Colchester Bantam metric scewcutting on imperial only machine

[Anonymous]

Posted by MICHAEL WILLIAMS on 25/07/2012 23:13:08:

Purely as a matter of interest if another plug in gear box or another leg on the tumbler implemented a Pi ratio then DP pitches could also be cut with simple ratio gear trains . See if anyone can work that one out.

Simple enough, use a ratio of 22/7, or as it is implemented on my lathe, 88/56, give or take a factor of 2. The ratio 88/56 equates to PI/2 within 0.04%. The ratio 355/113 is a better approximation to PI, but possibly a bit less practical in terms of gears.

Regards,

Andrew

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Andrew :

I was interested to see if anyone could explain why the fact of having a pi ratio available made cutting DP pitches easier . As you say taking the 22/7 ratio literally gives at least one workable solution for the actual gears . With four gears the ratio can be made as near as needs be exact .

General :

Most of these funny ratios should be realisable with epicyclic gear boxes .

In general – and even for the ordinary set ups of changewheels – spotting that there is a special factor needed like 1.27 or pi makes calculation of trains much easier .

Regards ,

Michael Williams .

 

Edited By MICHAEL WILLIAMS on 27/07/2012 09:24:21

[Anonymous]

Posted by MICHAEL WILLIAMS on 27/07/2012 08:52:10:

I was interested to see if anyone could explain why the fact of having a pi ratio available made cutting DP pitches easier.

So is that because you do not know, or are you merely teasing the rest of us to see if we know?

Suppose we want to machine a worm to mate with a given gear. Clearly the worm is a thread and so can be screw cut in the conventional way, if we knew what the 'thread' pitch was. Working in DP, the gear has N teeth on its pitch circle diameter (PCD), where N is a positive integer. If we 'unwrap' the pitch circle we get N teeth along a straight line, where the distance from one tooth to the next is the pitch. However, the length of the line is PI times the PCD, which will be an irrational number. Hence the pitch will also be an irrational number. Using a gear pair that removes the effect of PI (at least to a good approximation) simplifies the remainder of the gear train calculation by taking out the irrational factor.

Regards,

Andrew

On edit: Of course if both the worm and gear are under one's control you can cut the worm at a rational pitch and adjust the DP of the gear to suit.

Edited By Andrew Johnston on 27/07/2012 13:13:17

[VersabossParticipant @versaboss]

Posted by Andrew Johnston on 26/07/2012 13:39:55:

…….. I'd be interested in seeing the other pitches.

Regards,

Andrew

Andrew, I put a photo of the threading chart in my album(s). I think when enlarging it to the max. of what this site allows, it is quite legible

Greetings, Hansrudolf

[Anonymous]

Hansrudolf: Thanks for posting the chart. It seems odd that a Swiss lathe should have an imperial leadscrew. I would also surmise that somewhere in the drawing office some-one had a fetish for the number 5, given the sequence of change wheels. The set of imperial thread TPI seems fairly comprehensive, as does the metric, except for some of the metric pitches below 1mm. May be it was assumed that one wouldn't be screw cutting small diameter metric threads? How big is the lathe?

Regards,

Andrew

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Andrew :

So is that because you do not know, or are you merely teasing the rest of us to see if we know?

Sorry – I missed your little quip on first reading .

I knew the answer but asking this sort of question in other spheres has sometimes generated thought provoking responses which have proved quite useful .

Your explanation is of course correct .

Regards ,

Michael Williams .

[Sub MandrelParticipant @submandrel]

According to my ageing Machinery's Handbook: The external (metric) threads used for screws conform to Tolerance Class 6g limits (medium fit) as given in BS3643, "ISO Metric Screw Threads". The internal threads for nuts conform to tolerance class 6H limits (medium fit) as gi8ven in BS 3643: Part 2.

As any discussion of 0.2% vs. 0.02% is essentially meaning less unless we know what sort of tolerance we need to work to, does anyone have access to these standards and can they elucidate us as to how accurate we need to be to make threads to them?

Neil

[VersabossParticipant @versaboss]

Posted by Andrew Johnston on 29/07/2012 11:46:01:

May be it was assumed that one wouldn't be screw cutting small diameter metric threads? How big is the lathe?

Big danger of going off-topic here

However, about the small threads you are possibly right. I can imagine that other (maybe even smaller) threads couldn be done with combinations not on the table.

This lathe is very small, comparable maybe to the Myford ML10. Up to now it has not received the honour of being shown on T. Griffith's pages (my fault, really, as I wanted to do this since years), I put a picture of the headstock in my album. The lathe has the quitr unique feature of a movable2-part bed. Normally, the centre height is 100 mm. By pulling the upper part out, you can obtain a large gap with centre height 200 mm (or a much longer bed length). It has also a very clever backgear, giving a very large speed range.

It is nor very rare in Switzerlans; I know of 2 others, one of them not far from my place.

Greetings, Hansrudolf

[Anonymous]

Hansrudolf: That sounds a very interesting lathe; any chance of posting a picture of the whole machine?

Neil: I assume you mean meaningless, rather than just meaning less? Possibly a more interesting question is what accuracy could be achieved if we assume the gear train is exact. In other words how good is the leadscrew? From a chart I have for Harrison lathes (kindly supplied by KWIL) for between centres of less than 2 metres it quotes less than 0.015mm over any 50mm, ie, 0.03%, and less than 0.04mm over any 300mm, ie, 0.013%. I've also found the following **LINK** on the accuracy of ground taps. For instance the pitch error for a 1mm pitch thread should be less than ±8µm measured over 9 pitches, ie, 9 mm. I make that 0.089%. Most of the thread tolerance stuff in my Machinery's Handbooks concentrates on diametric errors, not specifically pitch errors. May be that is because a pitch error translates to a diametric error?

Now we have some numbers let battle commence!

Regards,

Andrew

Edited By Andrew Johnston on 29/07/2012 22:05:45

[Sub MandrelParticipant @submandrel]

Hi Andrew,

Thanks for that. The 0.0125% error of the 63-tooth wheel is smaller than the allowable error in the leadscrew. tempting to think the '300mm' allowable error was chosen because they made their metric leadscrews on an inperial machine using a 63-tooth wheel

Over a 10 mm length of 1mm pitch thread, that's an error of about three wavelengths of red light.

Maybe too much for Cherry Hill, but good enough for mortals like me.

Nuff said?

Neil

P.S. don't forget UK miilimetres were a different size to US ones once, until we standardised on 1"=25.4000r mm as well.

Neil

[VersabossParticipant @versaboss]

Posted by Andrew Johnston on 29/07/2012 22:05:09:

Hansrudolf: That sounds a very interesting lathe; any chance of posting a picture of the whole machine?

Some photos of the Gral lathe in different configurations are now in my album.

Greetings, Hansrudolf

[Anonymous]

Hi Hansrudolf,

Thanks for posting the pictures. It looks like a lovely lathe; in pretty good condition too. Does it have a removable gap piece? The gap seems quite wide. If there isn't a gap piece I wonder how you get on turning near the chuck; it looks like the saddle will come off the bed ways?

Best Regards,

Andrew

[smithadonParticipant @smithadon]

Hi all,

In case anyone lands here (as I did) looking for the opposite – cutting imperial threads on a metric Bantam, I’ve made a wee calculator for my Bantam 600.

It gets very close to all desired pitches or TPI and gives you the gear lever positions as well as the driver, driven and idler gear combinations.  It assumes you have the gear set that came with the lathe.  The worst error is 0.36% most are under 0.2%

I’ve added it here as it could easily be adapted to do the opposite (metric from imperial) for the OP.  The formulae can be stolen, you’d just need to change the gears you have available and incorporate your formula from the imperial manual rather than the one I used.

https://www.dropbox.com/scl/fi/75s8ggy17xjxell9u3mvf/Colchester-Bantam-600-Thread-Cutting-Gear-Calculator-v.1.7.xlsx?rlkey=4coyzzl7yjczldpdeqgpju0zn&dl=1

Share at will.

 

Joe

[Author]

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