BF20LV Draw bar thread confusion

[marvin_schwartz]

This little puzzle has been has been bugging me all day long.
Finally I solved the puzzle and feel better now.
If you are interested read on.

Firstly I drew all three threads to scale M12, 1/2UNC-13 and 1/2BSW-12 they look like the first picture.
Not until you lay them over each other do you see what is happening. See the second picture.

You can see that over an inch or so UNC1/2-13 and M12 are within 0.45 mm which is whay my M12 drawbar worked and feels snug. Even though 1/2 is a slightly bigger hole 12.7mm instead of 12mm.
Also shows why combinations of UNC/BSW and M12/BSW will jam up after 4 or 5 turns.

Now you know.

 

[goldstar31]

Speaking as someone- with a 'lot of draw bars' trying to adopt one thread size for another merely advances the evil day of having to replace 'fretted' threads

As you wrote-- Now you know!

Norm