Broken tap in a lump of aluminium

I was stupid enough to break off a 3/16 whitworth tap in a large, part machined, block of aluminium. The tap has snapped off just above the
start of the flutes so the usual aid of a walton extractor won't work. A little Googling suggested that sulphuric acid should shift the tap without damaging the aluminium. However 3 days fully immersed in concentrated H2SO4 has done nothing.
What options do I have before I give up and find a spark erosion man?
Thanks
Charles
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Charles wrote:

IIRC what I've read works is nitric acid or a saturated alum solution.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Mar 29, 8:33pm, David Billington

Concentrated sulphuric acid does not dissolve steel very fast. Much better to dilute it to around 30% concentration. Use it cold since at high temps the aluminium will dissolve. Nitric acid tends to passivate both steel and aluminium so reaction rates will be very slow. Alum is quite a good choice. Use this warm to get acceptable rate of dissolution. Mike
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
In article

Mike is right, concentrated H2SO4 will passivate the iron if cold (may attack it hot, but then Mike's warning applies). I have used it at around 20% concentration to dissolve out taps from Al alloy. Two points though: (1) it stained the Al, and (2) since you have a small amount of acid and a (relatively) large amount of metal, you may need to change the acid a few times - it's not quick, and you may also need to prod it frequently to get H2 bubbles out.
I would strongly advise caution in letting nitric acid into your workshop - the fumes will attack your machinery quite rapidly. Hydrochloric acid is even worse (remember HCl is actually a gas, and the concentrated acid is actually an approximately 36% solution of this in water).
David
--
David Littlewood

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Tue, 29 Mar 2011 12:16:05 -0700 (PDT), Charles

If you can get it across to me by tomorrow evening (Wednesday), I'll see if i can get it on the sparker in the toolrom on Thursday.
Peter
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Thanks for the offer Peter but I'm off to Nottingham today forst thing (ie soon). I'll be in touch if all else fails.
I'll try the less concentrated solution of H2SO4. Given that the final machining has yet to be done on this piece would a 10% solution and an anodizing set up be even better? I'd just need to find some lead for the cathodes.
Charles
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
    --FWIW if there's enough tap sticking out of the hole it's a good chance to practice your TIG welding skills: attach a 'handle' and back it out. Example here: http://www.flickr.com/photos/steamboat_ed/4818677773/in/set-72157624880666386 /
--
"Steamboat Ed" Haas : Steel, Stainless, Titanium:
Hacking the Trailing Edge! : Guaranteed Uncertified Welding!
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
The answer was:
Put the aluminium (anode) in a 12% solution of Sulphuric acid. Old car battery Large coil of lead solder (cathode) Jump leads.
Two days of that and the tap was gone
Charles
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Did the aluminium get affected at all? Possibly anodised?
AWEM
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Apr 9, 8:56am, "Andrew Mawson"

.
Yes, it got lightly anodised but that isn't a worry with a part machined lump like this. If it had been a concern I could have dipped the "not to be anodised" parts in wax.
Charles
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.