Three multi-tasking machines - all machines are equipped with a second spindle - are providing an ideal solution for the machining of intricately contoured orthopaedic workpieces.

First used for orthopaedic implants in the 1960s, Ultra High Molecular Weight Polyethylene (UHMWPE) is now the primary bearing material for orthopaedic applications due to its high abrasion resistance, biocompatibility and toughness. However, while these properties provide significant benefits in terms of the design of hip, knee, elbow, wrist and finger joint replacements, UHMWPE is difficult to produce and challenging to machine. Perplas Medical, which as part of the United Industries Group manufactures UHMWPE from base powder using a pressure sintering process, is widely recognised as a major contributor to material developments aimed at improving wear resistance and device performance.

More recently, however, it has added to its role as a specialist producer of UHMWPE ram extruded rods, compression moulded 'press sheets' and direct moulded components with the machining of prototypes and small batch production.

Typically, this caters for customers lacking an in-house machining facility or unable to justify an initial investment in the specialised tooling and fixturing needed to guarantee a satisfactory outcome.

The Perplas site at Bacup, Lancashire, which supplies around 300 tonnes of UHMWPE annually to OEMs and their sub-contractors, is now home to an impressive temperature-controlled machining facility.

'We have to date invested around GBP 2 million,' says Neil Anderton, Medical Engineering manager, 'With the intention of better understanding this complex material and assisting our customers wherever possible.' A major part of the investment has been in CNC machine tools supplied by Yamazaki Mazak UK, with the first of four Mazak machines, a Super Quick Turn CNC turning centre, installed in May 2003.

The SQT 200M has since been joined by three multi-tasking machines - two Integrex 100 IIIS models and a Quick Turn Nexus 200MSY, all three machines equipped with a second spindle.

These machines, says Anderton, provide an ideal solution for the machining of intricately contoured workpieces.

The Integrex 100 IIIS has an 11kW, 6000 rev/min main spindle and an 11kW, 6000 rev/min second spindle, with tools located in a 5.5kW single B-axis spindle that has 225 deg of movement and is capable of five axis milling at up to 12,000 rev/min.

The QTNX 200MSY has a 26kW, 5000 rev/min main spindle and 7.5kW/6000 rev/min second spindle, with a 12-position drum-type turret equipped with a 5.5kW, 4500 rev/min tool spindle.

Machine control is via the Mazatrol Fusion 640MT (Integrex) and Mazatrol Fusion 640T (Nexus), both using a conversational programming system that, compared with optional EIA/ISO programming, significantly shortens programming times and program lengths for complex workpieces.

'Although this is a highly sophisticated machining facility we have not set out to compete with our customers' own machining facilities or to replace their existing sub-contracting arrangements,' says Anderton, 'We do, however, offer expertise and advice to customers lacking experience in machining a material that demands a radically different approach to that of traditional metal cutting.' Although accounting for less than a quarter of the content of each orthopaedic implant, components made from UHMWPE play a vital role in restoring pain-free movement to damaged joints.

And with new designs of implant being introduced on an almost daily basis and an ageing population fuelling the demand for joint replacements, Perplas Medical can expect further calls on its unrivalled knowledge of this unique material and experience of machining it to aerospace tolerances and a high standard of surface finish.