When considering the purchase of a CNC lathe or turning center, it is important to ensure that the workholding system is matched to both the machine’s capabilities and the type of work that it will be doing. For example, the workholding system for a large-bore oil-patch lathe that will be machining pipe threads is probably not going to be the right solution for a high-speed multitasking machine making hip joints. Many CNC lathes and turning centers come equipped from the factory with a standard workholding system consisting of a three-jaw power chuck, a hydraulic actuating cylinder and a drawtube. The hydraulic cylinder is fixed to the rear end of the machine’s spindle, while the chuck is mounted to the front side, or working side, of the spindle. These two components are typically connected together via the hollow steel drawtube that passes through the spindle bore.
The opening and closing of the chuck jaws is accomplished through the axial motion of the hydraulic actuator’s piston. As the piston slides back and forth, the drawtube moves in unison, driving a wedge-shaped plunger within the chuck body. This plunger converts axial motion to radial motion, causing the jaws to open and close. The amount of gripping force that the jaws apply to a workpiece is directly related to the actuator output force and is easily regulated with a pressure valve.
As a universal workholding device, a three-jaw chuck functions well for many common turning applications. It can hold a range of part sizes, is capable of operating at reasonable rotational speeds and achieves good accuracies—particularly when the jaws are bored in place.
However, there are many applications where a jaw chuck is not the best workholding option, which has led to the development of a range of alternative solutions. The most common among these is the CNC collet chuck.
A CNC collet chuck mounts to the working side of a CNC lathe spindle in the same manner as a three-jaw chuck and uses the machine’s existing hydraulic cylinder and drawtube for actuation. The installation of a collet chuck usually requires the fitment of an intermediate adapter between the chuck and drawtube to compensate for thread and position disparities. This adapter is custom machined for the specific lathe/chuck combination and is normally included in the price of the collet chuck.
Choosing the Right Collet Chuck
Choosing the right collet chuck begins by identifying the spindle nose of the machine on which the chuck will be mounted. All CNC lathe spindle noses follow international standards that clearly define the mounting interface dimensions. Common spindle nose configurations for the North American market include A2-6, A2-8, and 140 mm.
The next step is to determine the chuck’s required capacity, based upon the size of the parts that need to be held. For bar feeding applications, it is customary to match, or slightly exceed, the machine’s drawtube bore size.
Once the spindle mount and required capacity have been identified, the selection can be further narrowed based on specific features and benefits of the various models, including pullback versus dead-length operation; external clearance dimensions; RPM capability; and setup time requirements.
When properly matched to a machine, bar feeder and application, CNC collet chucks provide an economical and effective way of holding material less than 4 inches in diameter. Because collet chucks are generally matched with a bar feeder, some of which have magazine storage and are designed to maintain consistent gripping force at high speeds, they are the workholding choice for lights-out operations.
Collet chucks’ compact size improves tool clearance, maximizes Z-axis capacity and aids in rigidity. Readily available round, square, hex, internal expanding and emergency collets offer broad flexibility.
Pullback Versus Dead-Length Operation
As its name implies, a pullback collet chuck operates by drawing a collet back into the chuck body. As the external taper on the collet makes contact with the internal taper of the chuck, the collet compresses to grip the workpiece. A downside of this simple design is that as the collet draws back, it often starts pulling the barstock or workpiece with it, resulting in variable Z-axis positioning. On first operation jobs, this part movement can usually be remedied with a simple facing cut, but may present more of a problem with secondary operation work or subspindle backworking operations where length features must be accurately controlled.
Dead-length collet chucks (also known as as Accu-Length) have been designed to overcome the issue of part movement during the closing operation. With a dead-length chuck, the collet is fixed to the chuck body, and a tapered sleeve pushes forward over the collet to compress it. Because there is no axial movement of the collet with this design, workpiece positioning remains constant. As machine tools become more sophisticated with features such as live tooling, multiple spindles, multiple turrets and in-process gaging, there has been a shift in the market away from pullback-style chucks in favor of the more advanced dead-length model of collet chuck for both main and subspindle operations.