A real machinist would say they tapped it correctly, but just picked up the wrong bolt.The great thing about machining is that you learn new stuff every day. Today I learned that if you're tapping a 1/4 x 20 hole and it's taking forever, you should check to see if you grabbed a 1/4 x 28 tap by mistake.
I had a collection of such fubars. Double start threads will throw you for a loop too With a casual glance, they look like the correct thread.Somewhere I have what was supposed to be the threaded locking lever for a Craftsman bench top drill press: I had tried to get it installed for about 10 minutes before I looked at it closely and saw that the rolled threads were actually just grooves. Craftsman sent me a replacement and I used to keep the oddball sitting around to frustrate others.
When anyone asked why I had it on my desk, I would hand it to them along with a nut that should have threaded on and offered them $10 if they had the dexterity to thread the nut on the lever.I had a collection of such fubars. Double start threads will throw you for a loop too With a casual glance, they look like the correct thread.