Gibbs Gears Precision Engineers produces around 200,000 of its gears at its Stoke Mandeville subcontract and development machine shop on a Citizen M32-V CNC sliding-head mill-turn centre.
These gears are produced in single-cycle operations involving turning and gear hobbing, with certain gears also requiring the additional milling of drive hexagons.
On the back of the Citizen installation, Gibbs Gears was able to secure a contract that resulted in the retention of gear production in the UK and reversed plans by the customer to source these volume parts offshore and, as a result, close its UK gear machining facility.
‘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 18mm 0.8 module,’ said Reece Garrod, managing director.
During the negotiations, technical director David Worthington approached the supplier of sliding-head machines and had sample components produced.
Due to the locality of Citizen Machinery UK at nearby Watford, he ordered the Citizen M32-V.
Worthington said: ‘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,’ he added.
The type of contract that Gibbs Gears is now servicing tends to be more demanding and margins are being squeezed, especially in the early days of projects, but, according to Garrod, the use of technology such as the Citizen gives added flexibility for change and the development of processes that help to boost productivity and performance.
Gibbs Gears has invested approximately GBP600,000 over the last 18 months, with CNC replacing a considerable number of manual gear cutting machines.
The business is also said to be reaping success in subcontract machining involving five-axis turning and milling, for which it has now added the capability to produce very complex 32mm 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 reduces cycle times.
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.
‘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,’ added Worthington.
Most gears are made from EN34 steels.
However, to meet the latest medical contract, component 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 10mm, 13mm and 18mm, to further reduce lead times and speed changeovers investigations are under way 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 in diameter by 39mm in length with a 2mm bore and 0.3 module gear teeth to 18mm in diameter by 19mm in height with an 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.
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 a single pass.
By roughing using two passes at 1,100rev/min and 0.09mm/rev feed rate, the gear is finish hobbed with the same tool at 3,000rev/min and 0.02mm/rev feed.
Within the machine software, the hob is automatically returned to the start position, which means that 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.
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.
A further advantage 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 synchronised and aligned to a particular gear tooth or another key element of a component.
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