Neil,
Not sure if your thread profile sketch is just illustrative or intended to be more accurate, but I can see how you would potentially have a lot of problems with those threads, tearing during the turning process would be assured and problems with galling and interference in use !
The internal and external thread profiles actually need to have different clearances, and hence the thread profiles (in a perfect world !) would be different, but as a minimum the crest and root of the thread profile are flat or curved but never pointed.
Check out the drawing here for better detail.
http://en.wikipedia.org/wiki/ISO_met...#Basic_profile
They have shaded the male and female sections showing the profiles and how they mate together against the line drawings of the thread profile.
So the outside diameter of the male threaded part(eg bolt) would be smaller than the notional metric size (eg M12 bolt diameter is less than 12mm)
Likewise the inside diameter of the female threaded part (eg nut) would be larger than the notional metric size (eg M12 nut internal diameter is greater than 12mm)
There are simple tables in all the machining handbooks and elsewhere that you can use to get all the correct dimensions for the starting diameters and depths of cut.
Thread cutting can be a bit of an art form and to accurately cut them you ideally need specialised metrology equipment - thread micrometers, shadow graphs if you are grinding your own tools !
Cheers
Rally
Quote:
Originally Posted by Auster1b
Thanks Rally,Geoff, I did not want to do all the work and I did leave a lot unsaid. 
Furthering the discussion and an inquiry from the other thread about set over, given everything else being equal, this is an accepted technique for cleaner threads;especially on small machines or difficult materials. Just takes a bit more to setup.
Unless it is for something very special, I will just grab an off the shelf cutting tool and holder and accept there will always be a compromise with tool geometry especially with Al., and then practice technique to help overcome the compromises. More swarf - yippee. 
I love making swarf and always challenge myself to do the best I can regardless of what I am making. Practicing precision on a non precision project helps me learn technique with less tears, but no less frustration and scrap !
Just drew this up to help describe the technique, Neil |