Production Dynamics has been improving manufacturing processes since1990. The original product introduced by the company was a mechanical-grip/air-released, self-contained collet chuck. The Full Bore Chuck has established itself as a tool whereby the productivity of CNC lathes can be increased in both part count and conformity with the customer's tolerances. Also, the chuck allows a lathe to run larger size bar stock or front loaded parts because a conventional draw tube and actuator are not required, which frees up the entire spindle bore.
Cycle time is critical in many production situations in today's competitive marketplace. The collet chuck typically reduces cycle time by three to five seconds by virtue of the fact that the chuck actuation requires less than one second. Numerous customers use the chuck in applications with cycle times in the range of 15 to 30 seconds. In one case, a six-second part-to-part time was achieved in a special machine application. This represents approximately the time required by a conventional chuck actuator to open and close, without any machining time.
The chuck design is intrinsically safe, because grip force is generated by die springs internal to the chuck rather than an external actuator. Chuck close confirmation is available to ensure that the chuck is gripping prior to cycle start. Because the chuck is self-contained, it does not require an external actuator. Air is used only to open the device and spring force holds the workpiece while it is being machined. Thus, the part remains clamped, even if air pressure is lost.
Higher spindle speeds are available since a collet chuck does not have heavy jaws that lose grip force as the spindle speed of the machine increases. Clamping force is adjustable by tightening or loosening the collet using the spanner holes in the front of the collet. Removing pairs of springs inside the chuck can reduce clamping force.
The gland and gland housing are used to transmit air to the chuck. When the spindle is stopped, and the chuck is opened, a valve admits air into the gland housing, pushing the gland forward against the back of the chuck. The face of the gland seals against the chuck body and allows air pressure to build inside the chuck, forcing the piston forward. Rollers drop back, allowing the pusher to move back and release the collet. When the chuck is closed, the air in the gland housing is vented, the gland retracts, and the wedging action of the piston closes the collet.
The latest successful process enhancement using the collet chuck is a sub-spindle application in which finished parts are produced in an unattended operation. A 75-year-old Florida-based manufacturer of specialty lighting products utilizes a Mazak SQT 200 MSY twin-spindle turning center in a very effective manufacturing process. Because this company's products are used in luxury residential and commercial applications, it requires close tolerance, corrosion resistant parts with a superior decorative finish. An automatic bar feeder loads brass bar stock into the turning center.
In order to achieve the goal of producing complete finished parts in a "lights out" operating mode, a turning center was equipped with a collet chuck for the second spindle. This provided a particularly clean installation with good tool clearance in addition to the required secondary operation accuracy and fully adjustable grip force.
The chuck also contributed to the goal of producing finished parts without secondary operations by providing ID as well as OD gripping capacity plus the ability to "swallow" long parts for back end machining. For ID work, gripping the OD collet is replaced by a special collet, holding a Bristol mandrel and expanding sleeve. The system has been operational for three months with outstanding success. The manufacturer reports that the turning and milling capability of the Mazak lathe, coupled with the bar feeder and the sub-spindle workholding system, regularly allows six to eight hours of unattended finished part production.