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END USER: Thortex Inc., (503) 654-5726, www.thortexinc.com. CHALLENGE: Maximize tool life and reliability when threading cobalt chrome. SOLUTION: Replace solid-carbide taps with high-performance thread mills. SOLUTION PROVIDER: OSG Tap & Die Inc., (800) 837-2223, www.osgtool.com.
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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.
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.
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.”
Related Glossary Terms
- abrasive
abrasive
Substance used for grinding, honing, lapping, superfinishing and polishing. Examples include garnet, emery, corundum, silicon carbide, cubic boron nitride and diamond in various grit sizes.
- computer numerical control ( CNC)
computer numerical control ( CNC)
Microprocessor-based controller dedicated to a machine tool that permits the creation or modification of parts. Programmed numerical control activates the machine’s servos and spindle drives and controls the various machining operations. See DNC, direct numerical control; NC, numerical control.
- gang cutting ( milling)
gang cutting ( milling)
Machining with several cutters mounted on a single arbor, generally for simultaneous cutting.
- high-speed steels ( HSS)
high-speed steels ( HSS)
Available in two major types: tungsten high-speed steels (designated by letter T having tungsten as the principal alloying element) and molybdenum high-speed steels (designated by letter M having molybdenum as the principal alloying element). The type T high-speed steels containing cobalt have higher wear resistance and greater red (hot) hardness, withstanding cutting temperature up to 1,100º F (590º C). The type T steels are used to fabricate metalcutting tools (milling cutters, drills, reamers and taps), woodworking tools, various types of punches and dies, ball and roller bearings. The type M steels are used for cutting tools and various types of dies.
- interpolation
interpolation
Process of generating a sufficient number of positioning commands for the servomotors driving the machine tool so the path of the tool closely approximates the ideal path. See CNC, computer numerical control; NC, numerical control.
- machining center
machining center
CNC machine tool capable of drilling, reaming, tapping, milling and boring. Normally comes with an automatic toolchanger. See automatic toolchanger.
- milling
milling
Machining operation in which metal or other material is removed by applying power to a rotating cutter. In vertical milling, the cutting tool is mounted vertically on the spindle. In horizontal milling, the cutting tool is mounted horizontally, either directly on the spindle or on an arbor. Horizontal milling is further broken down into conventional milling, where the cutter rotates opposite the direction of feed, or “up” into the workpiece; and climb milling, where the cutter rotates in the direction of feed, or “down” into the workpiece. Milling operations include plane or surface milling, endmilling, facemilling, angle milling, form milling and profiling.
- milling machine ( mill)
milling machine ( mill)
Runs endmills and arbor-mounted milling cutters. Features include a head with a spindle that drives the cutters; a column, knee and table that provide motion in the three Cartesian axes; and a base that supports the components and houses the cutting-fluid pump and reservoir. The work is mounted on the table and fed into the rotating cutter or endmill to accomplish the milling steps; vertical milling machines also feed endmills into the work by means of a spindle-mounted quill. Models range from small manual machines to big bed-type and duplex mills. All take one of three basic forms: vertical, horizontal or convertible horizontal/vertical. Vertical machines may be knee-type (the table is mounted on a knee that can be elevated) or bed-type (the table is securely supported and only moves horizontally). In general, horizontal machines are bigger and more powerful, while vertical machines are lighter but more versatile and easier to set up and operate.
- pitch
pitch
1. On a saw blade, the number of teeth per inch. 2. In threading, the number of threads per inch.
- tap
tap
Cylindrical tool that cuts internal threads and has flutes to remove chips and carry tapping fluid to the point of cut. Normally used on a drill press or tapping machine but also may be operated manually. See tapping.
- tapping
tapping
Machining operation in which a tap, with teeth on its periphery, cuts internal threads in a predrilled hole having a smaller diameter than the tap diameter. Threads are formed by a combined rotary and axial-relative motion between tap and workpiece. See tap.
- threading
threading
Process of both external (e.g., thread milling) and internal (e.g., tapping, thread milling) cutting, turning and rolling of threads into particular material. Standardized specifications are available to determine the desired results of the threading process. Numerous thread-series designations are written for specific applications. Threading often is performed on a lathe. Specifications such as thread height are critical in determining the strength of the threads. The material used is taken into consideration in determining the expected results of any particular application for that threaded piece. In external threading, a calculated depth is required as well as a particular angle to the cut. To perform internal threading, the exact diameter to bore the hole is critical before threading. The threads are distinguished from one another by the amount of tolerance and/or allowance that is specified. See turning.
- titanium aluminum nitride ( TiAlN)
titanium aluminum nitride ( TiAlN)
Often used as a tool coating. AlTiN indicates the aluminum content is greater than the titanium. See coated tools.
- wear resistance
wear resistance
Ability of the tool to withstand stresses that cause it to wear during cutting; an attribute linked to alloy composition, base material, thermal conditions, type of tooling and operation and other variables.
