Beautiful horn, Tim. Any problems using the taps and die on the horn or the wooden plug? Many years ago I had special threading taps and dies especially for wood, but moved too many times!
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Standard machine taps and dies. You have to go slow, about a quarter turn and back off, clean out the scrap a lot, use a good lube, I use something called Dr. Jackson's Hide Rejuvenator. Sharp taps and dies are a must, as is tapering the piece to start a die, I use both a tapered tap and a bottoming tap.
Have a separate set of taps and dies for wood something like 6 TPI.
Tim C.