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Workholding options for multiple operations.

Workholding options for multiple operations

Marketing a variety of workholding devices brings us into contact with hundreds of shops throughout the country who have developed unique methods of applying these products. Examples of such applications are covered in this article, for which I thank our customers, who have permitted us to share their ideas with Tooling & Production readers. Also note that, although SMW products are used exclusively as illustrations in this article, similar workholding devices are available from other manufacturers in both the US and abroad.

Of the many types of modular fixturing systems available today, one that appears to be growing the fastest in popularity is essentially a replacement for the old standard machine vice. Basically, this device consists of a tooling plate whose work-clamping area covers the machine's travel limits. As show in Figure 1, modular clamping and locating units can be moved and removed to accommodate a wide range and mix of workpieces. Figure 1(a) shows a twelve-piece setup where identical pieces are fixtured in two different positions. Figure 1(b) shows multiples of two different pieces fixtured in different positions. Figure 1(c) shows the center-locating rail moved to the rear of the tooling plate and the clamp units configured to accommodate a single large workpiece. When the clamp units are tightened, they provide an angular (vector) force that produces both sideways and downward clamping forces. The clamping rail has adjustable locators that provide a two-sided nest, which resists machining forces, while providing accurate part location.

This SMW vector vice puts more workpieces on the machine table than a conventional vise. Thus, it increases the length of the machine cycle and frees the operator for other duties, such as running additional machines or inspecting previously machined pieces. The presence of more workpieces on the table also reduces tool change time per piece because each tool is used on several pieces prior to being changed, as opposed to being used on one or two pieces in the conventional machining vise setup.

Retrofittable pallet changers

A fixturing system that can be easily retrofitted to vertical machining centers is shown in Figure 2. It uses pallets along with the vector vise modular tooling system.

Figure 2(a) shows the operator setting up two large workpieces while a second pallet of smaller workpieces is being machined. This means that workpiece loading, unloading, setup, and clamping take place off-line while the machine is cutting other workpieces. In longer run situations, both pallets can fixture identical workpieces over and over again as volume requires.

The same pieces can also be re-loaded in different positions on the same pallet (refer to Figure 1) to expose different workpiece faces for machining. Runs as short as one piece are easily accommodated, as previously shown in Figure 1(c). The same resulting benefits as mentioned in the examples seen in Figure 1 can be expected.

Figure 2(b) shows the midpoint in a pallet change sequence. The machine table is equipped with a compact, low-profile pallet locating and clamping unit. A two-position index unit is mounted on the floor in front (or at the side) of the machine. To change pallets, the operator manually: Unclamps and removes the pallet in the machine, pulling the pallet out onto the index table; indexes the table 180 deg, placing the new pallet in front of the machine; pushes the new pallet onto the machine; locates and clamps the new pallet. The unit is now ready to resume machining. This sequence takes less than one minute.

Indexers and rotary tables

Earlier versions of indexers, and to a lesser degree rotary tables, simply could not hold the higher dimensional accuracy required, and fell out of favor as workholding devices. Further, as cutting tools and machines that allowed taking heavier cuts at higher spindle speeds were developed, many of these early units could not withstand the higher cutting forces involved. Accordingly, alternative processing methods were developed to circumvent these shortcomings.

Today, there are excellent indexers and rotary tables available that provide exceptional accuracy and rigidity. Figure 3 shows one of the SMW models that uses a face-gear coupling to establish indexing accuracy. The unit is accurate to [+ or -] 0.0001" on an 8.0" dia part and it can handle heavy milling cuts on overhung workpieces.

This device demonstrates how four sides of cube-shaped workpieces can be machined in one setup on a vertical machining center, compared to the normal four setups required to machine one face per setup. Eliminating setups avoids setup downtime plus it allows for greater accuracy because the workpiece is not relocated repeatedly. Note also that this workpiece has opposed bores that are also easily handled by the indexer, which has an accuracy of [+ or -] 5 arc seconds.

Figure 4 shows a short tombstone fixture mounted on an indexer. Four workpieces are held by the fixture, which is designed to present three faces of the workpiece to the machine spindle. Note that one side face of the forward piece is being drilled. This arrangement allows 12 faces to be machined in a single untended machining cycle.

Figure 5 shows a longer tombstone fixture supported by an outboard support. Three sides of each workpiece also are machined in this example. The fixture holds 16 pieces, thus 48 sides are machined in a single machining cycle. This frees the operator for other duties for extended periods of time. Tool change time per piece also is reduced substantially.

A power-operated chuck mounted on an indexer is shown in Figure 6. This arrangement allows fast power clamping of round workpieces, on which a variety of milling and other operations can be performed. The indexer shown also has a built-in collet closer in its spindle for chucking round, hexagonal, and square bar stock.

Figures 7(a) and 7(b) show another innovative use of an indexer to which a tombstone fixture is mounted. The tombstone fixture is designed to locate and clamp four small pallet fixtures, each of which contains multiple workpieces. Typically, the four small pallets in this example can be changed in approximately two minutes.

Quick-change chucks

Productivity increases of 100 to 200 percent are achievable on CNC turning centers by installing power-operated chucks, such as the SMW Model KNCS. This type of chuck offers several advantages that reduce changeover downtime. Figures 8(a) and 8(b) show the same workpiece being rough turned in hard jaws and repositioned for turning the second end using soft jaws. This jaw changeover downtime sequence is accomplished in one minute compared to about 20 minutes for chucks that use screws to clamp the jaws in position.

As seen in Figure 9, a jaw is removed from the body by inserting a hexagonal key into a socket next to the jaw slot. One-quarter turn releases the jaw, allowing its quick removal. After inserting the new soft jaw, a quarter turn of the key locks it into position. The procedure is repeated for each jaw. In this particular chuck, the hexagonal key cannot be removed until the new jaw is locked into position. This prevents a loose jaw from being thrown from the chuck when the machine spindle starts to rotate.

This same procedure is used to reposition hard jaws to accept a different workpiece diameter. However this particular chuck allows the same set of hard jaws to be used to clamp a wide range of workpiece diameters. This translates into reduced hard jaw inventory compared to conventional chuck requirements.

Self-centering steadyrests

A device designed to support a range of workpiece diameters during turning operations without requiring manual adjustment is the power-operated steadyrest. Figure 10 shows an SMW OmniRest(TM). The three arms contain rollers that automatically center on any diameter within the range and center the workpiece to its true center position to within 0.0002". Aside from eliminating time-consuming setup and re-adjustment, this unit provides other benefits.

As seen in Figure 11, the steadyrest can support the end of a shaft while the end surface is machined, allowing complete shaft machining in one machine cycle. It also can be used to hold the shaft during cutoff, or to center the shaft end for tailstock support. The steadyrest also can be mounted on a separate machine slide in a follow-rest application where it supports the workpiece as it follows the tool on successive roughing and finishing passes within the same machining cycle. Stationary mounted OmniRests also are used in various shaft-support applications. They can be used in multiples to support long shafts.

Workholding methods covered here are but a few of the possibilities. Applications are limited only by the ingenuity and imagination of the user.

PHOTO : 1. Multiple applications of a modular clamping device are shown. Photo (a) shows clamping

PHOTO : of 12 workpieces, six each in two different positions. View (b) is same device

PHOTO : reconfigured to fixture two different workpieces. Seen in (c), it is used to accommodate

PHOTO : a single large workpiece.

PHOTO : 2. Retrofittable pallet changer system allows offline loading, unloading, and setup, view

PHOTO : (a). Changeover from small to large workpiece is accomplished with only one minute

PHOTO : downtime, (b).

PHOTO : 3. Indexer allows machining four sides of a single workpiece' in one setup.

PHOTO : 4. Tombstone fixture on indexer allows machining on three sides of four workpieces.

PHOTO : 5. Tailstock-supported tombstone fixture for machining three sides of 16 workpieces.

PHOTO : 6. Power chuck mounted on indexer for machining flats and holes in round workpieces.

PHOTO : 7. Small pallet fixtures are seen in (a) mounted on a tombstone fixture attached to an

PHOTO : indexer. View (b) shows workpieces loaded and unloaded off line while machine is cutting

PHOTO : other work.

PHOTO : 8. First-end turning operation using hard jaws in quick changeover chuck, (a). Finished

PHOTO : end of same workpiece clamped in soft jaws is seen in (b). Jaw change takes only one

PHOTO : minute.

PHOTO : 9. Jaws are released or repositioned with one-quarter turn of special hexagonal key.

PHOTO : 10. Self-centering steadyrest automatically adjusts to different workpiece diameters.

PHOTO : 11. By supporting end of shaft with steadyrest, end face of workpiece can be machined in

PHOTO : same setup. Len Atkins President, SMW Systems, Inc, Santa Fe Springs, CA
COPYRIGHT 1989 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1989 Gale, Cengage Learning. All rights reserved.

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Author:Atkins, Len
Publication:Tooling & Production
Date:Aug 1, 1989
Previous Article:An enlightened management philosophy.
Next Article:Versatility of modular fixturing.

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