At the Nirvana Machine Shop on planet Perfection, every workpiece is clamped to a custom-built fixture mounted on a dedicated machine tool. Each workpiece is dimensionally identical to the one before and the one after. All the fixtures are totally automatic—instantly positioning, clamping, machining, inspecting, and releasing the part with the ultimate precision. Back here on planet Earth—where we are constrained by money, space, time, and other considerations—our machine tools are more likely to be multi-use and not dedicated.
Purpose-built fixtures are not the norm. Quite often, the setup is manual. And, our workpieces have variations: oversized, undersized, and parting-line shifted. Still, we look for effective solutions that get us one step closer to Nirvana. Or, at least, one step farther away from recurring headaches. A good step to take, according to experts, is to focus on workholding. As the saying goes, “If you can’t hold it, you can’t cut it.”
Execution is the Hard Part
The goal of workholding is to locate, orient, and hold workpieces in position to keep the tool in the cut. Sounds simple. The nuances, however, turn simple into complex. Many criteria—size, weight, and dimensional stability of the workpiece; cycle time; location and orientation; clamping forces; cutting forces—come into play. The speed with which a part can be changed over can also be critical. Fortunately, there are modern workholding technologies to address these complexities. But, with a variety of options available, how does a machine shop determine the right workholding solution, especially if it is “non-standard”? Manufacturing Engineering contacted several suppliers of workholding technology to learn what solutions they offer for reducing setup time and increasing productivity.
Penalties for Poor Workholding
Carr Lane Roemheld (now Roemheld USA) of Fenton, MO, is a global supplier of workholding solutions, supplying over 80 types of air and hydraulically powered workholding devices, precision machine vises, and flexible fixturing.
While workholding is critical in all manufacturing industries, shops doing high-volume runs will see a more pronounced penalty from poor workholding efficiency, according to David Vilcek, manager of workholding and assembly at Roemheld USA. Vilcek’s advice when it comes to designing workholding? “Stick to the KISS principle,” he said. “The simpler a system is to operate, the easier it is for workers of all skill levels to correctly employ it. I recommend starting with a solid zero point system [ZPS]. The quick-change interface of our Stark ZPS is probably the easiest way to get started in reducing setup and breakdown times, allowing you to go from chip-tochip with a new fixture in as little as 60 seconds, compared to up to an hour for manual.”
With a ZPS solution, operators can change out fixtures with a push of a button, no matter if it’s a mechanical,hydraulic, pneumatic, vacuum, or electric fixture. The Stark ZPS design constantly pulls the fixture pallets down, resulting in an extremely rigid and accurate system.
In addition to zero-point, the company offers a range of products: quick die-changes, quick mold-changes, hydraulic workholding, assembly, and handling. Most products are hydraulic, but others are pneumatic and there are even electric workholding systems with zero-point. “With electric clamping today, you can’t get as many parts onto the same footprint compared to hydraulic clamps,” Vilcek said. “That’s why our current state-of-the-art clamp is a blend of hydraulics and integrated electronics for real-time feedback.”
Real-time feedback is less common, but the company’s products can be fitted with position sensing and part sensing capabilities. “As Industry 4.0 is becoming a driving force, we are positioning ourselves to support these activities,” Vilcek said. “For example, we now offer a clamp that gives real-time feedback on its position, temperature, applied clamping force, and even reaction forces.”
Manually changing over workholding to produce a new part can take from 20 to 30 minutes and requires a skilled operator to ensure that workholding accuracies and repeatability are met. As the process is repeated several times over in an average workday, it is easy to see how revenue-producing spindle time is lost. Therefore, a straightforward ROI calculation for workholding is to ascertain how many more parts can be produced per shift (or day) with the proposed solution by examining cycle times (setup + machining + changeover). Cycle time savings equates to additional spindle time. There are, of course, other savings that can be calculated: reduction in operator injuries, reduced scrap, better finishes, fewer tool changes, longer spindle life, and longer tool life.
Roemheld USA’s Vilcek concurred. “We have had customers realize up to a 90% reduction in setup time by moving from a manual setup.”