EDM - not just for burning busted taps out

There was a recent article in Aviation Week saying that GE was using Electric Discharge Machining (EDM) to rough out turbojet blisks (combined rotor and blades) from Inconel 718, which is almost as hard as carbide. Apparently, GE got the idea in 2001 from the Chinese, who were using EDM for drilling of such alloys.

After roughing, the blisks are cut to final form using electro-chemical machining, and also polished.

GE calls the technology "Blue Arc", the blue being the color of the proprietary working fluid.

Joe Gwinn

Aviation Week, 9 May 2011, page 16, "Submerged Results - Novel technique speeds GE's blisk output", by Michael Mecham, The Inside Track (column).

I have not found this on the web; I assume it's behind a paywall.

There are patents, however. Search for "electric discharge" and inconel. I found applications 2010/0301016 and 0301017 for instance.

Reply to
Joseph Gwinn
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EDM was shunned on aircraft parts because it was thought that microcracks that would form in the cut which would cause failure of the part. Now with the final finishing of the parts EDM is becoming the choice for machining blades.

John

Reply to
John

Something must have got lost in the reporting. Inconel 718 is nowhere near as hard as carbide -- it's tough and gummy like most nickel alloys, but not especially hard.

GE wouldn't need to learn from the Chinese about making holes with EDM. I did quite a lot of work for GE's medium steam turbine division in the mid to late 80's, and they were cutting tubine parts with some pretty old conventional (sinker) EDM machines at that time, and the press tools I was working on were cut with wire EDM. I believe much of the work that was done on the sinkers was transitioned to laser cutting not long after.

Reply to
Ned Simmons

EDM has been used for drilling the cooling passages in aircraft turbine blades for around 40 years. And it was a pretty fierce burn done at high speed, which leaves a lot of microcracks.

However, that was on the inside of the blades, an area which is not in tension. They're close to the neutral axis, in fact.

With all due respect to _Aviation Week_ (published a couple of floors above _American Machinist_ when I worked there, whose editors often came to us when they needed to know something about metalworking ), the way those lines above read suggests that the writer isn't familiar with the technology. EDM has been used for drilling holes in Inconel, Hastelloy, tungsten carbide, and other refractory metals for, again, around 40 years. Making blisks with EDM probably is new. As you say, the microcracks would be a problem on any surfaces that are in tension.

Electro-chemical machining (ECM) of aircraft parts goes back even further -- at least to the early '60s, and maybe the '50s. It, too, was used initially for drilling cooling passages in turbine blades. It has an advantage over EDM in that it doesn't damage the surface, either with microcracks or with a heat-affected zone. It was largely abandoned for that work in the mid-'70s, when EDM took over. ECM is a messy process that presents some difficult tooling problems, and it needs its own room in a plant, with its own ventilation, or it will rust everything in the building.

But roughing with EDM and finishing with ECM would make good sense from the standpoint of roughing efficiency combined with finished-surface integrity. The tooling for the ECM must be very interesting to handle all of that geometry. Maybe they're using CNC positioning or something.

Reply to
Ed Huntress

It did get lost. I don't think the reporter had much experience with any of this. The issue with EDMing Inconel is its high melting temperature. It's a little slow to EDM.

Reply to
Ed Huntress

It has been interesting to hear the back stories, and the writer appears to be in over his head.

It sounds like AvWeek needs a Machining Editor, Ed.

Joe Gwinn

Reply to
Joseph Gwinn

They used to have one. A young engineer from McDonnell Douglas (Jerry Mayfield) worked for us at _American Machinist_ for two years and learned enough about manufacturing that _Aviation Week_ hired him away. He did a good job for them for several years. I was covering advanced materials and the aerospace industry at the time, and Jerry and I had a friendly rivalry going over who could break stories first.

Then Jerry left, and for the next decade or so, they were out of luck. _AM_ moved out of the McGraw-Hill building, then was sold to Penton in Cleveland, and AvWeek was on its own. I haven't been reading AvWeek for years, so I don't know what's happened since.

BTW, there was an EDM machine tool builder, Raycon, based in Ann Arbor, MI, that made most of its living building special machines for eroding those cooling passages in turbine blades. It was a good chunk of business at the time. There were three or four EDM companies, worldwide, that specialized in it. They used wire-like electrodes made of some refractory metal (tungsten, IIRC), burned with six or so of them at a time, and had automatic feeding and trimming of the wire ends. They were very clever.

Reply to
Ed Huntress

Their interest in the how of manufacturing appears to have declined, as it seems that Jerry was not replaced. Did Jerry leave under his own power?

I always wondered how they drilled those passages. It was clear that twist drills would be the hard way, to impossible.

By the way, how do they make the cooling passages in solid carbide tools? Embed a piece of steel wire before compaction and sintering, and dissolve the steel out with acid?

Joe Gwinn

Reply to
Joseph Gwinn

I never knew what actually happened there.

Ha! That's an interesting thought.

This is one of the growing list of things that I used to know, but forgot. If you're really curious, call a company that makes carbide drills with through-tool coolant and ask for an engineer. That's what I used to do.

Reply to
Ed Huntress

I suspected as much.

I'll try that. Thanks.

My other method is the patent literature.

Joe Gwinn

Reply to
Joseph Gwinn

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