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Sliding head lathe - Gibbs Gears

Story added 25 November 2009.

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 top-of-the-range 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.
 
Such was the significance to the business of the Citizen installation to the £3.75 million turnover company 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 very 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. 

Said 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 5 mm dia - 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.  Said Mr Garrod: “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.”
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 said: “Although the machine was over capacity with its 32 mm bar size for our immediate needs, when the largest gear we were quoting for was being produced from 18 mm 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.”
To which 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 that raised our head count 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 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.”

This year has also seen tremendous growth in new business from Europe and Scandinavia, which means exports now account for 40 per cent of production.  He said: “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. 

This has led to a re-investigation of UK sourcing with the ability to deliver more frequent smaller batch sizes with shorter lead times to help them meet changing demands in production.”

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 firm 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.  It also moved into new 20,000 ft² freehold premises in 2007.  The two adjacent units at Stoke Mandeville replaced a machine shop in Tring that was bursting at its seams and the company also recently acquired a specialist tool and grinding company  in Birmingham.

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 600 mm diameter and internal shaping up to 750 mm PCD.  The business is also reaping considerable success in subcontract machining involving five-axis turning and milling, for which it has now added the capability to produce very complex up to 32 mm  turned part machining on the 13-axis Citizen M32-V.  This machine is able to carry up to 72 tools, and cut with three tools simultaneously which considerably reduces cycle times.

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.  He said: “With the Citizen we go from bar, perform perfectly balanced turning and milling as required, then rough and finish hob the teeth. 
The parts, produced in batch sizes of between 250 and 5,000 pass seamlessly from main to subspindle and into the collection tray in cycle times of between 60 and 90 seconds depending on size.” He then adds: ”The only time we have to do any further work is if we have to broach splines or keyways, meet very 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 out of stainless steel.  Production control and setters work together to group parts into material types and while bar sizes tend to be 10 mm, 13 mm and 18 mm, to further reduce lead times 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 6 mm diameter by 39 mm long with a 2 mm bore and 0.3 module gear teeth to 18 mm diameter by 19 mm long, 8 mm 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. 

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 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 1,100 revs/min and 0.09 mm/rev feed rate the gear is finish hobbed with the same tool at 3,000 revs/min and 0.02 mm/rev feed. 

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 ed 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.  This 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.  He said: “We have gained important accreditations from some high profile OEMs.  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 research and development.  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.”

The knock-on effect is that the business now has four engineers involved with methods application and, far from seeing the use of gears in decline in favour of direct electronic drives, Mr Garrod maintains ”Power demands torque and that is what a transmission delivers.”  He is also finding that low carbon emission demands are beginning to open new doors and with the new levels of flexibility in the business, coupled to the plant and machine tools now installed with the inherent levels of expertise, things are looking good.  He said: “There is a whole new level of opportunity out there!”
 
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