The Continuing Adventures of a Home Shop Machinist  
  • 48-speed manual transmission
  • Getting an angle on it
The thread, as best I can measure it, is 0.685"-20 - but we'll be using the breech plug as a gauge to avoid some tricky measurements and calculations.  Set the change gears for 20 TPI.

Rotate  the compound to 29.5°, so the threading tool is mostly cutting on one side - this reduces the force on it, making for cleaner cutting.  Make sure the tool is centered vertically and the point is perpendicular to the center line.  Remember to set the threading dial in the lead screw!

With the point of the tool just touching the inside wall, reset both the cross slide and compound dials to zero.  Then run the tool in until it just nudges the bottom of the hole, then set the depth stop to stop the saddle 0.05" short of this.  Double check to make sure that you'll get at least 0.625" length of thread so the breech plug can seat firmly on the flat bottom rather than running into the end of the thread.