Cutting Metric thread on an Imperial lathe

[AjohnwParticipant @ajohnw51620]

This is the one that I use more often. All but one of the gears come with the lathe. They use one as a spacer.

However people on the yahoo boxford group come along lusting after the 100/127 gear on the basis that it's more accurate even though lathe screw cutting is not that accurate really anyway.

John

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[AjohnwParticipant @ajohnw51620]

I do have the 127 – 100 so if I need to set a compound train might as well use it. This works out as follows but there will be other gear pairs that will do the same thing.

Newer machines seem to have switched to a 120 – 127 gear. I assume that's to get round the mesh problems.

John

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[egaParticipant @ega]

Roderick Jenkins:

Many thanks for your excellent table and do please persevere in making it available in accessible form. I have a collection of copy letters to ME and MEW about the use of alternative stud gears to obtain metric pitches on the S7B but have never got round to collating them.

This idea goes back at least as far as MEW no 42 and is I suspect much earlier. Like you, I have not used my (shop-made) metric quadrant for some years – the last occasion may have been turning one of GHT's worms. The practice is so useful that I have wondered how one might "automate" the setup with pre-set stops so that meshing the new stud gear is effected tool-free and without resort to cigarette paper or workshop Esperanto.

[Roderick JenkinsParticipant @roderickjenkins93242]

Posted by ega on 11/10/2015 12:09:52:

Roderick Jenkins:

This idea goes back at least as far as MEW no 42 and is I suspect much earlier.

The earliest reference to the method I have found is a letter to ME Vol 171 iss. 3955, 1993 by the Rev. David Hoskin. There is no great magic in the method (apart from the original, brilliant concept). The new pitch is just the ratio of the replacement gear to the original 24 teeth gear multiplied by the selected pitch on the gearbox. Thus 24/34=0.706. Multiply by 12=8.471 tpi. Divide this into 25.4 gives a metric pitch of 25.4/8.471=2.99mm. Since 34 or, perhaps, 35 is the largest gear that will fit it was not much effort to calculate (with the aid of a spreadsheet) the various combination for gears between 20 and 35 and find the best fit for each metric pitch.

I must point out that there is a typo in the table above. In the Metric Fine for 12mm the box setting should be 28 not 20 – like all the other 1.25mm pitch threads. Corrected version here:

Cheers,

Rod

[egaParticipant @ega]

Roderick Jenkins:

Thanks for the helpful explanation and corrected table.

I have the Hoskin letter and notice that you have derived a better suggestion for 2 BA pitch than his 34T/44TPI; I think the 20T is the smallest standard gear which will fit. At the other end of the scale, I have squeezed a 38T in by extending the slot in the quadrant but I don't remember what pitch this was for.

As you say, the original idea whoever had it was simply brilliant and you have to wonder what the thinking was at Myfords when they came up with the expensive metric conversion set which I seem to remember does not quite provide an exact conversion to metric.

[AjohnwParticipant @ajohnw51620]

Posted by Bazyle on 10/10/2015 11:23:17:

On the CVA I believe it is arranged such that the gear train, including 127 has a 50 and gives 1.00 mm. Then the 50 is changed to give the metric intermediates eg 60 for 1.2mm, 70 for 1.4 etc which is fairly straightforward and uses standard gear numbers, and of course the gearbox can halve or double anything easily.

Edited By Bazyle on 10/10/2015 11:54:04

CVA also did gearing for mod and dp but i don't have any details of how they did it. And a fancy screw cutting indicator for metric threads.

John

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[BazyleParticipant @bazyle]

Useful info there from John for Boxford about the 44 option. It can also get you 0.7 (M4) using 80 on the gearbox (0.6985mm)

The challenge is to get the rest of the common threads by only swapping the mandrel gear as on the Myford table. Best I've managed is 3 more gears, 45,24,26 but mostly people may not need the .75mm (18DP if anyone is thinking of making them).

gear, gearbox, thread(mm)
45, 46, 1.242
45, 38, 1.504
24, 38, 0.802
26, 44, 0.7504
26, 22, 1.501

[AjohnwParticipant @ajohnw51620]

I was looking for sub micron accuracy Bazyle. Mad I know but fitting 2 gears isn't that bad. Part of the reason was to shut up the must have the 100-127 compound as it's more accurate when approximations crop up.

The gears I have which I assume are all standard are

18	19	20	24	32	36	40	44
46	48	54	56	60	80	100
					18/72	100/127

 

I will have another go at getting more relaxed tolerance ones at some point using standard gears but currently mustn't get even more side tracked from tidying up. I'm finding it hard to stick at it. Nearly 30ft of extra shelving and I am still having problems.

John

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Edited By John W1 on 13/10/2015 10:58:23

[Roderick JenkinsParticipant @roderickjenkins93242]

Posted by ega on 11/10/2015 15:04:44:

As you say, the original idea whoever had it was simply brilliant and you have to wonder what the thinking was at Myfords when they came up with the expensive metric conversion set which I seem to remember does not quite provide an exact conversion to metric.

It depends how "exact" you want to be. These are the Myford metric threads as cut by their metric conversion set:

2 gears are changed, there are 60/63 and 50/45 pairs in between. The penultimate column is the gearbox selector in TPI. There are, of course many more threads here than just those required to make metric coarse and fine fastenings. For most threads you only have to change the tumbler stud gear. Quite convenient but you have lost the ease of changing from thread cutting to fine feed that is there with the imperial set up.

It is, perhaps, instructive to compare the Myford solution with what is considered adequate on a mini lathe with an imperial leadscrew:

Rod

[AjohnwParticipant @ajohnw51620]

A rather large range of pitches have been used on metric thread Rod. If I remember correctly a site by MarylandMetrics or something like that list lots of them.

I suspect a lot of knowledge has been lost concerning lathe gearboxes and conversion gears. I started on the basis that the person who originally bought the lathe along with other bits and pieces may well have been aware of what to buy for what he might need as he may well have been around long enough. I don't think there is a better arrangement than the one I posted using the gears he bought. Haven't tried DP or MOD yet. Another example is just why boxford chose to use on specific value of gear for packing which they appear to have done – it allows some moderately accurate metric threads to be cut.

It's pretty natural that a USA designed gearbox and the gear range that was provided with it will be capable of cutting a number of things that aren't "USA" and companies seem to favour very close approximations rather than exact conversion gears. Even Colchester for some time. CVA did offer a 127 based conversion gear but as far as I know in that period few did.

My wabeco is metric. I decided to go for a true metric 2nd lathe for fun. Unfortunately due to a "continental oddity" I will have to make a screw cutting indicator. No thought needed at all if it had a 3mm lead screw. They have used 5mm – I think from a quick measurement when I bought it. Maybe they don't over there as it gets rather cumbersome to cut some this way as the indicator takes a long time to come round. The boxford one will cut a fair number of the usual ones.

The Pultra is an indulgence. I've wanted one since I finished my apprenticeship.

John

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[egaParticipant @ega]

Roderick Jenkins:

Thank you for "doing the math" and for the instructive comparison with the modern mini lathe – no need for four places of decimals there! Myfords clearly came up with a very effective solution to the problem posed by the impracticality of using a 127T gear and I feel it would have paid them to include something like your table in the QC gearbox manual.

[IanTParticipant @iant]

I don't cut too many threads on the lathe (mostly I just use screw-cutting tackle (taps and dies) for my size of work, so I'm no expert but….

I did cut a metric thread on something a little while back and I did it with a 127t gear (acquired from RDG some years ago) and the other Myford change gears. I do seem to recall initially thinking that I'd also need a 100t gear but managed in the end with an existing 50t gear somewhere in the train I think.

Anyway, I don't remember that it was that impractical to use a 127t gear apart from not being able to close the cover on the S7 but since I don't do it that often and I tend to stand to the right of the saddle – it wasn't really a great problem in practice.

Regards,

IanT

Just checked RDG – the 127t is now £26.50 – whereas mine was only about £12-£13 or so I feel. Maybe it was quite a while ago or maybe 'tool inflation' is a bit higher than I thought?

[egaParticipant @ega]

IanT:

I was interested to see that you were able to set up a gear train including a 127T gear. These were listed by Myford as late as 2002 at £37.75. My comment was based not so much on the high price of the large gear as on the same point that you make about the gear change cover (and specifically in relation to the S7 with QC gearbox as I have no personal experience of the other models). However, Martin Cleeve (WPS3 p43) seems to confirm that the same problem would apply to the ML7 and he used a physically smaller 127T gear of 30DP.

I have to say that none of the descriptions or photos of his machines that I have seen included a change gear cover and, like you, I would be perfectly happy to cut a thread with the cover open if I had to. On many machines, of course, this would be prevented by an electrical safety switch.

[AjohnwParticipant @ajohnw51620]

Posted by Bazyle on 12/10/2015 21:08:07:

Useful info there from John for Boxford about the 44 option. It can also get you 0.7 (M4) using 80 on the gearbox (0.6985mm)

The challenge is to get the rest of the common threads by only swapping the mandrel gear as on the Myford table. Best I've managed is 3 more gears, 45,24,26 but mostly people may not need the .75mm (18DP if anyone is thinking of making them).

gear, gearbox, thread(mm)
45, 46, 1.242
45, 38, 1.504
24, 38, 0.802
26, 44, 0.7504
26, 22, 1.501

I did get side tracked. These are for module and dp screw cutting. I suspect the 2 prime gears are essential. Some mod pitches match imperial threads to couple of thou which would be an easier option than trying to work out simple trains..

Module

And DP

I started on this some time ago so spent an hour finishing it. Haven't cross checked yet but should be ok.

If any one starts cutting the primes I demand a pair of free b's

John

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Edited By John W1 on 18/10/2015 17:55:51

[Kris BParticipant @krisb]

Does anyone know where you can get hold of the 33 & 34 tooth input gears for cutting metric threads on an imperial Myford ?

[John Stevenson 1Participant @johnstevenson1]

If Rolls Royce don't manage to break anything I plan on cutting some tomorrow and Thursday

[peak4Participant @peak4]

How much do they work out at John.

I don't have a gearbox at the moment, but I'm likely to end up with one after I've found a new house later on in the year.

Thanks

[AjohnwParticipant @ajohnw51620]

I think you need to be more specific peak4. Perhaps it's aimed at some other John.

Buying Myford gears – ebay or myford themselves.

John

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[Michael GilliganParticipant @michaelgilligan61133]

Posted by Ajohnw on 06/01/2016 10:25:04:

I think you need to be more specific peak4. Perhaps it's aimed at some other John.

.

Hint : Have a look at the last two posts on page 2

[Kris BParticipant @krisb]

Hi John,

Did you make any progress with cutting those gears ?

[John Stevenson 1Participant @johnstevenson1]

Got all the blanks cut and the damn hobber broke down.

Can you get away with about 15 degree helicals I say about as I reckon if goes from 10 degree to 20 degrees in a sweeping curve

Something has gone for a wander on the drive train

[Kris BParticipant @krisb]

Sorry to hear about your mechanical gremlins.

I am in no hurry – when you have everything sorted out, please let me know .

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