Sharpening Old Taps And Dies

Tried some taps, some in sulfuric acid (battery acid from the auto store) and some in hydrocloric acid (30% pool acid).

An hour in the sulfuric acid worked nicely. Noticeable sharper, no noticeable change in size with the mic, and a nice blackened finish which came off with a bit of work with the wire brush on the dermal tool (but I didn't bother on most of them)

Hydrocloric acid was much slower, cleaned them up nicely but only felt marginally sharper - left some overnight but may of needed a day or so to get the same effect as the sulfuric acid.

Havent tried using them - but to my untrained thumb they feel noticeably sharper so I would say a success.

After taking them out of the acid I just rinced them in a water/bicarbonate soda solution to neutralize any other left over acid then dried them and coated them with WD40 (equivalent) and left them to air dry.
 
When I was doing mold making we use to resharpen our taps with a round stone in die grinder. The stone is run by hand along the cutting edge while bottomed in the clearance grooves. It worked quite well and we always got a few sharpenings before the threads would get a bit tight. Just use a stone as close to the groove diameter as possible.
 
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