A focus on fixtures and workholding pays off
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.
Critical for High-Value Parts
Destaco (Auburn Hills, MI) developed and patented the first toggle clamp in 1936 and has been evolving workholding products ever since: manual, pneumatic, pneumatic swing, pneumatic power, and electric clamps; automated dump units; automated pick-and-place devices, and end-of-arm tooling for large parts.
The industries where its advanced clamping technology is especially critical, according to Thomas Stimac, sales director for distribution in North America, include automotive, aerospace, medical, and agricultural due to the high-value components being produced. Stimac recommends making the workholding as modular as possible. “Avoid techniques such as welding a piece in place that prohibit you from making changes on-the-fly,” he said. “We pre-machine screw holes and dowel holes on our clamps for quick mounting and for easy attachment of preconfigured tooling blocks.”
Often a customer will ask for the company’s recommendation on how to best hold a part. “Even though we do not supply fixtures directly, we have teams of application engineers at our Technical Center who can work with fixture suppliers to integrate our products into a complete workholding solution,” Stimac said.
Manual clamps are still very popular and account for about 50% of sales according to Stimac. The evolution of clamping products now includes pneumatic, hydraulic, and electric models. Destaco also supplies a line of variable-stroke products that can be positioned along the length of the workpiece and compensate for differences in the thickness of the part.
“As far as trends, the move to electric devices is gaining traction,” Stimac said. “As the Industrial Internet of Things [IIOT] gathers momentum, we are discovering new ways to monitor and fully utilize the data that becomes available—when the part is positioned, when the clamp is open or closed, if there is a variation in clamping force, when the machining process starts and stops, etc.”
Investigate Key Areas
Emuge Corp. (West Boylston, MA) was founded as a precision cutting tool manufacturer, but began developing workholding devices when it needed to hold parts for its own production and could not find the right solution.
“Efficient workholding is important for any piece being machined or inspected,” said David Jones, product manager for precision workholding at the company. When designing an efficient workholding solution, Jones said his company investigates key areas such as: how quickly workpieces can be cycled through the device; the time needed to change from one workpiece in a part family to the next; a requirement to accommodate completely new top tooling or interchangeable components and whether this can be done in-machine or outside; and the desired tooling life. “If the workholding is not rigid enough, the cutting tools can wear prematurely, increasing costs and cycle times,” he said.
As for “tips and tricks” to set up workholding, Jones had the following suggestions: “Most quality suppliers have indicating rings on the main elements, and these areas must be used to indicate in the tooling. If there is a tailstock, it is important to make sure that the headstock and tailstock are lined up properly. Another consideration for precision workholding is lubrication. When assemblies are micron accurate, excessive lubrication can present QC problems.”
Jones also had a caution: “Some workholding is large and robust and looks like it could be dropped from the top of the building and survive. But remember that you are often working with micron-sensitive devices that must be handled and stored with care.”
He also noted that the machine spindle should be considered when removing a workholding device from a machine. The machine sides should be cleaned of chips and swarf, and lubricated according to specs. If a chip gets caught between the mounted workholding and machine taper, serious problems can result.
When there are part families or similar workpieces, Emuge offers quick-change workholding and interchangeable components. This can include tool-less mounting and dismounting of devices activated by the press of a button on the machine control.
“If the machine side does not have an adequate drawbar or other connection possibilities, we have mechanical solutions requiring minimal operator effort,” Jones said. “There are also part changeovers where the workholding devices are so similar that simply exchanging elements is all that is required to run the next workpiece.”
Good Workholding, Big Benefits
Lang Technovation Co. (Hartland, WI) is the US subsidiary of a German company founded as a contract manufacturer and toolmaker. Today, the company produces workholding equipment, zero-point clamping systems, and automation.
According to Eric Nekich, manager of operations, technology, and inside sales, the right workholding can help standardize processes, save on material costs, increase safety, improve operator performance, and even increase part quality.
“A critical application for workholding is multiaxis machining, where accessibility to the workpiece without impacting holding power is extremely important,” Nekich said. “There are also applications where workpiece deformation, due to clamping forces or the material hardness, presents a workholding challenge.”
Nekich recommended the use of modular workholding components with high repeat accuracy for machining operations where setup and breakdown speed are desired. “This workholding can be broken down and quickly and precisely reassembled without having to re-indicate, locate, etc.,” he said. “Systems that utilize pull studs, clamping studs, retention knobs [call them what you will] are great because they eliminate the time-consuming loosening and tightening of bolts and screws.”
Lang Technovation products are engineered to reduce cycle times. “We started with simple products like Clean Tec chip fans and Vario Tec accessory jaws,” Nekich said. “Soon after that, we developed the Quick Point zero-point and Makro-Grip clamping systems. The Makro-Grip unit, with its patented form-closure technology, changed the game for holding hard materials for five-axis machining. More recently, we moved into the world of machine tool automation.”
Addressing future trends, Lang Technovation sees a growing role for machine tool automation systems, especially with the development of Industry 4.0. “I am also optimistic about the use of our products in more advanced systems like mill-turn machining centers,” Nekich said.
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-to-chip 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.”
No Margin for Error
Vektek LLC (Emporia, KS) started as a job shop, but expanded into workholding when the owners could not find reliable clamping devices for their hydraulic pump and cylinder assemblies they made. Today, Vektek is a global supplier of hydraulic and pneumatic clamping devices.
Vektek hydraulic power clamps are used by medium-volume to high-volume manufacturers. As Reese Johnson, customer support supervisor and applications specialist, explained, the sweet spot for his company’s products are applications where operators need to load and unload parts quickly with no margin for error.
“An operator no longer has to tighten 15 bolts to the right torque, but only has to put the part in the fixture and push a button, or flip a switch,” Johnson said. “Depending on the complexity of the parts on the fixture, this can save from several minutes to a half-hour for each cycle.”
Vektek can be involved in making recommendations for fixture design, but does not supply fixtures. “If a customer is building its own fixtures and asks our help in selecting the right workholding components, we are more than happy to give suggestions and recommendations,” Johnson said.
When it comes to suggestions for workholding design, Johnson offered the following caveat: “A lot of people think that if a manual fixture is not working well, they can switch to hydraulics on that same fixture and the problem will go away. That is simply not true. Workholding success is based on sound fixturing principles [3-2-1 locations, clamping spots, datum locations, etc.] regardless of whether it’s manual or hydraulic.”
Vektek’s products are sold factory-direct with direct technical support. “Our product line is mostly hydraulic, which is best suited for CNC operations due to clamping force and a small footprint on the fixture,” Johnson said.
Automated (or lightly tended) operations are becoming more routine. The level of automation for workholding is no exception. “Our products can be fully automated,” Johnson said. “We offer products with pressure confirmation, clamp positioning sensing, and even a wireless pressure monitoring system. If pressure falls below a designated level in a fixture, our system notifies the machine to enter its safe shutdown sequence so that a part will not be scrapped because the clamping force is too low.”
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.
“To optimize the investment,” said Stimac of Destaco, “the location and number of parts on a fixture, the orientation of the parts, the cutting forces, and the clamping elements required all must be factored in. But, the economic payback can be in a matter of days. Just moving from manual clamping to pneumatic, we see cycle times reduced by 50%. This might mean that instead of having two operators in a cell, the use of pneumatic clamps might only require one person.”
Roemheld USA’s Vilcek concurred. “We have had customers realize up to a 90% reduction in setup time by moving from a manual setup.”
For Jones of Emuge, parts with different geometries, and the trend toward producing parts complete, create a demand for fixtures that are not considered off-the-shelf solutions. “Part of our business is designing and building flexible fixtures for holding these high-value workpieces,” he said. “Given that these fixtures are specifically engineered, the payback comes from producing a high-quality part with a low maintenance, easy-to-use workholding solution. Final part quality will be better and the scrap rate lower—and there will probably be fewer secondary or offline operations required due to the higher part quality.”
There are several metrics that can predict the ROI for a new workholding solution, according to Nekich of Lang Technovation. “If the proposed workholding solution has a higher cost than a standard solution, this cost can often be justified,” he said. “For example, a common dovetail fixture may be less expensive than our Makro Grip vise. However, if you have to prep dovetails in a hard, expensive alloy, the cost of the vise is returned relatively quickly.”
For Vektek’s Johnson, the biggest savings from modern workholding come from reduced scrap and cycle times. “Moving from a strap clamp to a hydraulic cylinder might save three minutes per load,” he said. “Magnify this for a complex fixture—and I’ve seen fixtures with 200 hydraulic clamps—and the ROI for hydraulic versus manual clamping is huge.”