Bit of a conundrum with screwcutting on a Chester Lathe

This topic has 9 replies, 5 voices, and was last updated 26 November 2013 at 14:34 by John Stevenson 1.

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25 November 2013 at 21:06 #12221
Oompa Lumpa
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@oompalumpa34302
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25 November 2013 at 21:06 #136401
Oompa Lumpa
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@oompalumpa34302
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.
25 November 2013 at 21:10 #136403
John Stevenson 1
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@johnstevenson1
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.
25 November 2013 at 21:15 #136404
Oompa Lumpa
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@oompalumpa34302
Posted by John Stevenson on 25/11/2013 21:10:55:

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)
25 November 2013 at 21:39 #136407
John Stevenson 1
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@johnstevenson1
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.
25 November 2013 at 22:16 #136412
Hairy Pete
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@hairypete39644
There's an example of 0.75mm pitch shown on page 18 of the manual.
26 November 2013 at 11:48 #136461
Oompa Lumpa
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@oompalumpa34302
Posted by Hairy Pete on 25/11/2013 22:16:01:

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….
26 November 2013 at 12:33 #136468
Les Jones 1
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@lesjones1
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
26 November 2013 at 13:19 #136471
JasonB
Moderator
@jasonb
You can also go

60-H

80-51

H-72

Subject to the length of the banjo and it being 12tpi
26 November 2013 at 14:34 #136477
John Stevenson 1
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@johnstevenson1
Just change A from 60 to 30
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