Heads up: Moving beyond standard five-axis machining with novel configurations
Our exploration of multi-axis machining last year got into various configurations that went well beyond five axes, including mill-turn and additive-subtractive hybrids. These machines reached their high axis counts by incorporating multiple tool turrets, or several spindles, or additive heads on slides—sometimes in combination. 5-Axis CNC Milling
Only one unusual example actually applied simultaneous movement of more than five axes to an individual part, and that was with two different spindles. This year we’ll keep it simple: Should you consider more than five-axis movement with respect to one spindle engaging one part?
The answer—as it so often does—comes down to the part, according to Jimmy Trusty, co-founder and operations manager at KRC Machine Tool Solutions in Independence, Ky. Does the cost and difficulty of making all the required cuts in several setups (possibly on several machines) justify an investment in a machine with a sixth axis? To date, most companies answering in the affirmative are in the aerospace industry, with large, high-value, tight tolerance parts. But it’s a changing calculus, because the cost of six-axis solutions is coming down.
The key, Trusty explained, is the cost of the two-axis cutting head. A typical six-axis arrangement includes three linear axes, a rotary table, and a cutting head that swivels in two axes. For situations in which you want to lock the head at a specific angle for a heavy cut, these two-axis heads can be equipped with a Hirth coupling or a hydraulic clamping mechanism. Trusty said builders in Taiwan, Italy, and elsewhere now offer two-axis heads costing as little as $20,000, versus the $150,000 to $200,000 that is common in higher-end machining centers.
A machine tool builder probably couldn’t market a cheaper machine with a six-figure head, Trusty noted. But a lower-cost machine with a modestly priced two-axis head has a place. “As the cost of some of the machines comes down, (six-axis machining) is starting to be applied in other industries, usually to reduce setup,” Trusty said. “You can run a part on one machine instead of moving to two or three machines.”
Trusty specified that these lower cost heads do not offer the longevity or higher rpm of the pricier models, but work well enough for the smaller machines and smaller tasks on which they’re being applied. Automotive aftermarket applications are one example.
”There was always five axis in automotive, but it was rotary table five axis,” Trusty said. “Now you’re seeing the horizontal and vertical rotary table machines with a two-axis head, along with the three or four linear axes... . A lot of those machines can’t apply six or seven axis simultaneously, but they can position the part with two rotary axes so that the other two axes in the head can achieve its required destination, like getting into a pocket.”
It’s a frequent refrain in multi-axis machining: Most five-axis machines are generally not cutting with five-axis simultaneous interpolation. Instead, two axes arrive at a temporarily fixed position and then three axes move through the cut. Hence, the designation of “3+2 axis machining.”
Likewise, Trusty noted, most six-axis machines don’t move all the axes through the cut, although he’s never heard anyone use the phrase “3+3 machining.” But the 6th axis “gives you a whole new group of positioning capabilities,” declared Trusty. And there are indeed situations in which you couldn’t make the required cut otherwise. There are even a few cases in which you may need six-axis simultaneous interpolation, he recounted.
Take KRC’s six-axis Helix horizontal machining center. Virtually everything can be customized, while the standard configuration boasts 78” (1,980 mm) of cross-travel in X and 147” (3,734 mm) in Y, with a 72” (1,822 mm) diameter table (B-axis). Vertical Z-axis travel is 66” (1,676 mm) and it carries a two-axis head that rotates 360° parallel to Z, plus swivels ±110° perpendicular to Z. The standard spindle offers 46 hp (34kW) and 12,000 rpm with an HSK100A interface, and you can get up to 30,000 rpm.
KRC delivered one such machine to an aerospace customer that wanted the capability to move all six axes simultaneously. This isn’t typical, Trusty said, but noted one part necessitated it. He described the application as a “very intricate aircraft engine part about three feet (10 m) in diameter. It’s intricate enough that the machining process required six axes just to rotate in and around some pieces of the part.”
Beyond the flexibility derived from a sixth axis, Trusty said it’s also fairly standard to offer multiple cutting heads on larger machines, often with the ability to switch them out automatically. Because each head may differ in features such as speed and torque, as well as axis configuration, having multiple heads effectively adds to the geometric flexibility of the machine as a whole.
For a perhaps extreme example, look to the latest of four machines that Starrag USA Inc., Hebron, Ky., delivered to Metalex Manufacturing in Cincinnati. This new monster, a Droop+Rein T series portal-type machining center, came with 12 machining heads, and Metalex is already considering adding two more.
The 12th head, which Metalex specified just before delivery in order to mill large aluminum rings for an aerospace customer, swivels ±203° and has an integrated motorized spindle operating at up to 18,000 rpm. Starrag described Metalex’s selection of heads as a “wide range.” The heads can be changed out automatically, according to the supplier, and all ride on a C-axis with infinite rotation. Starrag reported that the machine includes “two continuous universal fork type milling heads,” for “heavy machining with up to 1,770 ft-lbs (2,400 Nm) of milling torque.”
The machine’s size is also extreme, with a work area of 748” (19,000 mm) in X, 354” (9,000 mm) in Y, and 118” (3,000 mm) in Z. Starrag considers the “continuous mobile crossbar” to be the sixth axis (W), and cited a travel of 217” (5,500 mm) for “even more flexible, universal work.” The X-axis bed carries two tables, each 236 x 315” (6,000 x 8,000 mm) in size.
The layout is such that personnel can work on them separately. For example, one table can be used for machining, while the other is used for setup or inspection outside the work zone. Conversely, the tables can be used in tandem, extending them to 709” (18,000 mm), to support extra-long workpieces.
What’s more, each table has an integrated circular rotary table of 236” (6,000 mm) in diameter, powered by a 149-hp (111 kW) main drive. This enables rotary machining of components weighing up to 200 tons (181,000 kg). Thus Starrag also supplied turning tool holders from its Dörries product range. Finally, the rotary tables can also be used as a C-axis for precision positioning when milling and drilling. In short, Starrag’s sales director for large projects, Heiko Quack, “is convinced this machine is unique in its size, configuration, and multi-functionality.”
Breaking out of our self-imposed “one spindle at a time” limit, let’s consider the KRC Fusion series. As Marketing Manager Caitlin DeVore explained, these are gantry-style machines that KRC can configure “for a vast range of sizes and for multiple machining purposes. This system offers large format CNC machining with each system tailored as a turnkey solution and configured to match the specific and unique process requirements for each customer.”
Fusion machines feature “a movable bridge and dual vertical masts on parallel rails suspended above the work area.” Each mast carries a machining head, and each head can move in either five or six axes. In addition to milling/routing, a Fusion “machining head” can be an ultrasonic cutting system, abrasive waterjet, an inspection system, an ablating laser, or printing system. DeVore said customers can even add technologies years after machine delivery, as needs arise.
Plastic Prototype “Fusion series hybrid machining centers offer great flexibility as they are highly configurable and employ a variety of additive, subtractive, and adaptive machining technologies within one work center,” she summarized. “This multi-process machining center can complete trimming, drilling, routing, surface milling, marking, and inspection processes all in one machine.”