Modular Workholding Works Wonders

When it comes to improving productivity, built-in workholding should be at the top of a company’s to-do list, according to Colin Frost, president of St. Louis-based Carr Lane Manufacturing Co. Moreover, he asserts, such solutions are needed from setup through running time.

Modular fixturing is always a series of compromises.” It’s hard to argue against “ideal,” except to say that creating a custom fixture for each workpiece takes time and money, and many short run jobs wouldn’t justify either. What’s more, the myriad solutions now available within the broad category of modular workholding leave the smart buyer with few, if any, regrets.

For example, Wheeling, Illinois-based SMW-Autoblok Corp.’s active pull-down chuck can clamp at the center of the stroke. This offers the ability to adapt to dimensionally different parts where a part family can be covered “just by changing the tops, within the physical limitations of the chuck,” according to Larry Robbins, president of the company’s commercial division.

The chuck is unique in being both sealed and “active,” such that it “creates three features in the same mechanical movement,” Robbins adds. “With a sliding jaw chuck, you pull on a wedge in the back. The degree of incline on the wedge dictates jaw stroke and clamping force. With a ball lock chuck, the bottom of the jaw is a ball. You rock within a captive pocket that has keys on each side that limit your movement. The amount of movement is built into the chuck itself.

“Our chuck is special,” Robbins continues. “We can change these keys and change the amount of movement, the amount of jaw stroke, and the amount of pull-down effect, all by changing that little key inside the chuck.”

This fits with SMW’s general goal of making standard, off-the-shelf, products easily adaptable to customer-specific applications. “The one thing people have to remember, no matter what they buy, (is) it should be flexible, it should be adaptable, it should be convertible from one project to another,” Robbins says. “I can go from a three-jaw chuck to a mandrel, a collet chuck, a four-jaw chuck and back to my three-jaw chuck. All within less than a minute, never changing anything but the face of the components.”

SMW is the only company that hermetically seals all its chucks, vises and grippers as a standard feature, Robbins claims. “We do it with the thought of not only high-pressure coolant, but the variations in contaminants because of multitasking machines. Now, you have turning, milling and grinding on the same machine.”

This produces different types of chips and swarf, adding to the risk that essential grease is washed away. “When you don’t have grease, you create metal-to-metal contact and friction,” Robbins explains. “Your grip force can drop off by 80%. So your predictable 20,000 lbs. (9,071 kg) of grip force might now be 1,600 lbs. (725 kg). I’ve seen parts literally launched out of a chuck come through the sheet metal on the machine and embed themselves in a concrete wall. We try to avoid any catastrophic failure for any reason, so all our gear is hermetically sealed.”

Adapting one modular workholding combination to another to change part types takes time. But this time can be minimized—and moved off the machine. The “top tooling” that actually holds the workpiece (e.g., vises, clamps and collet fixtures) can be distinct from the base elements that sit directly on the machine table, explains Mike Antos, product manager at Cleveland-based Jergens Inc. And you can build a quick change interface into those base elements.

For example, with Jergen’s Ball Lock system, an operator manually connects the top tooling to the sub-plate in seconds, simultaneously locating it with 0.0005” (13 µm) repeatability. The company’s Zero Point system, like Schunk SE & Co.’s VERO-S or SMW’s ADP, is automatic and repeats within 0.0002” (5 µm). Either approach can eliminate the need to remeasure anything in the machine. Schunk also offers a version that boasts 2 µm repeatability, and all VERO-S systems are made of stainless steel and sealed against chips and swarf.

“Let’s say part A is held in a vise and Part B is held on a high-density fixture using micro clamps,” Antos says. “By building those on plates, with a system like our Ball Lock or Zero Point, you can change out from part A to part B, turning hours of setup to minutes, or minutes to seconds. That’s where the time savings comes in.”

What’s more, he adds, the repeatability of the interface eliminates variables. “You’re not relying on somebody to manually tram a fixture in and indicate it on the machine. You eliminate the variables in the process where something could go wrong. Things like an operator having a bad day. Or an inexperienced employee filling in for the usual setup person.”

It’s no wonder that more than half of Jergens’ workholding sales are in quick change systems, as Antos reports.

The zero-point systems described use pneumatic or hydraulic pressure to open the clamping mechanism in the base. They default to the locked position, grabbing the stud(s) in the workholding. Thus, they can be activated remotely and lend themselves to automatic part loading.

But Antos highlights another case in which such zero-point systems save the day: “We see Zero Point used a lot in clamping a large workpiece where the customer puts the studs right into the workpiece. Or with a larger fixture where it’s physically impossible for an individual to go back and insert and tighten down a Ball Lock shank. So being able to remotely actuate a valve to release the stud is an advantage beyond automatic loading.”

A perhaps more surprising automation solution, and one that doesn’t necessarily require zero-point fixturing, comes from Schunk in Morrisville, N.C. It’s a spindle gripper that turns your machining center itself into a three-axis part handling robot. Surprise number two? It does so for $3,500-$4,000, a fraction of the cost of a stand-alone robot, reports Brad Evans, Schunk’s team leader for stationary workholding. Surprise number three? It’s a 15-year-old idea that is only recently getting traction.

As Evans explains, the gripper uses through-spindle air or through-spindle coolant to activate. (Alternatively, you can use spring loading to grip and air/coolant to release.)

This hindered adoption, because 15 years ago, many machines in the U.S. didn’t have through-spindle air or coolant, especially in the job shops where this type of automation would be most welcome. Also, older machines generally didn’t know the rotational orientation of the spindle. It’s not enough to know the spindle’s position in X-Y-Z. You also have to know the “clock position” of the grippers to safely pick up the part. Happily, that’s much more commonplace now, Evans observes, as the older technology washes out of shops around the country.

Evans describes a typical user as a job shop with a 40- to 60-part overnight run. You’d load a pallet with the night shift raw parts, “hit ‘cycle start’ on the program, and leave,” he says. The tool changer would put the grippers into the spindle, the machine would move over to the pallet, orient the grippers, lower to grab the part and take it to the automatic workholding, etc. And just as the tool changer switches between cutting tools and grippers, it could also be changing gripper types. Thus, for example, you could go from a two-finger griper for rectangular parts to a three-finger gripper for circular parts in the same automated run.

Some ID clamping situations don’t even require knowing the spindle’s rotational orientation, Evans adds. “With a part like a ring, we can ID clamp such that the part is simply sitting on two lips, as opposed to the gripper grabbing it. So it’s just a matter of the spindle lifting up and down.”

Adding a rotation unit to the machine table can also automate several operations. For example, Evans says, “You’d load the part into one vise to perform OP1. Then the gripper would move the part to rotational unit, which flips it. Then the gripper would move the part to a second vise configured to hold it for OP2.”

While the spindle gripper doesn’t require fixtures with a zero-point base, Evans urges implementation for fast changeover, just as Antos explained. “Our premise has been to enable the customer to eventually transition to full automation without having to completely redo everything,” Evans says. So an investment in VERO-S zero-point fixtures for the spindle gripper approach would carry over to automation with a full six-axis robot.

“For instance, we are able to run pneumatics, hydraulics or vacuum through our VERO-S modules,” Evans continues. “So that you can have a pallet of tandem vises, pneumatic or hydraulic that couple directly to a VERO-S equipped plate. … So when the operator loads it, he doesn’t need to hook up any fittings. There’s no chance of him plugging the ‘in’ to the ‘out’ and the ‘out’ to the ‘in.’ It couples right through the pallet to the VERO-S. And then if they choose later to go to a six-axis robot, the robot just puts the pallet on the machine, the VERO-S automatically interfaces to your pneumatics or hydraulics, and runs the vises.”

If faced with the challenge of holding brittle components, composites, parts that would be distorted by standard workholding solutions such as clamps or jaws, or parts that are hard to hold due to their shape, consider the adhesive solution offered by Blue Photon Technology and Workholding Systems LLC, Shelby, Mich. Blue Photon offers a proprietary adhesive called BlueGrip that forms a strong bond with virtually any material after just 60-90 seconds of exposure to ultraviolet light.

How strong is this bond? The tensile strength of each one is up to 600 lbs. (272 kg) with the largest of Blue Photon’s grippers, the preferred method for connecting the workpiece to the fixture. These “grippers” are more like posts with a transparent core. They screw into any modular or made-to-order fixture. The adhesive is applied to the workpiece end and then cured from the back using LED heads. In contrast to the high-tensile strength of the bond, breaking the connection can easily be accomplished with a quarter turn of the gripper using a hand wrench, or by applying hot water or steam. Among other options, a “universal fixture” allows the user to configure a workholding system to hold near-net shaped parts.

As suggested, Blue Photon’s approach can be easily implemented with manual handling. But it’s also possible to automate the entire process, from dispensing the adhesive to putting the part and the fixture into a hot-water tank via a conveyor or robot to undo the clamp.

“You can use dry-ice blasting on a robot to remove any residual adhesive and it’s a very clean fast process,” says Dan Billings, Blue Photon’s president/CEO. The company offers a spindle-mounted cleaning tool (with a consumable abrasive) to remove residual adhesive from the grippers.

“This tool will come down and use the CNC spindle to clean the adhesive off all the grippers to get ready for the next operation,” Billings explains. Because Blue Photon’s system can “act as the interface between the part and a zero-point system, users can get more done on the part in less time,” he adds. “By removing clamps that are in the way, more machining can get done, especially on five-axis machines.”

For perhaps the ultimate in workholding adaptability, versatility and control, SMW has introduced a line of electromechanical chucks, vises, grippers, zero-point tables and tombstones, under its e-motion banner. These systems transmit both power and data through an inductive coupler, not a cable, and they offer an unprecedented level of control and feedback.

“We now offer auto adjusting technology on every chuck,” Robbins notes. “Every jaw will be considered its own NC axis. Envision a four-jaw chuck where each jaw is individually controllable either by itself or on a self-centering feature. And every jaw is sensing, sending data about its position and the force it exerts. If it finds the part is off by seven mm in X-Y at 237° from zero, every one of those and NC axes has the ability to work in conjunction with each other to do simultaneous interpolation to correct for this. Just like a CNC table moves at an angle to compensate.”

Robbins lists still more features “I can have high-low clamping on any component without having special accompaniments like a high-low cylinder. … I can set the expected jaw positions for my part, and because the system knows the actual positions at all times, it will send a fail signal if the jaws close more than the expected amount, indicating that the part is missing or miss-loaded.”

He continues: “It goes through a 25-millisecond safety check, telling you the part is still in position and still clamped to the right specification… Once the chuck, or vise is energized, if I have loss of energy it has no effect on the clamping. It will not come unclamped. I can actually energize the chuck, put a part in, and store it on the shelf.”

Robbins says the new electromechanical robot grippers can control and repeat their opening and closing positions to within 0.010 mm, with a similarly predicable clamping force. “I can also give you something no one else offers, 360° rotation. I can use these grippers to not only position a part, but also for assembly, screwing components together… I can even use them in a cleanroom environment, or in packaging food or drugs, because there’s no lubrication. I can touch the raw part and have zero worry about contamination from outside components.”

It’s an exciting development that Robbins boasts is unique to SMW.

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