Ease into 3+2 machining

The aerospace and medical industries have long embraced simultaneous multiple-axis machining on machine tools with five or more axes, but more shops that provide general-purpose machining services have begun producing components on multiaxis platforms in the past few years, said Jeff Wallace, general manager of national engineering at DMG Mori USA Inc. in Hoffman Estates, Illinois.

He pointed out, however, that the common approach for general machining is 3+2, or five-axis positional, machining, in which a three-axis machine is equipped with a rotary table or tilt-rotary trunnion. He estimates that about 70% to 75% of the multiaxis work is performed via 3+2 rather than simultaneous when cutting up to five sides of a prismatic workpiece in one fixturing.

“One of the main advantages over a multiaxis platform is just the ease of operation to have that ability to get to multiple sides of a workpiece as compared to flipping it how many times it might be needed to get to all sides of that workpiece,” Wallace said.

A VC-500A/5X VMC is equipped with a multifaced. workholderImage courtesy of Mazak

On a standard three-axis machine, the only part face that the machine could cut with a cutting tool in its normal position would be the horizontal face that looks directly at the spindle, said Errol Burrell, machine center product specialist at Okuma America Corp. in Charlotte, North Carolina.

“With 3+2,” he said, “you could put that same cube in a vise and rotate those faces to be normal to the spindle and to the tool — all five faces, including the one that is the default to the tool, which is flat on the table. To be honest, five-axis simultaneous should be used as the last resort.”

Burrell said even manufacturers with the capability to do simultaneous multiaxis machining frequently don’t.

“Probably 90% of the work is 3+2 even though they have the capability of doing simultaneous five-axis,” he said. “Most of the work is 3+2 on the same platform.”

Mike Kerscher, technical/application specialist at Mazak Corp. in Florence, Kentucky, estimates that the percentage of multiaxis machining being the 3+2 variety is at 80% or “maybe higher.”

“Our customers figure out that they don’t have to index their part four times to get it made,” he said. “They can put it on a single machine with more functionality and to machine features that may or may not be at odd angles, even if it is a true cubic part that they can get to four or five of the six sides in one workholding. That’s the game plan.”

Wallace said even makers of automotive parts, which usually specialize in serialized production in which one operation is performed on one machine before moving down the line to the next machine, are “starting to wake up and pay attention too. They are starting to look at multiple-axis machines to do multiple operations in one workholding setup.”

He said he sees this transition increasing as more electric vehicle components are produced.

“I don’t think that I’ve had a customer in the past 24 months that didn’t request a multiaxis platform,” Wallace said.

Moving Parts

One of the biggest benefits of 3+2 machining is the reduction of setup time.

“On average,” Wallace said, “and this is just ballparking, you are going to save easily 20% to 25% in setup time, if not more, just because you don’t have to handle that part multiple times.”

The third-generation DMU 50 is a universal milling machine with a swivel rotary table. Image courtesy of DMG Mori USA

A 3+2 approach also boosts throughput. Kenzie Roaden, Mazak’s Advantec product manager, said after machining one side, it’s common for parts produced on conventional three-axis vertical machining centers to sit for hours or even days until features are machined on additional sides.

“Maybe it’s just one other feature,” he said, “but the operator has to finish the run of parts on the machine and then change the setup and start again. That part sits in limbo, and that’s the problem.”

While setup time goes down with a 3+2 arrangement, the time spent machining a part isn’t necessarily impacted.

“It’s all about moving the part through a factory more quickly,” Roaden said. “It has little to do with cycle time and more to do with throughput.”

In addition, Kerscher said part accuracy can increase when 3+2 machining compared with three-axis machining because 3+2 significantly reduces tolerance stackup.

“Whenever we touch it, take it out and put it into another piece of workholding,” he said, “we have a potential for tolerancing issues.”

Also, 3+2 machining offers the opportunity to improve accuracy by being able to position a workpiece so that short, rigid cutting tools are applied, such as when producing undercuts in cavities and steep walls.

“Rather than using some custom, extra-long tool or holder and risk vibration,” Burrell said, “you can use standard, off-the-shelf tools.”

Those standard offerings not only provide precision benefits but save money.

“There is easily a 10% to 15% decrease in tooling costs because now I don’t have to have special tools,” Wallace said. “I can use more off-the-shelf, standard catalog tooling because now I have the freedom to reach into my part at different angles, and I don’t need an especially long tool or special to make that feature on that part.”

Nonetheless, Kerscher said the 3+2 environment can pose challenges to accessing a part if a cutting tool has to reach across workholding devices, such as clamps.

“That is a concern or something to be aware of that there is a potential for the tools to get a little longer,” he said.

Programming Paths

Roaden said a high level of skill is required to program multiaxis simultaneous contouring, such as having in-depth knowledge about inclined plane rotations and the multitude of G codes to access multiple part faces.