Sliding head lathe mills components mounted on a mandrel in a main spindle collet on a Star SV-32 CNC sliding head automatic lathe. The Bright Screw Company sometimes uses one of its seven Japanese-built Star sliding-head lathes, an SV-32, for milling operations. Nothing unusual in this, but for the fact that the components need to be held on a mandrel which is secured in a collet in the main spindle.

Although not a regular procedure, the Leeds company does have jobs where it considers the approach to be worthwhile because of the high accuracy capabilities of the lathe, supplied through sole UK agent, A and S Precision Machine Tools.

Darren Grieg, the company's production engineering manager, comments, 'We find Star machines to be reliable, accurate and easy to set up, and the milling capability is very good.

For one application where we use the SV-32 purely for milling we are able to set up the job and complete it more quickly than on any of our conventional milling equipment, which justifies the application.' It was in the mid 1990's that The Bright Screw Company began to use CNC sliding-head technology.

Although the company had previously employed single-spindle automatics it had never before used Swiss-type machines.

However, the CNC sliding-head format was recognised as being ideal for a contract that demanded large volumes of components with a second operation requirement.

Since investing in its first Star sliding head lathe, output from the machine shop has trebled.

'We had experience of the advantages of twin-spindle mill-turning using a fixed-head, 42 mm bar machine,' Mr Grieg recalled.

'However, this was too cumbersome for smaller diameter work, while our auto's could not achieve the desired quality performance as we would have needed a second drilling operation.

For that reason we elected to buy a 20 mm capacity sliding-head machine with a sub-spindle as we calculated that it would have the capacity to completely machine a high proportion of our small bar work.' Despite its name, The Bright Screw Company manufactures very few screws nowadays.

For some years it has operated as a precision subcontract machinist.

Customers are drawn from the electric motor, switchgear and utilities metering sectors.

It also manufactures specialist fasteners for architectural and other applications, including the Millennium Dome and Hong Kong airport.

Materials can be anything that the customer specifies while batch quantities range from 1-off up to 80,000.

General machining tolerances are down to 12 microns.

Facilities include sliding head turning from 3 mm to 32 mm, chucking work up to 300 mm and conventional bar work up to 65 mm.

The company also has a milling section for components up to 600 mm cube.

'We were drawn to our first Star, an SR-20 model, for a variety of reasons,' recalls works manager, Malcolm Hughes.

'Not least of these was the fact that the main spindle is on the left, like a conventional fixed-head machine; this was quite important at the time as we had no previous sliding-head experience.

We were also keen on the platen type tooling system as it implies good levels of rigidity, while the constant rating of 8,000 rpm on the main spindle and 6,000 rpm on the sub spindle were in line with our requirements.' The success of the initial application can be gauged from the fact that it was followed within a few months by a Star SA-12, and then by a Star SA-16.

In general terms, the benefits arising from the investment were reductions in operations and hence lower lead times, and the ability to completely machine a high proportion of components in one operation had consequential quality benefits.

A feature common to all of the company's machines is conveyorised parts off-load and through-spindle ejection of completed parts to maximise efficiency.

'The machines' productivity allowed us to supply customers' just-in-time and kan ban requirements while their capability to run with minimal attendance meant one setter/operator could take care of three or four machines,' Mr Grieg commented.

'Also, set-up and programming are very straightforward.' The company's fourth Star was an old KNC-32, purchased from a customer as part of a contract to produce his components.

Experience with that machine led The Bright Screw Company to replace it with the first of its two Star SV-32s.

They are mainly used for shaft work on which a variety of milled flats and cross holes may be required.

The largest component put up on the machines is a 794 mm long valve stem which has to be fed through and supported by the synchro spindle during machining.

At the other extreme, the smallest component in regular production measures 1.07 mm diameter, 20 mm long, and is machined from 3 mm bar.

Machining cycle times vary from 10 seconds to 11 minutes depending on component size and material, while machine changeover between different batches can take from five minutes to two hours.

'The machines are highly reliable and consistently achieve 95 per cent availability,' said Mr Hughes.

'Likewise the FMB bar magazines work well with the machines; they constitute a very good package.' During 2000 the company added two more Star machines to its sliding head machining resources, being an SR-20R and an SR-32.

These benefit from the latest Star technology improvements such as higher spindle speeds, faster rapid traverses and increased tooling capacity.

Mr Grieg concluded, 'The machines were bought to service increasing demand for components from our customers.

Since they have been installed, the extra speed has certainly been noticeable.

They are also easy to program and there is a lot of scope to overlap machining operations.

One of the reasons why we have stayed with Star has been the manufacturer's commitment to continuous improvement of the machines and this is certainly evident with the latest models.