Part 1 of this three-part medical machining series from Sandvik Coromant explores why cutting tools fail and what to do about it. Part 2 takes an in-depth look at cutting tool selection, how to keep cutting tools cool, and management of the cutting processes.
Broken hips, skull fractures, spinal injury, and worn knees and elbows. The human body is an imperfect machine, subject to trauma and degenerative disorders that only decades ago may have permanently impaired us or worse. Today, treatment of these and other unfortunate conditions often requires little more than a brief visit to the hospital and some physical therapy. Once-debilitated patients are good as new.
The majority of the pins, screws, and implants used to repair humans are made from metal. Titanium and cobalt chrome alloy, 300-series stainless steel – and a host of proprietary alloys are machined, forged, and formed into a variety of life-changing, often life-saving devices. But compared to many metals, these tough alloys are difficult to process. CNC machine tools must withstand extreme forces when carving up parts such as a Steinmann pin or threading a cannulated screw. Cutting tools wear down quickly, and accuracy suffers as a result.
Titanium and cobalt chrome are especially challenging to cut, belonging to a group of materials known as heat resistant super alloys (HRSA) which are widely used in the aerospace and energy industries. The subset of HRSA employed for medical purposes is also biocompatible, able to remain in the human body for months or even decades. And, since surgeons are very precise people and the products they use are complex, parts that go in those products must be extremely accurate and tightly regulated. Added together, these factors make HRSA tough to manufacture.
Cutting edgeWhether parts are made of 22Cr-13Ni-5Mn stainless steel or Ti-6AL-4V titanium, the cutting tools used to machine them must be strong and wear resistant. Fortunately, the causes of tool failure when machining medical grade materials are well understood and can often be remedied through application of high quality carbide grades, proper feeds and speeds, and sound machining practices.
One common failure mode is built-up edge (BUE), which occurs when a layer of workpiece material welds itself to the cutting tool. When this sticky glob of metal breaks away, it usually takes some of the underlying carbide with it, causing edge chipping or breakage. BUE is especially problematic on alloys with high amounts of nickel, such as 300- series stainless steel, a favorite among medical instrument makers.
The best way to avoid BUE is to increase the lead angle of the cutting tool. For example, face milling with a 45° tool rather than a square shoulder cutter greatly increases tool life. An indexable milling tool equipped with round inserts can withstand substantial abuse and generally provides the lowest cost per cutting edge, provided the machine tool can handle the higher cutting pressures of such a tool. An increase in cutting speed can also help solve BUE problems, but this in turn creates additional heat in the cutting zone and may lead to greater tool wear, making achievement of optimal cutting conditions a delicate balancing act.
Notching at the depth of cut line is another typical problem with HRSA machining. When taking a 1/8" deep cut with a shell or face mill, for example, a distinctive notch forms in the face of the insert the same distance from the cutting edge. Similarly, edge chipping due to shock as the cutter enters and exits the workpiece is fairly normal. Round inserts and increased lead angles help in these situations as well, but just as important are smooth, consistent cutter loads. (See sidebar, pages 12-13, for more details.) Also, climb milling should be used whenever possible. This generates a chip that is thick at the start of the cut and gradually tapers to nothing as the cutter exits the workpiece. This chip-thinning technique reduces heat and prevents rubbing and cutter deflection, all of which improve tool life and part quality.
Even after these problems have been resolved, tools simply wear out more quickly when cutting HRSA. It’s tough stuff. Work hardening around the cutting area is common, as is cratering of the tool nose, caused by the abrasive action of workpiece material as it passes over the cutting tool surface. As a rule, sharp tools with a positive shearing action are recommended, but this can be like using a steak knife to cut concrete; it doesn’t take long before even a razor-sharp edge is reduced to that of a butter knife. Look for cutters and inserts with a very slight hone or edge break. This prevents the micro chipping that often precipitates more catastrophic tool failure. Also, multi-layer PVD coatings such as TiAlN provide a hard but lubricious surface that greatly extends tool life, yet retains the up sharp edge needed to machine HRSA.