High-density workholding can squeeze more productivity from your machining center

Courtesy of Mitee-Bite

Clamping multiple parts on a 5th axis with Mitee-Bite cam-action fixture clamp.

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

An Allmatic eight-position monoblock tombstone from Dapra Corp.

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

Machining multiple aluminum parts at Joe Gibbs Racing using KSM-6 rail vises from 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

Schunk’s Vero-S quick-change pallet system.

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.