Get into gear

Gibbs Gears Precision Engineers produces over 600,000 gears a year at its Stoke Mandeville subcontract and development machine shop. Of these, some 200,000 are produced on a Citizen M32-V CNC sliding head turn/mill centre in single cycle operations involving, turning and gear hobbing with certain gears also requiring the additional milling of drive hexagons. Solutions reports.

Such was the significance to the business of the Citizen installation that it became a key element to help ride out the growing effects of the recession that at the time (September 2008) was beginning to bite into industry. On the back of the Citizen installation, Gibbs Gears was able to secure a competitively priced contract that resulted in the retention of gear production in the UK and reversed plans by the long-served customer to source these volume parts offshore and, as a result, close its UK gear machining facility.

Says managing director Reece Garrod: “We took a flyer on buying the machine and came up trumps. When we originally found out the customer was possibly closing its UK operation for gear machining we made an approach and, with the help of Citizen Machinery UK, quoted against its targeted overseas suppliers for the range of 30 fine pitch spur gears between 5mm diameter - 0.3 Module and 18 mm - 0.8 Module.”

As Gibbs Gears were working on the premise of using the latest single cycle machining technology, with the ability to run unmanned at night, it found it was very competitive in meeting the proposed offshore pricing strategy of the customer. “We could easily ensure the level of quality and we gave the customer confidence that we could meet the tough demands on delivery to line,” Mr Garrod explains.

Lifesaving decisions

During the negotiations, technical director David Worthington approached the suppliers of sliding head machines and had sample components produced. Due to the locality of Citizen Machinery UK at nearby Watford, coupled with the service and support given that helped to win the contract, he ordered the Citizen M32-V.

He affirms: “Although the machine was over capacity with its 32mm bar size for our immediate needs, when the largest gear we were quoting for was being produced from 18mm material, the machine guaranteed the additional rigidity for heavier and greater consistency for hobbing teeth without vibration. It also gave us the added flexibility to produce further subcontract components when needed.”

Mr Garrod adds: “In the event, installing the Citizen was a life saver. Had it not been for winning this contract, over the coming few months our business could have been slashed by half due to market conditions. However, our fortunes changed – two competitors went out of business which helped to raise the order book and, by the New Year, we benefitted from a considerable increase in new business. Such was the effect that we had to increase our workforce by taking on a few employees from the gear customer that ceased production raising our headcount to 40.” In fact, he qualifies the change in circumstance by showing how the last financial year, closing in August 2009, had achieved sales equal to the previous year of £3.75 million.

But he maintains: “We are not crowing. It was a very worrying time as we quickly lost some of our traditional customer base linked to construction and marine, which took a significant number of gearboxes and transmission gears. But we've added key customers from the MoD, off-highway, medical and aerospace which are all demanding a higher class of gear. Meanwhile, we are still supplying the likes of the petrochemical, agricultural and power tool sectors.”

Exports up

This year has also seen significant growth in new business from Europe and Scandinavia, which means exports now account for 40% of production. “The level of the pound has really worked in our favour coupled with our growing capability, and we are now finding UK customers that were previously shipping orders overseas are reacting in our favour because outsourcing abroad is not so economic, and involves considerable levels of added expense, control and risk,” Mr Garrod says.

The type of contract Gibbs Gears is now servicing tends to be more demanding and margins are being squeezed, especially in the early days of projects, but Mr Garrod insists the use of technology such as the Citizen gives added flexibility for change and the development of processes which helps to boost productivity and performance.
Gibbs Gears, a family-run business for over 60 years, has invested some £600,000 over the last 18 months, with CNC replacing a considerable number of manual gear cutting machines. The company thrives on its highly specialised core knowledge of gears and transmissions producing spur, helical and bevel gears, worms, rollers, rack and pinions, serrations, splines, sprockets and timing pulleys.

Gears are machined from 0.3 to 10 module, 10 to100 DP hobbing capacity is up to 600mm diameter and internal shaping up to 750mm PCD. The business is also reaping considerable success in subcontract machining involving 5-axis turning and milling, for which it has now added the capability to produce complex, up to 32mm, turned part machining on the 13-axis Citizen M32-V. The machine is able to carry up to 72 tools, and cut with three tools simultaneously which considerably reduces cycle times.

Case for automation

Mr Worthington describes the Citizen as the perfect example of automating the gear cutting process. Previously the customer's in-house machine shop produced the 30 spur gear types on a CNC lathe followed by autoloaded hobbing cycles. “With the Citizen we go from bar, perform perfectly balanced turning and milling as required, then rough and finish hob the teeth,” he reveals.”

The parts, produced in batch sizes of between 250 and 5,000 pass seamlessly from main to sub-spindle and into the collection tray in cycle times of between 60 and 90 seconds depending on size. “The only time we have to do any further work is if we have to broach splines or keyways, meet special process requirements such as heat treatment, grinding or special surface finishing.”

Most gears are made from EN34 however, to meet the latest medical contract, components parts are also turned and hobbed from stainless steel. Production control and setters work together to group parts into material types and while bar sizes tend to be 10mm, 13mm and 18mm, to further reduce leadtimes and speed changeovers, investigations are underway to standardise on one material size and use the rigidity and twin tool turning capability of the Citizen to quickly size the part.

The class 8 spur gears range from 6mm diameter by 39mm long with a 2mm bore and 0.3 module gear teeth to 18mm diameter by 19mm long, 8mm bore size and 0.8 module. Using carbide hobs, cutting speed trials have been performed to strike an economic balance between floor-to-floor time and effective tool life. 

Rough with the smooth

In developing the process, Citizen Machinery UK's application engineers used the machine's synchronised hobbing software to provide the flexibility to experiment with the number of passes and feeds and speeds to not only minimise cycle time, but also to obtain the level of quality in surface finish on the tooth form to meet the classification required. As a result, it was found to be significantly quicker to rough and finish the gear than hob the teeth in single pass. By roughing using two passes at 1,100rpm and 0.09mm/revolution feed rate the gear is finish hobbed with the same tool at 3,000rpm and 0.02mm/revolution.

Within the machine software, the hob is automatically returned to the start position which means it provides the flexibility to action the roughing and finishing operations at the most cost-effective and practical point in the overall machining cycle. Speeds and feeds can also be independently selected to obtain the desired cycle time or surface finish and are not locked into set mechanical ratios between hob and spindle rotation.

Through a macro, Citizen Machinery UK was also able to introduce to the program a hob shift via one of the two Y axes of the machine which enabled the cutter to be automatically moved a distance of one tooth pitch following the processing of five gears. Not only does this ensure even wear and effective control over the tool, but it also helps to maintain a consistent tooth form without burrs. Around 2,000 parts are produced per hob before it is reground or replaced.

Further advantages from the Citizen software on the M32 is hob phase which allows features such as the milling of the drive hexagon on one type of gear shaft to be perfectly synchronised and aligned to a particular gear tooth or another key element of a component.

Mr Garrod sees the business as being rejuvenated over the last nine months. “We have gained important accreditations from some high profile OEMs,” he comments. “The type of work we now produce is totally different and following the shake-out in our industry, we have also become more deeply involved in R&D. With the type of equipment now installed, we are finding we are being drawn into being a fast reaction prototyping operation helping in the development of new generation gearboxes.”

Citizen UK
www.citizenmachinery.co.uk Gibbs Gears
www.gibbsgears.com