Cutting Tool Engineering
February 2012 / Volume 64 / Issue 2

Thread milling tops tapping

By CTE Staff

Medical part making combines a variety of challenges, including handling advanced, expensive workpiece materials; responding to demand for high precision and reliability; dealing with lead-time pressures; and respecting the life-critical nature of the parts themselves.

Thortex Inc., Portland, Ore., routinely overcomes those challenges. The company was founded in 1990 as a provider of sintered porous coatings for orthopedic implants. Later, customer requests for additional work prompted the company’s evolution into a provider of machining, heat treatment, passivation, finishing and laser marking services. Thortex also developed and patented a metal-injection molding process for titanium that meets ASTM standards.

Thortex makes medical parts ranging from implants and surgical instruments to orthopedic screws. Much of the company’s work involves the manufacture of stem and shell components for artificial hips. Some parts are made in volume while others are produced in single-digit lots.

Implants usually are machined from biocompatible materials such as titanium and cobalt chrome. Cobalt chrome’s wear resistance makes it a prime choice for long-lasting hip-replacement components, but, according to Kerry Smith, engineering manager at Thortex, it is a difficult-to-machine material. “For example, if you tried to thread cobalt chrome with a HSS tap, you would lose the tap right away,” he said.

One step in the stem component machining process involves threading an approximately ½ "-deep blind axial hole in the component’s larger end. The threaded hole serves as a place to screw in an instrument that guides the implant into the bone cavity.

Thortex used to proceed cautiously, first cutting the threads to about half depth with a ¼-20 or ¼-28 solid-carbide tap on a Mori Seiki machining center and then manually finishing the last few threads to avoid tool breakage caused by tap wear or a chip jammed in the hole.

“The cobalt chrome material is so hard that the tap would wear a little, and then all of a sudden just give out,” Smith said. “When we break a tap off in a part, not only do we lose the tap, but the part is gone as well.” Tapping the last four or five threads by hand enabled the operators to get a sense of the tool’s condition. “When the tap started to wear, they could tell because it would be a little tougher to crank it in there,” he added.

However, that strategy added another step to the process, and the tap wore out after only 20 to 25 pieces. To address the problem, distributor Palm Abrasive and Tool Inc., Vancouver, Wash., suggested replacing the carbide taps with EXOCARB thread mills from OSG Tap & Die Inc., Glendale Heights, Ill., and milling the full thread depth. OSG says the micrograin carbide and TiAlN coating of its EXOCARB thread mills provide long tool life in difficult-to-machine materials.

Thread milling is a CNC-enabled alternative to conventional tapping. A thread mill is smaller in diameter than a tap for the same size thread, mills a thread via CNC circular interpolation and can produce multiple diameters of the same pitch thread. In addition, thread milling requires less torque than tapping, easing the load on the tool and enabling operators to continually compensate for tool wear via adjustments in the thread-milling program.

Despite some initial skepticism on the part of Thortex shop floor personnel regarding thread milling, especially in cobalt chrome, the decision was made to “test drive some of the tooling and see what we could get out of it,” Smith said.

Hip Stem.tif
Courtesy of OSG Tap & Die

When threading axial holes at the top of hip-replacement stem components like these made from cobalt chrome, Thortex switched from conventional taps to OSG EXOCARB thread mills (below) and increased tool life by more than 10 times.

OSG Thread Mill.tif

Results were excellent. “The guys on the floor were getting between 400 and 500 pieces with a thread mill,” Smith said. “That is substantially more than what they were getting with the tap.”

Tool breakage clearly was not a problem with the EXOCARB thread mill. Even if it were to occur, Smith said, “Since it is a smaller diameter tool than a tap, it is easy to back out. Picking that hole back up with another thread mill would not be done, because it would be almost impossible to not cross thread it. But because it is easy to back out, it would allow us to go back in with a tap and finish that part and save it.”

Smith noted that upgrading the threading operation was not an isolated instance. “We are always looking to improve processes,” he said. “We keep it in mind on a regular basis.”

In medical part making, the goal is more than simply making the part. “We’ve got a custom part and somebody is sitting in hospital bed somewhere—in pain—waiting for it,” Smith said. “We get it done, get it out to them. We do that in a timely fashion and it is a good feeling.”

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