Bars Don't Just Feed Into Lathes

Ever thought of fitting a fixed table, moving column vertical machining center with a bar feeder? Maybe it is as far out an idea as connecting a pallet shuttle to a turning center? Far out? No, maybe you already know both things have been done.

A French company—no longer with us—tried the second idea in the early 1970s. The first idea is less complicated, and a Swiss company—Willemin-Macodel, in Bassecourt—has been building VMCs with bar feeders for some years. Then at EMO, in Hanover, Germany, in September 2001, Hermle of Gosheim also showed its "new concept": a VMC with a bar feeder. As far as I know, apart from homegrown, in-house, do-it-yourself lash-ups, these are the only two regular makers of bar-fed VMCs.

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The idea with both machines is that you can machine a five-sided component from bar stock in "one hit" much easier than loading individual billets to a chuck on an NC rotary table. Some of Renishaw's engineers had thought about this in the 1970s and began to five-side machine components for probe bodies and other jobs from bar in a horizontal NC chuck mounted on a Hitachi Seiki machining center table. Robots later took over the load/unload jobs.

In Japan, workshops buy in faced—and centered—billet stock from the their metal suppliers. The billets are supplied in trays ready for simple "pick-and-place" robots to load into CNC lathes, turning centers and machining centers. Bar feeders are rare.

Returning to our bar-fed VMC, once the job has been machined, a fast tungsten carbide circular saw can cut off the job from the parent bar much quicker than parting it off. Many of you probably are thinking that you can machine all six sides in your twin-chuck turning center.

True, but the VMC provides more milling spindle power—40 kW and 16,000 rpm on the Hermle S100M—to machine bar of 9/16 inch to 4 inches. That is more milling spindle power than you would get with a milling head on the average mill-turning center with a 4-inch bar capacity.

The VMC can readily perform simultaneous three-axis machining. The user would need a turning center with Y-axis facility to imitate the VMC. The Hermle and Willemin machines can also perform 3 + 1 and 3 + 2 axis CNC machining and, say the makers, offer high rigidity and fast machine spindle orientation.

Then there are time cycles. In machining jobs where the only turning function is parting-off and one or two circumferential grooves, check to see if the grooves can be interpolation milled so as not to tie up a turning center. It depends on the groove tolerances. Interpolation milling may not produce a smooth enough circumferential profile shape.

Another consideration is tool change time. There are very few mill-turning centers—with 40 kW milling head capacity—that can exchange tools with a tool magazine as quickly as the standard VMC. Also, if the potential customer already has VMCs in the shop, it is a fair bet that existing tooling can be used in the bar-fed VMC.

So what jobs are machined in bar-fed VMCs? Willemin's W418 five-axis VMC will feed two bars and can turn and grind jobs as well as machine them. In Europe, the machines are turning, milling and grinding the cup faces in four-wheel drive vehicle drive shaft couplings from bar of 90 to 100 mm in diameter. Willemin has more than 100 machines installed in the United States. Hermle demonstrated the S100M at EMO—with a prototype live workholding chuck with turning speeds up to 4,000 rpm—machining tool bodies. The Hermle machine uses short bar stock up to 1,100 mm, fed from a seven-bar magazine. The main difference between the two appears to be that Willemin uses a swivelling head while the Hermle machine uses a three-jaw chuck workpiece carrier mounted on a vertical slide and swivels through a 270-degree B axis.

Bar-fed machining centers? Someone's using them—maybe they are worth a look!