by John Zaya
Effective ways to hold workpieces in multi-axis machining
Finding smarter ways to hold the workpiece, especially in the age of multi-axis machining, is becoming more of a difference maker than ever.
One increasingly popular way to realize efficiencies is with zero-point workholding systems. They maintain impeccable accuracies and strength while allowing
for extreme modularity and quick changing of fixtures/parts, unlocking exactly the kind of creativity and flexibility shops need to stay ahead.
However, we find that it’s not typically until the setup cost reaches a tipping point, generally 20 to 25 percent of the total cost to manufacture a part, i.e. hours/day and minutes/fixture, that we see customers inquiring about a change to zero-point. That’s a long time to wait and a lot of efficiency to lose.
What’s more, zero-point clamping isn’t only a setup accelerator in the traditional sense. It can also speed up in-process time since you can build a single fixture and machine five sides of the workpiece in one clamping—simply move the part from the mill to the lathe to the grinder to inspection with no loss of accuracy.
How zero-point works
Most systems operate in a similar fashion, relying on air and/or hydraulic pressure. With BIG Kaiser’s Unilock, for example, an air line opens chucks built into a base plate. Simply bleeding air pressure out of the chuck compresses die springs that drive multiple clamping pins against a tapered knob. We always recommended using filtered, oiled and regulated air to keep the mechanics in good working condition.
Air lines are typically cut into the baseplate. Many customers do this themselves, but turnkey, supplier-engineered setups are available too. Clamping forces range from a few hundred pounds to several thousand with the help of options.
Integrating with existing equipment
Zero-point workholding setups can be taller than traditional ones. Most machines have plenty of working volume, but on smaller centres, keep in mind that stacking base plates, chucks, a fixture and the part may require sacrificing some of your available Z stroke.
The addition of some extra weight is also an important consideration. On a vertical turning lathe, for example, you would need to adjust for balance since it’s part of a rotating mass. Similarly, on a five axis machine with a trunnion, the chucks add momentum that will need to be accounted for to maintain accuracy.
Mounting to the table is straightforward on vertical machines since they typically use T-slots. On horizontals, there are more variations like grid holes and edge locators which can be a little more complicated, but not disqualifiers. Adding a knob to the underside is simple and there are plenty of options like timing and guide pins that make it easy to use your existing fixtures and plate. However, if you’re existing plate or fixtures have taken a beating over the years, replacing them is a wise move.
Chuck selection and layout
The relation of the part to the edge of the chuck and the distance between chucks are the first two things I look at when customers propose specific applications. Depending on the material of the fixture plate, the part or fixture should not extend too far beyond the diameter of the chuck. With a steel plate that’s at least 20 mm thick, I can generally go 40 mm off diameter without losing stability. If it’s an aluminum plate 25 mm or thicker, 20 mm off the diameter should be about the limit. Thicker plates are an option, but material costs start getting a little higher than most customers want to bear.
When it comes to multiple chucks, plate material and thickness are again key. General guidelines for maintaining accuracy and rigidity: if the diameter is less than a 138 mm chuck, spacing should max out at about 200 mm; for bigger diameters, we recommend a maximum spacing of about 350 mm.
I’ve seen applications with as few as two chucks and some with dozens. They can be aligned in simple linear organizations to more complex circular and combination patterns where parts can be quickly and easily repositioned. SMT
John Zaya is product manager, workholding, with BIG Kaiser.