Sliding-head lathes speed up machining

Drilling and sawing procedures during small bone surgery require a hand-held power tool rotating at up to 20,000 rev/min, so component parts must be accurate to avoid vibration and noise.

However, there is a trend towards smaller, more compact hand tools of greater complexity, which places stringent requirements on manufacturers when they are machining the rotational parts.

Subcontract machinist Bushell and Meadows has solved the problem by transferring much of its turned parts production from fixed-head lathes to two new SV-32 sliding headstock, multi axis, mill-turning centres supplied by Star Micronics GB.

The first machine was installed in January 2007.

It became a vital part of production so quickly that the company decided to purchase a second identical model within six months to increase capacity and introduce back-up on the shop floor.

Both of the 11-axis CNC machines are fitted with electrostatic mist extraction and high-pressure coolant delivery to assist swarf clearance, as a lot of stainless steel is machined, notably 17-4 PH and AISI 440C.

Mike Attwood, operations manager at Bushell and Meadows, said: 'None of our fixed-head turning machines has a counter spindle whereas both of the Star machines have that facility.

'It means we can produce components on them in two or three operations that previously needed six or seven separate set-ups.

'The savings in machining time alone are considerable, around 20 per cent on average, as tool movements are quicker on sliding-head lathes.

'Faster spindle speed, overlapping machining operations and simultaneous front- and rear-working also contribute to cutting idle times.

'Even more dramatic are the reductions in work-in-progress and lead-times when supplying our sole medical customer and associate company, De Soutter Medical.

'Parts that used to take us several days to turn around now come off a Star SV-32 in six or seven minutes and can go straight to dispatch if necessary.' 'Although the Star lathes are of 32mm bar capacity, at Bushell and Meadows they are employed mainly for turning spindles up to 20mm diameter by 20 to 40mm long in batches of typically less than 200-off, and sometimes as low as five-off when producing prototypes.

All mill-turned parts were previously machined on fixed-head lathes, which meant that thin, long shafts had to be supported by a tailstock.

Even so, tool push-off in the centre of the slender components was a problem, slowing the cycle and making it difficult to hold tolerance.

Now that machining is performed close to the guide bush on the Star sliding-headstock lathes, not only is component support unnecessary, but also accuracy is not compromised.

Medical parts account for around two-thirds of production, with the remainder destined for the aerospace and defence sectors.

A fair amount of aluminium and titanium is consequently machined on the Star SV-32s, in addition to stainless steels.

Cycle times for De Soutter's medical components are typically between six and nine minutes, as extensive slotting, keyway milling and cross hole drilling are needed.

Tolerances are tight, sometimes down to 10 microns total for bores and bearing diameters on motor parts.

Good surface finish is also important for the aesthetics of the medical products as well as for reducing the risk of harbouring bacteria.

Mr Attwood added: 'Choosing Star GB for our first two sliding head lathes was not difficult, as it was the only potential supplier that successfully completed a 10-part run-off of our sample stainless steel parts.

'We feel it was a good choice, as the company provided turnkey solutions that included not only oil mist removal and high-pressure coolant, but also programs and tools for the first six jobs.

'Installation and commissioning were rapid, and deliverable components were produced from day one.' Due to the complexity of Bushell and Meadows' components, programs are prepared off-line with the help of Delcam Partmaker CAM software, which Star GB also helped to source as part of the project.

Running under Windows, Partmaker is a knowledge-based system that uses divide-and-conquer computer algorithms to simplify and speed programming.

The system remembers the tooling, spindle speeds and axis feeds necessary to machine individual part features in different materials and stores the processes for rapid recall when a job is repeated.

A bi-directional DXF interface allows data exchange between Partmaker and Bushell and Meadows' CAD system.

Optimising the synchronisation of the individual parts of a program in the main and counter spindles can be a laborious job to carry out at the machine control.

With Partmaker, different 'what if?' synchronisation strategies can be simulated on screen to see the impact on overall cycle time, so the program is already optimised before it reaches the Star lathes and there is no need for further editing at the control.

The ability to analyse a solid model of the machined part allows any errors to be spotted before cutting starts, virtually eliminating the production of scrap.

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