Workholding for CNC efficiency.

Quick change is the challenge to production engineers charged with bringing CNC machine tools up to speed. It's not enough to install fourth axis rotary table, for example. You must also tool it up to receive workpieces instantly and, hopefully, preset.

You can do that with the "Fourth-Axis Fixture Quick Change" system from Rimeco Products Inc, Oak-wood Village, OH. Designed for use on VMCs, as well as lathes, it adds quick-change convenience, so the operator can set up one fixture while another is being machined. Once a fixture is set up, the operator can change it in less than 10 sec. Also, once set up, the workpiece is precisely located for all machines that use the system.

Two models are available, fitting any indexer with a 1.25"-dia center hole or larger. Two pins spaced 2.5" apart align the components, and a third prevents reverse mounting. The larger (standard) model has an 8"-dia piston that is 3/4" thick. It develops 3600-lb clamping pressure with normal 80-psi shop air, but can handle much more pressure if available. The smaller unit, with 6"-dia piston, develops 1800 lb force. An important feature, you can make your own fixtures.

Modular fixturing for the fourth

Gordon Coope, applications engineer at Stevens Engineering Inc, Phoenix, AZ, tells T&P, "The increased popularity of both subplates and fourth-axis rotary tables has led to tooling that improves overall productivity of both types of accessories. For instance, you can avoid constant alignment of headstock and tail-stock. A fourth-axis unit is mounted once to an adapter plate, then that assembly is mounted on the machine using cap screws and pull dowels to ensure repeatable location.

"Modular tooling also serves on fourth-axis faceplates and for fixturing that mounts between the head-stock and tailstock. Components such as modular vises and other accessories that have been used in modular applications previously are now being used on the fourth axis."

SMW Systems Inc, Santa Fe Springs, CA, introduced high-performance electric-servo-driven indexers at IMTS 94. Model ET-200 offers high rigidity, high accuracy, and heavy-duty brakes. A large-diameter spindle mounted in preloaded anti-friction bearings helps provide good rigidity under heavy machining loads. Face-gear coupling results in 5-arc-sec accuracy.

Troyke Mfg Co, Cincinnati, OH, provides special rotary tables including horizontal-vertical models in sizes from 9" dia to 30" dia. DMT-Cross slide tables offer up to 15 sq-in working surface and cross-slide accuracy within 0.001".

Buck-Matsumoto, Logansport, IN, has introduced a new line of economy and high-precision rotary tables that includes models with tilt tables and dynamic grip-force gages. The gages check the grip force of three-jaw chucks, ensuring that proper forces are set for today's high-speed lathe operations.

Horizontal workholding

Royal Machine and Tool Corp, Berlin, CT, provides Quad and Dual Vertical Combination chucks for holding up to 16 parts (QVC) or eight parts (DVC) on HMCs. The firm customizes the bases for individual machine tools, and the chucks permit virtually continuous spindle use with greatly reduced toolchanges and almost no downtime for parts changing. Master jaws on the QVC and DVC can be set in either a self-centering or compensating mode.

The firm also offers multiple hydraulic pallet fixtures that hold several large workpieces on tombstone-like structures mounted on indexing tables. Systems employ dedicated fixtures built to customer requirements. Standard hydraulic manifolds reduce chip traps, and automatic locating and clamping sequences employ air or spring operation for retraction.

Vise systems

Modern fixturing brings new design and status to the simple vise. For example, Interlen Products Corp, Encinitas, CA, introduced a modular tombstone assembly (PTA) that allows machinists to take advantage of a tombstone with the setup speed of a pallet system. The MTA can be vise mounted and used on a horizontal machining center, or side mounted in the vise and used on a VMC.

The key word is vise, and the Interlen line includes a Vise-Mount[TM] pallet system with a clamp receiver that goes between vise jaws. When the operator tightens the vise, the action pulls the receiver down and mates it with the top of the vise (not the jaws) to provide a rigid working surface.

Self-centering vises have become vital to modern workholding systems. Kurt Industrial Products Div, Kurt Mfg Co, Minneapolis, MN, has introduced the SCD600 self-centering vise for holding castings, forgings, flame cuttings, or other rough-surface parts. Both jaws of the vise move simultaneously as the screw rotates. Working on the principle of a three-jaw chuck, it centers the part accurately, regardless of workpiece size. Center-position accuracy from minimum to maximum opening of the jaws is 0.0006". Repeatability of centering same-size parts is 0.0002". Although the centerline of the jaw relative to the centerline of the key-way has been preset at the factory within 0.005", an operator can make a 1/8" adjustment to match all centerlines when mounting several vises on one machine table.

Kesel NC high-pressure hydraulic vises from JRM International Inc, Rockford, IL, will integrate into NC systems. Available as manual-hydraulic or pure hydraulic units with jaw widths from 3.5" to 6", the vises boast fixed-jaw deflection of less than 0.000 080" throughout the clamping range. The 6" vise provides clamping pressure up to 11,250 lb. A patented tie-bar system prevents the fixed jaw and base from deforming, even at maximum clamping pressure. The spindle is designed so that repeat-ability is [+ or -]0.0002" no matter who operates the vise. Other Kesel products include hydraulic self-centering vises, self-leveling vises, extended-opening models, double vises, compensating jaws, modular vises, and hydraulic pumping units.

Chick Machine Tool Inc, Warrendale, PA, offers more than vises. The full line includes M-System workholding systems for CNC machining centers. Each starts with a light, compact high-strength aluminum base, then adds cover plates, QwikChange[TM] machinable fixture jaws, clamps, and other items to make up a complete workholding system.

Airlox[TM] Production Products, Queensbury, NY, introduced a pneumatic vise with 20" maximum jaw opening and very high holding forces of 90 times air-line pressure, standard, or 120 times as an option. President Craig Seeley notes, "That's 6 tons of holding force from 100-psi shop air." Model S-13-20 can integrate with CNC machines via an M code for automated use.

The Pivot workstop, from Bock Workholding Inc, Pittsburgh, PA, mounts on a vise or machine table with a single screw and does not need to be removed when machining operations are performed. Its swing arm allows it to move out of the way to allow machining on all available sides of the work-piece. For repeat setups, the Pivot can swing back into place with a high degree of accuracy. Three-axis positioning of the unit allows for rapid setups and high repeat accuracy.

Clamps - manual and powered

Clamps complete the bottom line in many systems. Even with pallet changers, tombstones, etc, finally the workpiece must meet the fixture, and clamps most often hold it in place. There are so many! For instance, Carr Lane provides Swiftsure swing clamps, air or hydraulically powered units for flexible fixturing systems. The clamps typically produce 495 lb of force at 5000 psi. Other products include tooling blocks, hydraulic vises, vise jaws, and ball-lock mounting systems.

In its broad line of workholding devices, De-Sta-Co, Troy, MI, includes pneumatic swing cylinder clamps that sense open and closed position. Models 8200 and 8300 provide the safety factor of knowing exactly when the clamps are fully open and fully closed. Low-voltage Hall-effect switches mount in a groove in the side of the cylinder and are actuated by a magnetic piston ring as it moves up and down. A programmable logic controller (PLC) then reads the open and closed condition of the switches. The fast-acting clamps produce up to 160 lb (73 kg) of clamping force from shop air pressure. Inlet air pressure can be as low as 30 psig, or up to 120 psig maximum. A four-way valve controls the clamp.

Fixture building can be easy and cost effective with low-profile Loc-Down mounting-bar systems, according to Technical Tooling Inc, Hendersonville, NC, source of Kopal[R] tooling. In fact,the firm claims you can create dedicated fixtures with modular versatility. For both vertical and horizontal setups, the clamps mount on precision-ground bars, making it possible to clamp single and multiple workpieces in a wide range of sizes. Operators can secure parts of any length by simply spacing the clamps on the bars every 10".

For dedicated fixtures, mount the clamps directly to a tombstone or a subplate using standard bolts and T-nuts. The clamp system works safely with bar stock, machined surfaces, and raw castings with draft angles, according to the manufacturer. It holds workpieces securely, positioning them with repeat-ability of 0.0004", eliminating the need for additional jig and fixture requirements.

The Ball-Unit gripper assembly from Fairlane Products Inc, Fraser, MI, is a workholding device that houses a replaceable gripper and swivels to self-adjust to any draft angle or irregular shape up to 10 deg. A complete assembly consists of a stainless steel ball with a pocket area to accept the gripper, and a socket-head cap screw to hold the gripper in place. The gripper itself can be high-speed steel, carbide tipped, or solid carbide. Gripper sizes range from 3/8" to 3/4". An O-ring facilitates swiveling action and keeps contaminants out.

Gas springs normally used in press dies can also serve for workholding. Dadco, Detroit, MI, makes this suggestion for its nitrogen gas springs. In fact, new 3/4"-dia and 1"-dia models work in place of conventional springs in many job-shop applications. The springs come in preset-force models from 50 lb to 450 lb on contact, with stroke lengths ranging up to 80mm.

Another way to provide clamping action is to use shape-memory alloys (SMAs). As explained by Charles Whitehead, president of Linear Motion Industries Inc (LMD, Birmingham, AL, the shape-memory clamp is based on the 30-year-old idea of using alloys that respond to temperature changes and mechanical deformation. He tells T&P, "After a low-force distortion, a heated SMA will return to its original shape and generate tremendous forces while remembering to do so. Among the several means of actuation, electrical-resistance heating seems to have application in workholding. The advantages will be simplicity of control designs."

Indeed, Mr Whitehead proposes hydraulic clamps with SMA actuation for agile workholding. Systems could entail direct electrical operations of clamps, or use of an SMA-driven booster riding on board a pallet. An electrical signal to the shape-memory device would actuate a booster pump, which, in turn, would actuate several hydraulic clamps or vises on the pallet. Fascinating? Yes, but only in the research stage at present.

Hydraulic systems

Is hydraulic workholding becoming more common? Where? Tom Campbell, production automation marketing manager, Enerpac, Butler, WI, tells T&P, "It's becoming more widely recognized for its reliability and overall benefits for in-creased efficiency and reduced downtime.

"Along with growing awareness has come a broadened scope of applications. Effective workholding has long been practical in the automotive and aviation industries, but now you'll see applications in medical-equipment and electronics-testing fields, plus a number of other industries. Regardless of the industries involved, the more widespread use of hydraulic workholding is bringing the common desired results of increased profit and overall efficiency."

Where's hydraulic technology going? Mr Campbell says, "We're seeing a trend toward manifold mounting of cylinders, which brings a cleaner environment and greater overall efficiency. Through manifold mounting, hydraulic flow passes through internal, unexposed drilled passages in the fixture block to the activating side of the cylinder.

"Machine tools with palletized designs represent another trend. To accommodate, we have a series of positive-locking cylinders. These combine the nondegrading locking forces of mechanical clamps with the speed and consistent high pressures available with hydraulic clamps. The cylinders are hydraulically activated and mechanically locked, so that, when the hydraulic connections are removed, the clamps can't be loosened. Thus, positive locking over a long period is ensured.

"Other developments include automatic couplers for FMS systems - and Enerpac's introduction of an activator wand and FMS booster package to enhance operating performance and economy two ways: reducing the prospects for external contaminants penetrating the circuit, and eliminating the most common source of FMS system hydraulic fluid loss.

"A development that corresponds with the greater variety of workholding applications is the availability of small, reliable, cost-efficient machine-side power units, such as our new Turbo pumps with restart capability to maintain desired circuit pressures."