A look at Swiss-type turning technology
By Jim Destefani
Swiss-type turning machines were originally developed in the late 1800s to manufacture small precision clock parts. Over the ensuing 120 years or so, the machines have evolved from complex, cam-operated beasts that required skilled operators and setup personnel to fast, flexible CNC tools able to handle part volumes from small lots to high-volume production.
A couple of things have remained constant over that time, though. One is the type of parts produced on these machines: relatively small, complex components requiring a high degree of dimensional precision and surface finish. Another is the complexity of the machines themselves: some Swiss-type turning centers feature 11 or more axes under control, with combinations of gang tools, driven tools, and subspindles. Attachments for tapping, thread whirling, and other operations, as well as bar feeders and other material handling equipment that enables untended operation, further complicate matters.
That complexity may be daunting to new users, but it enables Swiss-type machines to provide "done-in-one" capability on par with other types of multitasking machine tools. And, the ability to produce parts complete is a big factor in the growing appeal of the machines.
"People talk about multitasking," says Mark Saalmuller of Tornos Technologies (Brookfield, CT). "I like to say Swiss machines have been doing that since 1880. But as tolerances get tighter, it's hard to make parts to size if you handle them more than once."
Swiss machines come in turret (Tsugami TMU, left), gang-tooled (DMG Speed 20), and combination configurations.
According to Saalmuller, some users are even taking advantage of the multitasking capabilities of Swiss machines to produce small parts that might otherwise wind up on a machining center. "We're seeing some companies use these machines for parts that would typically be done on a VMC or HMC, but they're small and fixturing is a problem," he explains. "These are parts that might be 3/8" [9.5-mm] square. On the Swiss machine, they feed the bar and hit it with milling tools, then grab it with the subspindle, cut it off, and finish the back side. We've had applications where the user never even turned the main spindle on."
Tornos' MultiDeco 32/6c provides a good illustration of the complexity and multitasking capability of current Swiss-type machining technology. Designed for automotive applications, the six-spindle machine allows simultaneous multiple operations on both the front and the back of the part. The machine is fitted with five back-operation stations, and is said to be the first Tornos machine to incorporate a manipulator and automatic palletization system.
Swiss-type machines are also known as sliding-headstock lathes, and it is this feature that sets them apart from conventional turning centers. Rather than feeding the tool into the workpiece, Swiss-type machines feed the work (in the form of a bar) through the headstock. Cutting is done within a couple of millimeters of a carbide guide bushing that supports the bar. Tool pressure does not deflect the material, and vibrations aren't transmitted to the cutting area. For these reasons, typical jobs for Swiss-type machines tend to be parts with high length:diameter ratios that would be prone to deflection and chatter if produced on a conventional turning machine.
Machines are available in gang-tooled, twin-turret, and combination configurations. According to Jeff Boulden of REM Sales Inc. (E. Granby, CT), gang-tooled machines tend to be lower-cost than twin turret or combination turret/gang machines. Twin-turret machines tend to be used for heavier cuts, and often are capable of heavier cutting operations than gang-type lathes. Because twin-turret machines can hold more tools than gang-tooled machines, they're well suited to production of complex parts that require milling, drilling, and other operations.
Boulden says some twin-turret machines also are available with "half-station indexing," which effectively doubles the number of tool positions available by indexing to a position midway between turret flats. Special holders allow mounting of tools in these positions.
Combination machines are well suited for machining prismatic parts that require, for example, off-center milling operations, and fill a niche between gang-tooled and twin-turret lathes, according to Boulden. The base price of a combination machine is similar to that of a twin-turret machine, with cost savings resulting mainly from reduced tooling requirements, he adds.
Because they're used mainly for small-diameter bar work, Swiss-type machines tend to be relatively low in horsepower. Capacities are increasing, however: some manufacturers offer machines able to handle bars to 1.5" (38-mm) diam. An example is REM Sales' Tsugami TMU1, which can handle 1.5" bars with 15-hp (11-kW) front and rear spindles and a 7.5-hp (5.5-kW) spindle that can power all of the back spindle tools to 10,000 rpm. The nine-axis machine features linear guides on all axes, a 16-tool front turret, and Fanuc 18i-TB two-path control. A 60-tool toolchanger is standard, while an optional 118-tool ATC allows the machine to handle up to 142 tools. All tools in the ATC can be either fixed or driven, allowing use of a variety of tools and reducing setup time.
At the other end of the bar diameter scale, subminiature parts are becoming more common. Some builders are focusing on enabling their machines to handle smaller bar stock for medical, automotive fuel injector, and other applications. An example is the R04 from Marubeni Citizen-Cincom Inc. (Allendale, NJ), which is designed to handle bars as small as 0.5-mm diam. Features of the gang-tooled machine include a 20,000-rpm spindle and linear motor drive on the slides, which is said to eliminate deflection and backlash while providing high-speed capability.
Regardless of bar size, stock roundness and straightness are important. According to REM Sales' recommendations, Swiss machining can improve bar roundness by up to 60% or perhaps more using a driven guide bushing. For cold-drawn bar stock, the company says bar diameter tolerances of +0.000/-0.003" (+0.00/-0.076 mm) will enable users to hold tolerances of 0.001" (0.025 mm) to six-sigma standards. For parts with total diameter tolerances of less than 0.0005" (0.013 mm), the company recommends centerless ground bar stock.
Also checking in with linear-motor drive technology is the Speed Linear series of Swiss-type machines from DMG America (Chicago). The linear drive is said to provide axis acceleration from 0 to 1654 ipm (42 m/min) in only 0.04 sec. Available with from five to 11 axes under control, the machines feature integrated spindle motors that can operate at speeds to 12,000 rpm and can work autonomously and with different cutting speeds. Fanuc's 160i TB CNC is standard, and the machines can put three tools in the cut simultaneously. DMG claims a process tolerance of <0.00006" (1.5 µm) at the part OD.
Another advantage of Swiss-type machines is their ability to run untended, and one of the main ways to make that happen is to use a bar feeder. "When you take a Swiss machine and you put an automatic bar feeder behind it, you've automated your process," says Richard Waite of Methods Machine Tools (Sudbury, MA). "Once you know what tool life is, you can run untended. It's an easy way, even for a very small shop, to pick up additional productivity."
According to Waite, who's been involved with sliding-headstock machines for 14 years, 95% of the Swiss-type machines from Maier Werkzeugmaschinen (Wehingen, Germany) that his company sells are now equipped with bar feeders. "It used to be the other way around," he says. Maier Swiss machines feature a modular design that allows the company to offer five series, increasing in complexity from the four-axis A series to 12-axis E machines. "The majority of our sales volume is taken up with D and E series machines," Waite reports.
According to Waite, the Maier machines also have a unique configuration in the way the turret faces the guide bushing. "Everyone else is at 90º," he says. "Our setup allows us to have a very large Y axis, which gives us the capability, with a standard mill/drill head, to do functions on any angle we want." Waite adds that a Y2 axis for back work provides up to nine tools (six driven), and that E-series machines can have three tools in the cut simultaneously.
Sliding or Fixed Headstock? You Decide...
According to Klaus Voos, VP, Index Corp. (Noblesville, IN), roughly 60 - 70% of the parts currently being produced on sliding-headstock machines don't need to be, because they aren't long and thin enough to require the support of a guide bushing. "Many shorter parts can be made faster and with better quality on a fixed-headstock machine," Voos says.
"The formula we normally use is, a part with length that's more than 4 X diameter needs some kind of support," he explains. "For small parts, that means a Swiss machine. But anything with length less than 4 X diameter can be done on a fixed-headstock machine."
Examples of parts that may be better produced on fixed-headstock lathes (left) and sliding-headstock machines, according to Index Corp.'s Klaus Voos.
For this reason, Index offers users a choice of fixed or sliding-headstock versions of its Traub turning machines. The sliding-headstock TNL 26 and fixed-headstock TNK 36, for example, were developed on the same machine platform, and use identical main and counterspindles, turrets, front and back-working attachments, and toolholders. Operation, toolholders, and programming are also the same, and Index says the machines can actually complement each other on the same line.
Voos also offers one final benefit of the fixed-headstock design: It eliminates the long remnants--often 6 - 8"--that are generated by Swiss-type machines. Such long remnants can become costly if the work material is, for example, titanium.
This article was first published in the August 2005 edition of Manufacturing Engineering