One of the founders of the company I work for has a Coles hit'n'miss engine.  It has a drip oiler on a long pipe that reaches down through the water tank to lubricate the piston.  The pipe had 3/16-40 Model Taper Pipe (MTP) threads on the end that had gotten irrepairably bent.  I offered to fix it.  I decided to cut off the bad threads and splice on new threads.  I was averse to buying an expensive MTP thread die for a one-time use, and I don't have a taper attachment on my lathe.  Instead, I milled the threads using a thread mill in a Deckel FP2NC CNC milling machine with Deckel Dialog 4 control.  The associated photos tell the story.  The threads came out better than any I could ever dream of cutting with a die.  The Dialog 4 can do helical interpolation, but not spiral interpolation, which is needed for taper thread milling.  To work around this issue, I made a mathematical model of the pitch spiral of the thread and broke it up into helical segments, four per turn.  Each segment had a slightly different radius, but the end points of each segment coincided.  Although the centers of the segments are mathematically off the center of the intended spiral, the offset is less than 0.00005", so negligible.  I used Mathematica software, which I used to model the spiral and also spit out ready-to-go g-code.  I programmed these helical segments into the Deckel by downloading from my laptop.  I used cutter radius compensation on the contour, with the radius set as the pitch radius of the cutter, not the outside radius.  The pitch radius varies with the thread pitch.  The threads came out perfect on the first try.  It continues to amaze me what this 20-year-old second-hand Deckel can do.