Hello Gareth,

​Welcome to the Forum, I'm sure you will find all sorts of solutions here

​To answer this one, you can over drive the leadscrew in the ratio 4:3 to get a 6 tpi thread pitch. Go carefully as the normal recommendation is not to exceed the leadscrew pitch as that strains things, but with care you can get away with it

​The Mandrel gear should be 40 teeth, the leadscrew gear 30 teeth with suitable idlers to link them. Use the lowest back gear for powered cutting and take off only a few thou at a time per cut. Cutting fluid helps

Regards
Brian

Edited By Brian Wood on 13/07/2017 18:31:28

Hi Gareth, welcome,

Brian's expert advice is encapsulated in his recent book 'gearing of lathes for screwcutting'. I have ordered a copy since my screwcutting knowledge is minimal, and as an indirect thanks to the author for being a great source of info on this forum.

DaveD

It is often better with such high leads to put a handle on the leadscrew and use that to turn the whole job so that the gear train is still overall a reduction.

Although you said it was your first lathe is it your first experience of using one? You might want to start by making a mandrel handle anyway rather than using the motor and cutting a more regular thread like 18tpi. To start screwcutting on a difficult job might not be such a good idea.

Brian,

​The Dore Westbury column is a very coarse pitch indeed at 3/4 inches and by any standards should be regarded as a special case for hobby sized lathes.

​I have successfully cut a 5 tpi square thread into half shaft steel without great drama using a Myford lathe, but I did first rough out a standard Vee form thread to reduce the material quantity somewhat, taking care to be sure the shoulders of the vee remained inside the material that would be cut away with the square form tool. It helps a great deal if you have cut a decent run out groove at the end of the thread as well.

You do need a sharp tool, cutting fluid and light cuts for success

Regards Brian

When you are attempting to cut threads coarser than the leadscrew you are trying turn the leadscrew faster, ie gearing UP, this is where the straining begins.