Removing a broken tap

I thought this may be of interest to the group as it's been a topic a few times inthe recent past. I recently discovered a model steam
engine casting I'd buggered as a teenager some 35 odd years ago by breaking a 7BA tap in a hole. Given the prices Stuarts are charging these days I thought it might be worth trying to recover the situation.
I don't know anyone near me who might do spark erosion and briefly considered a home-brew attempt, but some research indicated it would need a fair investment in time and basic equipment just to 'have a go'. However, along the way I also noted some of the details for electro chemical machining, which seemed to offer an alternative with potentially less overhead to start with.
So armed with a battery charger and a couple of pints of salt water (no idea what concentration, it was sea-water) I set to work. The tap was broken off just 'under flush' as one would expect. Initially I used a piece of 1.5mm2 core from some lighting wire, cut off flush at the end ie only the end of the wire exposed. I held it against the end of the relevant hole. Switched on the charger and there were immediately some bubbles from the hole and then nothing. I concluded that the bubbles effectively drove the electrolyte out and then the current stopped so I shaved the sides of the insulation to allow the brine to flow. Continued bubbles - promising. I left it for half an hour and came back 'just to see if anything had happened' and was staggered to find the hole had been opened up to the diameter of the insulation an down to the top of the tap. So I swapped the 1.5mm2 for a piece of single filament from some telephone wire and re-started the charger.
Having seen the effect I made sure I came back every 15~20 mins to stir the sludge in the hole and push the tip of the wire further down and ensure it was in the centre of the hole and not resting against the side. In this way I disolved the entire tap in under 2 hours.
Given the extremely rudimetary setup I had and the complete lack of accurate control of either the current or the electrode position I consider this an amazing success. I may just have been very lucky, and I would make a somewhat more engineered rig for use if I need it again. I don't often break taps, but such a simple in-house means of sorting the problem seems much better than having to hunt round for tool makers with spark eroders or the restrictions of sulphuric acid provided it's in brass etc. Just make sure the work-piece is the anode or you'll build-up the broken end, not disolve it!
Richard
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    --Well I hate to tell ya but I've got a little EDM machine, but it might be a bit of a commute to California! ;-) FWIW it's a fun bit of kit to add to your shop if you can fine one. Besides tap busting it's possible to modify it to use it as a sinker and I've made a few injection molding dies this way. IIRC there were a series of articles in either ME or MEW many moons ago describing how to build one.
--
"Steamboat Ed" Haas : "Hold on! we're passing
Hacking the Trailing Edge! : through the moronosphere!"
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Richard wrote:

That sounds very promising. I have an aluminium air-raid siren housing with broken off screws in it, waiting for an idea like this. What metal is your Stuart casting made from?
Best wishes,
Chris
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Sorry, should have said, the Stuart casting is iron. As I frankly didn't have a great deal of hope in the first instance I made very little attempt to control anything - the wire was lodged in place with a dob of plasticine and a rubber band and tended to move about a bit. I would put in a bit more effort next time. After the first go it was immediately obvious that the finer the electrode, the finer 'hole' you'd machine. Blindingly obvious really.......
I was careful to cut the 'electrode' off flush with the insulation so that current could flow only out of the end and not radially. Given your situation of something you don't want to make worse, I'd try with a steel screw in any old bit of ally first.
If you want to read a bit of technical stuff on the subject look at this: http://www.osti.gov/energycitations/servlets/purl/15015141-slv9mp/native/15015141.PDF
It _is_ quite theoretical, but skipping through it picks up a fair bit of useful pointers.
Richard

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Hi Richard
What made you go for sea water - I thought the fluid should be non conductive?
Tim
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wrote:

with
bit.
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at
this:http://www.osti.gov/energycitations/servlets/purl/15015141-slv9mp /nat...
bit
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metal
Yes that point made me think it was mainly electrolysis rather than edm that disolved the tap. True EDM needs a dialectric that breaks down releasing an explosive discharge, with a conducting electrolyte that wouldn't happen.
AWEM
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On Wed, 11 Jun 2008 08:34:00 +0100, "Andrew Mawson"

Yep...this is consistent with John S's favourite ploy for getting broken taps out of ally - send the part off to be anodized.
Regards, Tony
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Electro CHEMICAL machining (not EDM) is an electrolytic process and needs a conductive electrolyte. I went for sea water 'cos there's lots of it 100 yds from my front door and I didn't have to negotiate with SWMBO s to how much Saxo I was nicking. The reference quoted says sidium chlorate (not chloride), but they were looking for efficiency and towards an industrial process.
As I said, it was a rough and ready 'try it to see what happens' and I was surprised how successful it was.
Richard
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On Wed, 11 Jun 2008 17:13:09 +0100, Richard

Mage sure you don't replace SWMBO's Saxo with sodium chlorate <G>
Regards, Tony
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Hi Richard
It looks very interesting - I use a electrolytic process for rust removal and it works very well. I used washing soda (Sodium carbonate) to make the water conductive. Any chance of some pics ??
Tim
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I cant get the article (the url is ok) - it always times out - can anyone email it to me please.
Tim
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In article

Tim,
You have mail (a bit long!).
David
--
David Littlewood

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I'll take a couple of photos and post them on PhotoBucket or somewhere. The 'equipment' was a plastic bucket, two crocodile clips and a battery charger (13.8V regulated supply actually) so I won't bother with that.
Interesting you mention the rust removal use - it is evident that the area immediately beside the hole in question is noticeably less dull that the rest. It wasn't badly rusted, just tarnished and maybe still some foundry scale.
If you've not got the article yet let me know and I'll email it to you. One thing to note is that they propose working with quite small clearances (<0.5mm) and the electrode works almost like a form tool or indeed spark erosion, hence my clipping the wire off flush with the insulation. It was my intention to largely drill 'down' and not 'out' from the end of the wire/electrode.
I'm not certain, but I think the electrode will work similarly to an electrostatic discharge so that a point will drill much more quickly than radiused end - even if the nominal diameter is the same a few mm up from the tip. That said the phone wire filament is only about 10 thou dia so it's pretty 'pointy' even when blunt IYSWIM.
Cheers Richard
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Thanks I did get the artical via email from David Littlewood.
The big difference with the rust removal is that the work piece is the cathode (negative) and the sacrificial metal is the anode . Also the anode is usually iron.
Tim
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Here are some photo links
http://i297.photobucket.com/albums/mm222/RichardandNovo/RichardsToys/7a_06.jpg
http://i297.photobucket.com/albums/mm222/RichardandNovo/RichardsToys/7a_04.jpg
http://i297.photobucket.com/albums/mm222/RichardandNovo/RichardsToys/7a_02.jpg
The accidental counterbore, caused by too large an electrode initially is about 3/32" deep - I'd guess the top of the tap was initially about half that below the surface. It's a bit irritating, but not an unrecoverable situation and hey, I've got the casting to use again. There's quite a bit of remedial work to be done in general, I'm rather more critical now than I was as a 15 year-old.
The total depth I disolved is difficult to say as the tap is obviously no longer existing, but I would estimate at least 1/4" probably a little more. It's too long ago now to remember, but it would have almost certainly been a 2nd tap and looking through an eyeglass I could see full threads on the top of the tap before I started. So the whole of the taper was in the hole.
The overall red rust was caused by the salt water and brushes off easily with a wire brush. The slightly bright area around the hole is due to the electrolytic process.
Richard
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