by Kip Hanson
Considering a five axis machining centre? The choice between horizontal and vertical isn’t as clear cut as it once was
Like many in the industry, I’m a big fan of the traditional, four axis, rotary table-style horizontal machining centre (HMC) design and consider them a far better investment than their vertical three axis brethren. Here’s why:
Compared to a vertical machining centre (VMC), horizontals provide better chip flow. This eliminates re-cutting, increases tool life and provides for more predictable machining processes.
Horizontals don’t suffer the cantilever effect experienced on the typical C-frame VMC, making them more rigid and accurate.
Most HMCs come equipped with relatively large tool magazines and a pallet changer, both of which are relatively easy to expand, and both of which are a prerequisite to any level of unattended machining.
Spindle utilization rates on a well equipped HMC are often twice that of a vertical machining centre.
Further, mounting a tombstone on a horizontal’s rotary table gives easy access to three sides of the workpiece, and allows multiple parts or multiple jobs to be run in the same machining cycle.
Granted, horizontals cost more—often much more—than VMCs, but the cost delta is darned easy to justify, especially if you’re machining microwave oven-sized and smaller parts and have enough work to keep the machine busy. This last part is important, because many newbie horizontal machining centre owners—especially when part of a flexible manufacturing system, or FMS—soon discover they’re chewing through work far more quickly than expected. As in, time to hire another sales rep quickly.
Let’s get small
Yet I’m also a big fan of five axis VMCs. The ability to machine all but the bottom of a workpiece in a single operation reduces work-in-process, improves part quality, slashes fixture and labour costs and opens doors to work that would otherwise be unattainable. In fact, if I actually owned a job shop and didn’t just write about them all day, I like to think that my production floor would be filled with HMCs (preferably attached to a linear pallet system), five axis VMCs, and multitasking lathes. Throw in a wire EDM, a Swiss-style CNC lathe or two, and there’s practically nothing that a well-equipped machine shop couldn’t produce.
And yet, if HMCs are so great, why not skip vertical machining centres altogether—five axis or otherwise—and go completely horizontal? Good question, and as with most things manufacturing, the answer depends on the application. Let’s start with size. Practically all of the five axis horizontals on the market today have tables measuring 500 mm and above (most start at 800 mm). By comparison, there’s no shortage of five axis VMC builders offering 300 mm and smaller tables.
Why is this important? If your shop makes only smallish parts—say those the size of a coffee cup and below—you’ll probably want an equally smallish machine. If not, clearance can become problematic (as it often is with five axis machining, regardless of the machine type or brand) and the generally lower spindle speeds of a horizontal might also be an obstacle. Granted, this is a bit of a generalization, but the takeaway is clear—for smaller work, a five axis VMC is the clear winner in terms of usability.
But hold on…
So what about chip flow? Palletization? Tool capacity? Must my imaginary shop give up the advantages of a horizontal just because it’s making orthopedic implants or other small, complex parts that are best machined on a five axis VMC? Not at all. According to John Einberger, product line manager at Makino Inc., there’s no reason to sacrifice these and other benefits if you buy the right machine tool.
“Due to their larger masskinematic layout differences, horizontals may well have the advantage in terms of rigidity, but for smaller parts and lighter chip loads, you need a more nimble machine,” he says. “Our DA300 five axis vertical, for example, has a standard 20,000 rpm HSK-A63 spindle, 60-tool magazine and is easily adapted to a pallet system. This makes it quite popular with medical manufacturers, aerospace job shops, die and moldmakers and pretty much anyone that wants many of the advantages of a horizontal machining centre but with a compact footprint and higher spindle speeds.”
As far as chip flow, he admits that HMCs have long had the upper hand over vertical machining centres, although with the DA300 this isn’t the concern it once was. “The chip flow advantage of the typical HMC becomes less important with many most five axis VMC applications, as the workpiece gets rotated around in multiple orientations and the chips can fall away into the machine’s center chip trough,” Einberger says. “Add it all up, and five axis VMCs start to make a lot of sense for many applications, even in higher volume production situations.”
It’s all relative
Errol Burrell, machine centre product specialist for Okuma America, sees things a little differently. He’s quick to point out that all of the machine tool builder’s MU-series five axis VMCs are just as accurate as their horizontal counterparts. All can be equipped with extended tool magazines and high speed spindles. And all can be adapted for use with an automatic pallet changer or FMS; in fact, Okuma’s newest addition to the MU family—the 1.4 meter-wide (55 in.) MU-S600V—comes with built-in robotics for passing parts from machine to machine.
“The benefits of five axis vertical and horizontal machining centres are identical,” he says. “Choosing one over the other largely depends on the application, the size of the part, the available budget, and often, personal preference.”
That said, Burrell notes that five axis HMCs are especially popular with the aerospace industry, in part due to the perception of improved chip flow, the other due to larger part size.
Andy McNamara, national sales director at Doosan Machine Tools America, agrees. “In terms of five axis machining centres, it’s almost all aerospace right now,” he says. “Our DHF 8000 has a nodding head design that lends itself well to larger parts, where you want to avoid swinging a heavy mass all over the place, and comes standard with a 6000 rpm geared head spindle for heavy cuts in aerospace superalloys. And our 30,000 rpm, 100 hp HFP 1540 has a 4000 x 1500 mm table (157 x 59 in.) that’s ideal for any shop doing high volume aluminum structural components. With that in mind, we’ve also been seeing a lot of activity with our DVF 5000 and other five axis verticals, a fair amount of which is in the aerospace sector.”
McNamara points out that five axis machining centres, whatever their spindle orientation, are popular with aerospace shops because the materials are expensive and the tolerances tight. “I worked with a customer just yesterday where the stock itself cost $16,000, and that’s for aluminum,” he says. “If it had been Inconel or titanium, it would have been several times that amount, and that’s before machining. You can’t have a lot of mistakes in this arena, so you want the ability to machine as much of the part in one operation as possible. That’s what five axis brings to the table.” SMT
