Pinions are turned in one set-up on its latest Star sliding-head lathe, supplied in August last year to Mellor Electrics by A and S Precision Machine Tools.
Pinions are turned in one set-up on its latest Star sliding-head lathe, supplied in August last year to Mellor Electrics by A and S Precision Machine Tools. One of Europe's leading motor/gearbox manufacturers is Blackburn-based Mellor Electrics, which delivers 25,000 units per month to customers in the vending machine industry and 5,000 other types of DC motors in addition to transformers. To help it keep pace with increasing production levels, bearing in mind that most products contain between 8 and ten turned components, the company has started gear cutting as well as turning pinions.

Pinions are turned in one set-up on its latest Star sliding-head lathe, supplied in August last year by A and S Precision Machine Tools.

Said works engineer, Trevor Atkinson, 'We now completely machine 30 per cent of our pinions on the Star SR-20R and find that we consistently achieve +/-2.5 microns accuracy on shaft diameter and on the OD of the gear teeth.

This is far more difficult to achieve when we turn the part and then mount it on a mandrel for a second-operation on one of our six dedicated gear cutting machines.' The gear cutting cycle takes about 40 seconds on the SR-20R, twice as long as the turning operations, so it is not feasible for Mellor Electrics to complete all pinions on the lathe.

However, 100 per cent of those pinions destined for a peristaltic pump in an air conditioning unit are machined in one hit on the Star, as maximum accuracy is essential to ensure quiet running.

Low noise is not so much of an issue in a vending machine.

The company started to move towards sliding-head technology in 1998 when its old fixed-head turret lathes, both peg-board and microprocessor-controlled, became due for replacement.

The servicing requirement had risen dramatically and tool wear was adding significantly to the cost of keeping the machines running.

Three 50mm bar machines were replaced with a conventional CNC bar and chucking lathe to make the gear wheels, as these diameters are outside the scope of sliding-head capability.

However, the four 25mm capacity fixed-head lathes used for pinion production were replaced by one Star CNC machine, an SA-12, on which the cycles were so quick that there was no loss in production capacity.

Mr Atkinson had been made aware of the benefits of sliding-head lathes through a friend who had worked with him at the Royal Ordnance factory in Blackburn and had subsequently set up as a subcontactor specialising in sliding-head work.

During busy times, Mellor Electrics had put work out to him and were impressed with component quality and speed of production.

Continued Mr Atkinson, 'We decided that we would invest in this type of equipment and looked at all the main suppliers.

We were in some ways a special case as our components are relatively simple but quite large for sliding-head turning, and there is a tendency for larger capacity machines to be more complex and consequently higher priced.

'Not so with the Star SA-12, which was of an ideal specification for our type of work.

Moreover, although nominally a 13mm bar machine like the others we considered, it was the only one capable of turning 16mm stock.

All that is necessary is to turn the end of the larger bars down to 14mm, the size of the largest collet.

Effectively we got a 16mm capacity machine with all the facilities we needed at a competitive price, and we did not even need a swarf conveyor as the open design of the machine and large coolant tray means we can easily do without.' The turning capability of the SA-12/16 allowed Mellor Electrics to machine over 90 per cent of its pinions, whereas only 50 per cent of them are 12mm diameter or under.

The benefits to the company were therefore considerable.

However, a 20mm lathe was needed to produce the remainder of the pinions as production volumes were increasing, so Mr Atkinson decided on the SR-20R from the same supplier, this time with gear cutting capability.

He pointed out an additional benefit of the Star lathes: many commonly used functions like setting zero and automatic cut-off for bar end preparation can be activated at the flick of a toggle switch on the CNC system.

Other controls require a complicated and time consuming excursion into the menu for even the simplest operation.

Batch sizes at the Blackburn company are typically 2,000- to 3,000-off for pinions, although some additional components such as whipper motor shafts used for mixing and frothing vending machine beverages are produced in larger quantities.

Cross milling and drilling is needed on about half of the components machined on the SA-12.

Both Star machines are equipped with a full synchronous sub spindle and this is used all the time as nearly every component needs reverse-end machining including reaming of the pinion bore.

As a postscript, Mr Atkinson talks of the rigidity of Star lathes, as sliding-head machines are not generally noted for this characteristic.

The old fixed head lathes were breaking drill bits, for example, after only 200 components.

Now, 2,000 components are produced on a Star lathe before it is necessary to exchange a drill.

In his opinion, there is no better practical illustration of a machine's dynamic stiffness, a feature which is easy to claim but difficult to substantiate.