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Year on year, global demand for precision turned medical parts continues to increase, especially in the areas of orthopaedic and dental implants. This rise is due partly to an aging population – in essence, the longer we mortals live - the more ‘spare parts’ we will need! This is creating new challenges for all companies involved in the sector – including cutting tool manufacturers. Solutions reports.
Unsurprisingly, competition for this high end work is intensifying. The often lucrative, precision turned medical parts business will continue to gravitate towards providers who are not only able to deliver high accuracy and quality in difficult to machine metals, but also to those who can provide a prototyping capability, a rapid turnaround and display the willingness to deliver in small quantities.
Working with the expensive and difficult to machine materials associated with the sector and manufacturing in short batch runs leaves little room for error. In order to remain competitive and profitable, manufacturers must get it right the first time.
The good news is that advanced tooling is now available that is designed specifically to help those demanding medical jobs run faster and more efficiently. The correct choice of tooling – even drop-in replacements – can often double machining rates or edge life, improve chip control and eliminate part distortion on thin walled or difficult to grip workpieces.
For example, whilst producing a small precision part at a Swiss dental product manufacturer inserts were failing prematurely and unpredictably, often by chipping and edge breakage. Edge life averaged 90 pieces but the range varied so widely as to render the term ‘average’ meaningless. Consequently the operation required constant operator attention.
With the ‘drop-in’ switch to an Iscar PVD coated Pentacut insert housed in a rigid screw lock tool holder, edge life more than doubled to 250 pieces per edge and surface finish was also significantly improved. The failure mode became gradual as did edge wear, both predictable by the slow deterioration of surface finish. The adaptation of the most applicable Iscar products meant that the operation could run unattended. Key to the improvement was the greater clamping rigidity that held the insert in place.
Three common denominators
In this case, more than half of the original metal weight was machined away, illustrating an often overlooked aspect in medical component turning – though the parts may be small, the volume of metal to be removed is relatively large. Whether on miniature Swiss automatics or small turning centres, turning medical components often involves extreme stepped diameters machined from solid bar stock.
Another common denominator in medical part turning is that the material used will more often than not be difficult to machine. The most frequently used materials are ‘gummy’ stainless steel and other nickel-based alloys; long chipping titanium and high temperature alloys; plus hardened steels. With regard to material selection, biocompatibility, corrosion resistance and extreme high strength will inevitably take precedence over machinability.
Workpiece geometry can add to the challenge. Thin walled tubing, especially aluminium on other non-typical medical parts, is prone to distortion unless the cutting edges used are extremely sharp. Many workpieces will be asymmetrical with complex curves interfering with fixturing and support. These factors necessitate the use of minimum cutting forces so that the workpiece is actually cut and not distorted. In particular, RPMs must be adjusted in order to maintain correct surface speeds.
Proven advances, smaller sizes
Fortunately, significant advances in tooling that has been developed for difficult materials in larger scale applications are now available in the smaller sizes needed for medical component work. Advanced tooling coatings reduce friction, decrease machining heat and help eliminate the microscopic stress raisers that can lead to sudden edge failure. Improved carbide grades stand up reliably to these punishing materials, even with interrupted cuts. More aggressive chipbreakers and through the tool coolant delivery improves chip control and evacuation whilst also reducing re-cutting even in the deepest bores. Moreover, secure clamping systems are able to keep the insert securely in position and eliminate the micro vibrations associated with insert movement in the seat pocket.
Coating breakthrough improves performance
Three years ago, Iscar introduced the Sumo Tec post coating treatment which was designed to make the insert coatings smoother and more lubricious thereby mitigating the three key enemies of insert life – friction, heat and stress raisers. At the time the technology was offered for a limited selection of milling and turning inserts. Experience in the field demonstrated that the Sumo Tec treatment improved efficiency by an average of 35%, taking into account both throughput and edge life. Since then Iscar has steadily expanded its capacity for the treatment and extended it to a wider array of inserts including the smaller variants that are used in implant and prosthesis work. Today more than 40% of Iscar carbide inserts come with the Sumo Tec treatment as standard.
Matching grades to applications
Iscar has developed a number of carbide grades that it claims are improving medical turning across the industrialised world:
Grade IC807 is a submicron grade with a TiAN PVD coating and Sumo Tec treatment. It is ideally suited to medium speed machining in austenitic stainless steel, heat resistant alloys and hardened steel. Through the Sumo Tec post coating treatment, it has also demonstrated excellent resistance to built-up edge in stainless work.
Grade IC808, with a hard, fine grain substrate and the same coating and treatment as IC807 delivers high resistance to wear and chipping on a wide variety of materials and interrupted cut applications.
Extra tough grade IC328 with TiCN coating and the new IC830 with the Sumo Tec treatment have proven successful in milling, parting, grooving and unstable turning applications in plain, alloy and stainless steels.
Grade IC928 performs well in heavy roughing work and for interrupted cuts in all of the common metals used in implants. It features a tough substrate with a PVD TiAlN coating.
The choice of chipbreaking geometries can also make a big difference on small medical parts that are manufactured in difficult to machine materials. For general turning and good finish in all metals including aluminium, the sharp edges, high rake angles and polished tops of the Iscar AS type chipbreaker designs are said to be more than adequate.
For more severe applications however, such as parting and grooving, thin wall tubing in softer, gummy materials, the more aggressive Iscar J, JS and JT chipbreaker geometries may be necessary. These chipbreakers feature sharper cutting edges along with the high rakes for improved efficiency.
Give it a whirl
For threading on slender parts, a multi-tool process called whirling is able to perform the job without deforming the workpiece. Multiple tools, symmetrically spaced, balance the cutting forces as no single point tool possibly can. Iscar now offers inserts specifically designed for this process. Typical successes have included thread whirling titanium bone screws.
Secure chip evacuation
Within the area of internal turning, historically chip control and evacuation has been a persistent problem, especially on gummy, long chipping metals. A complete range of Iscar Mincut grooving tools now help alleviate this problem owing to the use of through tool coolant delivery. The toolbars feature a rigid clamping system that prolongs edge life and further improves process security. Available for bore diameters down to 8mm, they have proven safe for grooving, undercutting, threading and channelling in deep holes. The Mincut range is also capable of face grooving depths of up to 5.5mm.
Similar improvements are available for implant milling. Recently on a cobalt chrome material knee prosthesis, a switch to a special Iscar cutter with eight flutes reduced cycle time by more than 40%.
It is clear that medical component machining will remain a growth industry for the foreseeable future. As with other high end manufacturing disciplines the medical sector will remain demanding and increasingly competitive. Advanced tooling, now more widely available in the required smaller sizes, can make a big difference to efficiency and profitability. Often all it takes is a drop-in replacement.