Medical devices are complex with extremely tight tolerances.
Tornos 12-axis turning center with control system with a central clock — electronic camshaft — and virtual electronic cams manages all axes simultaneously.
High thread whirling device. (Inset) Orthopedic Screw with high pitch.
MEDICAL DEVICE MANUFACTURERS FACE TOUGH challenges. Their customers are demanding ever smaller, more complex parts produced with extraordinary accuracies from difficult to machine materials, such as titanium. On top of this, they must operate under the close scrutiny of regulatory agencies that require extensive and costly compliance documentation.
Orthopedic devices are designed to conform to the complex shape of bones and joints, so the machining of these parts is also complex. Devices machined from bar stock require a lot of material to be removed, resulting in an expensive process because of the low machinability rating of many of the materials involved. As a result, some parts are cast to near net shape, and that often requires fixturing that is complex and expensive. Another issue that adds to the complexity of machining is the tight tolerances required—0.002 in., or less—for most devices.
These pressures have given rise to new technologies to help shops that manufacture medical parts to cope and compete. Agile 12-axis turning machines tools, new insert grades and innovative thread-whirling machines are capable of producing complex parts to extreme tolerances, while innovations in EDM result in the production of high-quality parts at faster rates by eliminating the problems that were inherent in earlier technology.
TROUBLES WITH TITAMIUM
Stainless steels and titanium are the materials most used for medical implants. Stainless steels typically are used for devices that will not stay in the body permanently. Titanium typically is preferred for medical implants because of its light weight, high strength and biocompatibility. Also, titanium implants are compatible with magnetic resonance imaging and computed tomography imaging procedures, so they do not interfere with those procedures if the patient needs them after the implant is made.
Titanium 6AL-4V ELI is the standard material used for the manufacture of hip joints, bone screws, knee joints, bone plates, dental implants, and surgical devices. However, cobalt/chromium alloys are coming into use more often because they are stiffer, tighter grained and cleaner than titanium.
Machining titanium alloys requires cutting forces only slightly higher than those needed to machine steels, but titanium alloys have metallurgical characteristics that make them more difficult to machine than steels of equivalent hardness.
Titanium has a work-hardening characteristic that eliminates the stationary mass of metal (built-up edge) ahead of the cutting tool. That makes for a high shear angle in machining that causes a thin chip to contact a relatively small area on the cutting-tool face. Because of this work-hardening characteristic, feeds should not be stopped while tools and workpieces are in moving contact. The high bearing forces produced by machining in this way, combine with the friction developed by the chip as it rushes over the bearing area to result in a great increase in heat on a localized portion of the cutting tool. Heat generated by cutting titanium does not dissipate quickly because it is a poor conductor. Therefore, most of the heat is concentrated on the cutting edge and the tool face.
The combination of high bearing forces and heat produces cratering action close to the cutting edge, resulting in rapid tool breakdown.
To make matters worse, titanium alloys have a strong tendency to alloy with or to react chemically with the materials in cutting tools at tool-operating temperatures, and they have a tendency to gall as chips weld to the cutting edges of tools.
These difficulties multiply as tools start to wear, so tools used to machine titanium and its alloys should be watched carefully to make sure they are sharp, and they should be replaced before they dull. The rule-of-thumb in machining titanium and its alloys is that if you see any change in the machining process, you should change the tool immediately because it is likely that it is becoming dull
Another reason to keep tools sharp is that titanium can catch fire when cutting with worn or broken tools. The metal generates oxygen when it burns, so the fire can become self-sustaining. Therefore, many shops that machine titanium do not run "lights out," and they equip machines with fire-suppression systems.
With its relatively low modulus of elasticity, titanium has more "springiness" than steel, so work tends to move away from cutting tools unless heavy cuts are maintained or proper backup is employed. Slender parts tend to deflect under tool pressures, causing chatter, tool rubbing and tolerance problems. Consequently, rigidity of the entire system is very important, as is the use of sharp, properly shaped cutting tools.
12-AXES CONTROL MEANS LESS WORKPIECE HANDLING
The need to reduct the cost of producting complex parts is especially keen in the medical industry. This has given rise to advanced machine tools with up to 12 axes of motion that allow for total positioning capability in any spatial envelope, while increasing the number of operations that can be performed on a workpiece with a single setup and without repositioning or handling.
For example, 12-axis turning centers produced by Tornos Technologies US Corp. (www.tornos.com) produce complex parts in a single setup with all 12 axes operating simultaneously. In addition to machining complex parts, the machines provide close tolerance work and fine surface finishes. The control in Tornos' 2000 series turning centers is coupled with software that runs on a Windows PC for complex part programs. The software is written so that the central clock in the control is used as an electronic cam, making the machine mimic the action of a conventional, camoperated machine.
Axis paths are calculated by data processing and stored by the control as data tables. Each axis has its own control chip—analogous to an "electronic cam"—that stores only its toolpath as a step table, a sequence of moves in one or two axes. A clock signal generator reads and executes steps every eight milliseconds. One data table is used for the toolpath axes, another for spindle speed, rotation and stops, and a third for machine functions. Data tables are programmed offline for each part.
The central clock synchronizes the reading of the multiple, individual toolpaths. The cycle in a camoperated machine is limited to 360o. In the Tornos machines, replacing the cams with stored programs eliminates physical, angular limits on the machine. The control reads data in parallel— not one at a time —so the machine can have four tools cutting simultaneously.
INSERTS FOR TURNING
To increase productivity and reduce tooling costs, Sandvik Coromant (www.coromant.sandvik.com/us) recently introduced a line of round inserts for turning hip joints. When used for internal turning of the spherical cup in a ball and socket hip joint, the company says these inserts provide a balance of security and productivity, while optimizing the roughing process when machining direct from castings.
In roughing applications, the inserts' round shape imparts a strong cutting edge and resistance to excessive notch wear resulting in fewer tool changes. Because lower temperatures are generated with these inserts, operators can increase feeds and speeds to maximize production. The company also offers toolholders with positive, D-style inserts for finishing and spherical turning. These also can be used for internal turning in applications where accessibility is limited.
Sandvik Coromant's round inserts are compatible with the company's CoroTurn 107 boring bars and its EasyFix method of achieving the correct cutting-edge center height.
The company says the round inserts and D-style inserts are good for machining titanium and cobalt chromium implants. When machining with round inserts and cobalt chromium, the company recommends grade GC1030. Grade GC1105 is the best grade for D-style inserts and cobalt chromium. Grade H13A achieves the best result while machining titanium for both round and D-style inserts.
MEDICAL COMPONENTS continue to get more complex to allow for easier use by surgeons. However, this complexity requires new attachments for special turning and milling operations to generate the complex shapes required. For instance, Tornos has designed a new thread whirling unit to whirl the high helix angles used on some bone screws.
Unlike thread cutting and tapping, thread whirling produces clean contours without burrs. Thread whirling can be done for external thread cutting and internal tapping. The process is carried out on an automatic lathe and requires a high-frequency spindle turning at speeds to 30,000 rpm.
During internal tapping, the spindle axis must run parallel with the part being machined. The internal whirling process is 60 percent faster than conventional tapping. Also, the tools used have a longer useful life, and Tornos says that more than 2,500 titanium parts can be tapped without tool breakage. Cutting speeds are high, so machining time is shorter. There are no burrs or residual chips and the thread cutting depth can be more than three times the diameter of the thread. It is even possible to machine to the bottom of a blind hole.
For external threads, a device fitted to the end of the lathe rotates and inclines in relation to the thread pitch angle being produced. Machining is done by a bell-shaped tool comprising three cutters of the same section as the thread being machined. The spindle driving the whirlthreading tool revolves at high speed — up to 12,000 rpm — while the part simultaneously turns in the opposite direction at slow speed. The feed rate is synchronized with the two rotational speeds and the process continues until the required threading length is achieved. The hard metal tool must have the same shape as the thread being produced.
The surface of the threads produced is perfect because the tools rotate at high speed in the opposite direction to that of the part, thus avoiding the undesirable face lands that sometimes are found with conventional threading done with milling processes.
The whirling process also eliminates the long withdrawal of the bar from the guide channel, so it helps to avoid seizure due to an excessively long projection.
A BETTER WIRE EDM PROCESS
Wire electrical discharge machining (EDM) is a popular process for producing intricate, highly accurate medical devices because the process is not affected by workpiece hardness and can be used to machine hard, difficult-to-cut alloys. However, the process affects titanium in two ways: First, it changes the color of the natural highly corrosion-resistant oxide coating on the surface of titanium parts from gray to a bluish tinge. Secondly, and more troublesome, during the EDM process tiny droplets of copper and zinc from the wire are redeposited on the surface of the part being machined. That backplating requires an expensive cleaning process because copper is not biocompatible.
Charmilles Technologies (www.charmillesus.com) has developed a new EDM generator — called CleanCut — that the company says cuts faster than previous technology and makes sharper shapes. The generator also reduces the undesirable surface effects of EDM machining. By generating alternately positive and negative discharges, the CleanCut generator eliminates the bluing of the surface of titanium parts. Also, Charmilles says the generator significantly reduces copper pollution of titanium surfaces reducing postmachining cleaning costs.
The drive to make medical parts smaller and stronger has led Phillips Plastics Corp. (www.phillipsplastics.com) to develop a process for micro molding titanium medical parts as small as 0.0001 cu. in. While the process follows the same guidelines as most other injection molded metals, titanium generally produces a rougher surface finish and thinner wall sections. Advantages of the process include high material usage and low waste, high cavity-to-cavity repeatability and tight tolerances of +60.001 in.
SATISFYING THE FEDS
The Food and Drug Administration's stringent Quality Systems Requirements govern the practices of medical device manufacturers. These quality standards require manufacturers making medical devices to document every action that is taken on a part while the manufacturer has custodial possession of it. All material — bar stock and castings — is serialized and all documentation must match. If at any time the serial number on the paperwork does not match the material, the entire lot must be scrapped.
To comply with FDA regulations and maximize machine tool productivity, Odyssey Medical in Memphis, Tenn. (www.odysseymed.com) goes to great lengths to validate and improve process quality. Odyssey has implemented stringent controls for process prove-out and an in-process technique called Precontrol for monitoring part tolerance and process stability. Precontrol augments standard practices of control charts by triggering the operator to make adjustments to machine offsets well in advance of trouble. Tim Gooch, director of technical services at Odyssey, explains, "Part specifications control the acceptability of the part, but Precontrol controls the actions of the operator." The technique divides the tolerance band into green, yellow and red areas with the middle 50 percent of the tolerance band being the operator's target operating zone. Any two consecutive parts that exceed this tighter tolerance trigger the operator to adjust the machine offsets. Consecutive parts that are off on opposite sides of the tolerance indicate that the process is unstable requiring adjustments to the process. Parts outside the tolerance zone are, of course, scrapped.
Contributors to this article include: