Swiss-Style Turning: The Prescription for Medical Parts Manufacturing

Medical component manufacturing requires a machine flexible in its design that it can create any geometric shape that can be conceived.  Swiss-style lathes represent a key process for manufacturing many medical parts.  Bone screws are one type of medical component that uses Swiss machines.  This is due to the productivifty of the machines and ultimately being more cost-efficient for the manufacturer.  It is estimated around 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. 

A Swiss-style lathe is known for its guide bushing and a sliding headstock.  The strength of the Swiss machining process is the material supported by the guide bushing.  Stationary tools cut the bar stock as it passes through the bushing.  One of the main advantages of this machine 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 in some cases eliminates chatter.  Medical component manufacturers can use these machines for a wide variety 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 drastically 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, part of the gang tooling arrangement on Swiss machines and the C axis, which is standard on most machines, allows milling to 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. 

Deep depths of cut are common with Swiss-style aches since turning must often be done to finish depth in one pass.  The 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 can wrap around the part and prevent coolant from reaching the cutting zone.  Over the past 10 years, high-pressure coolant has become a mainstay on many Swiss machines.  This coolant is a must to help control stringy chips when drilling deep holes.  High temperature alloys and titanium, which are common in medical components, require high-pressure coolant because of the high temperatures produced in the cut and by directing chips away from the cutting zone.

Medical components can be difficult to produce for a few different reasons:  materials and geometry.  Materials such as titanium, one of the primary materials for bone screws, and plastic have unique characteristics such as 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.  Swiss machines have a distinct advantage, there are other challenges such as bone screw designs evolving and the use of multi-start threads.  In multi-start threads, 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, triple-start thread three times, etc. Two-start threads are the most common but more frequently bone screw designs include three-start threads and some engineering requests are being made for four-start bone screws. 

The process of cutting double-start threads on a Swiss machine begins with insert design.  The insert design is 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 can be 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.  Multiple-start threads are cut with inserts that have the multiple forms ground into the insert.  Tooling manufacturers are striving to keep up with the advancements. 

Medical parts are often delicate which can create problems when they are picked off with the subspindle for backwork.  It is very important but difficult to apply just the right amount of pressure to hold the part in the subspindle.  Once the optimum gripping force is determined, it is not easy to communicate to other machine operators how the adjustment was made.  Another challenge is making dental implants because of their small size. 

Swiss-style machines have been combining turning and milling in one machine for a long time.  Now machine tool manufacturers have introduced laser options to be even more precise.  It will now be possible to make stents on Swiss machines due to the cutting of solid stock for the tubing.  This can be handy in prototyping because the inside diameter of the tubing can be bored out to that size to make small batches.  Swiss turning machines have evolved greatly from making small components in the watch industry to now being used in medical manufacturing. 

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