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 mainbearing 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, lessemissions,” 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.”
About the author: Robert Schoenberger is the editor of Today's Motor Vehicles and a contributor to Today's Medical Developments and Aerospace Manufacturing and Design. He has written about the automotive industry for more than 17 years at The Plain Dealer in Cleveland, Ohio; The Courier-Journal in Louisville, Kentucky; and The Clarion-Ledger in Jackson, Mississippi. firstname.lastname@example.org
Automotive original equipment manufacturers (OEMs) often create engine variants for special models or adjust engine sizes without going through complete system redesigns. Such changes occasionally lead to new problems such as engine noise or performance losses. Impco Microfinishing Global Sales Director Mark Hendel says the company works with OEMs to support the design changes that occur during a product’s life cycle, for example the decision to increase a 2.7L engine to a 3L displacement by increasing the piston stroke.
“That put the horsepower up, and the load went up on the bearings. So, the process that we had for the main and pins broke down,” Hendel says. The increased power was putting too much pressure on the edges of the bearings, creating high edge loading.
After studying several failed parts offline, Impco engineers altered the microfinishing process to slightly alter the bearing profile, creating a shaft that performed with the higher power levels.
“That’s how things typically work around here. Customers make changes, and they come back to us with problems,” Hendel says. “We work out solutions.”
The downsize-and-boost strategy for modern engines has forced several automakers to abandon cast-iron crankshafts for tool steel models. The torque and horsepower increases generated by turbocharging put too much load on the crank for many cast-iron models to handle.
This change has had a ripple effect in powertrain machining with several machining center producers offering new models to rapidly produce shafts. Harder materials require more powerful spindles, different cutting tools, and some changes to workholding.
Impco Microfinishing Global Sales Director Mark Hendel says that shift has made it easier, to some degree, to offer custom surface finishes via microfinishing.
“We get a cleaner cut from a harder material. Softer material smudges a bit more and binds up in the abrasive film,” Hendel says. “The harder the material, the more consistent the results.”
One reason for the increased popularity of the process is the consistency of the results – shafts processed on the machines tend to have identical bearing profiles, regardless of when and on what grinding machine they were produced. And while Impco engineers would like to take credit, much of that repeatability has come from improvements in grinding.
Variability in grinding can lead to variability in end results – garbage in, garbage out. No amount of microfinishing will fix a part that arrives out-of-spec from grinding. Impco Microfinishing Global Sales Director Mark Hendel says such incidents are rare, thanks to improved grinding abrasives that generate significantly more consistent pre-finished parts.
“With the movement from aluminum oxide to cubic boron nitride (CBN) grinding wheels, grinding processes and surface finishing values are getting much more repeatable,” Hendel says. “They’re helping us hold a tighter tolerance.”