I've never even tried to cut threads on my lathe, I've always used taps and dies and am familiar with what size drill to use for say an M6 and what size a piece of stock should be turned down to to take that die for example. Looking at those charts, I were trying to work out the formula/figure that you could apply to any size of diameter of stock but I don't think there is? This must be down to the differences in the depth of the teeth between the coarse and fine sizes I'm thinking?? I want to have a go at making a capsule with screw on lid, let's say you have a thick walled tube with a 17mm inner diameter for example, how deep would you cut a thread into that for a start? Then what would you turn the lid down to before you start to cut the thread onto that? I have to change/swap out cogs on my lathe for thread cutting, would I be right in saying that it wouldn't really matter which size I actually went for, 0.5 pitch for example because at the end of the day, only that lid and body of capsule needs to fit together.