GBP 25,000 has been slashed from annual cost of producing 1,000 steering shafts per month for a major international client since the supplier changed to a CNC turn-mill centre.

Steel shafts manufactured by Pailton Engineering, the international steering systems manufacturer, were previously produced in a cell comprising six machine tools - a saw, dedicated facing and centering machine, CNC lathe, spline rolling machine, gear hobber and a machining centre. Now the shafts are machined almost completely on a Star SV-32 sliding-head, 9-axis turn-milling centre, followed only by the gear hobber. The latter operation could be completed in-cycle on the Star, but it is more efficient for Pailton to produce the part in two hits in order to balance workflow and maximise utilisation of the sliding head auto.

According to operations manager, Neil Foster, valuable savings in cost and time have been achieved in the manufacture of this core product, which ends up in a large variety of bespoke steering columns.

Lead-time from order to delivery for a complete column assembly has been cut - sometimes to less than four weeks, including prototyping.

Most shafts are produced from 25mm diameter EN8 bar, batch sizes being relatively small at up to 3,000-off, as the columns are mainly for buses, trucks and off-road vehicles.

Typical shaft length is 300 to 500mm, although they can be over 1m long.

A specific project for which quantifiable savings are available was described by Steve George, cell leader at the Coventry, UK, factory.

A steering shaft for a major international client is made in two sections, both ends being completed in one hit on the Star SV-32 instead of on six machines.

Monthly batch size is 1000-off.

Floor-to-floor time for one end of the column has been cut from 86 to 20h, while the reduction at the other end was 79h down to 20h.

The time saving was a combination of having eliminated five set-ups as well as all inter-machine component handling, and having reduced the overall machining time.

The cycle time savings were a result of fast machine movements, and simultaneous front and reverse end working using both spindles, with two and sometimes three tools in cut simultaneously.

An impressive GBP 25,000 has been cut from the annual cost of machining these components, based on internal rates for shop floor staff.

The saving is large because six operators were previously needed to load and unload machines as well as to move the parts around the cell.

Now the process is virtually automatic on the Star, 24h/day, including 'lights-out' working overnight.

The CNC sliding head automatic lathe was installed in June 2005, since which time it has had a big impact on reducing production costs of a number of other, simpler components such as washers, threaded inserts and bushes.

The machine's capabilities have also prompted changes to the design of some components so that they may be machined more efficiently.

A good example is a new air column pin, which has been redesigned so that it can be manufactured at a rate of 130/h on the Star to within 10 microns total tolerance on some features.

A similar pin used to be produced at 40/h on a fixed-head lathe.

An instance where lights-out, sliding-head turning has shown an even larger improvement over fixed-head CNC turning is in the production of a pivot bush, whose production rate has gone up from 25 to 110/h.

The main source of this saving is the elimination of manual deburring and depipping, which is now completed in-cycle for free on the sliding head automatic.

Consistent with its philosophy of continuous improvement in terms quality and efficiency, Pailton's choice of a Star in preference to other makes of sliding-headstock lathe came down to the machine's weight, rigidity and high power, necessary for hobbing and spline rolling, as well as to the 310mm headstock stroke, more than on other machines, allowing long shafts to be produced faster.

Comparative trials demonstrated that the Star produced four times as many longer shafts per hour as the nearest competitor.