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Swiss Mix: Niche Units Evolve Into Machining Centers

Ed Sinkora
By Ed Sinkora Contributing Editor, SME Media
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The Index Traub TNL20-11 can engage up to four tools simultaneously for shorter cycles and apply up to 58 different tools for complex parts and part families without tool changes.

B-axis milling … lasers … low-frequency vibration cutting … multiple tools in the cut, on multiple axes … thread whirling … wobble broaching. Who said Swiss-style machining was for millions of simple, round parts? As industry veteran Paul Cassella explained, “Today, Swiss-type machines are more like machining centers and the parts are more prismatic. I’ve seen setups where the only time we rotate the bar stock is during cutoff. Everything else is being machined by milling, cross drilling, or other similar operations.” So how have Swiss-style machines evolved? And what advantages do they offer versus traditional machining centers?

Bar Stock and Bushings

Cassella, the applied technology manager for Tornos Technologies US (Lombard, IL), pointed out that Swiss-style machines have the advantage of being able to work from bar stock, as opposed to having to create and feed blanks into a machining center. And because the sliding headstock allows them to consistently cut the part close to the guide bushing, Swiss machines deliver extremely good accuracy with high material removal rates.

One knock on Swiss-style machines is the need for ground bar stock in order to ensure that the bar neither binds in the guide bushing nor floats, ruining the machine’s ability to hold the diameter. But Cassella explained that if the part’s length-to-diameter ratio is under 3:1, “95% of the Swiss-type machines on the market allow you to remove the guide bushing, push the headstock forward, and work like a chucker. So for a short part you can work right off the headstock.”

What’s more, Cassella explained that now there are also self-adjusting bushings that “let you use unground stock and still hold very tight tolerances.” In particular, he pointed to the DunnAir system from Walter Dünner (Moutier, Switzerland), which accommodates variations of up to 0.5 mm (0.020″) in bar stock diameter. According to Dünner, the system maintains concentricity within 0.015 mm (0.00006″) when using a standard guide bushing and 0.005 mm (0.00002”) with an ultra precision bushing. If the operator machines the entire part using a standard bushing, the out-of-round is at most between 0.002–0.004 mm (0.0008–0.0016″).

Cassella said he hasn’t tested these claims himself but knows the DunnAir and similar systems are in at least limited use in the market. “Whether you use a sliding headstock and a guide bushing like a standard Swiss-style machine, or a fixed headstock and a collet to secure the bar, the physics dictate that the accuracy depends on the quality of the bar stock and how tightly you can adjust the bushing or collet to that bar stock. But a guide bushing will hold concentricity much better than a chuck or a collet on longer parts that require re-clamping,” he said.

Attila Catto, president of Tornos Technologies US, summarized guide bushing tolerances as follows: “A guide bushing can hold 75% of the bar stock tolerance. So if a customer wants to hold 0.0003″, the stock should be ground to 0.0005″. If the customer wants to hold 0.003″, any decent unground stock will hold those tolerances.”

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The Dünner DunnAir system accommodates variations of up to 0.5 mm (0.020″) in bar stock diameter and maintains concentricity within 0.015 mm (0.00006″) when using a standard guide bushing and 0.005 mm (0.00002″) with an ultraprecision bushing.

Multiplying Tools & Spindles to Divide & Conquer

Perhaps the most obvious difference between today’s Swiss-style machines and yesteryear’s (aside from CNC control) is the multiplicity of cutting tools that can be thrown at any given part. The new TNL20 from Index Traub (Noblesville, IN) is a notable example. In its simplest version, the TNL20-9, two turrets (each with eight tool stations) move independently in X, Y, and Z. Each of these 16 stations can have up to three stationary tools or two live tools, and either turret can operate on parts held in either the main spindle or the counter-spindle, separately or simultaneously. Plus, a back working attachment on the lower spindle features four toolstations. As a result, a back working tool can be cutting a part in the counter-spindle while a tool on each turret engages a part in the main spindle. And in case you’re not following the math, you can choose from among 52 tools in this configuration.

A second version, the TNL20-11, has an additional front working attachment on an autonomous X/Z slide and an interpolating swing axis (H). This attachment has six stations, three of which are live. By interpolating the H-axis move with the X-axis slide, the machine can position the cutting tool where needed for machining on the main spindle, enabling up to four tools to be in the cut simultaneously to reduce machining times. And since the holders in the turrets can again hold up to three tools, the TNL20-11 can deploy up to 58 tools, enabling the machining of complex workpieces or part families without tool changes or major setup effort. Also, since the turrets in both the TNL20-9 and TNL20-11 move in Z (in addition to X and Y), standard tools can be used to cut complex forms, further reducing costs and simplifying setup.

Tornos’ latest entry in this arms race is the MultiSwiss, which can be thought of as a hybrid between traditional Swiss-style machines (which have a sliding headstock) and traditional multispindle screw machines (which have fixed spindles in a rotating barrel). Instead, the MultiSwiss features the unique combination of multiple spindles that both rotate in a barrel and slide independently in Z. Like the Traub TNL20 machines, this Z-motion also gives the MultiSwiss the ability to machine complex forms with standard cutting tools. In its most complex configuration, the MultiSwiss can offer eight spindles (26-mm bar capacity) and eight tool stations, each of which can have up to four tools and move independently in X and Y. The counter spindle moves in X and Y and holds up to four tools, two of which can be live. (The highest possible tool total is actually 31 on the main stations and four for back operations.) So depending on the part geometry and the programming, you could theoretically have nine tools in simultaneous operation. The barrel indexes between tool stations in 0.4 seconds, thanks to a torque motor.

For a simpler layout that still offers tremendous flexibility, several builders offer nine-axis versions of the “standard” Swiss-style design by splitting the toolposts along the main spindle into two independent X-Y-Z movements, while retaining the counter-spindle and the back-work toolpost and its three axes. The Tsugami BW129Z is an excellent example, as Derek Briggs, presales engineering manager at Rem Sales (a division of the Morris Group in Windsor, CT), explained: “The nine-axis machine gives you the ability to apply two different stick tools on the part simultaneously. So, for example, you can rough turn and finish turn at the same time. Or you can pinch turn, or pinch mill, using opposing tools to counteract each other to reduce deflection.” Add the counter-spindle and you can have three tools in the cut at the same time and up to 28 tools to choose from in a given setup. Yet the machine takes up only 32.5 ft2 (3 m2), excluding the bar feeder.

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The Tornos MultiSwiss offers a unique approach that combines up to eight spindles that both rotate in a barrel and slide independently in Z, giving the machine the ability to machine complex forms with standard cutting tools. The eight toolstations surrounding the spindles move independently in X and Y and can have up to four tools each.

B is for Versatile

Another big change to Swiss-style machine design in recent years is the addition of a B axis, making it possible to approach the part at an angle. Besides enabling features (like an angled hole) that would otherwise be impossible in a Swiss machine, Cassella explained that “a B axis gives you the ability to machine irregular shapes via back machining, rather than clamping in the back spindle. That is important because the back spindle may require a greater clamping length than what is available with the desired part geometry. Some parts, including dental abutments, are quite difficult to clamp in the counter spindle, and we have often been forced to make the part longer in order to clamp it there and then cut that extra material off. So the addition of a B axis has been a great help because you can angle your cutting tool exactly where you need it, and with CNC control.”

A B axis is often an option on a machine, to include the Traub TNL20-9 discussed earlier (making it a TNL20-9B). In that case the upper turret is able to swing vertically 105°, so the tool can approach the part in the main spindle from any angle, including straight on as if it were in a front working fixture. Tsugami’s approach in its SS207 line is to use the area that would have been occupied by the rear tool post beside the main spindle to accommodate a fixture that swivels horizontally 135° and also moves in X and Y. The B-axis fixture drives three rotary tools on the front and three on the back, while stationary fixtures hold a variety of additional stick and rotary tools, including tools for machining the part in the counter spindle.

Cutting Threads in One Pass

Another key to the improved versatility of Swiss-style machines is the addition of specialized tooling attachments like thread whirling heads. Such heads use multiple inserts to cut a thread quickly and without burrs and require a high-frequency spindle operating at up to 30,000 rpm. Cassella said “Tornos developed thread whirling 25 years ago and it’s been a big factor in our success in the medical market for making bone screws. But now it’s a common feature on many Swiss machines.

“Depending on the screw, you may not even turn the part and can cut the thread in one pass, right at the bushing, using the Z axis to feed stock through the whirling head,” he continued. “The feed rate determines the pitch of the thread. On the other hand, there are new screws on the market with double lead, with one wiped out at the beginning, so the inserts are becoming quite elaborate. We’ve mounted up to three thread whirling attachments on our machines to cut the geometries required by some OEMs.” Cassella added that thread whirling is much faster than grinding and also superior to rolling, because rolling can trap impurities in the metal, making it unacceptable for most medical applications.

“There are a variety of thread whirling heads on the market,” explained Cassella. “GenSwiss (Westfield, MA) sells a unit made by Utilis, another Swiss company. We use Schwanog (Elgin, IL) heads quite a bit. And we still offer our original thread whirling heads, which have three circular cutters, each of which can be reground 30-40 times. The other systems use six standard turning inserts, so they cut faster, but that approach can be much more expensive because when the inserts are worn you throw them out.”

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Thread whirling heads use a high-frequency spindle and multiple inserts to cut a thread quickly and without burrs.

Cutting Splines, Keyways and Polygons

Another helpful attachment performs rotary broaching to cut non-round shapes. Cassella said a better name for the technique would be “wobble broaching, because the broach literally wobbles. It’s set at a 1o angle, which reduces the broaching force. We get these heads from different sources, depending on the customer’s needs. GenSwiss sells one made by the Swiss company PCM. Here in the US a lot of people use Slater [Clinton Township, MI] and Somma Tool [Waterbury, CT].”

A common application (again medical) is the hexalobe form often used in bone screw heads. But, interestingly, Cassella said that while some people broach this form, Tornos prefers to mill it with a high-frequency spindle from NSK, IBAG or Meyrat.

Automatically Breaking the Chip

As Cassella put it, “chips are a constant problem in Swiss turn machines, especially if you want to run untended.” Happily, the modern controls used on many Swiss-style machines, including FANUC and Mitsubishi, offer a capability that deals with this easily and automatically. Called “low-frequency vibration cutting,” the machine uses the X and Z axes to rapidly oscillate the tool axially in the cut. This vibration effectively creates tiny periods of “air cutting,” which break the chip. Cassella added that it’s best to use this kind of technology with synchronous motors, which have acceleration and stoppage times roughly four times shorter than asynchronous motors.

If your control doesn’t offer low-frequency vibration cutting, Cassella said M4 Sciences (West Lafayette, IN) offers a similar approach you can add to any machine. “They call it TriboMAM and it fits into your toolholding system like a collet extender. We’ve used it for gundrilling and it makes it a much faster process. It’s a thirty-thousand-dollar option, but it works and it’s very simple.”

Swiss Turning, Laser Cutting, and Laser Welding on One Machine Center

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The Tsugami LaserSwiss combines full Swiss machining capability with laser cutting and/or laser welding, creating entirely new manufacturing possibilities.

The addition of laser cutting capability to a Swiss lathe is potentially game changing and Tsugami Automation (another division of the Morris Group)started a “skunk works” laser project in 2012. The Tsugami LaserSwiss (based on the Tsugami S206 platform with six linear axes) was introduced in 2014 and combined full Swiss-style machining capability (with up to 36 tools) and a fully integrated laser cutting head. That laser could accurately cut thin-walled tube with a kerf width of only 0.002″ (0.0508 mm) without any burrs, backside dross, or slag.

As Dan Walker, Tsugami Automation’s business development director for the LaserSwiss, put it, “The LaserSwiss is like having a two-thousandths end mill that never gets dull. So we’re able to push the envelope in terms of size, to do things that you couldn’t normally do. And having the functions of milling, turning, thread whirling, and more in the same machine makes it possible to do everything in one operation.” And that “everything” now includes laser welding and laser marking.

Regarding welding, Walker explained that “There are many components, particularly in medical devices and instruments, that require the laser welding of one component to another. We can introduce a deep-drawn cap or a tip for an instrument into the machine via a magazine or a bowl feeder, clamp in the counter-spindle, bring it over to the tube that we’re cutting in the main spindle, and laser weld it into place. The item can be lined up to do a butt weld, or inserted into the tube and then welded, even with dissimilar metals. We’ve done torque testing and microscopic inspection and the quality of the weld is fantastic. It’s ideal for medical instruments, including biopsy tools.

“The welding head can also be used for what we call re-flow,” he continued. “For example, we can swage the material, making it into a spear or a domed end, and then laser weld it to eliminate any micro cracks that may occur during the swaging process. You can also laser etch the part in addition to cutting it. We have a current project on an industrial application that requires part marking. In this case, the machine will be programmed to produce parts of varying lengths from one tube. The customer wants the length, plus the part sequence, etched into each part. It could also be used for serialization, or adding logos. What it can’t do is QR code type marking. Those are best created by a laser marking system with a scan head.”

Other recent advances further improve the LaserSwiss’ cut quality and speed. “We’ve always had the ability to switch between two different assist gases,” explained Walker. “For example, switching between nitrogen and oxygen, or oxygen and argon. But now we also offer the ability to vary the pressure to anything from 5 to 350 psi (34.5–2413.2 kPa) through the part program and the CNC, to aid in the cutting. For instance, when you pierce metal with your initial cut you want extremely low pressure. Once you start cutting you bring the pressure up to aid the cut. Assigning the proper ratio is what achieves the nice clean cut.”

The ability to focus and defocus the laser head on the fly enables adjusting for material diameter changes, maintaining focus during four-axis cuts, and rounding the cut edges for applications that require it.

Tsugami Automation has also broadened the LaserSwiss portfolio to include models based on other Tsugami platforms (including several 32-mm versions), laser power supplies going up to 500 W, and a B-axis variant for even greater geometric flexibility. The B axis can be configured to include the laser cutting head plus several cutting tools, the laser welding head plus several cutting tools, or both laser heads. Walker also underlined the fact that while they purchase the laser power supply (an SPI high-stability laser rated for less than 1% variation), the Tsugami Automation team in Connecticut developed everything else, including the optics, laser head, control interface, and gas control system. “Everything that delivers the cut is streamlined and fully integrated,” said Walker. “It’s not a third-party thing and it’s not a retrofit. It’s a laser machine.

“The LaserSwiss is what I call disruptive technology,” he continued. “There are a lot of people out there who don’t know they need it yet. It’s changing the way that people think about manufacturing. We have customers that are actually designing parts around the LaserSwiss because we have capabilities that weren’t even possible prior to this.”

Low Frequency Vibration Cutting Produces Smooth Swiss Machining

The capabilities of Swiss-style machines continue to grow as new technologies are introduced. For example, Citizen Low Frequency Vibration (LFV) cutting technology from Marubeni Citizen-Cincom Inc. (Allendale, NJ) prolongs tool life and reduces problems caused by chips in the cutting zone.

The benefits of LFV include increased cutting tool life, reduced heat generation and reduced power consumption. LFV technology can handle a broad range of machining shapes and materials and is ideal for cutting difficult-to-cut materials, providing increased throughput and improved part accuracy, according to the company.

In LFV cutting, the servo axes are vibrated in the axial direction using a unique control technology where cutting is performed while synchronizing the vibration with the rotation of the spindle. Because air-cutting times are provided during cutting, this technique is also characterized by intermittent expulsion of fine chips. This has made it possible to resolve machining problems such as spiraling of chips, chip entanglement and built-up edge (BUE).

In addition to linear machining on faces, Citizen LFV technology can be used in other types of machining, including tapers, arcs, and drilling across various machining geometries and materials. It can be turned on and off by inserting G codes in machining operations that have proven difficult in the past, such as deep-hole drilling and micromachining.

  • According to the company, other benefits of LFV include:
    Malleable materials such as copper and plastic, which previously produced impossible-to-control chips, are easily controlled.
  • Chips are broken up into very small pieces so they do not become entangled with the work material or cutting tool
  • Machining downtime is reduced
  • Machining of very fine workpieces is possible
  • Chip control is programmable
  • Need for high-pressure coolant is reduced or eliminated
  • Machining temperature doesn’t rise, minimizing the possibility of distortion
  • Tool life is extended
  • Very fine cuts are possible
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