In my on going series of teaching myself to model machine parts in Inventor, I keep stumbling across the following scenario for both external and internal threads. My reference drawing are from the WWII era and more specifically are V2 rocket parts. When drawing threaded parts they designated them nicely with M14x1.5 or the like and proceed to draw the outer and inner limits of the threads. The part that gets me is they always throw on a an angled bit at the start and end of the thread. Usually one end's angle will be tangent to a fillet.
External thread example
Internal thread example
My current technique is:
- Draw up the section as complete as possible.
- Assume the threading dimension or limiting line is the point of tangency between the fillet and the 60 degree bevel.
- For the example designation of M14x1.5 assume the 14 is the diameter and 1.5 is the thread depth.
- Revolve the shape keeping threaded holes solid.
- Apply external threading through Inventor to the external 14 mm diameter surface OR make a threaded hole through Inventor with the designation of M14x1.5 to the depth required. For hole I then subtract the voided area at the bottom of the hole if there is one. (ie in case above the 22.2x3 area (with 1 mm corner fillets based on other similar examples in set).
Example of current approach
I thought the technique was fine until I happened to line up a threaded hole with the section I had drawn for it. The limit of the Inventor produced hole was somewhere between the thread limit lines I had drawn.
I experimented with drawing hole threads by drawing up the inner surface limits and then providing the threading information to the surface, but I believe the problem here is that the diameter of the hole no longer matches the diameter of the designation and therefore does not show up as acceptable threading option.
My question is, for reproducing these as a modeling exercise, what is the proper modeling approach?