September 2011 / Volume 63 / Issue 9|
By Kip Hanson
|High-density workholding can squeeze more productivity from your machining center. |
Courtesy of Mitee-Bite
Since the dawn of CNC milling, machinists have been trying to squeeze more parts onto the table and position them quicker. That’s because to a machinist, it’s all about reducing setup time and cranking out more parts per hour.
There are many ways to achieve this, but for shops still relying on the old tried, true and blue 6" machinist vise for workholding, the only option is to cram as many vises on the table as possible. On a typical 20"×40" machine, this means a maximum of four vises. For small lot sizes, this might be OK, but it’s not very productive. And while it’s possible to double up and clamp two smaller workpieces in a 6" vise, it’s not advisable because parts can come loose during machining and fly across the shop unexpectedly if there’s variability in part size. The good news is there are better ways of gripping parts.Twice the Vise
Twin vises have been around for a long time, and many companies make them. The various offerings allow a user to safely hold two parts per vise using hardened and ground step jaws for first-operation work, or machinable aluminum top jaws for secondary operations. These vises cost about twice as much as an old-fashioned machinist vise but are typically more accurate and have a smaller footprint. That enables placing up to half a dozen 6" twin vises on that 20"×40" machine, or 12 parts per cycle.
One company offering a unique twist on the twin-vise concept is Dapra Corp., Bloomfield, Conn., exclusive distributor of the German-made Allmatic Duo vise. Matt Milhomens, inside technical sales at Dapra, said, “We’re focused on supplying high-precision, quality vises to the machine tool industry, and offerng something different than other twin vises. With the Duo, the outer jaws are fixed, and the two center jaws move toward the outside.” And because one jaw makes contact first, the operator doesn’t have to attempt holding two parts in the vise while simultaneously cranking it shut—unless you have three hands, this is never an easy task.
Milhomens added that because of an integral pressure-intensification device, the Duo has enhanced clamping capabilities. Using a system of wedges, rollers and Belleville springs, the vise can provide more than 13,000 lbs. of clamping force. In the event of part distortion due to excessive clamping pressure, the system allows the operator to reduce the pressure to the required level with a simple adjustment.
For horizontal machining centers, Allmatic offers the monoblock multiple clamping system, a four- or eight-position tombstone with the same pressure intensification technology as the Duo. And, like the Duo, the system is available in standard jaw widths of 3½" and 5"; specials can be ordered.
Rich Tatem, service manager at Dapra, said, “They’re casting the base of the four vises in one shot for greater rigidity and accuracy than mounting four separate vises on a tombstone.”
Another company offering multiple vises is Schunk Inc., Raleigh, N.C. Brad Evans, Schunk’s product manager for workholding, noted that the company’s KONTEC KSM multiple clamping system is suitable for a variety of high-density workholding applications. The system is available in 2½" and 3½" widths and lengths up to 25". It features one fixed jaw and can accommodate multiple movable jaws, allowing six or more parts per vise.
“The KSM rail vise is an adjustable, multiple-position vise, using step or soft jaws,” Evans said. “It has a unique internal clamping mechanism. Because the serrations are inside the rail, by simply loosening a jaw, it can be moved smoothly and easily.”
Courtesy of Dapra
Another Schunk offering uses tandem clamping force blocks. “Tandem means both jaws travel in synch,” Evans said. “They’re self-centering, with a footprint starting at 2½" square. This means a 4" vise has a 4" footprint, so you can put up to 16 vises on a 16" pallet.”
Schunk also offers a variety of clamping mechanisms on its vises: manual, hydraulic and pneumatic, as well as a spring-clamp device with air release. “It works like air brakes on a tractor-trailer, and it takes only a fraction of a second to clamp/unclamp the vise,” Evans said.Zero Point
OK, all these vises are neat, and they certainly enable putting more parts under the spindle, but there’s more to the story. What about changeover? After all, a shop might increase workholding density enough to load 20 or 30 pieces per machine cycle, but if the machine is sitting idle while the operator loads and unloads parts, what has the shop gained? A machine equipped with a pallet changer at least avoids lost time because of part changeover, but still there’s fixture setup time to consider. This is one reason why companies developed “zero-point” locating systems.
With a zero-point locating system, machine shops can cost-effectively palletize workholders, reducing setup time and eliminating the costly time to change workpieces. As its name implies, a zero-point locating system defines a hard zero position for workholding. This is typically accomplished via a set of fixtures or pallet-mounted locating pins that lock into machine-mounted stationary bushings.
Schunk’s version is the Vero-S quick-change pallet system. “Vero-S is the foundation for quick changeover, with multiple parts per setup, closer together and more to a pallet,” Evans said. “It offers locational repeatability of 0.0002" and pallet load times of under a minute. All this means you can load parts offline, significantly increasing throughput.”
Courtesy of Schunk
Most zero-point positioning systems are modular, so users can purchase a complete custom system or just the components that best fit their needs. “The shops that build their own systems vs. buying off-the-shelf products is split about 50-50,” Evans said. “Some shops, especially moldmakers, will install our locating pins in the base of the tool. The Vero-S allows them to bring the mold back 6 months later for rework and locate it in the machine within 5 microns of the original position.”
These systems aren’t cheap. Each Vero-S pallet or fixture requires two or more positioning pins at $100 a pop, while the mating receivers—the business end of the system—start at $1,000 each.
But Evans defends the investment. “We’re doing workholding projects costing anywhere from $50,000 to $100,000 where customers are seeing payback in less than a year. And with our rail systems and other workholding products, we have saved ‘buku bucks’ for customers like Joe Gibbs Racing and ESAB Welding.”
Another company offering a zero-point system is BIG Kaiser Precision Tooling Inc., Hoffman Estates, Ill. Gerard Vacio, product manager, workholding systems at BIG Kaiser, said, “With our Unilock system, the operator has a much easier job loading parts into the fixture as he can take the entire fixture off the tombstone rather than having to stretch and bend over.”
Courtesy of Schunk
The Unilock can assist when other equipment is lacking. Vacio said: “A lot of people use our tooling when they don’t have dedicated pallet changers. For example, most shops will build two identical fixtures, then load up as many parts in those as they can. Unilock allows the fixture swap to occur in seconds, helping to keep the door closed and the machine running. High-density machining is the most common area where people use our product.”
Vacio added, “We see people using a pallet maybe 8" wide and 24" long with a whole bunch of parts loaded on it, and with less than $100 worth of quick-change hardware on the bottom of that fixture plate.” Adding a pair of Unilock chucks creates a simple manual pallet system capable of holding various workpieces for less than $5,000. To start smaller than that, BIG Kaiser also offers a starter kit with one chuck and a pair of 4", 5" or 6" pallets. So for a couple grand, it’s possible to create a quick-change pallet system for a VMC or HMC or even for mounting on a lathe.High-Density Move
Sounds nifty, but how do you grab that “whole bunch of parts” anyway? You certainly won’t be using 6" vises on a pallet of this size. While some shops might opt for dedicated fixturing using conventional locating pins, toe clamps and toggles, a better option is compact tooling designed for high-density workholding.
Courtesy of Mitee-Bite
One company that specializes in tooling for high-density workholding is Mitee-Bite Products Co., Center Ossipee, N.H. Dave Bishop, general manager of Mitee-Bite, said, “By effectively utilizing the work envelope and the machine’s functionality, some customers have increased output by more than 200 percent. Our methods are very simple and that’s a big draw.”
One example of this simplicity is Mitee-Bite’s cam-action fixture clamp, which is constructed of two components: an offset-head cap screw and a hexagonal washer (see photo on page 70). Starting at 5⁄16" across and less than 1⁄8" high, the clamp boasts more than 200 lbs. of clamping force; the largest of this series offers 3 tons of clamping power, all in a device the size of a pecan.
According to Bishop, there are many benefits to marrying high-density workholding with a quick-change pallet system, including reduced setup and tool change times per part; longer cycle times, which allow operators to do more-profitable tasks than feed a machine tool; additional machine capacity through faster job completion; and lowered part cost. His customers agree. D.J. Paulson, general manager of Straitline Components, a bicycle component manufacturer in British Columbia, said Straitline has been using Mitee-Bite for 4 years for high-density workholding on HMCs.
Courtesy of Mitee-Bite
Paulson said: “We needed to keep the machine running for as long as possible without interruption. The other criteria was that we needed the clamp to be as low profile as possible and still be able to apply positive down force while holding on to less than 0.090" of stock. Using a conventional vise setup, we had to severely limit the cutting parameters to reduce the risk of parts being thrown from the vises. Because of the rigidity of the clamping setup, our machining parameters are now extremely aggressive and this, in turn, has decreased cycle times, improved tool life and surface finish and, most importantly, increased profit.”
Sounds great, but it’s not all sunshine and flowers. One drawback to this type of system is, compared to a vise, shops must design and build their own fixtures and probably need more of them. The good news is most workholding suppliers offer CAD drawings of their tooling components, so it’s a fairly straightforward process to incorporate them into a fixture design.
Bishop noted that another drawback to high-density workholding is an unattended machine might cut air for a long time when a cutter breaks. And, Bishop warned, inspectors might not like doing first-piece inspection on 100 parts.A Different Way
To overcome the expense of building fixtures, Raptor Workholding Products designed a workholder line that doesn’t need them. Brian O’Rell, president of the Simi Valley, Calif., company described Raptor’s unique workholder as one born of necessity. “The original design was for the Matsuura MAM72-35V with a 5" pallet,” he said. “When I first bought the MAM, I didn’t have anything to hold my parts.” To address this problem, O’Rell designed and built the first prototype of the Raptor, which resembles something more suited to a CNC lathe than a machining center. “It’s all about grabbing a part and doing as much work as possible before letting it go,” he said about its unique shape.
Courtesy of Mitee-Bite
Using the Raptor requires first prepping workpieces by milling a dovetail shape into the bottom of each part blank. This dovetail fits into the Raptor’s jaws, which, according to O’Rell, gives tremendous gripping power without distortion while holding as little as 0.046" of material. “One customer used two of our RWP-001S units to hold a 386-lb. block of aluminum and machined the whole thing in one operation, but I don’t necessarily recommend that,” he said with a laugh.
“Our product fits a different need, one where you wish to run different parts, and all of them are prepped the same way,” O’Rell said. “We serve a variety of customers, but particularly those with 4- and 5-axis machines.”
Courtesy of Raptor Workholding Products
O’Rell knows his product works, because he and Raptor partner Dave Fisher of S&H Machine Inc. in nearby Burbank use it in their own shops. O’Rell said: “It doesn’t matter to me whether I’m cutting aluminum or stainless, I just prep it all the same way. Over the weekend, we ran some 15-5 stainless, some ¾"-thick cover plates that were about 5"×7" long, and some cylinders that started out around 2"×4". We used the Raptor for everything. If we can’t run it on a dovetail, we don’t run it.”
Over the past 3 years, O’Rell and Fisher have built the original Raptor into a product line with various sizes and configurations, as well as the Raptor Nest and Raptor Tower, all of which can accept basically any type of workpiece—without the need for a fixture. “We have one customer who’s looking at using 30 pallets, each with a six-part Raptor Nest. Another customer, which manufactures military products, is using our RWP-4111. That allows them to run one long workpiece or maybe six to eight smaller parts.”
If faced with high-volume machining of smaller workpieces, consider ditching that old 6" vise in favor of a workholder that maximizes a machining center’s capacity. This means not only fitting more parts on the table, but loading quickly to keep that spindle turning. “Your saws might not be able to keep up withproduction,” warned Bishop of Mitee-Bite. That sounds like a good problem to have. CTEAbout the Author: Kip Hanson is a manufacturing consultant and freelance writer. Contact him by phone at (520) 548-7328 or e-mail at firstname.lastname@example.org.
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