28 June 2012
Iscar tools support efficient medical parts turning
Iscar cutting tool developments are supporting successful medical part turning, explains the company
Production of a small precision part at a Swiss dental product manufacturer saw inserts failing prematurely and unpredictably, often by chipping and edge breakage, but a drop-in switch to an Iscar product has solved the problem.
Originally, edge life averaged 90 pieces, but this range varied so widely as to render meaningless the term 'average'. Consequently, the operation required constant operator attention. With the 'drop-in' switch to an Iscar PVD coated Pentacut insert, held in a rigid screw lock toolholder, edge life more than doubled to 250 pieces per edge, while surface finished was also significantly improved.
The failure mode became gradual, as did edge wear, both predictable and visually monitored via slow deterioration of surface finish. Key to the improvement was the greater clamping rigidity, with the insert held firmly in place. The operation could also run unattended.
In this case, more than half of the original metal by 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 very 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 demand minimal cutting forces, so that the workpiece is actually cut and not distorted. In particular, spindle speed must be adjusted in order to maintain correct surface speeds.
Significant advances in tooling that have been developed to tackle difficult materials in larger-scale applications are now available for the smaller tools that medical components demand.
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.
Three years ago, Iscar introduced the SUMO TEC post-coating treatment, which was designed to make the insert coatings smoother and more slippery, 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 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 ones that are used in implant and prosthesis work. Today, more than 40% of Iscar carbide inserts come with the SUMO TEC treatment as standard.
Turning to carbide grades, developments here are now improving medical turning across the industrialised world.
Grade IC 807, a submicron grade with a TiAN PVD coating and SUMO TEC treatment, it is ideally suited to medium speed turning in austenitic stainless steel, heat-resistant alloys and hardened steel. Thanks to the SUMO TEC post-coating treatment, it has also demonstrated excellent resistance to built-up edge in stainless work.
Grade IC 808, with a hard, fine-grain substrate and the same coating and treatment as IC 807, delivers high resistance to wear and chipping on a wide variety of materials and interrupted cut applications.
Extra-tough grade IC 328, with TiCN coating and the new IC830 with the SUMO TEC treatment, has proven very successful in milling, parting, grooving and unstable turning applications in plain, alloy and stainless steels.
Grade IC 928 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 design are adequate.
• For more severe applications 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.
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 offers inserts specifically designed for this process, with successes including thread whirling titanium bone screws.
In the case of internal turning, historically chip control and evacuation have been a persistent problem, especially on gummy, long-chipping metals. A complete range of Iscar Mincut grooving tools now alleviates 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 8 mm, they have proven safe for grooving, undercutting, threading and channelling in very deep holes. The Mincut range is also capable of face grooving depths of up to 5.5 mm.
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%.
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