[ATM] removing that broken tap or drill

[Charles Mitchard <charlesmitchard@iinet.net.au>]

with all this talk of threading in sticky aluminium maybe its time to 
mention how to remove that broken tap.
I broke one today.
As I had a sulphuric bath available (failed anodising test) I removed a 
broken 4mm tap from a casting by anodising it.
The steel gradually dissolved till I could unscrew what was left.
Took 4 hours at 6 volts, 8 amps.
The aluminium did not dissolve but did discolour slightly.
I connected the work to + and a lump of lead to -.
The steel bubbled more that way so I assumed it was correct. (it worked)
There are also tap removers available that use 3 prongs to slide down the 
flutes so you can back the tap out. Trouble is the shop that sells them is 
never open when you need it.
I have heard that boiling in a saturated alum solution (aluminium 
sulphate??) will also dissolve the steel but I dont have experience with that.
There is also a special welding rod that fuses with the tap, the flux fills 
the threads without damaging them, you then place a nut over the hole and 
keep welding to fill the nut then unscrew the nut and the tap comes out. 
again I cannot vouch for this process but it sounds cool. A search should 
find the name of these rods.
Regards
Charles


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