working out screwcutting gears.

[Roderick JenkinsParticipant @roderickjenkins93242]

Thats better! With a 24/50 pair (threw me for bit there John) and a 1.4mm pitch leadscrew we get:

The imperials are exact and the metrics improved. Interesting way of doing it!

Rod

[Michael GilliganParticipant @michaelgilligan61133]

Excellent work, Rod

Just one awkward question: How do you reconcile 1.4mm pitch with the 'measured' 13tpi

… Is it the effect of that two speed-gearbox ?

MichaelG.

[Roderick JenkinsParticipant @roderickjenkins93242]

Posted by Michael Gilligan on 18/10/2015 21:49:15:

Just one awkward question: How do you reconcile 1.4mm pitch with the 'measured' 13tpi

I can't make it work with a 2mm leadscrew

[AjohnwParticipant @ajohnw51620]

There should be a graduated handle on the end of the lead screw. The numbers each side of the zero might offer a clue.

Trouble is that it could be anything with an unusual drive like that even 13 tpi but I'd guess it's metric.

One thing for sure 8.5 tpi is too far out anyway.

John

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[BazyleParticipant @bazyle]

It is still possible that there are two versions of the lathe as the OP has said the table on the lathe does not agree with the manual. 1.4 pitch is weird. Perhaps the op could run 'screwcutting' with a felt tip tool on a long work piece with a specific set of gears to confirm Rod's calculations.

[AjohnwParticipant @ajohnw51620]

The best option is probably a photo of the dial on the handle on the end of the main lead screw showing the zero in the middle and the numbers each side of it. Extreme but I found that was the only way of finding out if some one was selling a metric or imperial lathe. Naughty might be able to tell or maybe not. There are distinct differences. On the other hand the markings might be approx – not exactly unknown but in the past it's been 2.5mm = 0.100 which it doesn't.

Or maybe Clark will reply to the email he sent.

John

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[mick70Participant @mick70]

the D gear on 40/20 tpi should be 42 not 27.

when set to 1.5 the 8tpi is correct.

will slowly work my way through the others over next couple of weeks and keep updating thread as when i check them out.

am very grateful for help so far.

[Roderick JenkinsParticipant @roderickjenkins93242]

I think MichaelG is correct, there is further translation gearing associated with the 2 speed lever. A 2mm pitch leadscrew seems to fit with Naughtyboy's measurement. This, together with introduction of 20:14 (or similar ratio) gears gives a very good result:

I've added the third place of decimals so we can see that the metric gears are now exact and the imperials are very good approximations. If this is how the gearing is arranged then hats off (and apologies) to the designers!

It really needs a 30 tooth gear in the train to give a reasonable 8tpi.

What a fun puzzle

Rod

Edited By Roderick Jenkins on 19/10/2015 10:33:51

[AjohnwParticipant @ajohnw51620]

The parts diagrams only show a dog clutch – no high low feed lever. It does mention that there is a 50T gear on the spindle but zero else.

There are lots of variations on this one about but no luck finding another make's manual. I found one that did 4 – 120 tpi, others 8 to ??. Some have gearboxes, some have the milling head with and without a separate motor. Some even have power cross feed.

I think I saw one that quoted an 0.001" over all accuracy but they might just mean the dials.

Actually I had an early one – put me off Chinese lathes for ever but maybe they are better now. They had just updated it by chopping off the tailstock nose to gain an extra 2" between centres. Fine but it needed a morse extension socket to work up to a centre which reduced the centre distance to less than it was before. Head stock out of alignment but could have been shimmed – I think. Tail stock the killer. Hand scraped, it had been but probably 1/2 mm to go and not anything like aligned anyway.

John

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[Paul Relf-DaviesParticipant @paulrelf-davies37806]

Hi all,

I've been working on a simple (web-based) program to calculate change gears for my own use…it is still a work in progress, but using it, with the OP's gear, it has come up with the following combinations:

It should be noted that my program is designed to expect an imperial lead screw pitch, but should handle the 'imperialification' of a metric pitch, so these figures used a pitch of 7.69TPI, as opposed to 3mm.

I make no guaranteed that these combination will actually fit together…I'm still working on that feature!

Simple:

A : D -> Result (Error)
60 : 63 -> 8.0745 TPI (0.9313% )

Compound:

A : B / C : D -> Result (Error)
50 : 48 / 36 : 39 -> 7.9976 TPI (0.0300%)
50 : 56 / 42 : 39 -> 7.9976 TPI (0.0300%)
50 : 39 / 36 : 48 -> 7.9976 TPI (0.0300%)
50 : 39 / 42 : 56 -> 7.9976 TPI (0.0300%)
56 : 63 / 40 : 37 -> 8.0024 TPI (0.0301%)
56 : 37 / 40 : 63 -> 8.0024 TPI (0.0301%)
60 : 54 / 32 : 37 -> 8.0024 TPI (0.0301%)
60 : 37 / 32 : 54 -> 8.0024 TPI (0.0301%)

(under the covers, the program is just working out the resulting TPI for a given lead screw and set of gears for all possible combinations, and then the result with the specified target (in this case 8TPI) and filtering out those where the error is too great.

The calculation used is as follows (using the 1st compound set as an example)

(Driven / Spindle) x (Leadscrew gear / Driver) x Leadscrew TPI

(B/A) x (D/C) x Leadscrew TPI

(48/50) x (39/36) x Leadscrew TPI

0.96 x 1.083 x 7.69

1.04 x 7.69

7.9976, which is 0.03% under the target of 8TPI

I hope this is of use….(and I'd be very grateful if anyone could point out any errors…I'm still 'enjoying' the gear-combination calculation learning curve!)

cheers

Paul

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