Hold strong: 5-Axis Machining

In recent years, you’ve likely been hearing a lot about the advantages of automation in machining operations. Related to that topic is an important one that gets far less attention: how workholding devices fit into the modern automation cell and the benefits they bring there. Besides improving accuracy and repeatability, workholding can give a big boost to machining efficiency and the output of automated systems.

Common workholding devices include vises, into which blanks are manually loaded outside the machine tool. In automated systems, the loaded vises are picked up by a robot arm and placed into the machine.

One example of this is the RoboTrex automation system from Lang Technovation Co. in Hartland, Wisconsin. The system contains a pair of trolley carts that hold vises loaded with blanks. Loading takes place offline while the machine is running, explained General Manager Jon Dobosenski. Once filled with loaded vises, the trolley cart in the loading area is ready to be wheeled into the machining area, where it takes the place of the other cart, which is filled with vises that hold parts that have been machined. During the machining process, a robot arm picks each loaded vise out of the cart and places it into the machine, then puts it back in the cart when machining of that part is finished.

A robot locates vises in a cart and keeps track of which are ready for machining and which are finished. Image courtesy of Lang Technovation

With this dual-cart system, he said, “we can keep the automation process consistent and maximize usage time on the machines.”

Workholding devices can include features that make it easier for them to be picked up by robot end effectors. For example, devices sold by Mate Precision Technologies in Anoka, Minnesota, have 8 mm tapped holes that can hold studs or other objects that can be grabbed by an end effector.

“Anything that you want to grip, you just attach it and it works with the end effector system that you have in mind for your automated environment,” said Frank Baeumler, vice president of workholding.

Automation Location

To ensure location accuracy when a robot places vises in a machine, they can be attached to a so-called zero-point clamping system that also interfaces with the machine table. A combination of zero-point holes and clamping studs is meant to guarantee quick, firm and accurate clamping.

“You can move precision vises in and out and know exactly where each part is every time you (load) it into the machine tool,” said Dobosenski, whose company sells both vises and zero-point clamping systems.

Lang Technovation’s zero-point clamping plates come in nearly 50 versions. He noted that plates for bigger parts have larger clamping studs to provide enough holding force to assure accurate machining.

“Once you get into automation,” Dobosenski said, “(a zero-point clamping system) helps the whole cycle because you know how
the material is being held and that you’re going to get repeatable results.”

Accuracy and Repeatability

A variety of such systems are on the market, and the overwhelming majority of them provide location repeatability in the 0.0002″ to 0.0003″ range when fixtures are moved in and out of a machine tool, said John Zaya, product specialist at Hoffman Estates, Illinois-based BIG DAISHOWA Inc., which sells workholding products. He said this is particularly important in applications that involve automation. When repeatability requirements are extremely demanding, he said a robot arm has trouble meeting them by itself.

A robot (top) pulls a fixture loaded with material from the racking, then the loaded fixture (above) is placed on a custom tombstone fixture in the machine. Image courtesy of BIG DAISHOWA

If a shop relies strictly on a robot in these situations, “the robot programming will require constant upkeep,” he said. “You have to keep going back in and tweaking the program points where the robot picks and places things.” However, “an accurate location system for the pull studs ensures that the process has a good level of repeatability.”

In part, accuracy and repeatability of automated processes depend on the cleanliness of mating surfaces.

“All it takes is one loose chip in the wrong place to (produce) a fixture that is not seated completely and correctly,” Zaya said. “If you get a chip in between the two mating faces, then that means that chip is not letting the fixture sit flat.”

As a result, the locating feature is not properly centered.