THROUGH THICK AND THIN

There once was a time when clamping a thin-walled, easily deformed part in a chuck was often difficult. When the jaws tightened, excessive grip force could cause distortion that altered the shape and accuracy of the workpiece. A simple example of this occurs when the concentricity of a round hole in a part is compromised by the clamping force of the chuck, becoming oval when the piece is removed.

That was then. Now a new generation of chucks prevents distortion in thin-walled parts and helps reduce setup time even further. For example, the ROTA NCR Precision 6-Jaw Compensation chuck from Schunk (Morrisville, NC) can be used on all lathes. Sizes are standardized, with diameters up to 1200 mm for clamping with large lathe chucks.

When clamping thin-walled parts, this system uses a central chuck piston with three inner pendulums positioned at 120 deg, each connected to two base jaws, to center the workpiece between six contact points that adjust in pairs. Since the clamping forces align towards the chuck center, deformation of the workpiece is virtually nonexistent because the master jaws oscillate in pairs for concentric clamping, with a pendulum connected to each set of two base jaws.

This is ideal for applications where clamping the part from the outside is easier, but must be restrained due to excessive pressure. The ROTA NCR can help resolve this problem, which is actually not the pressure itself, but the direction of the clamping force.

Pendulum jaws are effective when clamping thin-walled parts because workpiece deformation reduces through the uniform arrangement of the clamping points on the perimeter. For example, a 20 mm long section of steel pipe measures external Ø 150 mm and internal Ø 120 mm, with a clamping force of 6000 daN. The mathematically determined value of deformation is 0.008 mm when using a 6-point pendulum, versus 0.14 mm with 3-point clamping. 6-point pendulums are common, but for extremely thin pieces, pendulum jaws with 12 clamping points can be used.

Another solution to deformation is the Magnos magnetic workholding system from Schunk, which uses electro-permanent magnets to clamp ferromagnetic parts. This system arranges square magnetic poles as north and south positions in a chessboard formation. They are magnetically independent and consist of steel cores surrounded by nonreversible, permanent, neodymium magnets with reversible AlNiCo magnets located below them. This arrangement creates a uniform magnetic force that gently clamps thin-walled or sensitive parts and reduces warping, twisting, and crushing of the workpiece.

To solve excessive grip force, Hardinge (Elmira, NY) offers force-limiting step chucks that prevent deformation of thin-walled parts by automatically determining and using the necessary grip force. The chuck assembly adjusts grip force while eliminating the need to manually adjust drawbar force. Because actuation is independent from the machine drawbar, users can operate at normal pressure, eliminating any loss of closing cycle time associated with a low pressure setting.

Used in Hardinge CNC lathes with a drawbar stroke of ½ in or less, these chucks are available in 16C and 20C sizes for gripping thin-wall tubing, pipe, castings, housings, and soft or delicate materials that cannot withstand standard chucking methods.

Another common problem can occur when one takes on the tasks involved with limited space applications. For a solution to these types of issues, many turn to the Spieth Type MSF Locknut, offered by Advanced Machine & Engineering of Rockford, IL.

The MSF series features a reduced width made possible by new design geometry. The geometry for the mechanism of the locknut is based on Spieth's diaphragm principle, in which the load and locking parts are constructed from one piece. The ?vise-like? locking feature also allows the locknut to remain locked without additional components such as pins and keys to hold it in place. During the locking procedure, the thread flanks are loaded in the direction of the load, providing a connection with high axial rigidity.

The new Quick Change Clamping Chuck from Unilock, offered by BIG Kaiser (Elk Grove Village, IL), addresses low profile applications in water. This stainless steel 150 mm x 100 mm x 40 mm tall zero point clamping chuck is ideal for environments where limited travel range creates problems with overall stack height.

By using the same clamping knob and ram system as larger Unilock chucks, this system is compatible and rigid. Its standard 40 mm clamping knob attaches directly to workpieces or fixtures with up to a 12 mm thread. A large retention thread relative to the chuck's small size is great for high-torque applications. The chuck uses an integrated mounting pattern to support many existing wire EDM tables. Waterjet users will find it superb as a zero point clamping system.

This chuck accepts Unilock clamping knobs from the top or bottom side, allowing parts to be flipped upside down. Accurate positioning of better than .0002 in repeatable TIR is backed by highly rigid spring-based mechanical clamping. Adding an air pressure boost increases spring clamping force by more than 30 percent.

The Unilock Workholding System addresses repeatability of location from one workpiece blank to another, and the transfer of work from one machine tool to another by eliminating inconsistent workpiece loading and unloading. A workpiece reference location is only established once. Chuck alignments are established from its center line. That data is captured electronically and kept in the machine's CNC program. Work can then move through several stages of processing without having to relocate the reference point.

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Kyndall Brown is the assistant editor of Fabricating & Metalworking Magazine.