Good news! They're rebuilding the raft lines in America, and magnets, switched on and off by a 400-V DC electric current, are helping to do the job. We checked out a system at Cleveland Track Material Inc, Cleveland, OH. No question about its workability and safety. The firm is taking deep cuts in track plates held securely by a Quadsystem QS Series electroper-magnent-magnetic chuck from Tecnomagnete Inc, Morrisville, NC.
Cleveland Track's Chris Witham, in charge of automation and systems upgrade, tells T&P, "We're using a Sundstrand mill with 50-hp spindle motor to cut slots 5 9/16[inches] to 6 3/8[inches] wide using at least 40 hp. Carbide cutting inserts with positive geometry work at feedrates of about 32 ipm, with depth of cut ranging from 1/4[inch] to 1/2[inch]. The track plates are 8[inches] wide x 1[inch] thick x 20[inches] long, and the workpiece material is A36 plate steel at 220 to 280 Brinell hardness.
"Formerly, the operator could load only one plate at a time on a rather cumbersome hydraulic clamping system. Now, the magnetic chuck accepts five workpieces in one load. We simply put the plates on the chuck surface, line them up with a back gage, and turn on the switching power for a few seconds.
"After we turn off the chuck's power supply, we can even disconnect the electric cable to the chuck if we want. The workpieces will stay in place until we power up the chuck again for a few seconds."
In the future, Cleveland Track plans to use several magnetic chucks in a long line to mill 20-ft to 40-ft lengths of T-rail (railway) tracks. Present techniques bolt and clamp the T-rail on manual planing and milling machines. H James Andrews, VP manufacturing, tells T&P, "Soon, we will retrofit a CNC unit on the Sundstrand mill and reduce our need for planing, which is a slower process.
"For side milling, we often reduce one end of the T-rail to a knife edge for use in railroad switchgear. Presently, the rail must be clamped at a precise angle on the long table, so that a straight milling cut will do the job. We use a laser for lining up the rail, and accuracies over the entire length must be held within 1/32[inch] total tolerance. Planing the top of the rail, of course, is simpler."
Mr Andrews continues, "In either setup, top or side, with CNC and magnetic chucks, there will be almost no fixturing; the control will provide the taper cut when needed. For side milling, the magnetic chuck will also hold a support guide on one side of the T-rail, but the other side will be free of clamps for unobstructed machining.
"Looking at the original cost of hydraulics and the further cost of upkeep and maintaining hydraulic cylinders, we expect the magnetic chucks to pay off in a short time."
Referring to the past, Mr Andrews says, "It's important to note that the chucks we use now are a new breed. We tried magnetic chucks 20 years ago with poor results. They were bulky and hard to control, and they suffered from power failures and limited holding power. If the part had any canter, it was difficult to hold. Today's magnetic chucks overcome this problem, pulling the workpiece down fiat to the surface. The magnetic chucks have more than enough force to hold the parts securely for milling."
In other applications, a magnetic chuck can hold a ring-shaped part without any fixturing - and without distortion of the workpiece. Tooled with simple pole shoes and other modular components, magnetic chucks can hold odd-shaped parts. Pole pieces or fixed extensions can hold plates above the surface of the chuck table for drilling through-holes. Also, mobile extensions (sometimes called floating or self-shimming poles) hold warped parts more securely than conventional tooling systems.
On the other hand, the magnets can flatten some warped parts for better machining. In many cases, workpieces held by magnets are less subject to vibration, and thus able to accept full power for milling or other machining operations with carbide tools that are sensitive to chatter. Magnetic chucks can present five surfaces for machining, and Quadsystem cubes serve on machining centers and pallet changers.
Of course, there are limitations. Magnets won't hold brass, aluminum, and other nonferrous metals. And not all ferrous metals have the same reaction to magnetic force. Certain alloys may not receive a strong pull. And the work-piece must be thick enough to get full attraction.
For workpieces that are not good magnetic candidates, consider using steel subplates with conventional clamps. You can set them up off the machine table, then change the plates quickly on the magnetic chuck. Use simple guide posts or brackets to line up the plates for good repeatability.
For grinding, O S Walker, Worchester, MA, offers a line of permanent-magnet chucks including models with the Electroperm feature, which provides fail safe holding by using momentary pulses of current to energize or deenergize the magnets. Because there is no heat buildup, you can leave complicated setups on the chuck overnight.
Welding is changing the nature of modular fixturing. Now we see tables with alternating threaded and dowel-pin holes on large top surfaces as well as the sides. According to Robert W Ellig, president of Bluco Corp, Carol Stream, IL, "Conventional welding fixtures are not economically justifiable for very short production runs or prototype work. Here, an operator may manually hold a component or fixture it with a bewildering arrangement of C-clamps, bar clamps, and framing squares on cast-iron platens. If there's time, he may build up a temporary fixture on a tack table."
Bluco suggests the Demmeler Modular Fixturing System as an alternative to either building expensive dedicated fixtures or assembling makeshift temporary types. The system consists of a welded 3D worktable that provides a sturdy platform for mounting a variety of standard angles, blocks, and other accessories in a manner similar to building modular tooling for machining.
Heavy ribbing ensures stability and flatness to within 0.0004 inch per foot. You can join the tables for larger fabricated assemblies. Mounting holes in a grid pattern are used to attach the various fixturing elements. The holes are 28 mm or 1.1[inches] in diameter and spaced 100 mm or 3.9[inches] apart across the face of the 3D worktable. The holes and grid pattern are accurate to within 0.0015 inch per foot and are included in all the structural elements furnished with the system.
A special positioning and clamping bolt attaches the fixturing elements and workpiece positioners to the worktables or to each other. The hardened bolts provide tons of clamping force while withstanding up to 25 tons of shear force. They precisely position and hold the individual elements. Fast-acting toggle and threaded clamps hold the pieces for welding.
You may have heard that new CNC machine tools driven by electric motors and ballscrews will run at higher cutting speeds and therefore exert less force against the workholding fixtures. That's an excuse to go to modular tooling instead of hard tooling.
However, there are a couple of holes in this argument. First, modular tooling can be very rigid, indeed. But, second, the new machines may serve for jobs that require higher precision than ever asked before. Today, however, we're seeing a third factor. Everything is so efficient that setup engineers put several fixtures on one table. This can boost productivity, but may lead to catastrophic weight problems. The electrical servodrive systems, especially on smaller machines, may not cope with the extra mass, which will slow them down, lead to early wear, and possibly cause inaccuracies if position feedback uses ballscrew-driven encoders.
Thus there's a need for lighter fixtures. However, because tooling cubes and special vises are popular, there's even more weight. You have the weight of the tombstone, the tooling subplate, clamps and angle brackets, and possibly a multistation vise and even a magnetic chuck.
What to do? Abbott Aluminum Inc, Manhattan, KS, offers aluminum tooling cubes, said to solve the weight problem. Also, some engineers say that the aluminum bends slightly during heavy cuts not enough to hinder accuracy, but just enough to make tools last longer.
Doug Marcoux, manufacturing engineer at Precision Shooting Equipment (bows and arrows), Tucson, AZ, speaks of custom aluminum extrusions and forgings. He notes that most conventional hydraulic clamps, fixtures, and modular-fixturing components are designed for castings, not for small precision parts. Using conventional off-the-shelf tooling, he holds 16 to 20 parts per side of an Abbott tooling cube. But using hard-tooled workholding fixtures designed in-house, he can hold up to 80 work-pieces of the same type and size.
With this kind of lead, weight is certainly an important factor, and the lightweight aluminum cubes help make the tooling successful. Furthermore, the machining is fast. Mr Marcoux uses a Hine balancer operating at 750 rpm to balance tools that will run at 25,000 rpm.
Gordon Coope of Stevens Engineering Inc, Phoenix, AZ, notes that his hollow Silo tooling columns meet the challenge of tooling for horizontal-spindle CNC machines. These require tooling structures optimized for stiffness, accuracy, and weight. The Silo column is basically a rigid framework for mounting either tooling plates or subplates to its faces. It's a tombstone with removable faces; faces quickly mount with two pull-dowels and a few cap screws.
Mid-State Machine Products, Winslow, ME, includes a special three-sided cube in its line of vertical and horizontal tombstones. Dropping a side, of course, cuts the weight. Also, the firm will core out the inside of columns to meet special requirements for low weight.
The computer can help put together any fixturing system, and is especially helpful for finding components in detailed modular systems. Almost every modular fixturing system is backed by a CAD system or computerized catalog.
Carr Lane Mfg Co, St Louis, MO, offers an extensive tooling library called the Tool Designer's Assistant[R]. It's a CAD database containing precise scale drawings of all the tooling items in the standard catalog. With more than 7000 components, it is said to be the most comprehensive standard library available from a tooling manufacturer.
Carr Lane can supply the tooling library in dozens of ready-to-use software formats, 3D and 2D, for virtually any PC, workstation, or mainframe system. Once installed, it helps save the expense and hours of building a standard library from scratch. Qu-Co, Union, OH, offers modular fixturing components, all presented in a CAD database designed to offer automated fixturing. It can help you select just the components you need to build a particular fixture without going overboard! It is user friendly and improves fixturing efficiency while reducing drafting time. Available in 2D and 3D wireframe images, the library has 675 predrawn modular-fixturing elements.
Presentations are full-scale drawings, and each element includes two to five views, depending on complexity. You can pull drawings from the computer to make your final drawing, calling components by Qu-Co catalog part numbers. Drawing files can be used with systems that include an IGES translator, and software is available in AUTOCAD and DXF versions.
Bluco Corp offers modular fixturing for short-run and prototype jobs. Modules suit both horizontal and vertical machining centers as well as CMMs, and a complete CAD database aids documentation of the four system sizes plus the new modular fixtures for welding.
Most modular systems employ alternating dowel pins and threaded holes to mount the components. For an alternative system using T-slot mounting, consider the CAD planning system for Halder modular jigs and fixtures from Flexible Fixturing Systems Inc, East Granby, CT. As the name implies, there is an added degree of flexibility in the ability to slide the mounting hardware to any point on the fixture plate for rigid clamping.
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|Author:||Miller, Paul C.|
|Publication:||Tooling & Production|
|Date:||May 1, 1995|
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