The Five-Axis Frenzy

Dec. 28, 2009
Shops have a lot to consider before diving into 5-axis machining.

The number of shops that have caught on to the benefits of 5-axis machine tools has grown by leaps and bounds in recent years. Still, there are many shops that, for the first time, are adopting 5-axis machining, and for them, choosing a machine can be a daunting prospect – especially when one machine tool builder reported having found hundreds of different possible machine configurations available for achieving 5-axis machining capability.

However, shops making their first foray into full simultaneous 5-axis machining or 5-sided machining typically opt for either a vertical or horizontaltype machining center. And thankfully, with those two types, the choices are a bit more manageable.

Within the realm of verticals and horizontals, there are basically four common configurations for five axes, and those four can be categorized into two groups. Group one configurations achieve a fifth axis through workpiece/ machine table movement, and group two configurations through tool/machine spindle movement.

The table-movement group includes the A over B axis (dual trunnion table) and B over A axis (knuckle)-type machine configurations. Configurations within the spindle-movement group are the tilting-head and C-axis head-types.

Each type of 5-axis machine configuration handles certain types of workpieces better than others, which is the reason that practically every builder of 5-axis machines will agree that choosing the appropriate machine configuration is extremely “part driven.” So, shops need to know the size and weight range of the parts they will be running, what material (soft or hard) those parts will be made from, what level of accuracy needs to be held, and what types of surface finishes are required.

According to Dave Lucius, vice president of sales at Methods Machine Tools Inc., the exclusive U.S. importer of Matsuura machines, the best configuration for rigidity and accuracy when machining hard materials is one where the machine’s table moves rather than its spindle. Machining hard materials is Matsuura’s forte, he said, which is why the builder focuses heavily on A over B-axis dual-supported trunniontype machines with vertically oriented spindles for 5-axis machining. Matsuura also produces B over A-axis knuckle-type machines, but recommends that shops move to a dual-supported trunnion-type machine when part weights exceed 100 lb.

Lucius explained that Matsuura machines get their rigidity and robustness from vertically oriented spindles together with trunnions that are basically mounted to a machine’s core base. He also said there are benefits concerning work envelopes and machine automation.

“On trunnion-style machines, most of the work envelope is usable because it doesn’t have to be big enough to provide enough room for a moving spindle to maneuver around a workpiece. So when considering a part’s size, a trunnion- style machine’s work envelope for that particular size will be smaller, as compared with the required envelope a tilt spindle/C-head type machine will need to accommodate that same part size. With a trunnion type machine, shops can get closer to filling the work envelope with part size because the machine design eliminates the interference issues associated with a moving-spindle machine,” said Lucius.

To date, Matsuura’s largest dualsupported trunnion machine provides a work envelope measuring 24.8 in. sq., handling parts weighing to 880 lb. And, the company soon will be presenting a larger horizontal version dual-supported trunnion style 5-axis machine.

Responding to the belief that trunnion-style machines are difficult to automate, Lucius pointed out that just the opposite is true. “Automating a trunnion-type machine is actually easy to do. For example, the Matsuura MAM72-35V can incorporate a simple pick-and-place robotic system, some with as many as 180 pallets. The machines are designed for fully integrated, simple off-the-shelf automation. In fact, the bulk of 5-axis machines Matsuura installs are ones with high-quantity pallet systems.”

Commenting on 5-axis machines, Tracy Ellis, machine group sales manager at Heller Machine Tools, stated that all types of 5-axis machine designs provide accuracy, but there can be physical drawbacks to each. For example, he pointed out that an A over B-axis trunnion-type machine design might limit operator accessibility.

Workpieces and workholding, on those types of machines, must rest between the two (A-axis) trunnions, forcing operators to work around the trunnions. He also said that shops can fit only so much workpiece mass and weight between the two trunnions before having to move to a larger machine. But, he added that the trunnion machine design is a very reliable one.

“In my opinion, I don’t think a shop should go over an 800-mm pallet size in an A over B axis trunnion machine. When workpiece mass/size approaches 2,500 kg/one meter or larger, it can be difficult to handle on a trunnion machine because the trunnions would have to be spread out to the point where table sag might become an issue,” said Ellis.

He agreed that A over B-axis machines are adaptable to pallet changing. However, the pallet changer has to be either a shuttle or linear type, which may not be as fast as a rotary-type pallet changer. Pallets must be lifted and shuttled and a new one pushed on, as opposed to lifted, rotated and set down with a rotary system.

Ellis added that pallet changing is possible and commonly done with B over A-axis type machines, but there can be an overhead connection to the machine’s B axis that can get in the way, especially with aftermarket addons. Builders can overcome this problem by making the connection integral to the table.

Heller builds tilting head and C-head type five-axis machines with B axes as the rotary tables and the other rotary axis as either a swiveling workhead C axis or a tilting A-axis workhead. Machine sizes run to 1.6 m by 2 m, and if a shop needs something larger than that, Heller recommends opting for a portal or gantry type five-axis machine.

Tilting head and C-head designs keep a machine’s work area accessible and ergonomically friendly while allowing for a large work envelope, high table load capability and good adaptability to pallet changing. Operators can easily reach workpieces, workholding, the spindle and tooling, and the machines’ tables carry the full load and workpiece size of traditional horizontal machining centers.

With a B over A-axis design, the machine’s indexer takes up a large portion of the work envelope, and with an A over B-axis trunnion machine, there are potential collision zones with the trunnion, explained Ellis.

For its 5-axis machine models, Heller offers a choice of three different workhead designs. There’s a powercutting, high-torque universal head, a speed-cutting universal head, and a speed-cutting tilt spindle for highspeed cutting.

The power cutting heads use Heller ‘zero’ spindle system spindles, cartridge- type spindles that have a coupling between the spindle cartridge and the motor. This allows the cartridge to be changed in about two hours, as opposed to motorized spindles where the task is a lot more involved.

Tilting head machines from Heller sport higher speed spindles, primarily for cutting aluminum and cast iron. The company also offers a high-speed spindle on its C-head machine. The power cutting C-head delivers ample torque having a gear-driven headstock in 40 and 50 taper versions.

Makino’s approach to various five-axis machine designs considers both the positioning of the workpiece relative to the spindle and the positioning of the cutting tool/ spindle relative to the workpiece.

On some horizontals, for example, Makino uses a table-on-table design, where the B axis is under a horizontal pallet that has a vertical rotary C axis. The design provides stiffness and rigidity, and such operations as drilling and boring can be performed as single-axis operations versus multiaxis movements. On its verticals and some horizontals, the company also uses tilt trunnion-style tables.

Makino also designs five-axis machines with the A and C axes on the spindle. However, those machines are for large parts, such as big flat plates.

“Traditionally, the drawback to a tilt trunnion design, or even the tableon- table one, has been the amount of rotational speed that can be generated on the rotary axis, whether B and C axes or A and C axes,” said Bill Howard, product manager at Makino. “A relatively high table speed (rpm) is needed to get the most out of a highspeed spindle when machining, for example, the outside edge of a 30-in-diameter workpiece. Mechanical drives and elements can actually limit a machine’s rotational speed.”

He pointed out that Makino, and other machine tool builders, overcome these table rotation drawbacks by using direct drive motors that provide high rotational speed, resulting in higher machining feedrates.

Direct drive motors, such as those used on Makino’s D-500 vertical 5-axis machine with a tilt/trunnion table, eliminate backlash and provide the same accuracy and repeatability of mechanically driven tables. They also eliminate the need for mechanical components linked to mechanically driven tables, along with the wear and tear associated with those components.

Machine tool builder Mazak addressed 5-axis accuracy with a recently designed 5-axis machine that incorporates linear motor technology to increase accuracy when doing fine incremental machining. Such a design eliminates the need for ballscrews, along with the backlash and starting/stopping reversal errors that go along with them. And Chuck Birkle, marketing vice president at Mazak, said that more and more machine tool builders are turning to linear motor technology to boost overall machine accuracy.

According to Birkle, Mazak has no qualms about building machines with the fifth axis in either the tool or the table: more than half of the machines the company builds are five-axis.

On the company’s Vortex vertical machines, for example, the spindle provides the A and B axes, while its Variaxis verticals, use an A over B-axis design via dual trunnion table.

“One advantage to the trunnion table design is that relatively short tool lengths can be used because the table can maneuver up close to the spindle. There’s better spindle rigidity, yet no need to use long tools for reaching certain part surfaces, even on complex geometries,” said Birkle. “And, the configuration tends to simplify programming in that the tool tip is always in a known position. It’s the A and B axes that are the variables.”

No matter the type of machine design, today’s tougher part materials are placing a heavy burden on machine tool builders to increase machine rigidity and re-think how spindles are built.

“Four or five years ago, it was high-rpm machining of mostly aluminum. That’s changing," said Birkle. "There’s an increased use of hard materials, such as titanium, so the goal isn’t more rpm, per se, as it is more torque at lower rpm."

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