CNC production lathes have proved fully capable of meeting accuracy and productivity requirements on bearing components, which could easily pose quite a challenge to the uninitiated, says the user.
With a 20 year production-record at Barden Corporation in Plymouth, of which at least the last decade was spent running triple shift during the week plus through the weekend working, Traub has set a standard that is very hard to beat when it comes to justifying replacement machines. However, it was not a foregone conclusion when the FAG Group subsidiary was making its plant purchase replacement decision for its aging Traub TNS twin-turret and synchronized spindle lathes. But, in the final analysis, three of the latest Traub TNC twin- spindle, single turret machines still topped the justification charts.

Barden Corporation is one of the world's leading suppliers of high precision bearings for industrial applications.

The Plymouth factory manufactures a wide range of high precision ball bearings for an equally wide range of applications from 'instrument' bearings for use in aircraft gauge systems and dentist drills right through to clutch thrust bearings for Formula One racing cars, machine tool spindle bearings and bearings for turbomolecular pumps.

Roy Sansom, Barden's production engineer who is associated with machine selection says: 'We've used Traub machines for around 20 years and found them reliable, consistent and accurate.

For the last 10 years they've run under triple shift conditions with some weekend working which means they have operated virtually non-stop.' The latest Traub installations comprise two TNC 42EGs and a larger capacity TNC 65EG.

Both have twin-opposed spindles with single turrets which offer faster traverse rates and enhanced rigidity.

Each machine has the capability to maximise spindle speeds by using short bar magazine feeders which helps ensure better component consistency.

Barden's bearings conform to APEC 7 and APEC 9 precision requirements and are made from bearing quality steels such as SAE 52100, AISI 440C, M50 tool steel as well as a proprietary corrosion resistant material developed by FAG.

The turning section produces thin walled inner and outer bearing rings in batches of between 500 or 1,000 with the occasional demand for smaller batches.

Accuracy requirements during soft stage machining are defined by the need to minimise grinding stock allowance, as the rings are subsequently heat treated and then ground to finished dimensions.

Typical machining tolerances are + 0.037mm on the bore and outside diameter and + 0.05mm on overall width and these dimensions are monitored using electronic gauging and SPC process control.

'Components tend to be produced from bar or thick walled tube,' Roy Sansom explained.

'While we use fixed head lathes for parts from 15 mm to 80 mm diameter we use sliding head machines for smaller components and here, the capacities of the three new Traub TNC machines allowed us to align the machine size closely to our product mix.' He reflects how the company used to have quite a substantial second operation requirement which is why it originally bought Traub TNS machines with the twin-turrets.

'This requirement has been considerably reduced and out of some 1,400 jobs that could be loaded to the turning section only two now have threads and three have slots.

The remainder are fundamentally just plain rings, although the thin wall section and internal profile features makes the maintenance of tolerance quite difficult,' he says.

The main difference between the TNC 65EG and TNC 42EG is the spindle bore and the respective maximum speeds of 4,000 revs/min and 5,600 revs/min.

All three machines are equipped with short bar magazine/loaders, conveyorised parts take-off systems and cyclonic coolant filtration.

In addition to these new machines, the company has also retained one of its older Traub TNS 65/80 in order to accommodate larger diameter work.

Among the more demanding jobs are inner and outer clutch thrust bearing rings used on Formula One Grand Prix cars.

Whereas most parts are conventional rings, the outer for the F1 application is essentially an annular channel while the inner is produced as an L-section annulus.

Both are very thin walled components with tighter tolerances and flatness requirements on the faces.

The F1 parts are machined on the Traub TNC 65EG and following initial turning, drilling and boring operations, the L-section of the inner ring is produced by trepanning in order to minimise distortion-induced stress on the bore.

Finish turning is carried out at 1,500 revs/min and 0.05mm/rev feed which is followed by part-off with the component fully supported by the sub-spindle of the machine.

Material is fed from short bar magazine/loaders which were selected for a number of reasons.

They reduce the floor area demand and, help to comply with the company's material storage and health and safety requirement because a 1.5 metre bar takes up less store space and is much easier to handle.

In addition, with batch quantities tending to be smaller, the capacity offered by a four metre magazine - as previously used - is simply not required any more and because there is less 'whip' optimised speeds and feeds can be maintained.

As each of the machines have 10-station VDI turrets, changeover is very quick and repeatable.

Each turret can provide up to nine main and nine sub-spindle tool positions, plus one dedicated part-off station.

In practice, many of the parts produced on the TNC machines require common tooling so relatively few tool exchanges are needed between jobs which further improves uptime and utilisation.

Most tools are maintained in the pre-set condition however, when a new tool needs setting, the Traub optical tool setting system ensures it is very accurately replaced without any lost time involving trial cuts.

'As we were with the previous generation Traubs, we're very satisfied with the new machines,' Roy Sansom concluded.

'They have proved fully capable of meeting our accuracy and productivity requirements on a range of components which, though they may appear straightforward to produce, could easily pose quite a challenge to the uninitiated.'