly more precise in producing controlled diameters in recent years, allowing manufacturers to produce very good parts without further processing. Unfortunately, nearly perfect isn’t good enough in the smaller, boosted engines in favor today. Avoiding noise and reducing friction require removing microscopic features created during earlier processes – typically by high and low frequency chatter during milling or grinding. That process optimizes the bearing form, creating a functional surface for the rotating part.
“Certain OEMs in Europe are very focused on bringing powertrain surface finishes down to some very fine surface finish values. And they’re not just talking about it. They’re ordering equipment, and they’re doing it,” Hendel says.
Impco’s machines use abrasive films to smooth and shape features on rotating shafts. Arms with tooling shaped to fit over cylindrical bearing journals close around the journals, holding the abrasive tape in contact with the surface for a specified time. Tape abrasive levels vary in grit type, grade, and manufacturer, depending on the bearing form and surface finish requirements, with increasingly fine abrasives to achieve the final finish, he adds.
Creating a functional surface, typically a very smooth one, is the primary goal of the process. Some companies use microfinishing to generate unique geometric features to reduce edge loading, maintain lubricant coverage on rotating parts, to seal a surface, or to guide fluid flow. Impco’s engineering staff works with customers to develop custom processes to generate those features.
“The functional surface that we create is customized to the function of that feature,” Hendel says. “A main bearing diameter, an oil seal, or a thrust face – those are three different component features that need three different types of surfaces.”
Updating equipment, processes
With the need for high-volume microfinishing rising, Cebelak and Hendel say customers have been asking for ways to increase productivity and capacity with the goal of 100% repeatability for every surface on every shaft. With many vehicles sharing engines or variations of engines, quantities for many components are rising, even as the industry diversifies vehicle offerings.
Impco recently shipped its first Worldstar 1680 microfinishing system to Brazil where a supplier plans to make crankshafts for a global OEM. The CNC system automatically polishes critical cylindrical bearing journals for different sized crankshafts, without manual tool changes and repositioning for component pitch. Automating that function reduces changeovers from hours to minutes, processing a 4-cylinder crankshaft through three levels of microfinishing plus thrust-face finishing.
It can automatically position each of 11 pairs of arms in any position to accommodate various crankshaft sizes, allowing one piece of equipment to serve a large engine family. Each microfinishing station in the 4-machine system is loaded and unloaded automatically with an overhead gantry.
“We make surfaces perfectly round and perfectly smooth. That means less friction, more horsepower, less emissions,” Cebelak says. “With the downsizing of engines, the crank has to spin faster, not overheat, and have the right oil layer over all of the spinning parts. Our process allows all of that to occur without breakdowns.”