Forged synchroniser rings for manual transmissions demand high accuracy and they are being machined in CNC lathes in faster cycle times when compared with front-loading chuckers used before.
VTL Automotive turned to Pollard Mori Seiki CNC lathes to mass-produce a variety of forged Synchroniser Rings for manual transmissions. Compared with the output from front-loading CNC chuckers, the Pollard Mori Seiki CL Series CNC chuckers machine the rings in faster cycle times, to tighter tolerances and with virtually zero reject rates. 'Our main products are synchroniser rings and selector forks for manual transmissions, said VTL Automotive Managing Director, Bruno Jouan.

'We forge and machine some eight million rings/year for a number of OEMs, including Renault, Ford, New Venture Gear and Volvo.

Deliveries are in tightly controlled, sequenced, just-in-time batches and there is no room for rejects.' Forged high tensile brass is not the easiest material to machine, 'We forge individual rings from blanks, which have been precisely sawn from extruded seamless brass tube.

After heat treatment, the hardness of the forgings is up to 230 Brinell.

It is a hard material and can rapidly form a built-up edge on the turning tools if the tool geometry is not right.' 'The rings look a deceptively simple job.

But it is not easy to turn and bore these rings within a few microns and hold tight tolerances over a typical batch size of 5000-8000.

Also batch sizes and ordering patterns can fluctuate, so VTL Automotive have to have the flexibility to be able to change over quickly from one component to another.

Under the previous ownerships of Sagar Richards and laterly Valeo Transmission the Luddendenfoot factory was producing the rings successfully on twin-spindle, front-loading CNC chuckers.

By the early 1990s, the synchroniser ring designs were steadily evolving, and the trend was towards more sophisticated designs with even tighter tolerances.

'We felt that the existing machines would not be able to cope with the tighter tolerances demanded by our customers, we were not able to reduce cycle times and improve efficiency any further, as machining on the old chuckers was restricted by the clamping system then in use,' explained Sales and Technical Director David Clegg.

The synchroniser ring has to be located from the forged blocker teeth to allow tooth facing and OD turning be performed with the ring clamped internally, and then chamfering, boring and internal threading be carried out with the ring clamped externally.

In the late 80's and early 90's we had examined a number of ways of increasing machining output and tightening up tolerances.

'At one time, we operated front-loading chuckers, Wyvernmatics and Herbert Cridans,' related David Clegg.

'The work then involved lots of different operations, a one man-one machine strategy and work-in-progress stocks.

We looked for a new machining concept - we even considered rotary transfer machines - before acquiring our first Mori Seiki CL-20 from Pollard Mori Seiki in 1991, and a second in 1992.

After some development work, we acquired five CL-20s during 1995-96 and later, six CL-203s in 2000.' The company had entered into a joint development with Pollard Mori Seiki to develop machining methods for a new generation of synchromesh rings.

Pollard had had considerable experience in applying Roehm chucking systems to the Mori Seiki CL Series of CNC lathes, explained Pollard Mori Seiki's Area sales Manager, Rod Lockwood.

David Clegg added that the important considerations were the actual orientation and location of the precision forged rings in the chuck.

The limits for clamping pressures and spindle speed had to be determined before 'lobing' occurred in the rings (clamping pressure, coupled with the tooling pressure during boring could cause the rings to lobe outwards from between the three clamps).

On the twin-spindle chuckers the rings had to be 'tooth faced' first to create a location face.

The clamping mechanism was a three jaw pull back system, pulling back onto the location face which allowed boring, threading and chamfering.

This system was replaced by the earlier CL-20s.

Applying a tailored R”hm pullback chuck on the CL-203s with tailstock support meant that the rings were machined in one operation with a shortened the cycle time.

The first Mori Seiki CL-20 CNC production lathe, complete with a twin-gripper gantry loader was delivered in 1992.

The gantry loader would pick a forged ring blank from a palleted stack, transport the forged ring to an orientation device, from there to load to the chuck (removing the finished part first) and return a machined ring on a finished stack The orientation devices use vibration to rotate the ring until a certain radial position is registered and detected by a light sensor, the gantry loader would then re-grip the ring for insertion into the chuck.

The back face of the rings is turned in the same operation as facing and boring.

From then on production engineers at the company were involved with the machine tool, tooling and workholding suppliers to continuously refine, develop and 'tune' the production turning process.

VTL Automotive acquired Valeo Transmission (formerly Sagar Richards) through a MBO in November 2001, and the co-operation with Pollard Mori Seiki continued.

Today, VTL Automotive is operating 15 Pollard Mori Seiki machines, having added seven of the Mori Seiki CL-203s to the eight CL-20s production lathes.

A further two machines joined as an in line linked pair will be due for delivery in early 2002.

The latest installation of the CL-203s in Cell 5 is also a linked pair.

It consists of two machines joined in-line to form one cell and is served by a common gantry with two robots.

In operation, the sequence is similar to that of the CL-20s, except that a R”hm double-action chuck grips the ring externally, then internally in sequence.

After machining, one gantry robot places the ring in a turnover device, before the second picks it up and loads it into a collet chuck in the second CL-203 for back end facing and chamfering.

A simple 'handshake' between the two robot controllers is all that is necessary.

The first robot signals to the second that: 'I1ve moved out of the way, you can have it.' The whole 2-stage operation now takes 23 sec, floor-to-floor, and has eliminated the production buffer stocks that were necessary when using two separate operations and machines.

If needs be, the two linked CL-203s can operate as single, stand-alone machines.

Pollard Mori Seiki's and VTL Automotives engineers continue to 'tune' the twin machine cell as well as the machines in the other four cells.

All the tooling, including the tips, are 'specials' supplied locally; no standard tooling is used.

VTL Automotive are very strong on tooling and methods.

Regular tooling investigations are carried out, for example, including Pareto analyses, investigating break-even points between tool life and feeds and speeds, seeking further standardisation in tool types and relating machine and tooling parameters to, for example, surface finish tests.

Efficient cutting of hardened brass is dependent on tool geometry and the avoidance of edge build-up.

To give an idea of the precision machining involved, a typical ring can have an OD of some 80mm, an average wall thickness changing in section from 8 to 3mm and a depth of up to 10mm.

It has teeth and needs to be faced, chamfered, bored and internally threaded.

When being turned, threaded or bored, a ring will try to spring and lobe, but the OEM wants out-of-roundness held within 12 microns.

Individual detail relationships may have to be held within around 5 microns.

To do a job like this in forged and hardened brass, under strict SPC, and achieve near-zero rejects in a batch of 8000 speaks well of VTL Automotives methodology and the quality of the Pollard Mori Seiki CNC lathes.

It also speaks well of the calibre of the machine operators.

In an eight-hour shift, six operators man 15 Pollard Mori Seiki's.

They are totally responsible for quality - there are no formal quality audits - each of five cells are supplied with co-ordinate measuring machines (CMMs) roundness testers and surface quality measurement devices.

'Every operator has his own work specification, they know the CNCs and operate the CMMS as well as carrying out visual inspection,' says David Clegg.

The operators have the backing of a comprehensively equipped standards room, which watch over fixtures/gauges and gauging quality.

VTL Automotive has detailed 32 operators to manage its Pollard 15 Mori Seiki's on a three-shift/24h/5-day basis with overtime and weekend working as necessary.

'We get a very good back-up from Pollard Mori Seiki,' says David Clegg.

'The CNC controllers are a good system, it copes with everything we want of it.

We continually look at other makes of machine, sometimes we even consider other agencies for the same machines, as we always want to know if we are getting the best price.

Overall, both companies have a better than average working relationship.' 'We practice two main philosophies at VTL Automotive,' concluded Bruno Jouan.

'We concentrate on the maximum utilisation of capital and the constant reduction of piece part prices.

Efficient and close collaboration with a machine tool supplier like Pollard Mori Seiki is a major factor in adhering to our philosophies.' On its Luddendenfoot site, near Halifax, VTL Automotive employs 140 of which 100 are directly involved in the production of synchroniser rings and selector forks.