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.
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.
The Mechanics of Locating
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 from External Surfaces
Flat surfaces are common workpiece features used for location. Locating from a flat surface is a form of plane location. Supports are the principal devices used for this location. The three major forms of supports are solid, adjustable, and equalizing, Figure 3-3.
Figure 3-3. Solid, adjustable, and equalizing supports locate a workpiece from a flat surface.
Solid Supports are fixed-height locators. They precisely locate a surface in one axis. Though solid supports may be machined directly into a tool body, a more-economical method is using installed supports, such as rest buttons.
Adjustable supports are variable-height locators. Like solid supports, they will also precisely locate a surface in one axis. These supports are used where workpiece variations require adjustable support to suit different heights. These supports are used mainly for cast or forged workpieces that have uneven or irregular mounting surfaces.
Equalizing supports are a form of adjustable support used when a compensating support is required. Although these supports can be fixed in position, in most cases equalizing supports float to accommodate workpiece variations. As one side of the equalizing support is depressed, the other side raises the same amount to maintain part contact. In most cases adjustable and equalizing supports are used along with solid supports.
Locating a workpiece from its external edges is the most-common locating method. The bottom, or primary, locating surface is positioned on three supports, based on the geometry principle that three points are needed to fully define a plane. Two adjacent edges, usually perpendicular to each other, are then used to complete the location.
The most-common way to locate a workpiece from its external profile is the 3-2-1, or six-point, locational method. With this method, six individual locators reference and restrict the workpiece.
As shown in Figure 3-4, three locators, or supports, are placed under the workpiece. The three locators are usually positioned on the primary locating surface. This restricts axial movement downward, along the -z axis (#6) and radially about the x (#7 and #8) and y (#9 and #10) axes. Together, the three locators restrict five degrees of freedom.
Figure 3-4. Three supports on the primary locating surface restrict five degrees of freedom.
The next two locators are normally placed on the secondary locating surface, as shown in Figure 3-5. They restrict an additional three degrees of freedom by arresting the axial movement along the +y axis (#3) and the radial movement about the z (#11 and #12) axis. The next two locators are normally placed on the secondary locating surface, as shown in Figure 3-5. They restrict an additional three degrees of freedom by arresting the axial movement along the +y axis (#3) and the radial movement about the z (#11 and #12) axis.
Figure 3-5. Adding two locators on a side restricts eight degrees of freedom.
The final locator, shown in Figure 3-6, is positioned at the end of the part. It restricts the axial movement in one direction along the -x axis. Together, these six locators restrict a total of nine degrees of freedom. The remaining three degrees of freedom (#1, #4, and #5) will be restricted by the clamps.
Figure 3-6. Adding a final locator to another side restricts nine degrees of freedom, completing the 3-2-1 location.
Although cylindrical rest buttons are the most-common way of locating a workpiece from its external profile, there are also other devices used for this purpose. These devices include flat-sided locators, vee locators, nest locators and adjustable locators.
Locating from Internal Surfaces
Locating a workpiece from an internal diameter is the most-efficient form of location. The primary features used for this form of location are individual holes or hole patterns. Depending on the placement of the locators, either concentric, radial, or both-concentric-and-radial location are accomplished when locating an internal diameter. Plane location is also provided by the plate used to mount the locators.
The two forms of locators used for internal location are locating pins and locating plugs. The only difference between these locators is their size: locating pins are used for smaller holes and locating plugs are used for larger holes.
As shown in Figure 3-7, the plate under the workpiece restricts one degree of freedom. It prevents any axial movement downward, along the -z (#6) axis. The center pin, acting in conjunction with the plate as a concentric locator, prevents any axial or radial movement along or about the x (#1, #2, #7, and #8) and y (#3, #4, #9, and #10) axes. Together, these two locators restrict nine degrees of freedom. The final locator, the pin in the outer hole, is the radial locator that restricts two degrees of freedom by arresting the radial movement around the z (#11 and #12) axis. Together, the locators restrict eleven degrees of freedom. The last degree of freedom, in the +z direction, will be restricted with a clamp.
Figure 3-7. Two locating pins mounted on a plate restrict eleven-out-of-twelve degrees of freedom.
Analyzing Machining Forces
The most-important factors to consider in fixture layout are the direction and magnitude of machining forces exerted during the operation. In Figure 3-8, the milling forces generated on a workpiece when properly clamped in a vise tend to push the workpiece down and toward the solid jaw. The clamping action of the movable jaw holds the workpiece against the solid jaw and maintains the position of the part during the cut.
Figure 3-8. Cutting forces in a milling operation should be directed into the solid jaw and base of the vise.
Another example of cutting forces on a workpiece can be seen in the drilling operation in Figure 3-9. The primary machining forces tend to push the workpiece down onto the workholder supports. An additional machining force acting radially around the drill axis also forces the workpiece into the locators. The clamps that hold this workpiece are intended only to hold the workpiece against the locators and to maintain its position during the machining cycle. The only real force exerted on the clamps occurs when the drill breaks through the opposite side of the workpiece, the climbing action of the part on the drill. The machining forces acting on a correctly designed workholder actually help hold the workpiece.
Figure 3-9. The primary cutting forces in a drilling operation are directed both downward and radially about the axis of the drill.
An important step in most fixture designs is looking at the planned machining operations to estimate cutting forces on the workpiece, both magnitude and direction. The "estimate" can be a rough guess based on experience, or a calculation based on machining data. One simple formula for force magnitude, shown in Figure 3-10, is based on the physical relationship:
Please note: "heaviest-cut horsepower" is not total machine horsepower; rather it is the maximum horsepower actually used during the machining cycle. Typical machine efficiency is roughly 75% (.75). The number 33,000 is a units-conversion factor.
Figure 3-10. A simple formula to estimate the magnitude of cutting forces on the workpiece.
The above formula only calculates force magnitude, not direction. Cutting force can have x-, y-, and/or z-axis components. Force direction (and magnitude) can vary drastically from the beginning, to the middle, to the end of the cut. Figure 3-11 shows a typical calculation. Intuitively, force direction is virtually all horizontal in this example (negligible z-axis component). Direction varies between the x and y axes as the cut progresses.
Figure 3-11. Example of a cutting force calculation.
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.
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.
The examples in Figure 3-13 show conditions that may occur when locators are placed too close together if the center positions of the locators are misaligned by .001". With the spacing shown at (a), this condition has little effect on the location. But if the locating and spacing were changed to that shown at (b), the .001" difference would have a substantial effect. Another problem with locators placed too close together is shown at (c). Here, because the locators are too closely spaced, the part can wobble about the locators in the workholder.
Figure 3-13. Positioning locators too close together will affect the locational accuracy.
The final consideration in the placement of locators involves the problem of chip control. Chips are an inevitable part of any machining operation and must be controlled so they do not interfere with locating the workpiece in the workholder. Several methods help minimize the chip problem. First, position the locators away from areas with a high concentration of chips. If this is not practical, then relieve the locators to reduce the effect of chips on the location. In either case, to minimize the negative effects of chips, use locators that are easy to clean, self-cleaning, or protected from the chips. Figure 3-14 shows several ways that locators can be relieved to reduce chip problems.
Figure 3-14. Locators should be relieved to reduce locational problems caused by chips.
Coolant build-up can also cause problems. Solve this problem by drilling holes, or milling slots, in areas of the workholder where the coolant is most likely to build up. With some workholders, coolant-drain areas can also act as a removal point for accumulated chips.
When designing a workholder, always try to minimize the chip problem by removing areas of the tool where chips can build up. Omit areas such as inside corners, unrelieved pins, or similar features from the design. Chip control must be addressed in the design of any jig or fixture.
Avoiding Redundant Location
Another condition to avoid in workholder design is redundant, or duplicate, location. Redundant locators restrict the same degree of freedom more than once. The workpieces in Figure 3-15 show several examples. The part at (a) shows how a flat surface can be redundantly located. The part should be located on only one, not both, side surfaces. Since the sizes of parts can vary, within their tolerances, the likelihood of all parts resting simultaneously on both surfaces is remote. The example at (b) points out the same problem with concentric diameters. Either diameter can locate the part, but not both.
The example at (c) shows the difficulty with combining hole and surface location. Either locational method, locating from the holes or locating from the edges, works well if used alone. When the methods are used together, however, they cause a duplicate condition. The condition may result in parts that cannot be loaded or unloaded as intended.
Figure 3-15. Examples of redundant location.
Always avoid redundant location. The simplest way to eliminate it is to check the shop print to find which workpiece feature is the reference feature. Often, the way a part is dimensioned indicates which surfaces or features are important. As shown in Figure 3-16, since the part on the left is dimensioned in both directions from the underside of the flange, use this surface to position the part. The part shown to the right, however, is dimensioned from the bottom of the small diameter. This is the surface that should be used to locate the part.
Figure 3-16. The best locating surfaces are often determined by the way that the part is dimensioned.
Preventing Improper Loading
Foolproofing prevents improper loading of a workpiece. The problem is most prevalent with parts that are symmetrical or located concentrically. The simplest way to foolproof a workholder is to position one or two pins in a location that ensures correct orientation, Figure 3-17. With some workpieces, however, more-creative approaches to foolproofing must be taken.
Figure 3-17. Fool proofing the location prevents improper workpiece loading.
Figure 3-18 shows ways to foolproof part location. In the first example, shown at (a), an otherwise-nonfunctional foolproofing pin ensures proper orientation. This pin would interfere with one of the tabs if the part were loaded any other way. In the next example, shown at (b), a cavity in the workpiece prevents the part from being loaded upside-down. Here, a block that is slightly smaller than the opening of the part cavity is added to the workholder. A properly loaded part fits over the block, but the block keeps an improperly loaded part from entering the workholder.
Figure 3-18. Simple pins or blocks are often used to foolproof the location.
Using Spring-Loaded Locators
One method to help ensure accurate location is the installation of spring-loaded buttons or pins in the workholder, Figure 3-19. These devices are positioned so their spring force pushes the workpiece against the fixed locators until the workpiece is clamped. These spring-loaded accessories not only ensure repeatable locating but also make clamping the workpiece easier.
Figure 3-19. Spring-loaded locators help ensure the correct location by pushing the workpiece against the fixed locators.
Determining Locator Size and Tolerances
The workpiece itself determines the overall size of a locating element. The principle rule to determine the size of the workpiece locator is that the locators must be made to suit the MMC (Maximum-Material Condition) of the area to be located. The MMC of a feature is the size of the feature where is has the maximum amount of material. With external features, like shafts, the MMC is the largest size within the limits. With internal features, like holes, it is the smallest size within the limits. Figure 3-20 illustrates the MMC sizes for both external and internal features.
Figure 3-20. Locator sizes are always based on the maximum-material condition of the workpiece features.
Sizing cylindrical locators is relatively simple. The main considerations are the size of the area to be located and the required clearance between the locator and the workpiece. As shown in Figure 3-21, the only consideration is to make the locating pin slightly smaller than the hole. In this example, the hole is specified as .500-.510" in diameter. Following the rule of MMC, the locator must fit the hole at its MMC of .500". Allowing for a .0005 clearance between the pin and the hole, desired pin diameter is calculated at .4995". Standard locating pins are readily available for several different hole tolerances, or ground to a specific dimension. A standard 1/2" Round Pin with .4995"-.4992" head diameter would be a good choice.
Figure 3-21. Determining the size of a single locating pin based on maximum-material conditions.
The general accuracy of the workholder must be greater than the accuracy of the workpiece. Two basic types of tolerance values are applied to a locator: the first are the tolerances that control the size of the locator; the second are tolerances that control its location. Many methods can be used to determine the appropriate tolerance values assigned to a workholder. In some situations the tolerance designation is an arbitrary value predetermined by the engineering department and assigned to a workholder without regard to the specific workpiece. Other tolerances are assigned a specific value based on the size of the element to be located. Although more appropriate than the single-value tolerances, they do not allow for requirements of the workpiece. Another common method is using a set percentage of the workpiece tolerance.
The closer the tolerance value, the higher the overall cost to produce the workpiece. Generally, when a tolerance is tightened, the cost of the tolerance increases exponentially to its benefit. A tolerance twice as tight might actually cost five times as much to produce.
The manufacturability of a tolerance, the ability of the available manufacturing methods to achieve a tolerance, is also a critical factor. A simple hole, for example, if toleranced to ±.050", can be punched. If, however, the tolerance is ±.010", the hole requires drilling. Likewise, if the tolerance is tightened to ±.002", the hole then requires drilling and reaming. Finally, with a tolerance of ±.0003", the hole must be drilled, reamed, and lapped to ensure the required size.
One other factor to consider in the manufacturability of a tolerance is whether the tolerance specified can be manufactured within the capability of the toolroom. A tolerance of .00001" is very easy to indicate on a drawing, but is impossible to achieve in the vast majority of toolrooms.
No single tolerance is appropriate for every part feature. Even though one feature may require a tolerance of location to within .0005", it is doubtful that every tolerance of the workholder must be held to the same tolerance value. The length of a baseplate, for example, can usually be made to a substantially different tolerance than the location of the specific features.
The application of percentage-type tolerances, unlike arbitrary tolerances, can accurately reflect the relationship between the workpiece tolerances and the workholder tolerances. Specification of workholder tolerances as a percentage of the workpiece tolerances results in a consistent and constant relationship between the workholder and the workpiece. When a straight percentage value of 25 percent is applied to a .050" workpiece tolerance, the workholder tolerance is .0125". The same percentage applied to a .001" tolerance is .00025". Here a proportional relationship of the tolerances is maintained regardless of the relative sizes of the workpiece tolerances. As a rule, the range of percentage tolerances should be from 20 to 50 percent of the workpiece tolerance, usually determined by engineering-department standards.
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.
Factors in Selecting Clamps
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.
Positioning the Clamps
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.
Another consideration in positioning clamps is the operation of the machine tool throughout the machining cycle. The clamps must be positioned so they do not interfere with the operation of the machine tool, during either the cutting or return cycle. Such positioning is especially critical with numerically controlled machines. In addition to the cutters, check interference between the clamps and other machine elements, such as arbors, chucks, quills, lathe carriages, and columns.
When fixturing an automated machine, check the complete tool path before using the workholder. Check both the machining cycle and return cycle of the machine for interference between the cutters and the clamps. Occasionally programmers forget to consider the tool path on the return cycle. One way to reduce the chance of a collision and eliminate the need to program the return path is simply to raise the cutter above the highest area of the workpiece or workholder at the end of the machining cycle before returning to the home position.
Most clamps are positioned on or near the top surface of the workpiece. The overall height of the clamp, with respect to the workpiece, must be kept to a minimum. This can be done with gooseneck-type clamps, Figure 3-25. As shown, the gooseneck clamp has a lower profile and should be used where reduced clamp height is needed.
Figure 3-25. Using gooseneck clamps is one way to reduce the height of the clamps.
The size of the clamp-contact area is another factor in positioning a clamp. To reduce interference between the clamp and the cutter, keep the contact area as small as safely possible. A small clamping area reduces the chance for interference and also increases the clamping pressure on the workpiece. The overall size of the clamp is another factor to keep in mind. The clamp must be large enough to properly and safely hold the workpiece, but small enough to stay out of the way.
Once again, the primary purpose of a clamp is to hold the workpiece against the locators. To do this properly, the clamping force should be directed into the locators, or the most-solid part of the workholder. Positioning the clamping devices in any other manner can easily distort or deform the workpiece.
The workpiece shown in Figure 3-26 illustrates this point. The part is a thin-wall ring that must be fixtured so that the internal diameter can be bored. The most-convenient way to clamp the workpiece is on its outside diameter; however, to generate enough clamping pressure to hold the part, the clamp is likely to deform the ring. The reason lies in the direction and magnitude of the clamping force: rather than acting against a locator, the clamping forces act against the spring force of the ring resisting the clamping action. This type of clamping should only be used if the part is a solid disk or has a small-diameter hole and a heavy wall thickness.
Figure 3-26. Directing the clamping forces against an unsupported area will cause this cylindrical part to deform.
To clamp this type of part, other techniques should be used. The clamping arrangement in Figure 3-27 shows the workpiece clamped with four strap clamps. The clamping force is directed into the baseplate and not against the spring force of the workpiece. Clamping the workpiece this way eliminates the distortion of the ring caused by the first method.
Figure 3-27. Strap clamps eliminate deformation by directing the clamping forces into the supports under the part.
A similar clamping method is shown at Figure 3-28. Here the workpiece has a series of holes around the ring that can be used to clamp the workpiece. Clamping the workpiece in this manner also directs the clamping force against the baseplate of the workholder. This type of arrangement requires supports with holes that permit the clamping screws to clamp through the supports.
Figure 3-28. When possible, part features such as holes can be used to clamp the part.
If the part can be clamped only on its outside surface, one other method can be used to hold the part: a collet that completely encloses the part. As shown in Figure 3-29, the shape of the clamping contact helps control distortion. Depending on the size of the part, either a collet or pie-shaped soft jaws can be used for this arrangement.
Figure 3-29. When the part can only be clamped on its outside surface, pie-shaped chuck jaws can be used to hold the part and reduce deformation.
Selecting Clamp Size and Force
Calculations to find the necessary clamping force can be quite complicated. In many situations, however, an approximate determination of these values is sufficient. The table in Figure 3-30 shows the available clamping forces for a variety of different-size manual clamp straps with a 2-to-1 clamping-force ratio.
Figure 3-30. Approximate clamping forces of different-size manual clamp straps with a 2-to-1 clamping-force ratio.
Alternatively, required clamping force can be calculated based on calculated cutting forces. A simplified example is shown in Figure 3-31. The cutting force is entirely horizontal, and no workpiece locators are used, so frictional forces alone resist the cutting forces.
Figure 3-31. A simplified clamping-force calculation with the cutting force entirely horizontal, and no workpiece stops (frictional force resists all cutting forces).
When workpiece locators and multi-directional forces are considered, the calculations become more complicated. To simplify calculations, the worst-case force situation can be estimated intuitively and then treated as a two-dimensional static-mechanics problem (using a free-body diagram). In the example shown in Figure 3-32, the cutting force is known to be 1800 lbs, based on a previous calculation. The workpiece weighs 1500 lbs. The unknown forces are:
|F R =
|Total force from all clamps on right side
|Total force from all clamps on left side
|R 1 =
|Horizontal reaction force from fixed stop
|R 2 =
|Vertical reaction force from fixed stop
|R 3 =
|Vertical reaction force on right side
|Normal force = FL + FR + 1500
|= Coefficient of friction = .19
Figure 3-32. A more-complicated clamping-force calculation, using a two-dimensional free-body diagram.
The equations below solve for unknown forces assuming that for a static condition:
1. The sum of forces in the x direction must equal zero
2. The sum of forces in the y direction must equal zero
3. The sum of moments about any point must equal zero
At first glance, the example above looks "statically indeterminate," i.e. there are 5 variables and only 3 equations. But for the minimum required clamping force, R3 would be zero (workpiece barely touching) and FL would be zero (there is no tendency to lift on the left side). Now with only 3 variables, we can solve:
Solving for the variables,
FR = 1290 lbs
R1 = 1270 lbs
R2 = 2790 lbs
In other words, the combined force from all clamps on the right side must be greater than 1290 lbs. With a recommended safety factor of 2-to-1, this value becomes 2580 lbs. Even though FL (combined force from all the clamps on the left side) equals zero, a small clamping force may be desirable to prevent vibration.
Another general area of concern is maintaining consistent clamping force. Manual clamping devices can vary in the force they apply to parts during a production run. Many factors account for the variation, including clamp position on the workpiece, but operator fatigue is the most-common fault. The simplest and often-best way to control clamping force is to replace manual clamps with power clamps.
The force generated by power clamps is not only constant but also adjustable to suit workpiece conditions. Another benefit of power clamps is their speed of operation: not only are individual power clamps faster than manual clamps, every clamp is activated at the same time.