If you go for 1/2" acme then Rdg sell a tap for £12 , (whats a degree between friends )

What are you cutting with it? I used purly delrin for my mill and although it has no backlash whilst testing, under load cutting Ali it dose appear to move. I should have followed John S's advice and made them much longer so the load is spread out over more teeth

I'm just about to do an order to RDG so if I can find a use for 1/2" acme (need to check the lathe cross feed) I'll order one and make you some delrin nuts. (Think Ive got some black delrin left over)