Effects of electrical etching

[Durkee]

We had a problem with some custom tools that we made breaking for no apparent reason. We electrically etched the tool # on the shank. Is there a disturbance caused in the area by the electrical current that could cause the tool to be weaker there?

 

[NickE]

Are the tools failing after a long or short time of use?

Any surface defects will act as stress raisers to reduce sevice life.

nick

 

[Durkee]

I realized that we had created a riser after the first one broke out on demo. We took the tools back to the shop and ground the shank down in the area of the etching. The shank was already reduced in that area. We just took an extra .003" off and got rid of the etching marks. We have had this same style tool on a different job with same circumstances with good success. The only difference we can come up with is the etching.

Keith Durkee

http://www.toolgrinder.com

 

[Metalguy]

Sounds like you made some untempered martensite at the etching. The heat of the tiny arc is enough to raise the local metal temp. above the austenitizing temp, and the surrounding cold steel rapidly quenches it.

As you probably know, untempered martensite is very brittle. It you can't use some other marking method, then retemper the tools after you mark them.

 

[NickE]

Oh, you'd used EDM style etching, I guess I had assumed electrochemical etching, whick leaves no re-hardened zone.

nick

 

[Durkee]

Metalguy...I guess I forgot to mention that the tools were made from solid carbide blanks. Do you know if there would be a similar effect on carbide?

Keith Durkee

http://www.toolgrinder.com

 

[Metalguy]

Aha. Well, you won't have to worry about martensite.

One thing isn't clear to me. Did the etched tools break AFTER you ground off the etching or did the grinding save them?

 

[Durkee]

I guess I wasn't near complete enough in my first post. Sorry guys.

The tools still broke after grinding the etchings off. The etchings were done with an electrical DC etcher. Not electro-chemical. We have a new one that was never etched running right now, bet we also left the neck .015" bigger in diameter. We also broke this one in by running a couple of cycles at 50% before running full speed. We changed too many things at one time to know for sure.

Keith Durkee

http://www.toolgrinder.com

 

[tomwalz]

Any chance you can look at it under a microscope? Even a Radio Shack $10 lighted variable power scope can do a lot of good.

We tried resistance brazing WC years ago and never could do it without damaging the carbide. What you are doing may relate to that.

Several ideas come to mind.

1. If it is standard cobalt matrix cemented WC then there is a significant difference in electrical resistivity between the WC and the Cobalt. We cover this pretty well in
U. S. Patent 5,624,626 where we use current to remove either the Co or the WC. Removing the Cobalt would leave it weaker.

2. For some reason the term eta phase comes to mind. You may be affecting the carbon and creating an eta phase embrittlement I can’t explain why but this idea seems as though it might apply. Typically this is a sintering problem.

3. I would think that a simple stress crack is more likely.

4. Maybe just forget it and try a straight chemical etch.

Our last newsletter had some good pictures of carbide defects. If you don’t get the newsletter and haven’t heard me speak lately please email and I will send you info.


Tom

Mr. Thomas J. Walz
President
Northwest Research Institute, Inc. / Carbide Processors, Inc.
3847 S. Union Ave.
Tacoma, WA. USA 98409
800 346-8274
Ph (253) 476 1338
Fax (253) 476 1321
tomwalz@email.msn.com

http://www.carbideprocessors.com

 

[Durkee]

Good stuff on the site Tom. I am pretty well convinced that the etching was the problem. I just want to understand why. We cut a lot of form tools with the EDM and have not run into a problem like this. I would think it would show up there, too.

Thanks,

Keith


Keith Durkee

http://www.toolgrinder.com

 

[tomwalz]

Good question.

I have no idea if this makes sense but I think of EDM as passing the current by and / or though the part while etching forces the current into the part.

I first learned about etching as a problem when we tried etching our name into some of our ceramic tipped saw blades. We truly spent whatever it took to make the very finest saw blades we could. We didn’t even label them because we didn’t want to fool around with clearances measured in tenths of thousandths. However we were sold an electro-etcher absolutely guaranteed not to affect the plate. Well it warped the plate so that one or maybe two teeth out of sixty stuck out about 0.001” which left just beautifully clear marks in the wood when it cut. Of course we didn't check before we shipped so we had a recall nightmare for a couple dozen saw blades.

Tom