The most common form of locator is the pin type, Figure 7-2. Pin locators are available in two basic styles, either plain or with a shoulder.
Figure 7-2. Two common types of locating pin: plain and shoulder-type.
These locating pins come in a wide range of diameters. They are (normally) installed by either press fitting the
pin directly into the tool body, or slipping them into a bushing. This second type is retained by a lockscrew.
Round Locating Pins
Round pins can be used for both internal and external workpiece location. For internal location, the diameter of the pin must match the size of the locating hole. These locators come in many standard sizes, and are readily available ground to special diameters. For external location, the size of the locating pin is not as critical. Here, a standard pin size strong enough to resist machining forces is the best choice.
Plain locating pins are pressed directly into the tool body. They are normally used for workholders in short-to-medium production runs where there is no need for pin replacement. Plain locating pins provide the necessary horizontal location, in the x and y axes, for the workpiece. The vertical location and support, in the z axis, are provided by other supports.
The shoulder-type pins likewise locate the workpiece in the horizontal, x and y, axes. These pins have a shank larger than the head. The purpose of the shoulder is to prevent the pin from being pushed into the tooling plate too far. Unlike the plain pins, shoulder-type pins are made in two styles: press-fit type and lockscrew type, Figure 7-3. The press-fit type is pressed into the tool body in the same way as a plain-type locating pin. The lockscrew type, however, should be installed with a locating-pin liner bushing.
Figure 7-3. Shoulder-type locating pins are made in two styles; the press-fit type for permanent installation,
and the lockscrew type with a slip fit for renewable installation.
This liner is pressed into the tool body and affords the locating pins a hardened, wear-resistant mounting hole. The machined recess on the shoulder is for a lockscrew that holds the locating pin in position, Figure 7-4. The liner is primarily intended for workholders in long production runs or for applications where heavy wear is a concern. Locating pin liners permit the easy and accurate replacement of the locating pins as they wear, without damaging the mounting holes, or having to remove the fixture plate from the machine tool.
Figure 7-4. Liner bushings and lockscrews are usually used to mount renewable locating pins.
Bullet-Nose and Conical Locators
In addition to the round locating pins previously mentioned, other variations are available, including bullet-nose
dowels, bullet-nose pins, and cone locator pins, Figure 7-5. These end shapes are mainly for internal location
and allow easier loading of workpieces over the pins. Each of the locators is installed by press fitting into the
tool body.
Figure 7-5. These round locating pins have unique head shapes for specialized applications.
The most common application for these locating pins is the alignment of workholder elements, rather than
locating workpieces. A sandwich jig, for example, is made with two individual plates. Two locating pins ensure
the alignment of the top plate to the bottom plate when the jig is assembled. In these cases, the locating pins
are aligned with locating bushings, Figure 7-6. These hardened bushings help maintain locational accuracy
throughout the life of the workholder.
Bullet-nose round pins are ideal for aligning two pieces of a workholder. The pin’s shank diameter and the
locating bushing’s outside diameter are the same size to allow boring the installation holes in both pieces at the
same time, for greater accuracy.
Figure 7-6. With a sandwich jig, bullet-nose dowels and locating bushings ensure the alignment of the top and bottom jig plates.
Figure 7-7. Cone locator pins compensate for misalignment to allow quick assembly of two workholder pieces.
Cone locator pins are used with mating bushings, as shown in Figure 7-7. These medium accuracy locating pins compensate for a significant amount of misalignment to allow quick assembly of two workholder pieces.
Figure 7-8. Diamond locating pins are relieved to locate in only one axis. They are available in configurations to match round locating pins.
Diamond Locating Pins
Another style of locating pin frequently seen in jig-and-fixture design is the diamond, or relieved, locating pin. Like round locating pins, diamond pins are available in either the plain or shoulder-type, as shown in Figure 7-8. These locating pins are the most common form of relieved floating pin in workholders. To limit the pin’s contact area, the diamond locating pin is made with four machined flats. The exact width of the contact area varies with the size of the pin, and is usually equal to one-third of the diameter on each side.
Diamond pins are generally used as shown in Figure 7-9. Here the diamond pin acts as a radial locator to restrict movement of the workpiece around the concentric locator, shown by the round-pin locator. The diamond pin is positioned to restrict the radial movement of the part. Since a diamond pin locates in only one axis, the contact areas of the pin must be positioned as shown. Positioning the pin any other way would allow the part to move about the concentric locator. Plain diamond pins are available with three standard pilot diameters, designated as X, Y, and Z, with X having the tightest location tolerance, and Z the loosest.
Figure 7-9. Diamond pins are relieved to act only as radial locators, avoiding the redundant location that causes binding during loading.
Floating Locating Pins
Another locating pin that corrects slight differences between locating holes is the floating locating pin, Figure 7-10. This pin provides precise location in one axis, but moves up to 1/8” in the perpendicular axis. The body of the locator is referenced to the fixed and movable axes with a roll pin.
The floating locator performs the same function as a diamond pin. Due to the pin’s floating movement, however, this locator can be used for parts with looser locational tolerances between the holes. As shown in Figure 7-11, the floating locating pin is often used with a round locating pin.
Figure 7-10. A floating locating pin provides precise location in one axis and allows up to 1/8” movement in the perpendicular axis.
Figure 7-11. Floating locating pins are used with round locating pins to compensate for significant variations in hole spacing.
Locating Plugs
Locating plugs are simply large locating pins. Standard locating pins are usually available only up to 1.00” in diameter. Larger plugs are usually pressed into the tool body, then held in place with screws and sometimes dowel pins, as shown in Figure 7-12. Smaller plugs can be installed with a mounting diameter that has both a press-fit area and threads. In either case, the press-fit diameter locates the plug in the workholder. In modular fixturing set ups, or fifth axis applications, standard fixture keys are available in 25, 30, and 50 millimeter sizes. These are usually found in pallet applications, as the center locator. They are normally used with a radial pin to achieve proper orientation of the fixture plates, which are often round. For fifth axis applications, central locators are available with one end 25mm in diameter, and the other in a range of sizes from ½ “ up to 3.25” in both inch and metric diameters.
Figure 7-12. Locating plugs can be custom-made to locate workpieces on larger internal diameters.
As another option, a series of locating pins can take the place of a locating plug. As shown in Figure 7-13, the pins are positioned at three points, 120 degrees apart, around the internal diameter of the hole. This arrangement is usually much more economical than making a custom plug.
Figure 7-13. Instead of making a special locating plug, three locating pins, spaced 120º apart, can be used for locating large diameters.