Workholding for CNC efficiency.
The new fixturing mounted the engine blocks on end to place the cam and crank bores in a vertical plane. Taking advantage of that positioning, engineers created a fixture with a round vertical tower that runs internally through the cam bores. The block slides down over the tower and is then secured at the top by a plate that mounts against the face end of the block and fastens onto the tower ends.
By internalizing the fixturing, the arrangement allows 360-deg spindle access to bottom, side, and top features. An indexing table thus provides machining of crank bottom, cylinder banks, head decks, intake manifold, and related block features all in one fixturing. The integral construction provides high rigidity, lower center of gravity, reduced maintenance, and optimum design freedom. Finally, the new fixturing eliminates the need for a boring mill, broaching machine, and specialized grinders.
Not every problem requires an exotic solution, however. Some just need a reasonable investment in clamps and fixtures. "Nobody runs bare table anymore," says Gordon Coope, applications engineer, Stevens Engineering Inc, Phoenix, AZ. He tells T&P, "Nearly everyone is now mounting subplates with a standard grid pattern on their machines."
Beyond this, innovations range from redesign of simple clamps and vises to complex computerized modular fixturing systems. Consider the pallet changer, rapidly becoming a standard item in many shops.
Pallet pursues profits
C & H Machine, Escondido, CA, operated by Lyle Anderson and Charles Gohlich, lost business as a result of cutbacks in US defense spending. Mr Anderson sums up the dilemma this way: "In looking for work to replace our loss of government-related orders, we found out fast that we weren't the only shop with this problem...but we were fortunate to get involved in the development of prototype golf putters."
C & H devised experimental machining processes to produce the putter heads at minimum cost. As the final version of the product evolved, the firm received a contract to manufacture it at lower volumes during the market-introduction phase of the program.
"However," Mr Anderson tells T&P, "we knew that when our customers eventually started ordering them by the thousands we'd be faced with two problems. First, we'd have to get our costs down fast or we'd stand a chance of losing the contract to competitors who were struggling to stay alive.
"Second, we'd have to find a way to increase production on our existing equipment. We simply couldn't afford to buy more machines to meet the increased production schedule. Fortunately, we found the solution to both problems in a Workswitcher pallet changer from SMW Systems Inc, Santa Fe Springs, CA."
Basically, the Workswitcher lets the Fadal VMC 3016 machine parts on a nearly continuous basis by transferring load, unload, chip-cleaning, and setup functions from the machine table to the off-line pallet. The system normally employs two pallets, a power-operated locating-and-clamping fixture on the machine table, and a pair of shuttle tables. The shuttle tables index from right-or left-hand load-and-setup positions to center position in front of the table-mounted locate-and-clamp fixture.
The operator unloads and reloads the off-line pallet with raw work, or he sets up for a new job, while the on-line pallet is under the machine spindle. At the end of the machining cycle, the operator indexes the empty shuttle to the center position, unclamps the pallet on the machine, rolls it onto the empty shuttle, and then indexes the shuttle to the empty load-and-setup position.
Next, he indexes the fresh pallet to the center position, rolls it onto the locate-and-clamp fixture, actuates the clamping valve, closes the door, and starts the next machine cycle. The entire procedure takes about 1 min.
"Actually," says Mr Anderson, "we don't have enough production volume on our putter-head orders to keep the machine occupied full time. So we have to use the machine for other work, much of which involves quantities of less than five pieces.
"Although we bought the Workswitcher primarily for the high-volume putter head, we soon discovered that it was even more valuable for our short-run work because the machine is never down during setup. It's only down for the time it takes to switch pallets, change a few tools, and load the next machining program."
Mr Anderson concludes: "On our long-run jobs, the system increases machine output by nearly 40%. When we mix in short-run work, we come close to doubling machine output. We'll do even better when we finish building a pallet-transport cart that will insert additional off-line pallets into the system."
Mitee-Bite Products Co, Center Ossipee, NH, offers the Force manual pallet changer with 1.24"-thick base measuring 11.80"x15.75". The pallet weighs just 22lb and can be machined anywhere on the top surface. Repeatability is [+ or -]0.0001" with fingertip positioning to clamp the pallet to the receiver.
The firm has a line of edge clamps and other workholding devices to mount on the pallet, and these help set the system apart from other systems. Use of the pallet reduces the need for dedicated fixtures.
Midaco Corp, Elk Grove Village, IL, includes the new X-Tender-20 [TM] pallet system in its lineup to expand the X-axis capacity of VMCs by an additional 20". The variable-position, indexable-fixture table has an easy-to-push lever to unlock and precisely move the worktable from left to right in 10" increments. It allows machining of 40" long parts.
The Omni-Lock [R] system from Jakob Tooling Inc, East Dundee, IL, boasts compact design. Its overall height is just 3.52" with loaded pallet. The system accommodates small or large pallets, and it serves profile or creepfeed grinding as well as milling and other processes. Its locating components do not move, ensuring repeatability better than [+ or -]0.0001" under production conditions.
With or without a pallet changer, you may want to consider a tooling cube or tombstone. There are many variations and different ways to use them. Gordon Coope of Stevens Engineering says that his firm's tooling cubes represent a departure from traditional designs. "Tooling for horizontal-spindle CNC machines has evolved from tombstones into structures optimized for stiffness, accuracy, and weight.
Most fixtures mounted on tombstones have their own baseplates as a part of the fixture. But our Silo Columns go a step further. Each is essentially a rigid framework for mounting either tooling plates or subplates to its faces."
Mr Coope describes the Silo Column as a tombstone with removable faces. These may be either dedicated fixtures made from tooling plates, or modular fixtures built with standard components on a subplate. In either case, the fixture can then be repeatedly mounted to any face of the Silo Column with two pull-dowels and a few cap screws. It's important, of course, to make the components accurately to eliminate the need for probing or indicating when changing setups.
Mid-State Machine Products, Winslow, ME, includes a special three-sided cube in its line of vertical and horizontal tombstones with accessories such as T-fixtures, angle plates, hexagonals, octagonals, risers, subplates, and window frames. The tooling-cube line includes indexing styles; and many models let you custom order hole configurations.
Carr Lane Roemheld Mfg Co, St Louis, MO, provides blank tooling blocks that let you custom make your own columns. The firm's line of ready-to-mount components includes, pallets, plates, and extra-tall tooling blocks. Pallet sizes reach 630mm square.
Carr Lane Roemheld is among many companies providing modular-fixturing systems to compete with hard fixtures. The firm offers mini, standard, and heavy sizes, all using tooling plates and blocks with standard grid patterns and multiple-purpose holes at regular intervals. Modular fixturing lets you assemble a wide variety of workholding devices using a small assortment or reusable components.
Carr Lane finish grinds each block and plate, and every hole has an alignment bushing plus a threaded insert. (Other systems may alternate the bushing and threaded holes.) The multipurpose holes on the new mini size have 1.0000" spacing and 5/16-18 threads. Dual capability allows both JIS mounting (locating from two reference edges) and DIN mounting (locating from center and radial holes) with good accuracy.
Ray Okolischan, VP of Carr Lane, tells T&P, "In the hierarchy of workholding options, modular fixturing fills a significant gap between general-purpose workholders (standard clamps, vises, chucks, and other off-the-shelf components) and special-purpose (custom-made) workholders. Modular workholders are actually special-purpose workholders assembled from general-purpose components.
"Modular fixturing is a valuable tool for virtually every machine shop. Applications include: one-time jobs; infrequent production runs; prototype parts; replacement parts made to order; trial fixturing; and setups where permanent fixtures must be built or repaired. To meet this wide variety of applications, we offer systems with clamping forces from 900 to 3900 lb."
Qu-Co, Union, OH, also offers modular fixturing components, all presented in a comprehensive catalog. Catalogs are important in the modular game because there are so many elements available. A well-designed catalog can help you select only the components you need to build a particular fixture.
Qu-Co divides its modular fixtures into three sizes: a 5/16" system that holds small semifinished workpieces and those that will be inspected on CMM machines; a 1/2" system for medium-size parts that are either rough castings or semifinished workpieces; and a 3/4" system to hold large rough or semifinished workpieces. The catalog includes application photos, detailed line drawings, and technical information on solutions to fixturing problems.
Bluco Corp, Carol Stream, IL, stresses its modular fixturing for short-run and prototype jobs. Accuracy is stated as [+ or -]0.0004", and the firm says you can build new fixtures in 4 hr. The second time around, you can rebuild them in 20 min. Modules suit both horizontal and vertical machining centers as well as CMMs. A complete CAD database aids documentation of the four system sizes. Bluco also has modular fixtures for welding.
Stevens Engineering makes modular starter kits in several options. For instance, the Subplate and Vise Kit is suggested for small vertical milling machines used in the toolroom. Larger kits suit irregularly shaped parts and larger machining centers for low-volume production. Fixture setups may be designed on CAD systems or by the machine operator.
A special modular system
Yes, the computer can help build a modular fixture, but what if you go all the way and let the machine control actually locate components? A firm called MSCI, Allentown, PA, has a system that directs the CNC machine tool to build its own fixture, or at least eliminate the need for precision tooling plates normally used for locating and positioning fixture components.
Instead of precision plates, MSCI's system employs the machine tool's positioning ability for locating the fixture components. A special locating tool installed in the spindle includes an end face, ID pilot, and OD pilot. These allow precise positioning and direct viewing of the tool's coordinate position using the CNC.
The tool end face and ID/OD pilots position other system components, including multi-positional locators, edge locators, locating cylinders, and work supports. If the machine accuracy is [+ or -]0.0002", then the system can position fixture components within [+ or -]0.0006"
The system uses a baseplate with a grid of tapped holes, but these do not have a critical locating function--serving only to hold other fixturing components in place. Once you mount the baseplate on the machine table, its top surface can be milled flat to align it with the machine's X-Y plane. You can fabricate the entire baseplate in-house, saving a large amount of modular-fixturing cost.
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|Title Annotation:||computer numerical control|
|Author:||Miller, Paul C.|
|Publication:||Tooling & Production|
|Date:||Dec 1, 1994|
|Previous Article:||Mastering metrology.|
|Next Article:||Turning production up or down.|