A new range of resin-based finishing tools introduced by Tyrolit is capable of machining, for example, the ball of a cobalt chrome artificial hip joint to a mirror finish in three stages, rather than five or six. One UK customer, a major orthopaedic company, has more than doubled production output from its machines using the new stones, and reduced rework by 90 per cent.
For many years, the standard method for producing hip joint heads and cups was to turn, followed by laborious manual lapping. Then, with the introduction of dedicated machines (such as those made by Monnier + Zahner in Switzerland) capable of polishing as well as honing in one automated cycle, the finishing process for metal-on-metal and metal-on-plastic hip replacements was typically halved.
Tyrolit's silicon carbide, cup shaped wheels are commonly chosen for the automated process, successively removing the stock material and preparing the surface for a final polish. Grit sizes range from 180 to 1500 mesh, normally in a 5- or 6-stage process getting progressively finer, after which a diamond paste is typically used to achieve the polished surface condition.
However, medical firms have been pressing for cycle time reductions. Speed and feed increases obviously have their place, but Tyrolit concluded that substantial savings could only be made by reducing the number of tool changes, which in turn meant that each grade of stone had to be capable of removing a greater amount of stock. At the same time, the surface had to be left in a condition to be machined efficiently by the next, finer grade.
With its new style of ULTRA finishing tools, Tyrolit's research and development engineers in Austria have greatly advanced the technology. A number of successful case studies are already documented, including that of the UK orthopaedic company. In the latter application, a six-stage process with a cycle time of over seven minutes was reduced to a honing and polishing process requiring a total of three stages taking just 3 minutes 20 seconds. The financial saving is £250,000 per year.
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