Gear hobbers re-engineered to DIN 5 precision

A new, CNC- based, re-engineering package from Holroyd is enabling existing users of the company's range of mechanical gear hobbing machines to update them and benefit from DIN 5 levels of accuracy in the production of worm wheels and sets, at a cost up to 60% less than that of an equivalent new machine. Holroyd has a long and successful history of producing gear hobbing machines - and of exporting them to all parts of the world. The company manufactured its machines for about 25 years until the late 1970's when the market became saturated and the requirement for new machine stock had started to decline.

At that time the machines produced by Holroyd were mechanically superb.

They were constructed using all cast components and manufactured to a degree of accuracy that compares favourably with the most accurate machines on the market today.

'The highly stable cast base and structure that we routinely provided on our range of gear hobbing machines provides the ideal platform for updating the machines to achieve the highest levels of accuracy at a fraction of the cost of buying new,' says Holroyd Director, Steve Whitehead.

'We have already proved both the effectiveness of the conversion - and the cost savings - by updating one of our own production machines in our own factory.

Previously the machine was referred to as Class B precision, producing product to DIN 7 levels of accuracy.

Following re-engineering, the hobber is now a class A machine, producing to DIN 5 accuracy, a figure that enables the machine to satisfy over 80% of the market for worm wheels.

With most retrofit packages, improved accuracy on its own would more than justify the equipment outlay.

We have gone a step further, however, realising major improvements in set-up times, down from 2 hours to 20mins on the re-engineered hobbing machines.' Holroyd's original production of gear hobbers ran to five sizes of machines with table dimensions of 12in, 24in, 48in, 60in and 72in.

All of these machines were designed using the mechanical, 'multiple differential principle', whereby a single motor drive gives all motion differentials which are coupled mechanically.

In the new re-engineering package this motion is now controlled electronically by a Fanuc 16i CNC machine controller, working in conjunction with four brushless servomotors.

The Fanuc controller was chosen because it offers users the time saving advantages of plain language shopfloor programming.

As an example of this, program features are laid out in simple table formats that allow parameters for the finished worm gear to entered simply and quickly.

Set-up times are much reduced, as a result, leading to real gains in productivity.

The Fanuc CNC co-ordinates the motion of the four machine axes: Hob Spindle, Table Drive, Cross Feed and Infeed, via servomotors equipped with high-resolution encoders; the unit on the Hob spindle being used a reference to synchronise all the remaining axes.

The operation of these servomotors is optimised by the replacement, on the machine's linear axes, of the original plain screws by precision ground ballscrews.

Further mechanical improvement is also realised by the use of precision ballscrew end bearings, which have been adapted to mount worm shafts.

The use of these modified bearings prevents axial 'end wind', or a figure of eight movement encountered as the worm shaft is driven under load.

The complete process of stripping the original Holroyd machine down to its basic structure and then re-engineering with the new CNC package takes on average 26 weeks - saving around 3 months on delivery compared to buying new, and costs approximately GBP 300,000.

'This represents excellent value for money, generally, and especially so when compared to purchasing a new machine,' says Steve Whitehead.

'The buyer who chooses the latter route could be paying anywhere between GBP 500,000 to GBP 750,000 to achieve equivalent levels of quality, and be waiting around 9 months for delivery into the bargain.' At the present time the re-engineering package is available only on the five sizes of hobbing machine produced by Holroyd.

However, the company is prepared to quote for performing similar work on other companies' machines.