A 0.75mm pitch thread is half that of the Leadscrew, so the Saddle needs to travel half the pitch of the Leadscrew, per revolution of the Spindle.

That suggests a reduction gear ratio of two to one. Brian Wood, in his book, "Gearing of Lathes for Screwcutting", suggests 30 – Idler – 60, but any 2:1 ratio should do equally well, (20 – Idler – 40, 25 – Idler – 50, 35 – Idler – 70, or 40 – Idler – 80) as long as the gears mesh correctly. (Set the backlash, by running a piece of paper through each mesh)

The Idler is merely there to ensure that the drive passes from the Spindle to the Leadscrew, so that both rotate in the same direction, so no need to have two the same on the same shaft, unless it is impossible to mesh all three in the same plane.

Beware of using two idlers, unless you want to produce a Left Hand thread, because the Leadscrew then rotates in the opposite direction to the Spindle!

The calculation for any pitch, relative to the Leadscrew, is: Drivers divided by Driven.

(In this case 30/60 = 0.5 x 1.5mm = 0.75mm)

Several other books will give similar advice, (Martin Cleeve's "Screwcutting in the Lathe" No 3 in the Workshop Practice Series, covers the same ground, but in a more theoretical way.

Sparey's "The Amateur's Lathe" gives similar information. The gear set up for 16 tpi with a 8tpi Leadscrew would produce a 0.75mm pitch thread with a 1.5mm Leadscrew. It is the ratio between Spindle and Leadscrew that counts!

It may be worth buying a copy of Brian's book for future reference?

NO he is not paying me commission.

Howard

Edited By Howard Lewis on 25/08/2018 18:58:50

Edited By Howard Lewis on 25/08/2018 19:05:48

You can download my change wheel tables for mini lathes here:

http://www.stubmandrel.co.uk/change-wheel-tables-for-mini-lathes

Neil

You have the maths right. But if you try that, I doubt you will able to get C and D to mesh. The shaft holding D will collide with B.

IF your lathe is like mine, you can remove the B/C key and reduce the system to three gears, all in the same plane. Then it is as others have described – A needs to have twice the number of teeth as D. The simplest solution would be that with a single (any that fits) gear between

Pitch tables cover the most common/popular options. 0.75, while easy to achive, is less common (M4.5 or fine).

Other more complicated solutions would work.

Brian 's book is a real mine of information and it looks good too. I leave it out on the coffee table to confuse visitors!

Seriously it is money well worth spending, get a copy, you wont regret it.

Andrew.