Printer Friendly

Manual toggle clamp selection for fixturing.

New developments in manufacturing seldom highlight the benefits of proper workholding. However, clamping requirements must be considered carefully in order to reduce setup time and maximize productivity. Full value is realized only if all parts in a fixture are carefully evaluated for useful function. The cost of misusing a clamp often far exceeds the initial purchase price.

The choice of various manual clamp actions is extensive and includes: toggle-action and cam-action clamps, C-clamps, strap clamps, and T-slot clamps.

Manual toggle-action is much faster than tightening nuts and bolts. It is relatively inexpensive and readily available since it is manufactured in large quantities. Manual toggle-action clamps maintain a positive mechanical lock unless they are stressed beyond their ratings. Limitations of toggle clamps include the fact that they need a very consistent clamping point and exerting force is not easily determined.

Cam-action clamps depend on a low wedge angle to exert clamping force and to keep the clamp in a closed condition. Their main advantage is that they can adapt to varying work piece heights as is often found on rough castings and forgings.

C-clamps, strap clamps, and T-slot clamps have adjusting screws to apply clamping force. Advantages of these clamps are high exerting force and wide adjustabil-ity to adapt to different part thicknesses. A limitation is that they are not fast acting nor is the exerting force easily repeatable.

Determining holding capacity, number

When determining the holding capacity necessary for a clamping solution it is essential to understand the difference between holding capacity and exerting force. Exerting force is a measure of how much force (lb) the clamp applies to the work. This force varies and depends on the relative position of the clamp components, the adjustment of the spindle or the hook), part variation, and point of application on the bar. The maximum exerting force generated by a clamp, for any given set of variables, occurs just as the clamp goes over-center into mechanical lock. The magnitude of this force can still vary considerably depending on the variables noted. The exerting force should never exceed the clamp's rated holding capacity.

Holding capacity is the maximum force (lb) that the clamp will sustain when closed and locked without causing permanent yielding of any components sufficient to damage the clamp. Holding capacity applies only when the clamp is in its over-center condition. For hold-down action clamps, the holding capacity is rated at a point on the bar closest to the base. If the point moves along the bar away from the base, the holding capacity decreases. This relationship is not entirely linear so care must be exercised.

The number of clamps necessary is determined by dividing the resultant forces (lbs/ dN) applied against the clamp arm, by the holding capacity of the clamp. Manufacturers list maximum values for holding capacity. And take note that these values must be reduced as the clamping point moves away from the base.

Using this technique, if the number of clamps required results in a fraction, then round-up to the next largest whole number. This will also provide a safety factor. Remember that each clamp must be properly adjusted and positioned so that it will carry its fair share of the load.

Part tolerance, positioning, and clearance

Toggle action clamps are well-suited to applications where there are small variations in part thickness, because the over-center point of the toggle action is closely controlled. If part thickness varies on the low side, exerting force is reduced. If part thickness varies on the high side, the exerting force is increased and the clamp can become hard to operate and possibly overstressed. A Neoprene-tipped spindle will help adapt to part variation. Spring-loaded spindles can also adapt to variations, but the exerting force is limited to the spring force.

For large variations, a cam-action clamp would better suit the application. The repeatability of the overcenter stop point makes the straight line action clamp a good locator, if fitted with a pre-stop.

Recommended practice is to position parts against a fixed stop so that tool forces are resisted by the stop, not the clamp. Consider how the part will be loaded and unloaded. Make certain that the clamp bar, including the spindle, clears the area. Clamping points should be supported underneath. Otherwise, parts could distort, and the part could vibrate and move out of position. Also, consider the tool path. Many clamps have different handle styles available which provide a much lower profile. Handles are easily modified to clear machine movements.

Environment

Standard clamp materials are zinc-plated, low carbon steel for stampings and stainless steel for pivot points. Zinc plating is applied to the carbon steel. Corrosive liquids, gases, or high humidity can cause corrosion. Many standard models are available in 302/304 stainless steel for added corrosion resistance. In many cases, plating low carbon steel clamps with electroless nickel, cadmium, or chrome will enhance corrosion resistance and appearance.

Stainless steel clamps provide better service at high temperature (above 500 degrees F, 260 degrees C) than do low carbon steel clamps. The limit for plastic grips is 150 degrees E or 66 degrees C and for continuous service, neoprene on spindle assemblies is limited to 225 degrees F, 107 degrees C.

In addition to the mechanics of any application, there are human factors that must be considered. Operator safety is of paramount importance. Clamp style and positioning should be considered for possible pinch points, And it is critical to never exceed a clamp's rated holding capacity. Remember that the holding capacity of a hold-down clamp is the maximum load which the clamp bar will withstand at a point nearest the base.

The spindle should not be adjusted so tightly that the clamp becomes difficult to operate. One rule of thumb is that 60 lb (28 dN) is the maximum amount of force an operator can exert repetitively on a clamp handle. An average value during the course of a workday is probably closer to 30-40 1b (13 18 dN). If the clamp is too hard to operate, the operator's hand may slip and cause injury.

Similarly, a "'cheater" bar should not be used because it could slip. If a longer handle is necessary, it should be welded on. However, a longer handle call exert excessive force and overstress the clamp. In this condition the clamp may have a greater tendency to "pop out" of its mechanical over-center lock. This is especially true if there is vibration present and the handle is oriented so it Safety would tend to fall open. If positive locking is needed, a cross-hole can be drilled through the clamp assembly and a pin installed to prevent opening.

Also, various devices can be used to pre-load a clamp arm or plunger, if it is used as a locator. This creates an artificial load or pre-load on the arm or plunger as if it were clamping a part. Pre-loading helps keep the clamp locked.

Ergonomic considerations should also be evaluated. While operating a clamp action takes only seconds, many repetitive actions can lead to fatigue and related problems. It is best to keep the wrist straight when applying force to a clamp handle. Most clamp handles are designed for easy modification allowing the handle to be positioned in a "bent" condition, allowing the wrist to be kept straight. The optimal handle width diameter is 1 1/2" for a male and 1 11/32" for a woman. Many clamp models have contoured handle grips to better fit the user's hand.

When using squeeze action clamps, a release lever greatly eases opening the clamp. This also permits one-handed opening of the clamp. If exerting forces or cycle rates required are such that operator fatigue, and therefore safety, is involved, a better work holding choice could be a pneumatic/hydraulic cylinder-operated clamp.

To help customers increase fixture design productivity, De-Sta-Co Industries has placed its entire line of standard clamping products in a CAD database. The library is available online on the De-Sta-Co website. De-Sta-Co Industries, www.rsleads.com/312tp-202

Doug Ruffley is technical services director, and Tom Stimac is product manager industrial products at De-Sta-Co.
COPYRIGHT 2003 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2003 Gale, Cengage Learning. All rights reserved.

Article Details
Printer friendly Cite/link Email Feedback
Title Annotation:workholding
Author:Ruffley, Doug; Stimac, Tom
Publication:Tooling & Production
Geographic Code:1USA
Date:Dec 1, 2003
Words:1353
Previous Article:Creepfeed grinding: it's worth a closer look.
Next Article:Moving from process inspection to process control: CMMs take the lead.
Topics:


Related Articles
Workholding debate: roller cam vs toggle.
Getting it together with clamps: choosing and using the right clamp for your application can make all the difference in productivity and part quality.
Getting a grip on toggle clamps.
The virtuous vise and other shop 'soldiers.' (workholding)
Clamps get a grip on deep cut machining.
The toggle clamp connection.
Workholding/Toolholding Buyers Guide.
Workholding/Toolholding Buyers Guide.
Workholding.

Terms of use | Privacy policy | Copyright © 2018 Farlex, Inc. | Feedback | For webmasters