Wire-EDM machines high toleranced splines

Using wire-EDM, a transmissions manufacturer now soft-cuts an undersize spline, heat treats it, then finishes using wire or spark erosion to achieve the very close tolerances.

Manufacturer of high-performance transmission systems in Berkshire, UK, Xtrac, recently invested in two GF AgieCharmilles EDM machines for high tolerance machining work.

The machines are a FI 240cc (CleanCut) wire EDM machine, and a FO 35 die-sink machine.

They have been installed at Xtrac's 88,000ft2 purpose-built facility in Thatcham alongside three existing GF AgieCharmilles EDM machines (a Robofil 440 Cleancut; a Robofil 4030 Si and a Roboform 35P).

* Tighter spline fits - three to four years ago Xtrac was striving for tighter spline fits coupled with a more accurately finished spline form.

The initial objective was to improve spline fits on layshaft gears and dog rings.

Production engineering manager at Xtrac, Ken Howlett, said that the company was already happy with grinding the external splines to tight tolerances but was finding it difficult to maintain the required consistency on the internal spline.

He said that the problem lay with the 'soft' cut internal splines that experienced distortion during the heat treatment process.

Often these parts would be subject to variable degrees of shrinkage - and in some cases it was necessary to use three different cut sizes across the range to ensure the post-heat treatment fit requirements were achieved.

For layshaft gears, it would also require the purchase of three dedicated broach tools.

To solve the problem, Xtrac thought it would have to buy new, costly dedicated broach tools.

The company would also have had to develop a test programme to establish and quantify shrinkage rates.

These 'potential' solutions had their drawbacks, as Xtrac had to achieve tight manufacturing lead times and control its production costs.

Howlett continued: "EDM technology was able to deliver a better solution.

With EDM we are now able to 'soft-cut' an undersize spline prior to heat treatment, then finish the spline (after heat treatment), by either wire or spark erosion to achieve the very close tolerances required".

He added: "The EDM process has enabled us to remove the elements of risk and delay we were confronting, and has helped us achieve a predictable result".

AgieCharmilles told manufacturingtalk that whilst EDM is not a new technology its rapid development into becoming an accepted, viable and refined manufacturing method has accelerated quickly over recent years.

An early problem experienced with EDM technology was the so called 'white layer' (re-cast) left on workpiece surfaces after the EDM process.

AgieCharmilles said that this problem had now been all but eradicated.

The GF AgieCharmilles 'Si' or 'surface integrity' generator technology, and latterly, the digital 'CC' or 'CleanCut' generator have dealt with 'white layer'.

The two 'CleanCut' machines provide Xtrac with virtually the same levels of accuracy as the high-end 4030Si machine but with an improved cycle time.

Howlett said: "Historically EDM machines have had a niche focus in the production of very precise tools for the mould and tool industry.

With significant improvements to machine design and spark generation technologies, the process has achieved a wider use, most notably of course, in motor sport".

He said that EDM has proven to be an invaluable tool for machining complicated or intricate components.

For Xtrac, EDM has enabled the hard cutting of a diverse range of components that include change gears, final drive gears, dog rings, selector forks and output flanges.

Howlett said: "Working to a CAD file, EDM also offers us the flexibility and capability to manufacture low volume prototypes, or can help us manufacture urgent parts for customers without the need to purchase additional tooling".

He concluded: "It is our never-ending quest to refine and update our existing proven processes, coupled with the challenge to seek out new and emerging technologies that ensures we remain at the cutting edge of manufacturing technology, delivering the best possible and 'right first time' solutions to our customers.".

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