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Machining centers get the glory in most shops. They make the intricate cuts while CNC lathes often do little more than OD clean-up and facing. However, just the addition of a specialty chuck can give a wide range of new capabilities to a CNC lathe. It may even make the lathe more efficient than the machining center for many complex jobs.
Automotive supplier Hillsdale Tool (Manchester, Tennessee) illustrates one example. Its plant is now using a CNC lathe to machine a part that once required a machining center and a grinder. Overall production time savings from this switch amounts to about 30 percent. A self-balancing, offsetting chuck made this savings possible.
The part is a housing for a transaxle oil pump assembly. Its most prominent feature is a combination of non-concentric ring-shaped surfaces making up the cavity of the part. If these surfaces were concentric, they would produce a circular ring between them. However, the two surfaces instead leave a wall like a crescent to one side where they fail to intersect. In the finished assembly, inner and outer gears turn inside the two spaces to produce the difference in oil pressure.
A milling operation was used to finish these features on the casting, followed by grinding to achieve the 0.0004-inch flatness required for the part's face. Now, a Quick Turn CNC lathe from Mazak (Florence, Kentucky) accomplishes both jobs. To machine the non-concentric features, the lathe uses a custom chuck that shifts the workpiece off-center by 0.236 inch midway through the cycle. This chuck was developed by Sutton Tool (Sturgis, Michigan), a specialist in custom-engineered workholding equipment.
This workpiece shifting is no small feat. Turning with so much weight off-balance could lead to several consequences, particularly at the 1000 and higher rpm at which Hillsdale Tool runs these parts. Poor surface finish, poor accuracy, and premature spindle bearing failure are among the potential problems. And if the machine isn't well bolted down, it might bounce off the floor.
Sutton Tool's solution therefore involves a way to compensate for the imbalance. The chuck works in conjunction with a dynamic balancing system which continually measures the balance condition, and shifts counterweights on-the-fly to offset an unbalance that is too high. The system was developed by Balance Dynamics (Ann Arbor, Michigan) for high-speed milling machine spindles. This is its first turning application.