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Answers to Tough Workholding Challenges

 

Trouble hanging onto your parts? Here are some


By Jim Destefani
Senior Editor

 

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 high-speed 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 µm) TIR and repeatability to 0.00005" (1.3 µm) 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 through-machining 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.

Developed in Israel, manufactured in Switzerland, and available in the US from SIT LC Inc. (W. Bloomfield, MI), the patented device is interchangeable with F-type OD collet adapters that come standard with the machines. Using it requires no changes to the subspindle and adds no toolchange time, according to the company. Part transfer from the main to a subspindle is identical to transferring a part with an OD collet. The system is cooled by the tailstock, preventing chips from entering collet slots and also helping to eject finished parts.

For transfer lines and other applications that require movement of clamped parts or are subject to vibration, De-Sta-Co Industries (Madison Heights, MI) offers the Toggle Lock Plus manual clamp series. The patented clamps are said to help maintain over-center toggle position in applications where there is apt to be movement or vibration such as transfer lines. Features include bushings at major pivot points; stop point locations at the base; several ergonomic and safety improvements; and a long clamping arm for application flexibility. The clamp spindle contacts the workpiece in a vertical position to minimize marring or movement, and the clamping arm completely clears the workpiece for easy loading/unloading.

Workholding for inspection applications is not required to withstand machining forces, but it has its own challenges in terms of accuracy and other characteristics. An example is the series 37 Micro Vision system for holding small parts on video inspection systems.

From Omni Structures Int'l. Inc. (Grand Rapids, MI), the system is built around a transparent platform that allows rear illumination from a video inspection device to pass through unrestricted. Simple adaptable clamps secure the platform to the video machine, and multiple platforms can be connected into larger surfaces if needed. The company also makes similar systems with aluminum platforms for assembly, laser cutting, EDM, and surface plate inspection applications.

Odd workpiece configurations or dimensional variation of workpieces can create problems for workholding systems, and one way to resolve those difficulties is by using a chuck with long jaw stroke.

Case in point: ML series power chucks from Kitagawa/Northtech Workholding (Schaumburg, IL), which are available in single, two, and three-jaw configurations.

With up to 1" (25 mm) of stroke per jaw, the three-jaw configuration is said to be well suited to gripping of workpieces with flanges or other part configurations that require the jaws to open enough to clear a larger-diameter flange and then grip on smaller datums. The MLT two-jaw long stroke chuck can handle square, rectangular or unusually shaped workpieces, while the single-jaw MLV incorporates a custom fixed locator or jig for locating workpieces. All three configurations are available in 6 or 8" (152 or 203-mm) sizes.

For small prismatic parts, workholding density--getting a lot of parts into a single setup--and fast setups are often the key to productivity. SeraLock Towers and SeraLock Pallets from Kurt Mfg. (Minneapolis) can be configured into custom fixtures using modular components. Setups can be quickly changed to accommodate all sizes of parts within a family, and the systems' serrated mounting surfaces allow quick, repeatable accuracy when repositioning modular clamping elements. The spacing, or pitch, of the serrations allows the use of a variety of short-stroke clamping elements. The movable and stationary components of the clamp can be located relative to one another on the subplate or tower so that the needed stroke of the movable component need be little more than the pitch serration.

Moving pallets full of small parts in and out of the machine quickly and efficiently requires a system for accurately locating the work. Pallet clamping technology from Kosmek (Bridgeview, IL) locates and clamps pallets to machine tables quickly and with accuracy within a few microns. The company says its VS pallet clamp system is relatively inexpensive and reduces pallet or subpallet setup time to a few seconds, improving production flexibility and allowing economical lot size reduction.

The system's clamp is mounted onto the machine base or table. Mating collars are mounted to the underside of the pallet. Operators load a pallet or fixture by lowering it into an approximate position using rough guides. The pallet clamps are then activated, with both clamping force and dual-plane locating provided by an internal spring. An optional air sensor can confirm clamping. Clamps are released by applying hydraulic pressure.

Kosmek says the system is being used in flexible manufacturing systems in the automotive industry, and being retrofitted onto existing machines.

A similar setup from Jergens Inc. (Cleveland, OH) locates and locks fixture plates to machine subplates in seconds using a system of locating shanks, receiver bushing, and liner bushings. In the Ball Lock mounting system, receiver bushings are installed in the machine table or subplate. Liner bushings are mounted in the fixture plate. Locating shanks inserted through the liners and into the receivers provide accurate location within ±0.0005"(0.013 mm).

Each locating shank has three locking balls that are actuated by turning a setscrew. Once in place, the shanks are said to provide positive holding force of up to 20,000 lb (89 kN) per shank. Standard subplates and fixture plates as well as kits designed for specific machines are available.

Some small parts are just plain difficult to hold. Enter the Ice Vice Light from Horst Witte Gerätebau (Bleckede, Germany). The device enables secure and plane-parallel freezing of flat workpieces on its 40 X 40 mm clamping surface using only a little water and in just a few seconds. Holding force is even and effective over the entire contact surface, and the approach is said to be especially suitable for prototype and low-volume machining applications.

After the chuck is cooled to near freezing, water is sprayed onto the clamping surface and the part is positioned. Freezing takes the temperature down to -10C. Time needed for the operation depends on working environment, thermal conductivity, and workpiece size. The entire system is only 223 X 67 X 84 mm in size.

 

This article was first published in the February 2005 edition of Manufacturing Engineering magazine. 


Published Date : 2/1/2005

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