Distorting – The Facts

August 1, 2009

The fact that workholding distorts workpieces is a common challenge facing manufacturers. The forces applied while holding the part invariably alter its shape, and when it’s released from the holding device, it typically springs back to its original condition.

This becomes an issue when you are trying to produce precision parts and is prevalent when processing castings, forgings, powder metal, and thin-walled workpieces.

As an example, if you are holding the OD of a thin-walled part on a traditional three-jaw lathe chuck, the part will be squeezed into three lobes. The extent of the distortion depends on the material, the wall thickness, the shape of the preclamped workpiece, and the forces applied by the chuck.

Commonly, the first solution attempted is to utilize full grip, pie jaws to limit the distortion, but this method is limited because the clamp diameter held has to coincide very closely with the corresponding diameter of the chuck jaws. If there is any variance in the diameter of the workpiece, there will also be an increase in its distortion as the part is forced into the contour of the jaws.

Furthermore, the workpiece clamp diameter is not perfectly round, and so it will flex into the shape of the jaws and then spring back when the part is released. Often the part measures in specification while in the chuck, but as soon as it is released from the chuck, it is no longer acceptable.

The solution is to allow the jaws to contour to the part instead of forcing the part to contour to the jaws.

New Technology

SCHUNK recently introduced a lathe chuck that can help address this issue.

To minimize the distortion, the forces applied to the part have to be exerted over as much of the part perimeter as possible, be equal from all directions, and be directed toward the center of part rotation.

The ROTA NCR has a design that allows adjacent jaws to pendulum together to allow for nonround parts to be held with minimal distortion.

This compensation clamping method can reduce the distortion of the workpiece during clamping. The chuck’s pendulum compensation effect also can be changed to centric clamping, when OD-to-ID concentricity is the goal instead of roundness.

This 6-jaw chuck uses standard jaws that can be used throughout the full range of clamping diameters and is available in sizes from 165 to 1,200 mm.

For higher-RPM applications, the chuck can be provided with centrifugal compensation to better maintain constant clamping forces at higher speeds.

6-Jaw Versus 3-Jaw

A test was conducted comparing the holding of a part with a 6-jaw chuck to a traditional 3-jaw chuck. (See Chart) The initial parts were steel rings ground with an OD and ID to a tolerance of less than 0.005 mm. Parts with 6-mm and 8-mm wall thickness were tested.

The results show the roundness improvements that can be gained by utilizing this technology. These results are seen even in extremely thin-walled parts in which the deformation caused by a traditional 3-jaw chuck exceeds the workpiece’s elastic yield strength and causes permanent deformation.

This chuck uses standard off the shelf soft jaws. To take this roundness challenge one step further, the chuck can be coupled with pendulum jaws.

This combined system provides 12 points of contact and two compensation-pendulum joints per contact point. These jaws also can be provided with inserts that can be machined to ensure that all 12 pads are in contact with the workpiece.

For more information, visit www.ca.schunk.com.