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Getting it together with clamps: choosing and using the right clamp for your application can make all the difference in productivity and part quality.

Long a workholding staple in job shops, clamps are probably also among the most often misapplied workholding devices of any kind, says Doug Ruffley, chief engineer, De-Sta-Co Industrial Products Group, Troy, MI. And, he adds, "the cost of a misapplied clamp often goes far beyond the initial purchase price."

Penalties for improper clamp use can include costly damage to the machine or the workpiece and operator fatigue or even injury. "You need to consider ergonomics. The human cost in terms of repetitive motion injuries can really mushroom" if operators are expected to apply excessive force to manual clamps, says Mr Ruffley. "And of course, if the clamp lets go, there's the possibility of serious injury."

By following some general selection guidelines and consulting with clamp suppliers when necessary, users can get the most out of their workholding system.

Pick a clamp

But not just any clamp, says Mr Ruffley. The clamp you choose will affect process reliability, productivity, and part quality. Choices include both manual and power clamps in a variety of configurations.

The manual toggle clamp is still the bread and butter of De-Sta-Co's business, says Mr Ruffley, and manual toggle clamps come in several styles designed for various applications. Advantages of toggle clamps include good ratio of holding force to application force, rapid operation, positive locking action, and ability to be used in confined areas.

The most common toggle clamp styles are held-down with either vertical or horizontal handles, push-pull, latch action, and pliers. The first choice you need to make in clamp selection is what style is most appropriate for your application.

Next, you need to determine the holding capacity you need and the direction of forces that will be acting on the part. The last step in specifying manual clamps is to determine the number of clamps you need.

Force versus capacity

It's the second step that tends to cause the most confusion, says Mr Ruffley. One of the biggest problems he sees is that users confuse holding capacity and exerting force, two terms commonly used to describe clamps. Holding capacity is the amount of force the clamp will withstand without permanent deformation of any clamp components. When the clamp is in its closed and locked position ("overcenter"), holding capacity is the maximum force the clamp will resist.

Exerting force is a function of the relative positions of clamp components, spindle or hook adjustment, workpiece dimensional variations, and the point of application on the clamp bar. "You can adjust the spindle so it just barely touches the part so you'd have very low exerting force, but when the clamp goes into its locked position, then you'd have holding capacity.

"But users don't realize that the farther out on the clamp arm they go, the lower the holding capacity. So they extend the arm or clamp out at the end and expect the clamp to hold the rated maximum."

A rule of thumb in clamping is to try to minimize exerting force, both to minimize wear and tear on the clamp and to minimize workpiece deflection, says Mr Ruffley. "The problem is, with a toggle clamp, it's very difficult to determine how much force you are exerting unless you've got a load cell between the clamp and the part you're trying to hold." De-Sta-Co is in the process of doing just that in hopes of developing some generic guidelines to give people a better idea of how much force they need to apply in a given situation.

In the meantime, the trade-off between maintaining sufficient exerting force and allowing relatively easy clamp operation is still mainly a matter of feel. Mr Ruffley says the maximum force an operator can exert repeatedly on a clamp handle is about 60 lb, although a more realistic average is probably 30 to 40 lb.

Differences in operator strength are tough to account for. "If you have someone who has big hands, they're going to be able to apply a lot more force to the clamp handle than a smaller operator," he says.

Powering up

One way to avoid variability in clamping is to use power clamps, which rely on either pneumatic or hydraulic pressure for operation. "Consistency is one of the big advantages of power workholding over manual clamps," says Dann Rypka, product manager, Hytec, Owatonna, MN. "When an operator is feeling good in the morning, he might not have any trouble with clamps. But by noon, he might be getting a little tired, and he might exert only half the force he was earlier. And sometimes that might be OK, but in other cases that might affect workpiece quality."

Mr Ruffley says speed of operation, ergonomics, and operator safety also are important considerations in weighing power versus manual clamps. "When you get into quantities of clamps on fixtures, you don't want to spend a lot of time or have more than one person operating clamps. And certainly, you don't want people manually operating awkwardly placed clamps."

Ray Okolischan, vice president, Carr-Lane Mfg Co, St Louis, MO, believes the largest barriers to application of hydraulic workholding are cost and the need for a power source. "People really need to think of the workholding system almost as part of the machine tool," he says. "The power supply is a one-time purchase, and after that it almost becomes part of the machine."

But Mr Okolischan adds that the power clamp portion of Carr Lane's business is increasing. "I think people are overcoming that stumbling block of spending a little money on the power source and clamps. Up front it looks like a lot of money, but increases in productivity can be pretty dramatic. Nobody wants to spend more money than they have to on fixturing, but to get faster machining and better accuracy you really need to do it."

Choosing a power workholding system involves some of the same selection factors as specifying manual clamps, including determining the needed holding capacity, the direction of machining forces, and the number of clamps required. To those factors, Mr Ruffley adds calculation of the total hydraulic or pneumatic volume needed, selection of a power source, and selection of valving and plumbing. Suppliers of power workholding systems can help potential users sort out what they need.

Don't expect power workholding to solve all your problems, though, says Mr Rypka. "Quite often, people will have problems with fixturing, and they decide that hydraulic power workholding can solve their problems. That's definitely not the case. If you don't have good, sound fixture design, hydraulic workholding isn't going to help you," he says.

For information from companies mentioned in this article, circle the appropriate number:
Carr Lane Mfg Co 326
De-Sta-Co 327
Hytec 328

A cornucopia of clamps

The array of clamps available can boggle the mind. De-Sta-Co, for example, sells more than 300 configurations, and the Troy, MI, company is not atypical of full-line suppliers of both manual and power clamping equipment. Here's a sampling of available workholding solutions, including manual toggle clamps, power workholding systems, and some unique clamp designs for specific applications.

Toggle-Lock Plus clamps from De-Sta-Co have a locking release lever to ensure that the clamp stays locked in the over-center position in applications where accidental clamp opening due to vibration or other factors is a concern, such as when clamped parts are transferred on pallets or tombstones. Models capable of exerting up to 2500 lb of force are available.

Also from De-Sta-Co, Hydra-Dyne positive mechanical locking clamps can speed die changeover and part loading/unloading in applications requiring up to 14,200 lb of clamping force. The clamps are designed to provide more consistent exerting force than other manual clamping methods.

Enerpac, Butler, WI, specializes in hydraulic clamps, power sources, system components such as valves and couplers, and accessories for power workholding. The company's low-height swing cylinders, for example, are available with single- or double-acting operation and can be easily modified to swing to the right, left, or straight. During operation, the unit's cylinder clamp arm rotates downward 90 degrees in the direction selected, then applies a downward clamping force. Cylinders used in straight (nonrotating) applications can use all or any part of the total stroke. The swing cylinders use Enerpac's patent-pending clamping arms, which can be positioned or tightened on the swing cylinder without removing the cylinder from the fixture.

Hytec, Power Team Div, SPX Corp, also manufactures only power workholding devices. Typical of the Owatonna, MN, company's products is a swing clamp that uses a "live" roller mechanism rather than a ball and track groove to provide clamp rotation. During retraction, the cylinder rod rotates 90 degrees to swing the clamping arm into position. Clamping takes place as the rod retracts in a straight line, pulling the arm securely against the workpiece. The single-action, conventionally plumbed clamp offers a straight pull capacity of 3044 lb at 5000 psi.

Another Hytec product is a compact edge clamp designed to allow unrestricted machining access to the top of many workpieces. Only 1" high, the clamp not only locates the workpiece but clamps in two directions: horizontally against secondary locators, and vertically against the primary locating surface.

Carr Lane Mfg Co, St Louis, MO, is a full-line supplier of toggle clamps, hydraulic clamps, power supplies, valves, fittings, accessories, and other workholding devices. Its products include Swiftsure compact single-and double-acting swing clamps that operate at hydraulic pressures to 7500 psi and exert 700 lb to 6300 lb of clamping force at 6000 psi. The clamps also feature a proprietary safety clutch that disengages the clamp action if the arm strikes an object during its swing.

Source File

For information from these other suppliers of clamps and workholding devices circle the appropriate number:
ABS Imports Inc 329
Bluco Corp 330
Dapra Corp 331
DoAll Co 332
Enerpac 333
Fairlane Products Inc 334
Flexible Fixturing Systems 335
Hydra-Lock 370
Jergens Inc 337
Kurt Mfg Co 338
MAPAL Inc 339
Microcentric 336
Mitee-Bite Products Co 340
Northfield Prec. Inst. 372
Qu-Co 341
Rohm Products 373
SMW Systems Inc 342
Stevens Engineering Inc 343
Vektek Inc 344
COPYRIGHT 1993 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1993 Gale, Cengage Learning. All rights reserved.

Article Details
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Title Annotation:includes related article
Author:Destefani, James D.
Publication:Tooling & Production
Date:Aug 1, 1993
Previous Article:Machining MMCs: use your head.
Next Article:Vision readout has the right stuff.

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