Bit of a conundrum with screwcutting on a Chester Lathe

Well, as you all know, took delivery of my nice shiny Chester DB7 VS lathe the weekend. I am, for the money, really pleased with it.

So I have been playing with the settings and seeing what it can (and can't) do. The great thing about this machine is that I have a full set of Change Wheels and nothing has been bent, broken or cut out (yet!).

Thing is, I decided to investigate screwcutting and of course there is a lovely schematic bolted to the casing, you know the kind of thing - Wheels A-B-C-D use "whatever tooth wheel in the chart".

So I look along and find .7 and right after that .8 pitch - no .75 - which is a metric pitch I would want to cut. i am either misunderstanding the chart completely or this machine will not cut a .75 pitch screw in the Metric configuration.

The machine is the Imperial version just for clarification. I was wondering if there is a workaround?

graham.

If the driving gear for 0.7 pitch is 20 and the gear for 0.8 is 21 then you need a 20 1/2 gear to get 0.75.

Simples.

Thank you John, I can understand the logic in that but of course I just don't have such a thing. Under the circumstances I am going to call Chester in the morning. I really don't need to be learning gear cutting, yet!

graham.

(ps, sorry about the double post)

Graham, can you post a picture of the screwcutting chart please ?

It's probably something simple, can it do 1.5 ? if so then 0.75 is just the same ratio with an extra 2:1 reduction somewhere.

There's an example of 0.75mm pitch shown on page 18 of the manual.

Ah, snag is I do not have a manual. I will call Paul at Chester and enquire.

Meanwhile here are the pictures:

This second chart gives the wheel combinations for the common threads. Naturally I use 6-7-and 8mm Metric Fine, which are.75 pitch. of course I would....

I think this would work.

A B 80 60
C D 40 33
E F H 70

PS Is the leadscrew 12 TPI ?

Les.

Edited By Les Jones 1 on 26/11/2013 12:50:12

You can also go

60-H

80-51

H-72

Subject to the length of the banjo and it being 12tpi

Just change A from 60 to 30