Engineers who have designed small precision parts may already know something about Swiss-Turn Machines. Unlike most lathes, which feature a fixed headstock, Swiss-Turn Machines have a sliding headstock consisting of a collet to hold the work material and guide busing. During machining operations, metal or plastic bar stock is fed through the bushing while the cutting tool translates in and out to make the necessary cuts.
These machines started out as a way to make small, close-tolerance parts for the watch industry. Over the years, though, the machines have been put to use making precision-turned components for a wide variety of medical, electronics and industrial uses.
And precision is the key here. Modern CNC Swiss-Turn Machines can often hold tolerances within 0.0002 inches. They offer good surface finishes, too — about 16 Ra on stainless steel.
Usually, Swiss-turn parts are self-selecting. "Engineers pick Swiss-turn machining when they have to," says Ken Mandile, an engineer and president of SwissTurn USA, a contract manufacturing firm specializing in Swiss-style machining. He goes on to explain that the most obvious Swiss-turn parts are those whose tolerances are simply too tight for other types of lathes. Swiss-turn also gets the nod in cases where production volumes are too low to justify the use of a multi-spindle lathe, or "screw machine."
But there's more to Swiss lathes than these obvious cases. Mandile believes that most design engineers think of Swiss-Turn Machines only as a way to produce parts with a circular cross section. "But they can do much more than that," he says.
Today's Swiss-Turn Machines, with both the headstock and cutting tool under computer control, can produce complex milled features in addition to turning. Mandile says these milled features include accurate slots, holes in different orientations to the face of the part, threads and more. "The parts don't even have to be round," he adds, noting that SwissTurn USA routinely produces rectangular, square and even L-shaped parts. "We do a lot of parts that don't look anything like a turned part."
The nice thing about machining parts this way comes down not just to precision, but precision without any expensive fixturing. "You don't have to worry about fixtures, you just index the bar stock and the guide bushing holds it in place for milling operations."
Mandile acknowledges that Swiss-turn machining isn't always the best way to make precision parts. As volumes climb and tolerance requirements fall, other machining methods — or a combination of machining and grinding — may be a better fit. "It's hard to say exactly where the volume and tolerance cut-offs are because it's dependent on geometry, material and lead times," he says. SwissTurn USA has worked on some orders with millions of parts, as well as ones with hundreds.
The most cost-effective use of Swiss-style machining, in Mandile's view, comes when engineers design with a good understanding of the relationship among cost, tolerances and automation. For example, SwissTurn USA runs a lights-out automated operation on many jobs, which helps it compete against off-shore machining on the basis of fast lead times and a competitive, if not the lowest, cost. But the highest level automation isn't suitable for the tightest tolerances possible with Swiss-turn machining. "Anything under 0.0003-inches requires more operator supervision, which can drive up costs," Mandile says. Sometimes it makes economic sense to ease off the tight tolerances where possible. And when it's not possible to adjust tolerances, it can be more effective to start with fully automated Swiss turning and then employ a grinding operation to reach the finished dimensions.
"Engineers should carefully evaluate the relationship between automation and tolerances when they decide which machining method to use," Mandile advises.
For more information on Swiss-Turn Machines, visit http://rbi.ims.ca/4928-517.