Originally Posted by juergenwt  twocik - Advise. As mentioned before. Pick up some books and start reading.
If you want to keep everything solid metric you will have to use a standard metric thread. I do not know the OD of your thread, but depending on the OD needed you will need to stick to the metric thread that is close to it. The same goes for the pitch.
There is no table since the two systems are completely different.
If you need 20 treads per inch you will have to stick with standard inch thread. |
True, but what a mess.
Imagine someone years later having to identify this thread, nightmare.
Not sure if the 20 tpi has to be all that precise tho.
If it dose then I would use a OD in imperial units allso.
40mm / 25.4 = 1.5748 inch
maybe make a 1 9/16 x 20 thread or 1.562 x 20
if the 20 tpi could vary slightly, as mentioned before a
40mm x 1.25 thread would be suitable as well.
Both can be cut acurately on the 9 x 20 Lathe.
Good Luck whatever you decide.