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LOCATING AND CLAMPING METHODS

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Rolan Galenzoga

on 23 January 2014

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Transcript of LOCATING AND CLAMPING METHODS

Locating and clamping are the critical functions of any workholder. As such, the fundamental principles of locating and clamping, as well as the numerous standard components available for these operations, must be thoroughly understood.
The Mechanics of Locating
Positioning the Clamps
CLAMPING GUIDELINES
Factors in Selecting Clamps
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LOCATING AND CLAMPING METHODS
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BASIC PRINCIPLES OF LOCATING

To perform properly, workholders must accurately and consistently position the workpiece relative to the cutting tool, part after part. To accomplish this, the locators must ensure that the workpiece is properly referenced and the process is repeatable.
Referencing and Repeatability

"Referencing" is a dual process of positioning the workpiece relative to the workholder, and the workholder relative to the cutting tool. Referencing the workholder to the cutting tool is performed by the guiding or setting devices. With drill jigs, referencing is accomplished using drill bushings. With fixtures, referencing is accomplished using fixture keys, feeler gages, and/or probes. Referencing the workpiece to the workholder, on the other hand, is done with locators.

If a part is incorrectly placed in a workholder, proper location of the workpiece is not achieved and the part will be machined incorrectly. Likewise, if a cutter is improperly positioned relative to the fixture, the machined detail is also improperly located. So, in the design of a workholder, referencing of both the workpiece and the cutter must be considered and simultaneously maintained.
"Repeatability" is the ability of the workholder to consistently produce parts within tolerance limits, and is directly related to the referencing capability of the tool. The location of the workpiece relative to the tool and of the tool to the cutter must be consistent. If the jig or fixture is to maintain desired repeatability, the workholder must be designed to accommodate the workpiece's locating surfaces.

The ideal locating point on a workpiece is a machined surface. Machined surfaces permit location from a consistent reference point. Cast, forged, sheared, or sawed surfaces can vary greatly from part to part, and will affect the accuracy of the location.
A workpiece free in space can move in an infinite number of directions. For analysis, this motion can be broken down into twelve directional movements, or "degrees of freedom." All twelve degrees of freedom must be restricted to ensure proper referencing of a workpiece.

As shown in Figure 3-1, the twelve degrees of freedom all relate to the central axes of the workpiece. Notice the six axial degrees of freedom and six radial degrees of freedom. The axial degrees of freedom permit straight-line movement in both directions along the three principal axes, shown as x, y, and z. The radial degrees of freedom permit rotational movement, in both clockwise and counterclockwise radial directions, around the same three axes.

Figure 3-1. The twelve degrees of freedom.
The devices that restrict a workpiece's movement are the locators. The locators, therefore, must be strong enough to maintain the position of the workpiece and to resist the cutting forces. This fact also points out a crucial element in workholder design: locators, not clamps, must hold the workpiece against the cutting forces.

Locators provide a positive stop for the workpiece. Placed against the stop, the workpiece cannot move. Clamps, on the other hand, rely only upon friction between the clamp and the clamped surface to hold the workpiece. Sufficient force could move the workpiece. Clamps are only intended to hold the workpiece against the locators.
Forms of Location
There are three general forms of location: plane, concentric, and radial. Plane locators locate a workpiece from any surface. The surface may be flat, curved, or have an irregular contour. In most applications, plane-locating devices locate a part by its external surfaces, Figure 3-2a. Concentric locators, for the most part, locate a workpiece from a central axis. This axis may or may not be in the center of the workpiece. The most-common type of concentric location is a locating pin placed in a hole. Some workpieces, however, might have a cylindrical projection that requires a locating hole in the fixture, as shown in Figure 3-2b. The third type of location is radial. Radial locators restrict the movement of a workpiece around a concentric locator, Figure 3-2c. In many cases, locating is performed by a combination of the three locational methods.
Figure 3-2. The three forms of location: plane, concentric, and radial.
LOCATING GUIDELINES

No single form of location or type of locator will work for every workholder. To properly perform the necessary location, each locator must be carefully planned into the design. The following are a few guidelines to observe in choosing and applying locators.
Positioning Locators

The primary function of any locator is to reference the workpiece and to ensure repeatability. Unless the locators are properly positioned, however, these functions cannot be accomplished. When positioning locators, both relative to the workholder and to the workpiece, there are a few basic points to keep in mind.

Whenever practical, position the locators so they contact the workpiece on a machined surface. The machined surface not only provides repeatability but usually offers a more-stable form of location. The workpiece itself determines the areas of the machined surface used for location. In some instances, the entire surface may be machined. In others, especially with castings, only selected areas are machined.
The best machined surfaces to use for location, when available, are machined holes. As previously noted, machined holes offer the most-complete location with a minimal number of locators. The next configuration that affords adequate repeatability is two machined surfaces forming a right angle. These characteristics are well suited for the six-point locational method. Regardless of the type or condition of the surfaces used for location, however, the primary requirement in the selection of a locating surface is repeatability.
To ensure repeatability, the next consideration in the positioning of locators is the spacing of the locators themselves. As a rule, space locators as far apart as practical. This is illustrated in Figure 3-12. Both workpieces shown here are located with the six-point locating method. The only difference lies in the spacing of the locators. In the part shown at (b), both locators on the back side are positioned close to each other. In the part at (a), these same locators are spaced further apart. The part at (a) is properly located; the part at (b) is not. Spacing the locators as far apart as practical compensates for irregularities in either the locators or the workpiece. Its also affords maximum stability.
Figure 3-12. Locators should be spaced as far apart as practical to compensate for slight irregularities and for maximum stability.
Locating the workpiece is the first basic function of a jig or fixture. Once located, the workpiece must also be held to prevent movement during the operational cycle. The process of holding the position of the workpiece in the jig or fixture is called clamping. The primary devices used for holding a workpiece are clamps. To perform properly, both the clamping devices and their location on the workholder must be carefully selected.
Clamps serve two primary functions. First, they must hold the workpiece against its locators. Second, the clamps must prevent movement of the workpiece. The locators, not the clamps, should resist the primary cutting forces generated by the operation.

Holding the Workpiece Against Locators. Clamps are not intended to resist the primary cutting forces. The only purpose of clamps is to maintain the position of the workpiece against the locators and resist the secondary cutting forces. The secondary cutting forces are those generated as the cutter leaves the workpiece. In drilling, for example, the primary cutting forces are usually directed down and radially about the axis of the drill. The secondary forces are the forces that tend to lift the part as the drill breaks through the opposite side of the part. So, the clamps selected for an application need only be strong enough to hold the workpiece against the locators and resist the secondary cutting forces.
The relationship between the locators and clamps can be illustrated with a milling-machine vise. In Figure 3-22, the vise contains both locating and clamping elements. The solid jaw and vise body are the locators. The movable jaw is the clamp. The vise is normally positioned so that the locators resist the cutting forces. Directing the cutting forces into the solid jaw and vise body ensures the accuracy of the machining operation and prevents workpiece movement. In all workholders, it is important to direct the cutting forces into the locators. The movable vise jaw, like other clamps, simply holds the position of the workpiece against the locators.
Figure 3-22. A vise contains both locating and clamping elements.

Holding Securely Under Vibration,
Loading, and Stress. The next factors in selecting a clamp are the vibration and stress expected in the operation. Cam clamps, for example, although good for some operations, are not the best choice when excessive vibration can loosen them. It is also a good idea to add a safety margin to the estimated forces acting on a clamp.
Preventing Damage to the Workpiece.
The clamp chosen must also be one that does not damage the workpiece. Damage occurs in many ways. The main concerns are part distortion and marring. Too much clamping force can warp or bend the workpiece. Surface damage is often caused by clamps with hardened or non-rotating contact surfaces. Use clamps with rotating contact pads or with softer contact material to reduce this problem. The best clamp for an application is one that can adequately hold the workpiece without surface damage.
Improving Load/Unload Speed
. The speed of the clamps is also important to the workholder's efficiency. A clamp with a slow clamping action, such as a screw clamp, sometimes eliminates any profit potential of the workholder. The speed of clamping and unclamping is usually the most-important factor in keeping loading/unloading time to a minimum.
The position of clamps on the workholder is just as important to the overall operation of the tool as the position of the locators. The selected clamps must hold the part against the locators without deforming the workpiece. Once again, since the purpose of locators is to resist all primary cutting forces generated in the operation, the clamps need only be large enough to hold the workpiece against the locators and to resist any secondary forces generated in the operation. To meet both these conditions, position the clamps at the most-rigid points of the workpiece. With most workholders, this means positioning the clamps directly over the supporting elements in the baseplate of the workholder, Figure-3-23a.In some cases the workpiece must be clamped against horizontal locators rather than the supports, Figure 3-23b. In either case, the clamping force must be absorbed by the locating elements.


Figure 3-23. Clamps should always be positioned so the clamping force is directed into the supports or locators.
For workholders with two supports under the clamping area of the workpiece, two clamps should be used — one over each support, Figure 3-24a. Placing only one clamp between the supports can easily bend or distort the workpiece during the clamping operation. When the workpiece has flanges or other extensions used for clamping, an auxiliary support should be positioned under the extended area before a clamp is applied, Figure 3-24b.
Figure 3-24. The number and position of clamps is determined by the workpiece and its supports.
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