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Medical devices are complex withextremely tight tolerances.

Tornos 12-axis turning center withcontrol system with a central clock

electronic camshaft and virtualelectronic cams manages all axes

simultaneously.

Machining Titanium ImplantsJames Benes08/17/2006

MEDICAL DEVICE MANUFACTURERS FACE TOUGHchallenges. Their customers are demanding ever smaller,more complex parts produced with extraordinaryaccuracies from difficult to machine materials, such astitanium. On top of this, they must operate under theclose scrutiny of regulatory agencies that requireextensive and costly compliance documentation.

Orthopedic devices are designed to conform to thecomplex shape of bones and joints, so the machining ofthese parts is also complex. Devices machined from barstock require a lot of material to be removed, resulting inan expensive process because of the low machinabilityrating of many of the materials involved. As a result,some parts are cast to near net shape, and that oftenrequires fixturing that is complex and expensive. Anotherissue that adds to the complexity of machining is thetight tolerances required0.002 in., or lessfor mostdevices.

These pressures have given rise to new technologies tohelp shops that manufacture medical parts to cope andcompete. Agile 12-axis turning machines tools, newinsert grades and innovative thread-whirling machinesare capable of producing complex parts to extremetolerances, while innovations in EDM result in theproduction of high-quality parts at faster rates byeliminating the problems that were inherent in earliertechnology.

TROUBLES WITH TITAMIUMStainless steels and titanium are the materials most usedfor medical implants. Stainless steels typically are usedfor devices that will not stay in the body permanently.Titanium typically is preferred for medical implantsbecause of its light weight, high strength andbiocompatibility. Also, titanium implants are compatiblewith magnetic resonance imaging and computedtomography imaging procedures, so they do not interferewith those procedures if the patient needs them after theimplant is made.

Titanium 6AL-4V ELI is the standard material used forthe manufacture of hip joints, bone screws, knee joints,bone plates, dental implants, and surgical devices.However, cobalt/chromium alloys are coming into usemore often because they are stiffer, tighter grained andcleaner than titanium.

Machining titanium alloys requires cutting forces onlyslightly higher than those needed to machine steels, buttitanium alloys have metallurgical characteristics thatmake them more difficult to machine than steels ofequivalent hardness.

Titanium has a work-hardening characteristic thateliminates the stationary mass of metal (built-up edge)ahead of the cutting tool. That makes for a high shearangle in machining that causes a thin chip to contact arelatively small area on the cutting-tool face. Because of

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High thread whirling device. (Inset)Orthopedic Screw with high pitch.

this work-hardening characteristic, feeds should not bestopped while tools and workpieces are in movingcontact. The high bearing forces produced by machiningin this way, combine with the friction developed by thechip as it rushes over the bearing area to result in a greatincrease in heat on a localized portion of the cutting tool.Heat generated by cutting titanium does not dissipatequickly because it is a poor conductor. Therefore, most ofthe heat is concentrated on the cutting edge and the toolface.

The combination of high bearing forces and heatproduces cratering action close to the cutting edge,resulting in rapid tool breakdown.

To make matters worse, titanium alloys have a strongtendency to alloy with or to react chemically with thematerials in cutting tools at tool-operating temperatures,and they have a tendency to gall as chips weld to thecutting edges of tools.

These difficulties multiply as tools start to wear, so toolsused to machine titanium and its alloys should bewatched carefully to make sure they are sharp, and theyshould be replaced before they dull. The rule-of-thumb inmachining titanium and its alloys is that if you see anychange in the machining process, you should change thetool immediately because it is likely that it is becomingdull

Another reason to keep tools sharp is that titanium cancatch fire when cutting with worn or broken tools. The metal generates oxygen when it burns, sothe 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 worktends to move away from cutting tools unless heavy cuts are maintained or proper backup isemployed. Slender parts tend to deflect under tool pressures, causing chatter, tool rubbing andtolerance problems. Consequently, rigidity of the entire system is very important, as is the use ofsharp, properly shaped cutting tools.

12-AXES CONTROL MEANS LESS WORKPIECE HANDLINGThe need to reduct the cost of producting complex parts is especially keen in the medicalindustry. This has given rise to advanced machine tools with up to 12 axes of motion that allowfor total positioning capability in any spatial envelope, while increasing the number of operationsthat 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 operatingsimultaneously. In addition to machining complex parts, the machines provide close tolerancework and fine surface finishes. The control in Tornos' 2000 series turning centers is coupled withsoftware that runs on a Windows PC for complex part programs. The software is written so thatthe central clock in the control is used as an electronic cam, making the machine mimic the actionof a conventional, camoperated machine.

Axis paths are calculated by data processing and stored by the control as data tables. Each axishas its own control chipanalogous to an "electronic cam"that stores only its toolpath as a steptable, a sequence of moves in one or two axes. A clock signal generator reads and executes stepsevery 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 eachpart.

The central clock synchronizes the reading of the multiple, individual toolpaths. The cycle in acamoperated machine is limited to 360o. In the Tornos machines, replacing the cams with storedprograms eliminates physical, angular limits on the machine. The control reads data in parallelnot one at a time so the machine can have four tools cutting simultaneously.

INSERTS FOR TURNINGTo 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 companysays these inserts provide a balance of security and productivity, while optimizing the roughingprocess when machining direct from castings.

In roughing applications, the inserts' round shape imparts a strong cutting edge and resistance toexcessive notch wear resulting in fewer tool changes. Because lower temperatures are generatedwith these inserts, operators can increase feeds and speeds to maximize production. The companyalso offers toolholders with positive, D-style inserts for finishing and spherical turning. These alsocan 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 barsand 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 andcobalt chromium implants. When machining with round inserts and cobalt chromium, thecompany recommends grade GC1030. Grade GC1105 is the best grade for D-style inserts andcobalt chromium. Grade H13A achieves the best result while machining titanium for both roundand D-style inserts.

WHIRLING THREADS

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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 togenerate the complex shapes required. For instance, Tornos has designed a new thread whirlingunit to whirl the high helix angles used on some bone screws.

Unlike thread cutting and tapping, thread whirling produces clean contours without burrs. Threadwhirling can be done for external thread cutting and internal tapping. The process is carried outon 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. Theinternal whirling process is 60 percent faster than conventional tapping. Also, the tools used havea longer useful life, and Tornos says that more than 2,500 titanium parts can be tapped withouttool breakage. Cutting speeds are high, so machining time is shorter. There are no burrs orresidual chips and the thread cutting depth can be more than three times the diameter of thethread. 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 thethread pitch angle being produced. Machining is done by a bell-shaped tool comprising threecutters of the same section as the thread being machined. The spindle driving the whirlthreadingtool revolves at high speed up to 12,000 rpm while the part simultaneously turns in theopposite direction at slow speed. The feed rate is synchronized with the two rotational speeds andthe process continues until the required threading length is achieved. The hard metal tool musthave the same shape as the thread being produced.

The surface of the threads produced is perfect because the tools rotate at high speed in theopposite direction to that of the part, thus avoiding the undesirable face lands that sometimes arefound with conventional threading done with milling processes.

The whirling process also eliminates the long withdrawal of the bar from the guide channel, so ithelps to avoid seizure due to an excessively long projection.

A BETTER WIRE EDM PROCESSWire electrical discharge machining (EDM) is a popular process for producing intricate, highlyaccurate medical devices because the process is not affected by workpiece hardness and can beused 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 oftitanium parts from gray to a bluish tinge. Secondly, and more troublesome, during the EDMprocess tiny droplets of copper and zinc from the wire are redeposited on the surface of the partbeing machined. That backplating requires an expensive cleaning process because copper is notbiocompatible.

Charmilles Technologies (www.charmillesus.com) has developed a new EDM generator calledCleanCut that the company says cuts faster than previous technology and makes sharpershapes. The generator also reduces the undesirable surface effects of EDM machining. Bygenerating alternately positive and negative discharges, the CleanCut generator eliminates thebluing of the surface of titanium parts. Also, Charmilles says the generator significantly reducescopper pollution of titanium surfaces reducing postmachining cleaning costs.

MICROMOLDINGThe 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 smallas 0.0001 cu. in. While the process follows the same guidelines as most other injection moldedmetals, 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-cavityrepeatability and tight tolerances of +60.001 in.

SATISFYING THE FEDSThe Food and Drug Administration's stringent Quality Systems Requirements govern the practicesof medical device manufacturers. These quality standards require manufacturers making medicaldevices to document every action that is taken on a part while the manufacturer has custodialpossession of it. All material bar stock and castings is serialized and all documentation mustmatch. If at any time the serial number on the paperwork does not match the material, the entirelot must be scrapped.

To comply with FDA regulations and maximize machine tool productivity, Odyssey Medical inMemphis, Tenn. (www.odysseymed.com) goes to great lengths to validate and improve processquality. Odyssey has implemented stringent controls for process prove-out and an in-processtechnique called Precontrol for monitoring part tolerance and process stability. Precontrolaugments standard practices of control charts by triggering the operator to make adjustments tomachine 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 theactions of the operator." The technique divides the tolerance band into green, yellow and redareas 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 themachine offsets. Consecutive parts that are off on opposite sides of the tolerance indicate that theprocess is unstable requiring adjustments to the process. Parts outside the tolerance zone are, ofcourse, scrapped.

Contributors to this article include:Charmilles Technologies (www.charmillesus.com)Iscar Metals Inc. (www.iscarmetals.com)Odyssey Medical Inc. (www.odysseymed.com)Phillips Plastics Corp. (www.phillipsplastics.com)Rem Sales Inc. (www.remsales.com)Sandvik Coromant Co. (www.coromant.sandvik.com/us)Tornos Technologies US, Corp.(www.tornos.com)

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