Excellence is the Standard
There are many ways to hold on to your parts
By Jim Lorincz
The objective of effective workholding is always to present a part or workpiece, regardless of size, configuration, and material construction, to the machining process—securely, repeatably, and accurately, without creating interference issues. All workholding devices aren't created equal, but there's a good chance that they are being manufactured to increasingly stringent standards to perform required functions whether in production applications, a stand-alone machine, or a cell.
Workholding chuck manufacturers have wrestled with setting standards for the better part of a decade and a half, and have succeeded in establishing important benchmarks for the US. "We promote standards so that there is uniformity in the procedures of using workholding devices, of mounting jaws, and of developing jaw-fit standards," explains Sid Roth, president, SMW Autoblok (Wheeling, IL).
"Standards affect the most important considerations of safety and mechanical issues, such as how devices fit on the spindle, and what kind of training is required in using, repairing, and maintaining chucks," Roth says. "There is already a great deal of standardization, particularly in jaw mountings, for example. For safety, mechanical fit, and repair, we give good instructions wherever we can, and refer the user to the manufacturer's instructions."
Increased demand for a particular workholding device is often tied to the growth in popularity of a machine type. "Large closed-center chucks, ranging from 32 to 80" [800–2000 mm] in diam, are enjoying a huge surge in demand because of the large number of VTLs that have come on the market," says Roth. "They are designed for turning large parts, and are enjoying increasing use in the energy and off-road industries for machining large internal components for wind turbines with both their milling and turning capabilities."
VTLs typically take chucks in sizes ranging from 24 to 40" (630–1000 mm) in diam in a variety of types, including pull-down chucks, quick-jaw-change chucks, and standard three-jaw and six-jaw chucks. Which chuck is used depends on the type of machining being done. Roth points to some of the considerations: "In machining with VTLs, gravity is your friend so a lot of times you don't need the pull-down chuck. But where the user requires a high degree of parallelism between one face of the part and another face, the pull-down chuck jaws will grab the part and actively pull it down against the locator to ensure parallelism."
For short-run machining where part changeovers are required, quick-jaw-change chucks are best. "With quickjaw-change chucks, changeover from one part to another can be accomplished as quickly as one minute," says Roth. "For long runs of the same or similar parts, the standard three-jaw or six-jaw chuck is the best choice."
Five-axis trunnion-style VMCs require positioning workpieces at almost any angle for machining in a single setup. Interference with workholding devices can defeat the benefits of five-axis machining, including accuracy and reduced setup time, because machines have to be stopped so that workpieces can be repositioned.
Padgett Machine Inc. (Tulsa) is using the Kurt Low Profile five-axis vise from Kurt Manufacturing Co. (Minneapolis) on its trunnion-style five-axis VMCs from Haas Automation Inc. (Oxnard, CA) for machining complex aerospace and military components. Ed Padgett, president and CEO, explains: "There was always something in the way to restrict machining on all sides of the part in just one setup. With the Kurt Low Profile five-axis vise, the machine can hold and rotate the part a full 360°, and machine it on four facets in the same setup. It allows the machine to rotate and manipulate the part a positive 90° or negative 90° to get in and around areas of the part that require machining."
Padgett Machine's five-axis work for aerospace and military applications includes ribbed parts for the MD11's leading edge wing assembly and a wide variety of precision parts for the current fleet of B52 bombers under the government's continual maintenance program. These parts are machined from 4130 aircraft steel alloy, 7075 aluminum alloy, and 7050 high-fracture-resistant aluminum alloy. Most machined parts require substantial metal removal and close-tolerance drilling and boring operations necessitating rigid workholding.
The usual method using conventional vises would be to position the unmachined material as far down in the viseclamping station as possible to get the tightest "grip" on the broadest surface area of the part to make the most aggressive and fastest cuts possible. That method is satisfactory (taking into account the need to stop the machine and reposition the part) except that a trunnion machine spindle has restricted access to the deepest jaw areas of conventional vises and multiple facets of the material to be machined.
In contrast, the new five-axis Kurt vise elevates the part material so there is clear spindle access, while part clamping is done with no loss of rigidity. In fact, the actual clamping force with this new vise is at least 20% greater than that attained by conventional 6" (152.4-mm) vises. "Looking at the vise for the first time, you wouldn't think this is true, but the vise holds the part in an elevated position very rigidly, so machining deep pockets at many angles is fast and easy in one setup. We save setup time, plus it allows us to run the machine at top speed so we gain three ways—faster throughput, less setup time, and reduced machine downtime," Padgett says.
"Abbott Workholding Products [Manhattan, KS] basically has two main categories of products, which are quite different: chuck jaws for turning machines, and tooling columns and fixturing for HMCs," explains Doug Reed. "Chuck jaws are categorized as perishable tooling, because they are continuously consumed and reordered. Tooling columns fall into the category of capital equipment for tooling up HMCs."
Abbott manufactures more than 4000 aluminum, steel, and cast-iron straight and Pie Jaw chuck jaws, as well as master plates, segments, tooling columns, sub-plates, and a variety of accessories. Developed over 50 years ago, Abbott Pie Jaw chuck jaws can be used in place of rectangular jaws in more than 75% of all machine tool applications. They are made of 319 cast aluminum, cast iron, and steel. All tooling columns are made of A713 (Tenzaloy) aged to T-6 condition.
Abbott has noticed an increased market demand for custom jaws and special fixtures. "Our customers are looking for customized turnkey workholding solutions. They are looking for specific designs and coming to us to build them for them. We have a full engineering department and our own foundry, so that we can have some products cast if needed," says Reed.
Abbott Workholding's Master Plate, which is an alternative to quick-change chucks, will soon be available in a 8-12" (203–279-mm) diam version, with a more accurate mounting configuration for the segments and the master plate. Master Plates are currently available in 18 to 60" (457–1524-mm) diam. The Master Plate is the section that mounts to the chuck just as a chuck jaw would. On the opposite side, there is a mounting pattern for segments that target the specific diam range that you want to hold.
"Typically if you wanted to hold a part 40" [1016-mm] in diam, you would have to have a 40" Pie Jaw chuck jaw. The Master Plate allows you to target a range of sizes, and put the plate on a segment that can handle diam from 36 to 40" [914–1016-mm] range. All the interior material is useless to you, so you have to machine it away. The segment is already a certain ID and OD range for you, so it minimizes the amount of machining [and cost] in getting to the size range you are interested in. You don't have to pay for the entire wedge," Reed explains.
Vektek Inc. (Emporia, KS) manufactures a standardized set of components that are built into engineered workholding solutions for a variety of metalworking industries that require precisely cut parts. Vektek collaborates with fixture designers and users in the automotive and transportation, marine, power generation, medical, energy, aerospace, and defense industries to produce workholding systems that are capable of cutting parts to some of the worlds most stringent specifications. Hydraulic and pneumatic power clamping devices include swing, link, and edge clamps, work support, cylinders, and pallet fixtures accessories.
"Hydraulic clamping fixtures are used to hold any type of metal part that is being cut by a machine tool, typically prismatic parts that remain stationary while the machining takes place, as in milling," explains Rod Nelson, Vektek's vice president of International Sales.
"The industry is trending more toward manifoldmounted devices with gundrilled porting and flange-mounted devices," says Nelson. "We make both cartridge (thread in) devices to mount in threaded holes and flange mounted devices which mount with cap screws. It's much easier and less costly to drill a hole and mount the device over the top of a ported hole with a cap screw than to make a large complex mounting cavity," adds Nelson.
Vektek has improved its work-support sleeve designs and will introduce a double-acting Tuff Grip work support early this year. "The dramatic benefit of overcoming spring-related problems through positive hydraulic extension and retraction is revolutionary to the industry," says Nelson.
At IMTS, Vektek exhibited its in-port flow-control timing device. This device controls how individual swing and link clamps are actuated to contact the workpiece. "Problems, caused by the varying lengths of tubing connecting the devices, are virtually eliminated. A small adjustment on each of the flow controls allows clamps to be timed, so that individual devices are in sync with one another and are actuated simultaneously," says Nelson.
In the past, manufacturers were limited in their abilities to combine multi-sided machining of large parts while maintaining sufficient workholding forces, all in the same operation. When manufacturers need to machine all six sides of a cube-shaped part, more than two operations would generally be required. This is due to the need to place workholding devices around the perimeter of the part to secure it, which limits the machine access to features where clamps are present.
To solve this problem, Innovative Technologies Unlimited (ITU; Clarkston, MI) has designed in-hole and through-hole workholding devices, which are said to replace all types of perimeter workholding devices such as swing clamps, edge clamps, and slide clamps. ITU devices work through the fixture, and from behind the part, drawing it back against the fixture. This arrangement allows the machine access to features on the other five sides of the part.
ITU clamps replace bulky and intrusive swing clamps with the in-hole Diametrical Wall Clamp (DWC) or the through-hole Edge Gripping Clamp (EGC). The DWC works in blind-hole workholding applications by expanding grippers inside of a pre-manufactured diameter. The workpiece is lowered, manually or through automation, over the DWC and onto the locating surface. The DWC is then activated, either pneumatically or hydraulically, and the arbor expands to make contact with the wall of the diameter. As the arbor expands the grippers against the walls of the diameter, the part is also pulled down tight against the locating surface, ensuring good part location. The part is now secured into the fixture and ready for the manufacturing process. The EGC model works in much the same way as the (DWC), except that the clamps actually extend through the pre-manufactured part diameters. Once loaded the clamps are activated. They expand and pull down, gripping the backside of the manufacturing diameter and locking the part into the fixture.
Hold On To Your Work
Choosing the right clamping solution to hold your work is trickier than ever. Here are some tips from Emuge Corp.'s Workholding Div. (West Boylston, MA) that may lead you to either a ready-made or customized workholding solution.
Establish open communications. Be sure your vendor has an open ear beginning early on in the quotation process. It is critical that they understand the requirements for your entire operation. Some of this information may prove to be essential to the success of your workholding system.
Supply your vendor with complete operation specifications. Keep the vendor informed of any changes to these specs, even if you feel these modifications are not workholding- related. "When we stay close to our customer, we are usually kept informed without even asking. However, even when we are receiving timely updates, changes to the original specs may lead to some added cost or a delay in project delivery, because the design or build process may be too far along," explains Emuge's Dave Jones.
Thin-wall parts are a challenge. Operations with high transferable torque values on thin-wall parts can be met with Emuge's System SG (based on a series of short tapers), which spreads the clamping forces over the largest possible area of the workpiece.
Ensure all areas of your system are accurate. Not only should your new clamping system be precise, but look at your production environment and tooling, and make sure these areas are suitable for your application.
A clean workpiece yields the best results. The accuracy of the locating and clamping surfaces on the workpiece should also be considered. Make sure that clamping areas are free of debris build-up.
Consider the volume of work to be performed. If work volume is planned to be high, then a mechanical workholding solution can be the answer. In the case where your workpiece has a challenging bore-to-length ratio, a hydraulic system may be an effective alternative.
Design a solution with the future in mind, too. Your workpiece may be one of a series or family of parts. Properly designed, a workholding design may need relatively minor changes to accommodate future workpieces.
This article was first published in the February 2009 edition of Manufacturing Engineering magazine.