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By far the best way to remove difficult-to-reach broken taps is EDM. In a few cases I ask a friend of mine with acces to a wide selection of EDM equipment to do this. However I first try to use a HM drill. As most taps are HSS a HM drill will cut them up. As a regular full HM drill is rather costly I take a cheap stone-drill which has an HM bit at the top and I regrind it to cut HSS. This works quite often.
Another application for reground stone-drills is piercing the sometimes quite hard skin of castings.
 

Bentwings

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Yes, Guy Lautard recommends "pure" Nitric Acid in The Machinist's Third Bedside Reader. He also points to Brownell's Gunsmith Kinks V.1, which also recommends Nitric Acid as well as trying solid Carbide drills.
I don’t know about the nitric acid I’d guess it quickly corpses the seized area thus weakening it . Sounds like ifvyhe tap gets stuck that would bevthevtimevto apply it don’t keep turning until the tap breakes
 
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When a tap breaks in a not-too-big or complex workpiece of steel I simply throw the thing in the wood stove that heats my workshop. Next morning I can almost always get the broken tap out with a regular HSS drill.
 

L98fiero

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When a tap breaks in a not-too-big or complex workpiece of steel I simply throw the thing in the wood stove that heats my workshop. Next morning I can almost always get the broken tap out with a regular HSS drill.
What kind of taps are you using, M2 tool steels(1.3343) are annealed at approx 885°C (1625°F) and cooled at 72°C (162°F) per hour to 900°F
 

dazz

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Nitric acid works quite well-as I can personally testify-BUT (and it is a big BUT)-whilst it does not attack aluminium-it DOES attack other metals-and since very few people are tapping pure aluminium-in practice we are using a variety of aluminium alloys, the acid approach can lead to discolouration due to the trace components of the alloy reacting with the nitric acid. In some cases this may not matter-in other cases it may. In my case I ended up with a discoloured engine pan-(sandcast , low Si alloy-ended up mottled black and grainy) - which didn't trouble me especially, but for some components which might need to have cosmetic appeal -shiny, polished or anodised-you might run into issues.

ChrisM
I have also used Nitric acid but with all the problems above. Also Nitric acid is very dangerous. Take a lot of care if using it.
 
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The taps I'm using are Völkel hand-taps Made in Germany from HSS-E (with some Cobalt). Every once in a while I throw a piece of Aluminium in my wood stove which comes out melted. So the temperature in the wood stove is higher than say 700 degrees Celsius. This ain't 885 degrees Celsius, but apparently staying in there for about 12 hours and cooling extremely slow in the cooling ashes seems to soften the tap enough.
 

L98fiero

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The taps I'm using are Völkel hand-taps Made in Germany from HSS-E (with some Cobalt). Every once in a while I throw a piece of Aluminium in my wood stove which comes out melted. So the temperature in the wood stove is higher than say 700 degrees Celsius. This ain't 885 degrees Celsius, but apparently staying in there for about 12 hours and cooling extremely slow in the cooling ashes seems to soften the tap enough.
Wow, I didn't think a stove would get that hot!
 

awake

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The taps I'm using are Völkel hand-taps Made in Germany from HSS-E (with some Cobalt). Every once in a while I throw a piece of Aluminium in my wood stove which comes out melted. So the temperature in the wood stove is higher than say 700 degrees Celsius. This ain't 885 degrees Celsius, but apparently staying in there for about 12 hours and cooling extremely slow in the cooling ashes seems to soften the tap enough.
I think I may have done something similar before - melted the part away from the offending broken tap. Problem solved, tap removed ... lump of aluminum returned to the scrap bucket! :)
 

Sprocket

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I can't find a picture right now, but in aluminum, I have put a nut over the end of the broken tap then welded through the hole. I guess the combination of heat and something to hold onto works, because they turn out with the nut. It has to be close to the workpiece surface or above.
Doug
 

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