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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Foolproofing 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.
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.
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.
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.
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.