Product category:
EDM wire and diesinking, miscellaneous equipment
News Release from: REM Systems | Subject: Quick-Chuck 100 EDM tooling system
Edited by the Manufacturingtalk Editorial
Team on 17 July 2001
Tooling system improves EDM consistancy
Since adopting a quick change EDM tooling system, a turbine blade manufacturer says that the changing over of graphite electrodes has become easier and quicker.
Since adopting the Erowa Quick-Chuck 100 EDM tooling system, Exeter-based Howmet Castings says that the changing over of graphite electrodes has become easier and quicker Improved security in electrode location and repeatability of set-up have reduced set-up times
This article was originally published on Manufacturingtalk on 8 Nov 2002 at 8.00am (UK)
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Howmet Castings, a division of the Howmet Corporation, manufactures precision gas turbine components, to near net shape, using the 'lost wax' or 'investment' casting process.
Materials involved include nickel- and cobalt-based alloys.
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The components are supplied to most gas turbine manufacturers, including Rolls-Royce, Siemens, General Electric, Pratt and Whitney and ABB.
'Normally the pouring gates are located in the 'machinable' areas of the blade,' explained Howmet's Post Cast Production Engineer, Barry Mitchell.
'But as the blade specifications and surface complexity became more complex, the pouring gate had to be relocated at the blade root area.
The blade route area has more complex surface features which posed a machining problem.' Tooling accessibility for conventional machining and abrasive machining processes were severely limited in the blade root area.
Howmet had some experience of using die-sinking EDM techniques to finish-machine the abrasive ground surfaces or small details which could not be machined conventionally.
EDM appeared to be the best route for removing pouring gates from the blade root area.
After consultation with a leading EDM supplier, Howmet expanded its EDM department for the production EDM machining of the turbine blade pouring gates.
'Our interest lay primarily in achieving energy-efficient, high metal removal rates,' said Barry Mitchell.
'We were not so concerned with fine precision.
Tolerances on this kind of work are in the order of 20-50 microns.' He also explained that a high standard of surface finish was not required: 'What is important is to remove the re-cast layer to allow inspection for casting defects after EDM.' The company also sought a quick-change tooling system.
One of the main graphite electrode suppliers, Tooltech in Plymouth, collaborated with Howmet in introducing a quick-change tooling system.
Typically, a batch of blade castings are machined with EDM in multi-fixture set-ups.
EDM time can last two to three days.
Problems with the system eventually caused Howmet and Tooltech to seek a more reliable toolholder.
'It was important to us to know that the electrode was in the right position,' said Barry Mitchell.
'As electrodes may be replaced a number of times during one set-up, and one did not want to have to re-calibrate the tooling and fixture for every electrode change.' Problems began to occur after a while.
The high power used in the EDMs caused the single location stud in the tool holder to expand and 'lock-up'.
Expensive damage to the tooling and the fixtures could occur when trying to free the locked-up toolholder's.
'Bear in mind that we are running these jobs on high amps, normally up to 200A.
Also the machine and workholding suppliers do not have much experience of cutting 'exotic' alloys, they are more used to tool steels.
Our operators have developed their own EDM experience, particularly for aerospace alloys like Nimonic, for which graphite electrodes are ideal.' In a search for a better, more reliable and more accurate toolholding system, Barry Mitchellos department approached Erowa in Gloucester.
The result was the adoption of the Erowa Quick-Chuck 100 system.
'Electrode changeover is now accomplished in a couple of seconds and they do not 'lock-up' after long hours of high amperage cutting,' said Barry Mitchell.
He also added that the four spigots in the Erowa Quick-Chuck system provided a surer location and easier set-up.
In terms of saving on re-work and scrap, the Erowa Quick-Chuck system has virtually eliminated electrode damage.
Using the system has had the effect of assuring the consistency of the EDM gate-machining process.
The security of the Erowa Quick-Chuck tool-holding system, and easier tool changeover, has enabled Howmet to use larger graphite electrodes to accomplish with one large electrode what required two or three electrode changes before.
Howmet is now in the process of introducing the Erowa Quick-Chuck toolholding system to all its EDMs.
Typically, a pouring gate area in a Nimonic alloy blade can be semi-circular in cross-sectional profile, crossing two or three root ridge projections which have to be retained.
The volume to be removed is in the order of 210mm x 20mm x 30mm wide - a job taking about 45min in the EDM.
The largest blades can have up to three times the amount of metal to be removed, to a maximum depth of around 22mm.
'It is now our intention to use the Erowa system on all our EDMs as the old fixtures are modified and new tooling introduced,' said Barry Mitchell.
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