These spindles has been specially made in order to be able to replicate the lathe spindle thread. The spindles them selfs are made with a double start thread. So the one above will cut a 3.5 tpi pitch but looks like a finer cutting thread! Because its a double start thread, each pitch is 1.75 which add up to 3.5! Thats how these work! 3/6 x 16 can be made with the standard 16tpi spindle!