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.”

Newer CNC machines typically are capable of rigid, or synchronous, tapping. “These types of machines control the tapping cycle so the rotation of the spindle can be synchronized with the feed of the tap in clockwise and counterclockwise directions,” said Greenfield’s Moring.

A tension/compression holder can compensate when synchronized tapping is not available. “The tension/compression holder will make up for any variation between the actual feed rate and the rotation of the spindle,” Goodman said. “The holder has a spring in it, which compensates for any variation in the axial direction when the tap is rotating if the machine isn’t perfect.” 

Courtesy of DMG

Part of a stainless steel instrument for cutting the bones for a knee implantation. 

Chip packing can cause tap failure and degrade thread quality. Stainless steel produces stringy, hard-to-break chips. Titanium does not easily break into manageable chips either, while cobalt chrome does produce smaller chips.

However, when tapping through-holes, chip management can be simplified by applying a spiral-pointed, or gun-type, tap. That tool pushes the chips ahead of the tap—it “shoots” the chips forward. For a blind-hole, a spiral-flute tap pulls the chips back through the top of the hole.

The flute form is key to chip evacuation, according to Precision Dormer’s Maxey. “Our taps have a three-radii flute profile, which provides a constant rake along the flute length, leading to better chip evacuation and more accurate thread form.”

Fluid Success

Properly cooling and lubricating the tool/workpiece interface is critical to extending tool life and protecting the surface finish of the threads. “There is a lot of heat generated at the point of the cutting tool and the workpiece so you need to cool it down,” said Karen Eisenhauer, market manager for metalworking, Chemetall NAFTA, New Providence, N.J. “Heat generation results in friction, which results in abrasion and tool wear. You could actually harden the surface of the metal or distort the metal.”

Eisenhauer has both personal and professional experience with implants. “I have a titanium ankle joint and screws and plates in my right leg,” she said. “That’s why the medical parts industry must be so careful not only with the process and performance, but the fluid they use because the surface of that metal has to be perfect so the bones can graft themselves onto it.”

Lubrication is another function of the fluid and is important because of the pressures and metal-to-metal frictional properties when tapping. “With lubrication, we are trying to lay down a film that is going to protect the workpiece in order to produce a fine surface finish. We want a film that allows the tool to cut without producing a lot of heat,” Eisenhauer said.

To cool and lubricate, Goodman recommends a water-soluble fluid. Cutting oil provides the best lubrication but oil is an insulator, so it does not cool, and water effectively cools but does not lubricate, according to Goodman. “We’re looking for the cooling properties but we want enough lubrication to be efficient,” he said.

Courtesy of DMG

The DMU 40 machine tool is for producing medical parts like knees or bone plates and has a small footprint. 

Courtesy of DMG

The HSC 20 linear machine tool is for machining medical instruments and has a small footprint.

A concentration is typically 5 to 10 percent coolant and the rest is water. “We always encourage customers to bump up their coolant concentration as high as possible to increase the lubricity. If we can get them up to a 10 or 12 percent coolant concentration, they are a lot better off,” Goodman said. 

The water used in the fluid is critical when making medical parts and most manufacturers use ultrapure water, a purity grade higher than distilled water. “The difficulty with ultrapure water is that it produces a lot of foam, which is not acceptable in these types of systems,” Eisenhauer said. “It is soft water, so anything you have that has a surfactant base is probably going to foam a little more unless it is formulated not to.”

Chemetall’s TechCool 35075Z is formulated for materials used in the medical industry. And the fluid can be used for drilling and reaming as well as tapping. “Usually reaming and drilling are not as critical as far as surface requirements, so if the tapping portion is the most critical, increase the concentration when tapping because you want lubricity there,” Eisenhauer said. 

Every manufacturer treats parts differently after they are machined, but the goal is the same. The parts must be almost antiseptically clean to meet the industry’s stringent cleanliness requirements. 

“A lot of times it is customer-specific,” Eisenhauer said. “They have their own health and safety requirements and there are certain chemicals they don’t allow in their plant. For example, certain plants say no chlorinated materials and other plants do not allow biocides on site.” 

With water-soluble fluids, minimal residue remains by the time the part is ready to be cleaned. And the minimal amount is water soluble, so it is easily removed. 

And, of course, fluid customers run tests in-house before purchasing a fluid. They do machining operations, evaluate tooling and tool life, and subject the fluid to their cleaning processes to determine if it passes requirements.

In addition to fluid, optimizing all machining factors ensures economical and productive tapping. Because medical parts represent a considerable investment in raw material and substantial value-added processing, careful tapping is well worth the effort. CTE

About the Author: Susan Woods is a contributing editor for CTE. Contact her at (224) 225-6120 or by e-mail at susan@jwr.com.

Contributors

Chemetall NAFTA
(800) 526-4473
www.chemetallamericas.com 

DMG/Mori Seiki USA Inc.
(847) 593-5400
www.dmgmoriseikiusa.com

Greenfield Industries Inc.
(800) 348-2885
www.gfii.com

OSG Tap & Die Inc.
(800) 837-2223
www.osgtool.com

Precision Dormer
(800) 877-3745
www.precisiondormer.com