If you’ve ever seen industrial wind turbine components on the back of a flatbed truck rolling down the highway, you have a good idea of what a large, heavy, difficult-to-handle workpiece is. For example, with a single blade on the GE 1.5 mW turbine being almost as long as a football field, the entire blade assembly weighs about the same as 36 small cars.
Wind turbine blades may be an extreme case of big, heavy and awkward workpieces, but there are plenty of other, more common examples.
“Anything you can’t load in a machine to do the machining, or you need some assistance to put it into the lathe, would be what I would call heavy and difficult, especially if it’s horizontal,” said Larry McMillan, Great Lakes regional sales manager, Hainbuch America (Germantown, WI).
Kyle Downs, director of marketing and project management at SMW Autoblok (Wheeling, IL) also thinks a workpiece that requires help from a robot, gantry or lifting hook to load into a machine qualifies as heavy. But because of the nature of Autoblok’s workholding products, which are designed for spindle applications, Downs immediately thought of workpieces that are either asymmetric or have an unusual shape.
“We primarily do turning workholding, so anything that doesn’t have a balanced weight that’s going to throw a spindle turning at 2000 rpm or even 500 rpm off kilter, anything that has that asymmetry innately in the part, we have to look at counterbalancing in the workholding,” Downs said. “That’s something that we regularly see.”
He classifies any part that’s not round, rectangular or square as one that’s difficult to hold during machining.
“If there’s some out-of-round, that’s fine, but anything that starts to get way out of round or is a geometric shape that’s non-standard, that’s something we would have to account for and do differently,” Downs said.
Michael Gaunce, group manager of stationary workholding at Schunk (Morrisville, NC), has his own opinions about what constitutes a large workpiece.
“Part size and weight both play a factor, and the part features certainly come into play,” Gaunce said. “I would say that parts with a profile of 2 × 4 × 4′ [0.6 × 1.2 × 1.2 m] and up might be a good starting point for the size that might require special consideration.”
Gaunce pointed out, though, that special cases rarely fit neatly into a 2 × 4 × 4′ box. He said that most often it’s one or two of the three dimensions that drives the need for special consideration. For example, a fluid end for a fracking block is about 2 to 3′ × 2 to 3′ × 3 to 6′ (0.6 to 0.9 m × 0.6 to 0.9 m × 0.9 to 1.8 m).
“This might be the smallest component I would consider to be defined as “large,” Gaunce said.
Also, Gaunce said, certain aerospace structures such as fuselage components that are made of aluminum might be relatively light in weight, but are large and awkward to hold.Presenting the biggest challenge are parts that are large, heavy and awkward, such as near-net shape forged valve bodies for the oil and gas industry.
“They be quite massive: 3–6′ [0.9–1.8 m] in diameter, and, considering their odd shapes, can be quite difficult to handle and to clamp,” Gaunce said.
Like the fluid ends for fracking blocks, valve bodies and fuselage components that Gaunce cited, many parts for the oil and gas and aerospace industries are known to be large, heavy and/or awkward and present a workholding challenge. Others examples include:
Aerospace, defense, tool and die and transportation industry components sometimes require specialty workholding to accommodate their size, weight and shape. The specialty workholding is costlier than that required to hold standard parts and requires manufacturers to justify the higher cost or to make do with standard workholding.
“The issue with large parts is usually the volumes go down, so a lot of times people have to kind of ‘MacGyver’ it because they can’t afford the cost to fixture a part that they might only make 10, 20 or 30 of,” said McMillan, referring to Angus MacGyver, a television character with a knack for extraordinary problem solving. “Mainly the need is for support—with hydraulic clamps or magnets.”
Downs takes a different view, and said shops that want to make large, heavy and awkward parts don’t have a choice and must have the proper workholding. “A lot of times, when you have big or heavy parts, you’ll use a manual chuck and independent jaws and that setup will take 20 minutes to two hours to indicate something in as opposed to having something that is self-centering or automated,” he said.
Another option, he said, would be to make an upfront investment in automation to get more run time on a machine, a step that would also eliminate human error during setup. “Automating is almost always less costly in the long run, but it’s a very costly initial installation,” Downs said.
Gaunce is methodical in his approach and suggested that specialized workholding isn’t the first step. “Taking a step back, the shop should first consider what kind of machinery it is using to machine this component,” he said.
Then, Gaunce said, the next step is to minimize the number of operations and handling to ensure that the spindle runs a maximum amount of time. This should be done by buying the right machine tool for the application, such as a four-axis horizontal, or a five-axis machine.
Although the costs are at a larger scale, special workholding should still be justified in the same way capital equipment is justified. The shop must evaluate the time it takes to machine the part with its current machinery and workholding, and review what could be done with more specialized workholding.
“Investing into good workholding is a way to reduce cycle time, improve part quality and reduce setup time,” Gaunce said. “All of which contribute to a better efficiency of the machine tool, and less mistakes of course.”
If a shop looks only at better efficiency of the machine tool and improves the spindle utilization rate by 10–15% with better workholding, the flat price of the workholding may go up, but the time for return on investment is the same, he pointed out. If the time for ROI is the same, but the investment itself is actually higher, it will actually contribute more money to the bottom-line when the investment (i.e., workholding) is paid for.
If a shop decides to invest in specialty workholding, some of the next questions become: Who offers the right option with the best value? How do I decide?
“The type of workholding offered will depend largely on size/shape, material, and the ‘difficulty’ of the shape,” said Gaunce.
Companies are increasingly adopting direct workpiece clamping with a zero-point system like Schunk’s Vero-S quick-change pallet system, he said. The modular system has more than 1000 variants, according to the company’s web site.
With the Vero-S, in the very first operation the manufacturer works on the “bottom” side of a workpiece, or on the side that requires the least amount of work, Gaunce explained. A pattern of holes is machined on this side that will then be used for mounting clamping pins directly into the workpiece. These clamping pins are then clamped to the zero-point system.
“This zero-point clamping system could be a fixed-location type system for production on larger workpieces,” Gaunce said. “Or it could be made to be modular if more flexibility is required.”
The advantage of this process is that the prep operation can be done on any machine, even a simple three-axis vertical machining center. Then, using the clamping pins to hold the workpiece, the manufacturer has access to the remaining five sides of the workpiece.
For large steel parts, magnets offer the option of clamping across the entire surface of a workpiece rather than just with the point of contact where clamps are holding the part. “This is especially useful on large workpieces because when clamping points are far away from one another, it can cause vibration on the sections where there is no clamping,” Gaunce said. “Obviously, vibration is a factor to be avoided.”
Magnets also make material handling and clamping a simple process, he added. Loading the part is a matter of placing the component onto the magnet, then energizing the magnet with a hand-held remote.
For large aluminum parts, particularly in the aerospace sector, Schunk has standard vacuum workholding solutions, but companies usually build their own or contract for specialty fixtures, Gaunce said.
Hainbuch also offers high-speed change-over interface options between the machine and clamping device, including the centroteX system for lathes or even a mill.
“We can take, say, a 20″ [508 mm] chuck that would be doing large parts, and change over quickly from a large part to a medium part or even a small part,” McMillan said.For mandrels, Hainbuch has mandoteX for quick changeovers, which offers accuracy to within 3 mm.
“Some of the gears in the windmill industry weigh two tons, and that’s where our mandoteX helps,” said McMillan. “They also have different diameters.”
If a shop is interested in an auto-centering system, Autoblok has a number of self-centering chucks, but that’s not the only option, Downs said.
“We have other center-line adjustable products for when you have really heavy parts that you’re loading, where each jaw has a torque multiplier on it,” Downs explained. “Even though the chuck is self-centering, you can go in there with a wrench and adjust by hand on a steady rest or maybe a quick jaw change chuck if you have a really heavy part where the weight is working against a jaw or an arm on a steady rest.”
When a part has to be loaded with a lifting device, or if the part’s tough to balance, Downs said Autoblok is increasingly being asked to do part seat confirmation. The process involves pressurized air being forced through the spindle. The air is blocked when a part is seated properly onto the Z locators and a manometer measures the pressure being fed back into the system, confirming the part is loaded properly.
John Zaya, product manager for workholding at BIG Kaiser Precision Tooling Inc. (Hoffman Estates, IL), suggests loading and unloading parts onto the workholding fixture outside of the machine or adding his company’s Unilock clamping knobs to the part.
“When you load and unload parts outside the machine, that means the machine efficiency isn’t being degraded because of the amount of time it takes for the operator to load the workpiece, lock it into place and then indicate it in or find its location,” he said.
Also, if shops have to stack a lot of components or if the workpiece is very tall, BIG Kaiser has a new stabilizer system that’s also modular. It attaches to the worktable and the side of the workpiece to provide lateral support. This is important because as the workpiece gets taller and further away from the table, there are cutting forces pushing against it and the stabilizer helps to offset them.
Gaunce is another proponent of doing setup outside of the machine.
“This is something we’re seeing more and more, and especially with the large workpieces,” he said. “These machines are expensive, and they need to be kept running. Using a quick-change pallet system for your larger workholding devices can take what used to be a three-hour setup to a 15-minute setup. For a machine that costs $300 per hour, that’s $600 saved every time you do a changeover.”
Another tip Gaunce offered is to design for manufacturing, which means designing tooling holes in components that allow clamping pins to be attached.
“This will make your life so much easier in the long run,” he said. “These parts are already difficult to manufacture, so make it easier on yourself from the beginning by involving your designers.”
Downs has other tips:
Solid modeling software can help dynamically balance a part and find the way to get the centerline of its weight onto the axis it’s rotating on, or very, very close to that axis.
For longer parts, like shafts, think about how many points of support are needed. That’s where a tail stock on a machine or a second spindle might be used, and then if it’s heavy enough or long enough, a steady rest, or multiple steady rests should be used so the part doesn’t sag in the middle.
Make sure that loading systems and workholding systems are thought of at the same time. There must be no interference between their operations.
Make sure that all clamping and locating surfaces are qualified to a machine. It’s very important that anything that’s new should be cut and trued up to a machine.
And McMillan shared a tip about getting mechanical help for machine loading.
“For any heavy part, there are some great cranes out there now,” he said. “For changing these chucks out, you hit a button and it creates anti-gravity, so you can lift a 300 lb [136 kg] part with your finger.”
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