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Swiss-Style Turning: Rx for Medical Parts Manufacturing

By Dave Nelson Contributing Editor, SME Media
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The Laser System L2000 from Marubeni Citizen-Cincom makes possible conventional and laser machining on a single Swiss-style machine. The laser is faster than EDMing and makes possible multiple geometric shapes, minimal burrs and machine accuracies of 0.00001″ (0.0003 mm), according to the company.

Medical component manufacturing requires a machine so flexible in its design that it can virtually create any geometric shape that can be conceived. It is not just a matter of being able to produce these components, but of keeping up with the innovations as they emerge and responding to the challenges they represent—including both those that are economical and technical in nature. Swiss-style lathes represent a key process for manufacturing many medical parts.

Bone screws are one type of medical component that comes to mind when discussing Swiss machines. Why is that?

As a salesman for Productivity Inc. (Minneapolis), a distributor of machine tools and other metalworking products and services, Jack Chermak has focused on medical manufacturing since 1983, advising on process solutions and machine tool recommendations. “The most productive and therefore the most cost-efficient way to make bone screws is Swiss turning,” explained Chermak. “I’d estimate that 95% of all bone screws are made on Swiss turning machines today. This includes facial screws, dental implant screws and screws used to hold orthopedic bone plates in place.”

Traditional Strengths

A Swiss-style lathe is defined by having a guide bushing and a sliding headstock. The strength of the Swiss machining process is that the material is supported by the guide bushing. Stationary tools cut the bar stock as it passes through the bushing. The main advantage of this configuration is its ability to cut components with large length-to-diameter ratios. No taper is introduced to the part and the rigidity of the process significantly reduces or even eliminates chatter. Medical component manufacturers can use this capability for a wide array of parts.

It is common for Swiss lathes to have both sub-spindles and live tools. The sub-spindle allows operations to be performed on the back side of the part, which is inaccessible from the main spindle. Cycle times can be reduced by allowing work that might otherwise be performed on the main spindle to be done simultaneously on the sub.

Live tooling along with the Y axis, which is part of the gang tooling arrangement on Swiss machines, and the C axis, which is standard on most machines, allows milling be performed. Live tooling on a lathe combines turning and milling; on workpieces where there are both turned and milled features, the component can have all operations performed in the same chucking. This significantly reduces handling and improves the geometric location of the milled features to the turned features.

Since turning often must be done to finish-depth in one pass, deep depths of cut are common with Swiss-style lathes. These deep cuts must be made with slow feed rates in order to reduce the power requirement; this combination produces long ribbons of unbroken chips that may wrap around the part and prevent coolant from reaching the cutting zone.

As a result, over the past decade high-pressure coolant has become a mainstay on many Swiss machines. For drilling deep holes, high-pressure coolant is a must to help control stringy chips. Work materials such as high-temperature alloys and titanium, which are common in medical components, also require high-pressure coolant because of the high temperatures produced in the cut and in order to direct chips away from the cutting zone.

Getting the Right Start on Medical Parts

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A GenSwiss nine-pocket thread whirling cutter ring with double-lead, custom form, compound clearance ground inserts installed.

Medical components can be difficult to produce for a couple of reasons: materials and geometry. Work materials such as titanium, one of the primary materials for bone screws, and plastic have unique characteristics, including being poor heat conductors and producing stringy, difficult-to-break chips.

Complex geometries are also common with medical components, including high length-to-diameter ratios. While Swiss machines have a distinct advantage in this realm, there are other challenges involved with medical parts. Innovations in the medical field have driven changes in part designs and builders of Swiss-style lathes have had to innovate to keep up.

For example, bone screw designs have evolved in a number of ways, including the use of multi-start threads. In a multi-start thread, two or more threads start simultaneously and allow far more travel in one turn than single-start threads. A double-start thread will advance twice as far per revolution as a single-start, a triple-start thread three times as far per revolution, and so on.

“Two-start threads are commonplace,” said Chermak. “More frequently, bone screw designs include three-start threads, and engineering requests are being made for four-start bone screws. This puts pressure on whirling technology.”

The process of cutting double-start threads on a Swiss machine begins with insert design: two or more thread forms are ground on each insert. The thread whirling process involves cutting the thread in one pass in order to take advantage of the support provided by the guide bushing. Inserts for multiple-start threads must have the form for each start on the insert. This is a challenge for tool manufacturers because they must make thicker inserts to accommodate the additional forms. For every additional start, the insert must be made thicker.

It is more than just the inserts that make it possible to cut multiple-start threads, according to Scott Laprade, marketing manager, Genevieve Swiss Industries Inc. (Westfield, MA). “Multiple-start threads are cut with inserts that have the multiple forms ground into the insert,” he said. “But you must be sure that the attachment can be set to the helix angle increase required to cut that form.”

For example, a thread with a pitch of 2.6 mm means that, for a double-start thread, the lead is double the pitch, or 5.2 mm. The helix angle increases as the lead increases, so for a 6 mm-diam double-start thread, the helix angle is 16.8° and for a triple-start thread of the same diameter and pitch the helix angle is 24°. Many thread whirling attachments are not capable of that much of an angular adjustment. GenSwiss sells attachments that can adjust ±25°.

Double-start threads are very common and three starts are done as well. Four-start threads are not common yet, but there are a few applications for them. Tooling manufacturers are striving to keep up with these advancements.

Plastic Parts

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A GenSwiss thread whirling attachment adjusting to the required helix angle in-machine.

Swiss machines typically have bar capacities of 32 mm or smaller. These machines excel at small-diameter parts and medical parts are frequently smaller than 32 mm. One difficulty with small parts is that they can be delicate and difficult to handle. As a result, reduced handling can be critical in medical parts manufacturing.

This is what attracted New Age Manufacturing Inc. (Plattsmouth, NE) to Swiss turning. “We make a large number of parts that are used for analytical purposes,” said Dave Wood, owner of New Age. “We needed a multitasking machine that could handle the delicate work we do in plastic components. We have to be very careful when machining thin-walled parts not to scratch or deform them when we transfer the parts from front work to back. Swiss machining methods helped us handle these parts.”

One of the problems with cutting plastics, as well as many high-temperature alloys, is that the material is too malleable to allow chips to break. High-pressure coolant helps to a degree but not with every application. The chips that form during continuous cutting methods like turning, boring, grooving, drilling and threading produce long, continuous chips that are difficult if not impossible to break. This can make operations especially difficult on Swiss-style lathes because the guide bushing makes it impossible to take multiple passes on cuts longer than 20 mm. The full depth of cut must be taken in one pass. If the material is pulled back into the guide bushing after a single turn, the workpiece is no longer supported. The first pass is the finish pass. This often means the feed per revolution required is too low to force a chip to break, compounding the problems imposed by the material.

New technology from Swiss machine builder Marubeni Citizen-Cincom, called Low-Frequency Vibration (LFV), addresses this issue. With LFV, the control synchronizes a vibration of the cutting tool along the path of the cut with the rotation of the spindle to effectively break up the continuous chip. The vibration causes the cutting edge to enter and exit the cut more than once per revolution of the spindle. Milling is always an interrupted cut and because of this the chip breaks up into small pieces. LFV allows a turning operation to become interrupted in much the same way as milling. When the chips break, they fall away, thus removing the issues of “bird-nesting” chips. Chips are easily managed and coolant gets to the cutting zone, improving tool life.

Workholding Advances

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The Cincom L20 Series from Marubeni Citizen-Cincom is available with a Low-Frequency Vibration option that breaks chips to avoid spiraling, chip entanglement and built-up edge.

Medical parts are often delicate, which can create problems when they are picked off with the subspindle for backwork. It is important but difficult to apply just the right amount of pressure to hold the part in the subspindle. And once the optimum gripping force is determined, it is not easy to communicate to other operators how the adjustment was made. Machine tool accessory maker Masa Tool Inc. (Oceanside, CA) has developed a collet system that makes this process easier. The collet is set using a special tool with marked graduations that enable precise adjustments to the gripping force of the collet.

“You can document the micrometer-like adjustment for the collet closure in your setup plan,” said Matt Saccomanno, CEO.  “It’s done the same way every time, regardless of operator skill and without relying on feel.”

One of the more challenging Swiss applications is making dental implants because of their small size. Saccomanno said that Masa recently helped a customer broach a dental implant on the subspindle with the Masa collet system holding only a small land. Considering how must force is applied to the workpiece during a broaching operation, this might have seemed impossible to accomplish. The reason it is possible is that the Masa collet system applies the gripping forces directly over the area where the collet grips the part. The collet system has two main parts, a cartridge that replaces the standard machine collet and a smaller collet made by Masa that is installed into the cartridge. These components are held to a high accuracy and are made by Masa with proprietary grinding methods. Concentricity variation is held to 0.0002″ (0.005 mm).

“This particular broaching application used an over-grip collet,” Saccomanno said. “The process change moved an operation from the main-spindle to the subspindle and so allowed some simultaneous machining not possible previously. Before they used our system, the part was made entirely on the main spindle.”

An over-grip collet opens up to pass over a shoulder or some other feature on a part in order to grip the part past the shoulder. Masa makes over-grip collets that can open up to 4 mm over the chucking diameter. This is the largest difference in diameter in the industry and still holds 0.0002″ TIR, according to Masa.

Expanding Possibilities

Swiss-style machines have long combined turning and milling in one machine. Now machine tool manufacturers have introduced laser options in order to add even more capability. Adding a 400 W fiber laser to a machine with a C-axis adds a great deal of freedom to perform geometric forming on a part. If the laser is mounted on a B axis, the capabilities start to rival that of tube lasers. With a kerf as small as 20 µm, it is possible to make stents on Swiss machines. One advantage of cutting stents on a Swiss machine is that the tubing can be cut out of solid stock. This can be very handy when prototyping because the inside diameter of the tubing can be bored out to size to make small batches.

“Swiss turning machines have evolved greatly from their primary intended function of making small internal components for the watch industry,” Chermak said. “They have replaced cam-type screw machines in many applications and now have a strong emphasis in medical manufacturing.” In other words, there are a growing number of reasons for medical parts manufacturers to go Swiss.

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