Small machining centres often offer higher performance than larger general purpose machines.Click image to enlargeby Kip Hanson

Why average sized machining centres provide lower than average results when milling small workpieces

Swiss-style lathes are getting a lot of attention these days for their ability to turn small, complex parts. Everything from tiny bone screws to precision actuator shafts can be quickly and efficiently cut from bar stock no bigger around than a person’s pinky. The problem is, many small parts aren’t round. Micro injection moulds, miniature impellers, small firearm parts–these are often produced on machining centres big and beefy enough to cut a stainless steel doghouse. This is like trying to win the Grand Prix while driving a semi-truck.

Lean, mean, milling machines
“Small machining centres are lean,” says Bryan O’Fallon, product technical specialist at Haas Automation Inc. “They have compact footprints that allow multiple machines to be placed side-by-side for efficient use of valuable shop floor space. And in many applications, they offer considerably higher performance than larger, general purpose machines.”

The Oxnard, CA machine builder markets several styles of small machining centres, including the Mini Mill and Office Mill, the latter of which offers spindle speeds up to 50,000 rpm. This, together with smaller, less expensive spindle tooling, is a feature O’Fallon claims is popular with shops making small dental and medical products.

Shops purchase small machines for one of two reasons, he says: they need an entry level price point, or a solution to a specific machining challenge. In either case, they are likely to be pleasantly surprised with the results. “We have a number of Office Mill customers in the jewelry and watchmaking industries, but these machines are also quite popular for secondary operations on larger parts. Some shops use them to remove the dovetail feature commonly used to grip parts during five axis machining, for example, and for drilling, tapping, and even general purpose machining. Their high speeds keep cycle times to a minimum, which is always an important consideration for manufacturers.”

Sample of a part made on a Datron machine.Click image to enlargeA machine by any other name
A number of machine builders–Haas Automation among them–offer what are often called drill/tap machines. In certain high volume situations, that’s exactly what these machines do: drill and tap small holes very quickly. But in many shops, these compact machines are often repurposed into machining roles traditionally reserved for their big brothers, pinch hitting to meet the demands of overflow work, purchased because there’s too little available floor space, or simply because these machines are better at making small parts.

“It’s a rule of thumb: the smaller the part, the more accurate the machine making it needs to be,” says Sergio Tondato, director at Fanuc
America Corp., Rochester Hills, MI, who says “small” is a subjective term.

“What may be small for some machines might be considered large for others. Yet if you consider a ‘typical’ small part as anything under one inch cubed (25 mm), then you need a machine with plenty of spindle rpm and able to execute very small movements quickly. That in turn requires an extremely accurate positioning system, and a control that can process information quickly.”

Tondato says Fanuc’s Robodrill was designed with these attributes in mind. Shops that manufacture injection moulds might recognize these machine features as the same ones necessary for mouldmaking: intelligent look ahead during contouring, precise motion control, high spindle speeds and extreme accuracy. “The fastest growing segment for small machining centres is in the automotive market, but we also see strong growth in consumer products, electronics, medical parts and firearms,” he explains.

“And yes, many mouldmakers are using them to produce electrodes and small injection moulds, often at a much lower cost than with traditional machining centers. Fanuc general manager Lou Finazzo agrees, adding that shops sometimes buy machines for the wrong reason. “We work with customers all the time who are looking to buy a larger machine just so they can address the parts at the upper end of the size spectrum. ‘I’ve got to have a 40 or 50 taper, just in case I get some bigger parts,’ they say. What I say is the price versus performance ratio is far better when you buy the machine tool that most efficiently machines 80 per cent of your parts.”

Don’t judge a book by its cover
Neal Demazure, sales supervisor at Datron Dynamics Inc., Milford, NH, says looks can be deceiving. “You look at it and all you see is this small box. You have to remember there’s a twenty-two position tool changer in there, with linear and rotary scales, a 48,000 rpm spindle, and five micron accuracy. It has everything that comes with a big machine, but jammed into a little machine that’s purposely designed for small part production.”

He’s talking about the ultra small C5, a five axis machining centre designed for complex medical, micromechanical, dental implant, and similarly demanding parts. Demazure says he fights the same fight as other purveyors of super small machine tools. “Job shops in particular want machines that can deal with a wide variety of work. Many of them look at a machine like this and say, ‘why would I spend $200,000 on a machine the size of a phone booth, when I can buy a bigger machine that might not be as good at the small parts, but will give me far more flexibility?’”

His answer? “I’m a big advocate of the maxim, ‘there’s a job for every tool and a tool for every job.’ And I think manufacturing folks in general are used to general-purpose machine tools, a category into which the majority of machine tools fall. In many cases, using large machines to produce small parts is like using a sledgehammer to pound in a four-penny nail.”

Demazure says shops can make themselves far more competitive by right-sizing their machine tools to the job at hand. Not only does this approach provide more accurate parts more quickly than is possible on a “one size fits all” machining centre, but it saves valuable floor space, reduces electrical consumption, and offers improved tool life as well.

“Just yesterday I walked into a shop that’s been in business since 1970,” he says. “They have two dozen or so different machine tools shoehorned into this building and, like many shops, are running out of floor space. Nobody wants to move, or pay for a new building, right? So the name of the game is, how do you produce more stuff in the same amount of floor space?”

When you consider that the vast majority of parts today are smaller than a shoebox, the answer is clear. Mini machining centres are fast and accurate, consume minimal floor space, and are generally more cost effective than their jumbo counterparts. Simply put, stepping into one of these phone booths might just make you a Superman. Maybe it’s time to get small. SMT

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