Sliding head CNC autos produce instruments in one

Installation of two sliding headstock CNC autos with sub-spindles and extensive driven tooling has enabled the one-piece production of medical instrument components.

The medical device industry is continuously developing new designs and manufacturing processes to improve the cleanability of surgical instruments, ensuring they meet strict decontamination and sterilisation requirements after each use.

As part of the improvement process, manufacturers are adopting one-hit production techniques to reduce the number of separate components that go to make up an instrument, thereby minimising difficult-to-clean hiding places for bugs.

One such company is Symmetry Othy UK, based in Cheltenham, a subcontractor to the medical device industry and one of the top suppliers of implants, instruments and delivery systems to the global orthopaedic market.

It has seen a significant improvement to its capabilities for one-piece production of instrument components following the installation of two Star SV-32 sliding-headstock bar automatics with sub-spindles and extensive driven tool capability.

Sliding-headstock CNC auto's are normally to be found in high production environments turning out large quantities of parts 24 hours a day, seven days a week, with the machine running unattended for long periods.

Repeat orders are frequent, so set-up is fast and profitability is maximised.

Applications at Symmetry Othy could not be more different, as the company supplies products in small quantities, from one-off prototypes to 50-off batch sizes.

Repeat orders are infrequent and around 80 new part numbers are added every month.

To make matters worse, many of the parts are produced from 17-4ph stainless steel bar, a high chromium alloy that is difficult to chip and produces long, stringy swarf that has to be removed by hand, making operator attendance a necessity.

An example of one-hit manufacture developed by Symmetry Othy using the Star machines is the production of a grater reamer driver, a power driven instrument used in hip joint replacement.

Originally the driver was made in two parts requiring four separate machining processes.

The two parts were then assembled, creating a cavity at the join which was a potential bug trap and very difficult to clean.

Additionally, the two-part assembly created problems with concentricity, which is critical to driver function as accuracy is important when reaming.

Now the shaft is mill-turned complete from 32mm bar on a Star SV-32, eliminating contamination, cleaning and concentricity problems at a stroke as well as reducing the cost of production by eliminating second operations, manual handling and assembly.

Testament to the rigidity of construction of Star machines is the large amount of stainless steel that has to be removed during the machining operation.

The bar is turned from 32 mm down to 9.5mm diameter for much of its 250mm length to well within half of the +/-25 microns drawing tolerance.

Originally, Symmetry Othy's production manager, Chris Jones, thought that the reamer head was too complicated to machine in-cycle, so at the outset it was finished in a second operation, which still gave the benefit of one-piece construction.

However, with programming support from Star GB, a way was found to complete the part in one cycle and still hold 12 microns total tolerance on head detail.

Two thirds of the cycle time is spent sculpting the head and one third is turning, indicating the importance of the Star machines' live tooling for cross- and end-working.

Surgeons around the world use five different power tools to drive the grater handle, so Symmetry Othy has to produce a variety of five end fittings to suit the respective drives.

Such is the overall difficulty of producing these components that Symmetry Othy has retained volume production and supplies reamer driver shafts in quantities of 200 per month.

Another instrument component that benefits from sliding-head turning is a hand reamer which is now produced in one operation instead of fixed-headstock turning followed by milling on a 4-axis machining centre.

Overall the operation is not much quicker, the main benefits being fewer rejects owing to less manual intervention, and no tolerance build-up, resulting in better accuracy.

Examples of other jobs include a titanium waveguide for ultrasonic cutters used in spinal surgery that have to be turned from 12mm bar down to 2.6mm diameter for the majority of its 330mm length; and a trigger on which two pins are milled in the Star sub-spindle to a positional accuracy of 20 microns total.

In all, there are 700 part numbers in the Cheltenham factory and even though the Star lathes were installed as recently as the end of 2001 and mid 2003, already 40 per cent of components are machined on them.

Concluded Jones, "When we embarked on sliding-headstock turning, we were new to the technology and needed to learn how to program relatively complex parts quickly, as our small batch runs and infrequent repeat orders makes rapid set-up crucial to maximising profitability.

"Star GB's engineers were very good at providing after-sales support in all areas, including training, so we had no hesitation in going back to the same supplier when we needed a second machine.".

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