Zeroing in on Workholding

The path to better setups can be a zero-sum game; not all systems are created equally

Did you lose your "setup guy," or come to the realization that setups are costing you too much? Maybe you have the nagging sense you’re not properly accounting for setup, and, in any case, you know you need to cut costs.

According to John Zaya, a product specialist at Big Daishowa Inc. (formerly Big Kaiser) in Hoffman Estates, Illinois, these are the common motivations for considering a switch from a generic vise to zero-point workholding. The magic of this approach is that because the workholding repeats its position from part to part, it eliminates the need to probe or otherwise dial-in each setup. But how should you approach this, especially if your shop—like most—is relatively low volume, high mix?

Zero-point workholding with a pull stud base has been around more than 30 years. And while the size and shape of the studs, as well as the distances between them, have been standardized across industry, not all such systems are created equal. Although the quoted accuracy and repeatability of these systems are curiously consistent from vendor to vendor, there are differences in build quality, clamping force, and the degree of automation.

The majority of zero-point clamping systems, including Big Daishowa's UNILOCK, Schunk GmbH's Vero-S, and the systems from SMW Autoblok Corp. and Erowa Technology Inc., use a spring-loaded rod (or several in some cases) to push against the pull stud to lock it and pneumatic pressure to release. This has the advantage of enabling the machine to secure the base with the touch of a button, or even automatically; the disadvantage is the technology requires compressed air lines in the work zone.

One notable exception to automatic pull stud clamping is the DynoLock base from Mate Precision Technologies in Anoka, Minn., a relatively new entry to the field. Securing a vise to a DynoLock base requires manually turning a 6-mm hex wrench, ideally to 20 Nm of torque, explained Vice President Frank Baeumler. Each DynoLock base captures four pull studs (itself unusual), and the mechanism is unique in that it grabs each stud with a yoke that contacts half its circumference and pulls it toward the center.

Mate has more than 60 years of experience making precision fabrication tooling, and the attendant machining of "tool steels, high alloy steels, and exotics," Baeumler noted. But, he added, the company didn't have preconceived notions of how to build zero-point tooling and didn't think the usual approach made much sense.

Pushing against the pull stud with a rod or wedge limits the amount of contact area, Baeumler asserted, to "a fraction of what we achieve with a yoke around it. We also thought such a system should be self centering, but we’re the only company that pulls from the exterior diameter of the pull studs," he continued. "Everything in our approach drives to the center. That's how we get awesome accuracy and repeatability."

How accurate and repeatable? The DynoLock base boasts top tool on center accuracy of ±13 µm with repeatability of 5 µm. And it includes a precision ground center hole, making it easy for a machine probe to locate the base.

These are excellent numbers. But competing systems claim the same, or similar, results. Schunk cites 5 µm repeatability for the Vero S; Erowa lists 3 µm for its MTS2.0. You’d have to consider the long-term reliability of the different approaches (e.g., how well they seal themselves against swarf, how well they integrate with other fixtures, and automation considerations), to be confident of any decision.

It's difficult to make clear distinctions regarding clamping force. Mate said the force required to separate the top tool from the base is greater than 22 kN in its 52 system (52 mm being the distance between the centers of the pull studs) and 26 kN in the 96 base. Schunk cited a pull down force of 8 kN for the Vero S with normal spring clamping. But the company, like other "air-to-open" suppliers, also offers a "turbo" function that uses pneumatic pressure to increase the clamping force. In this case, turbo is said to deliver 28 kN.

You can also use more than one base to hold down a given fixture. In fact, Zaya said, "probably 75 to 80 percent of our sales…have two or more chucks" (Big Daishowa's term for the base). This, of course, requires multiple knobs on the fixture, and "the knobs serve different purposes. The first, primary knob, is what we call the SBA knob, or our round knob," Zaya explained. "This gives us our master datum location that has the repeatability in both X and Y axes. If you use two chucks, you need a secondary knob, which is called SBB. This is a diamond knob, meaning that the conical locating taper has been relieved and only makes contact on two points. So it controls the orientation of the fixture about the centerline of the A knob and aligns it parallel to your X or your Y axes—not both."

Beyond two knobs, Big Daishowa adds "SBC" knobs on which "the conical locating taper has been completely relieved along its entire diameter. So it only provides retention force, there is no location and no orientation function," Zaya pointed out.

For larger fixtures, you could simply add more C knobs, thus multiplying the retention force. As you’d expect, the "spacing from chuck to chuck, or knob to knob, needs to be fairly accurate," Zaya said. He added that the tolerance for most applications is ±10 µm, and "most modern machine shops can hold that level of accuracy. If they want higher accuracy, you get into jig grinding and fixture grinders."

UNILOCK doesn't rely solely on the threads for location, according to Zaya. "The knob has a locating pilot, which then goes into a precision locating bore."

For its part, Mate precision grinds a ring around the pull stud, and a corresponding ID in the bottom of the base. Put them together, Baeumler said, and "you’ve got the accuracy of the construction of the base, communicating directly to the accuracy of the construction of the pull stud, which is attached to the vise. You pull all that towards the center and you get awesome product repeatability and accuracy."

Users can often benefit from zero-point clamping by adding a compatible pull stud/knob to existing fixtures. "It's only very rarely the case that we can't modify a customer's current fixture to accept UNILOCK knobs," Zaya attested. How you do so depends on the fixture, your risk tolerance, and your preferences. Of the three methods, recounted Zaya, about two-thirds drill a hole through the fixture (from top to bottom) so a top bolt can pull the knob up into position. This gives you the ability to remove the fixture even if you lose air pressure, leaving the knob captivated in the chuck/base.

The other third choose one of two methods for securing the knob from the bottom, Zaya said, either because the fixture doesn't have enough room at the top for the preferred method, or they want to do everything from one side.

"When you attach from the underside you don't have to worry about flipping the part or fixture over in order to do a counter bore for a bolt," he explained "It's more of a convenience and/or cost savings strategy to do it from the underside."

Zero-point workholding contributes to automation for the same reason it aids initial setup. Whether you move a part into a repeatable fixture with a human or a robot, you can be as confident in machining it as the workholding tolerances allow. And, as we’ve seen, these tolerances are tight.

You can attach knobs to virtually any type of fixture. So you could be automatically loading individual vises, tombstones, or pallets that hold multiple fixtures. With the usual automatic spring to clamp, air to release arrangement, your machine could be programmed to automatically secure each fixture when the robot sets it down.

But, Zaya cautioned, you can't be confident in the clamp status unless you have an extra feedback loop. For example, the machine would "know" it called for the compressed air to release the chuck to receive the next part, but it wouldn't know that this actually occurred unless a separate circuit confirmed the open state. While Big Daishowa can add such features, Zaya noted, most customers don't lean immediately into this level of automation.

Most job shops start by getting zero-point tooling that eases setup and positions them for expanded automation later. Another middle road that's gaining popularity is to use zero-point tooling to build pallets offline, then load the pallets with a robot. In such an arrangement, Mate's manual clamping would also work. Baeumler referenced an IMTS demo in which Nikken's robotic part tender (the 10DER) moved pallets from a four-sided pallet tower to a machining center. In a nod to interoperability, the zero-point tooling on the pallets came from different vendors, including Mate.

In a perhaps ironic twist, Zaya also pointed out that five-axis machines (which are otherwise more versatile) aren't suited to automated zero-point fixtures. Because "in most five-axis machines, you’re dealing with multiple rotary axes. So having air lines running to the system is in most cases impossible."

Big Daishowa offers manual systems for such applications. Zaya added that five-axis machines usually present clearance issues as well. The best solution, he suggested, is "to elevate the part up off the table, giving you a lot more breathing room around the machine's headstock and spindle, as well as the table."

Let's close with the vise that actually holds the workpiece on the base. Ideally it would also be self centering, highly accurate, and repeatable—three benefits Baeumler claims Mate's DynoGrip delivers to a "best-in-class" degree. The DynoGrip clamps the part to within 15 µm of perfect center, repeatable to within 10 µm. Baeumler said that's due in part to Mate making the lead screw in house, using tool steel that's "properly hardened and coated with titanium carbo-nitride." The screw has a fine 1.5 mm pitch and a trapezoidal thread, he added, yielding a strong, steady push.

But the most important feature, according to Baeumler, is machining both the right and left sides of the lead screw from the same top dead center. "That way we control where it starts and stops, and we know where the timed top of the lead screw is," he explained. "We do the same for the ID of the pusher, so we know where that thread's top dead center is. Together these two things make it easy to get accurately to the center of the vise when assembled, because we control every element of variability."

Finally, Baeumler described the vise as having an "anti-lift" design, meaning that the pusher and jaw exert downward force on the workpiece as they clamp. That's in contrast to the natural physics of a vise, which tends to lift a part. With the better zero-point workholding vendors, the lift is toward ever better quality.

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