Answers to Tough Workholding Challenges

Manufacturing Engineering, Feb 2005 by Destefani, Jim

Trouble hanging onto your parts? Here are some

To machine parts accurately and productively, you have to be able to hold them securely and without distortion. Still, workholding is often almost an afterthought in many shops. Even manufacturers who have purchased new capital equipment sometimes fall into the trap of using the same old vises, chucks, or other workholding systems, when a fresh approach might allow them to take better advantage of new machine technology.

Then there are those parts and applications that drive engineers and shop-floor personnel crazy. Some workpieces may be fragile, or may vary in size, or have geometry or other characteristics that make them particularly difficult to hold accurately.

Applications that can pose special workholding challenges include dimensional measurement and other operations that require extreme accuracy, transfer line machining, and Swiss machining.

Here, then, is a look at a variety of workholding technologies designed to help you tackle these tough parts and applications.

Holding of thin-wall components for turning or grinding presents multiple challenges. Excessive grip force can distort or crush a fragile workpiece, playing havoc with concentricity after the part's released. Traditional ways to deal with delicate workpieces have included mandrels to grip the part ID as well as diaphragm chucks, which open and close based on the deformation by hydraulic fluid or air actuation of a thin-walled steel membrane.

The force-limiting step chuck is a new workholding device for thin-wall workpieces from Hardinge Workholding (Elmira, NY). 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. The chucks are said to prevent crushing or distortion of thin-wall parts by automatically determining and applying the needed grip force.

Currently for use in Hardinge CNC lathes with a drawbar stroke of 1/2'' (12.7 mm) or less, chucks are available in 16C and 20C sizes. Applications include gripping thin-wall tubing, pipe, castings, housings, and soft or delicate materials that cannot withstand standard chucking methods.

Diaphragm chucks remain a very effective way to deal with fragile workpieces and other workholding challenges. Northfield Precision Instrument Corp. (Island Park, NY) manufactures standard and special diaphragm chucks and air chucks for a variety of applications.

An example is a 7'' (178-mm) diam, 8000-rpm double diaphragm chuck used for highspeed balancing of driveshaft slip yokes at a Dana Corp. facility. Dana engineers wanted to balance driveshafts at road conditions, and the system, consisting of two separate diaphragm chucks built into one with two separate sets of jaws mounted one in front of the other, enabled them to do so.

Northfield says the long, thin workpieces required gripping at both ends to allow accuracy of 0.0001'' (2.5 pm) TIR and repeatability to 0.00005'' (1.3 pm) with extreme rigidity. Chucks were both air-opened and spring-clamped, so they would hold the parts even if air pressure was lost during the balancing operation.

If tool clearance is an issue in your operations, here are a couple of potential solutions. For OD turning operations, you may want to consider face drivers, which locate a shaft-type part between centers and hold the part in place using multiple knife-edge drive pins.

Madison Face Driver (Elkhart, IN) says its tools provide tool clearance that allows machining of the entire part OD in one setup, providing good concentricity and reducing setup and run time. The company manufactures standard elastomer and hydraulic face drivers to handle parts with ODs from 0.270 to 13.57'' (6.8-345 mm), as well as specials to handle other applications. Features include carbide drive pins for hard turning applications and quick-change spindle adapters that let users run several different drivers on the same machine with quick and accurate changeover.

For prismatic parts, magnetic chucks can allow five-side workpiece access to facilitate "done-in-one" machining. TurboMill MMC magnetic chucks from Walker Magnetics (Worcester, MA), for example, eliminate traditional clamping for surface grinding, profiling, milling, and drilling applications.

The chucks feature electronically activated permanent holding that exerts uniform force over the workpiece surface to minimize distortion and dampen vibration. They maintain holding power even if disconnected from the power supply, making them suitable for use with multipallet systems. Standard chucks are available in a range of sizes.

Tapped holes in the magnetic face facilitate positioning of riser blocks, which are used to locate the workpiece above the chuck and allow throughmachining operations without damaging the chuck.

Parts machined on Swiss-type turning machines are usually small and complex, and require machining on multiple surfaces. Designed to fit into the subspindles of CNC screw machines from Tornos, Citizen, Star, and other manufacturers, the Interchuck enables secure ID clamping of parts to allow machining of their OD.