Net-Shape Chucking Increases Hard Turning Accuracy

Heat treating a round, thin-wall part can distort the part's diameter. This lathe chuck grips an out-of-round part without forcing the workpiece perfectly round, which eliminates the spring-back effect after machining and removal from the chuck to enable a more accurate hard-turning operation.

Part distortion is a common, albeit unwelcome byproduct of the heat treating process. In the case of round, thin-wall parts such as bearing races, the distortion of the part’s ID and/or OD results in an out-of-round condition. When chucking such a hardened part for a turning operation using traditional methods, the clamping pressure can force the workpiece to become round. Once the part is removed from the chuck after machining, it will spring back to the original out-of-round condition. An additional grinding operation might then be required to achieve the desired roundness specification, increasing machining cost and waste.

The Shape-Compliant Chuck (SCC) from Hardinge (Elmira, New York) is designed to effectively hold hardened thin-wall or out-of-round parts in their post-heat-treat shape for turning, boring and grinding operations. By gripping the part without forcing its diameter to become round, the spring-back effect after machining and removal from the chuck can be eliminated. The SCC, which is only available on the company’s Quest lathes, was developed during a project sponsored by the National Institute of Standards and Technology’s (NIST) Advanced Technology Program (ATP). The ATP study focused on ways to integrate lean manufacturing principles into the manufacture of hardened steel components.

According to Dan Soroka, director of engineering for Hardinge Workholding Operations, the SCC centers the workpiece, provides adjustable gripping forces and offers multiple gripping contact points. “There are up to eight independent gripping locations at the radial position of the out-of-round part, with equal force on each leaf,” Mr. Soroka explains. “Because this chucking system maintains the part shape as close to the original state as possible, the finished un-gripped part roundness is improved an average of nine to 11 times versus the pre-machined values.” The SCC is also said to possess good damping characteristics.

The company recently performed a test on a hardened bearing race that had an initial bore roundness of 0.000913 inch. After a hard boring operation on one of its Quest 8/51SP HydroGlide turning machines with the SCC workholding device, bore roundness was found to be 0.0000835 inch.

Various SCC versions are available to support a range of part sizes for both ID and OD gripping.