Tapping into Medical

Tapping medical parts requires unique considerations not required in general-purpose machining.

Courtesy of OSG Tap & Die

OSG’s Exo-Carb VX carbide taps can thread cobalt chrome.

When tapping medical parts, two words come up repeatedly: small and challenging. Small is for the tiny size of many medical parts, and challenging is for the difficult-to-machine materials from which the parts are often made.

Developing strategies for tapping medical parts, then, typically involves many factors, including the cutting tool geometry, machine capability, toolholder, metalworking fluid and the parts themselves. Medical parts that are tapped include implants, such as hip and knee replacements, and surgical instruments.

Again, small is one of the operative words. “Typically, there are few medical parts that are going to need a tap over ½”, and most of them need a tap under ¼”,” said Ed Goodman, application specialist for OSG Tap & Die Inc., Glendale Heights, Ill.

Key medical part materials include medical-grade stainless steels, titanium and cobalt chrome. Most tap manufacturers have a product line specifically designed for each material. “When you talk about medical parts, some made from titanium might require a totally different tap than those made from stainless steels,” Goodman said.

Material Issues

A large percentage of medical implants are made of titanium alloys. “Its characteristics of being both strong and lightweight make it a very attractive alloy for use in the medical industry,” said Ray Moring, technical sales marketing specialist for Greenfield Industries Inc., Evans, Ga.

One characteristic that makes titanium difficult to tap is its high elastic memory. When tapping, the material closes tightly, or “shrinks,” around the tap, increasing cutting edge wear. “Titanium has a low modulus of elasticity,” Goodman said. “This causes the material to collapse around the trailing threads of a tap. This is a unique characteristic of titanium.”

Courtesy of DMG

An implant for the ankle made of titanium.

The solution is sufficient clearance. “A tap designed for titanium is going to have extra back taper, taper from the chamfer or the cutting edge to the back of the thread section. This provides the clearance necessary to successfully tap titanium,” Goodman explained.

“With that back taper on the tool, the collapsed titanium does not actually make contact with the back of the tool,” he continued. “The chamfer on the front of the tap is what is cutting or forming the thread. Adding back taper does not influence the cutting properties of the tap. It may make the tap a little weaker, but it eliminates the issues involved because of the ‘springiness’ of titanium.”

If the titanium does clamp around the tap and it is not able to rotate, the tap may break. Because tapping is often one of the final operations, a shop already has a lot of time and money invested in the part. If the tap breaks and can’t be removed from the part, the part ends up being scrapped.

The medical field uses various types of stainless steel, mostly for medical instruments. OSG reports that its VC-10 taps are for tapping some of the popular stainless steels, such as 17-4 precipitation hardenable and 15-5 PH. The taps are made from powder metal. The cobalt content in the taps provides additional hardness while the vanadium provides additional wear resistance. A version is also available for titanium.

Precision Dormer also offers P/M taps. “Typically, you use a tap that is produced out of a vanadium powder metal for stainless steel and titanium applications,” said Bob Maxey, director of operations for Precision Dormer, Crystal Lake, Ill. “Vanadium P/M has a homogeneous structure, which increases the toughness, strength and wear resistance. Solid carbide is not typically recommended for titanium and stainless applications because of its brittle nature.”

Taps for stainless steel have a sharp, positive cutting edge for shearing the material. “A tap for stainless steel might have anywhere from a 6° to 8° cutting edge,” Goodman said.

Courtesy of OSG Tap & Die

OSG offers VC-10 taps for tapping stainless steels and titanium.

One other note about stainless steels. While form taps can be applied in some stainless, “we can’t form tap in the medical industry because there is a small gap in the truncation on the minor diameter where bacteria can collect,” Goodman noted.

Cobalt chrome is one of the more difficult materials to tap because it is so hard, and, typically, a solid-carbide tap is used. But the tradeoff is carbide is brittle, so it chips easily.

“I just put some carbide taps in an application where the customer was doing tibial spacers that go under tibial trays for artificial knees in cobalt-chrome material,” Goodman said. “We needed our Exo-Carb VX tap, which is a carbide tap for hard materials.”

Cobalt chrome needs additional support because it is hard and brittle. Therefore, the tap has a neutral cutting edge; the cutting angle is close to zero. With a positive cutting edge, a carbide tap edge would just chip away.

Thin-film tap coatings enhance lubricity, heat and wear resistance and chip evacuation. “Typically, a TiAlN coating with a soft multilayer on top helps improve chip flow,” said Maxey.

Machines and Holders

Tapping medical parts is done on almost any type of machine tool—lathes, drilling machines and horizontal and vertical machining centers. Taps are especially susceptible to vibration, which can degrade thread quality and reduce tap life, so rigid machine tools and tap holders are needed.

“With difficult-to-machine materials, you need a rigid machine with high torque, stiff spindles and stiff guide ways,” said Uli Sutor, key account manager for DMG Pfronten GmbH, Pfronten, Germany, an affiliate of DMG Vertriebs und Service GmbH. (In the U.S., DMG machines are sold and serviced by DMG/Mori Seiki USA, Hoffman Estates, Ill.) In addition, linear motors provide faster, higher acceleration tapping than ballscrews, he noted.

Courtesy of Precision Dormer

Precision Dormer’s Blue Ring Shark spiral-flute taps are primarily for tapping stainless steels.

Sutor also noted that floor space is becoming an issue as shops ramp up production for the growing medical industry. “Most of the parts are very small so you can use a small machine,” he said. “For example, we have a medical machine for parts like knees or bone plates, the DMU 40, that is 3 square meters, which is half the size of what is normally used.” The company’s HSC 20 linear also has a small footprint and is suitable for machining medical instruments.

When cutting with these machines, “the most important thing is the acceleration,” Sutor said. “With such small parts, you need a very dynamic machine and ours provide a 2G natural surge acceleration force. You have a lot of different movement in different axes, so the axes are always accelerating and decelerating. For example, in a small spine part we are using the same velocity of the axes and the spindle as one of our competitors, but because we have the higher acceleration, we are 30 percent faster.”