Medical device manufacturers face many difficult challenges. Their customers demand the use of materials that are difficult to process, such as titanium, to produce tiny, more complex parts and require very sophisticated. The most important thing is that they must be manipulated in accordance with strict regulations and need to meet various and strict document requirements.
The design of the orthopedic device needs to conform to the complex shape of bones and joints, so the processing of these parts is also very complicated. These devices processed from bar stock need to cut off a large amount of material, so the process is very expensive due to the low cutting performance index of many materials. Therefore, some parts are cast close to the final shape of the parts, which often requires complex and expensive fixtures. Another issue of processing complexity is that most fixtures require tight tolerances-0.002 inches or less.
As a result of these pressures, new technologies have been introduced to help the workshops that manufacture medical parts respond to competition. Agile’s 12-axis lathe, new-grade blades and innovative rotary threading performance produce these complex parts with very high accuracy; many innovations in EDM have made the production of high-quality parts faster and eliminated many old machining techniques Intrinsic problems.
Stainless steel and titanium are the most used materials for medical implants. Stainless steel is usually used for implants that are not permanently inserted into the body. Titanium is usually preferred for medical implants due to its light weight, high strength and its biocompatibility. In addition, titanium implants can be used for magnetic resonance imaging and computed tomography imaging diagnostic procedures, therefore, implants will not affect patients performing these diagnostic procedures after implantation.
Titanium 6AL-4V ELI is a standard material used in the manufacture of hip joints, bone screws, knee joints, plate-shaped bones or organs, dentures and surgical devices. However, cobalt-chromium alloys will be used more and more often due to their hardness, tighter particle size and cleaner than titanium.
The machining force required for machining titanium alloys is slightly larger than that for steels. However, the metallurgical properties of titanium alloys make it more difficult to process than steels of considerable hardness.
Titanium has a work hardening property, this feature eliminates the consolidation of metal (crimping) in front of the cutting tool. This helps to increase the shear angle during machining, thus forming thin chips, which come into contact with the cutting tool surface in a relatively small area. Due to this work hardening, the feed should not stop during the kinematic contact between the tool and the workpiece. In this way, the large supporting force generated during machining, combined with the frictional force generated by the chips in the contact area, causes a large increase in the heat in the local area of the tool. The heat generated by cutting titanium will not dissipate quickly because it is a bad conductor. Therefore, most of the heat is concentrated on the cutting edge and tool surface.
The large bearing force and heat form a crescent crater around the cutting edge, resulting in rapid tool damage. To make matters worse, titanium alloys have a strong tendency to fuse into alloys or chemical changes with the materials in the tool at the working temperature of the tool. There is also a trend that when the chips adhere to the cutting edge of the tool, the tool surface damage.
When the tool starts to wear, these difficulties will be doubled. Therefore, the tool used to process titanium and its alloys should be carefully monitored to ensure that the blade is sharp and should be replaced before blunt grinding. The experience of processing titanium and its alloys is that if any changes are seen during the processing, the tool should be changed immediately, because the change means that the tool will become dull.
Another reason for keeping tools sharp is that when cutting with worn or damaged tools, titanium may cause fire. When burning, the metal produces oxygen, so the fire will spontaneously ignite. Therefore, many workshops that process titanium do not report a fire alarm, they are equipped with a fire extinguishing system on the machine tool. Titanium has a relatively low modulus of elasticity and is more elastic than steel, so it tends to deviate from the cutting tool during machining unless it is to maintain heavy cutting or be used as appropriate support. Slender parts tend to deflect under the pressure of the tool, resulting in problems such as knife tremor, tool friction and poor work. In addition, the rigidity of the entire system is very important, you must use sharp, correct shape tools.
12-axis control means less work load
In the medical industry, it is particularly urgent to reduce the cost of producing complex parts. This has led to the creation of advanced machine tools with 12 axes of motion, which can be enveloped in any space and positioned all at the same time, at the same time, increase the number of processing operations for one clamping without repositioning or loading and unloading.
Rotary processing of threads To make it easier for surgeons to use, medical parts are becoming more and more complex. However, this complexity requires new devices for special turning and milling operations to produce the required complex shape. For example, Tornos has designed a new revolving method for processing threads, which is used to process the large helix angle of some bone screws.
Unlike thread cutting and tapping, the rotary method produces a clean profile without burrs. Rotary thread machining can be used for external thread cutting and internal tapping. This process is performed on an automatic lathe and requires a spindle speed of 30,000 rpm.
When the internal thread is tapped, the spindle axis must run parallel to the machined part. The internal turning process is 60% faster than traditional tapping. In addition, the tool has a long service life. Tornos said that it can process 2500 parts without the tool being damaged. The cutting speed is high, so the processing time is short. Without burrs and residual chips, the depth of thread cutting can be greater than 3 times the diameter of the thread. It can even be processed to the bottom of the blind hole.
For the external thread, it is a device installed at the end of the lathe, which rotates and tilts according to the helix angle of the parts to be processed. The machining is performed by a spherical cutter, which includes 3 cutters with the same cross-section as the thread to be processed. The spindle of the tool that drives the thread by the rotary method rotates at a high speed (12,000 rpm), and at the same time, the part rotates at a low speed in the opposite direction. The feed rate is synchronized with the two revolutions, and the process continues until the required thread length has been processed. Hard metal tools must have the same shape as the thread being produced.
Because the tool rotates at a high speed in the opposite direction to the part, the surface of the thread to be processed is perfect, avoiding the surface of divergent requirements sometimes seen when machining with traditional thread milling methods.
Rotary processing of threads also eliminates the withdrawal of long bars from the guideway, thus helping to avoid jamming due to excessively long extensions.
Better EDM EDM, because it is not affected by the hardness of the workpiece, is a method commonly used in the production of complex, high-precision medical devices. It can be used to process hard and difficult-to-machine alloys.
