Thread cutting on a lathe

Hi

I am making a lathe backstop and on the drawing it specifies a 24x1.5mm screw thread on the outside of one component and inside a nut with no other info.

I have done a few external threads on my lathe and they have worked out ok but I have never done an internal thread. I think I am ok with the set up but what I am struggling with is how to work out the size of the bore I should make for the nut so it all fits together well

Does anyone know how to calculate this and if so can you let me know how it's done? Also should I make the nut first then thread the bar to fit or the other way round?

Thanks

Cutting Metric screwcutting or tapping sizes is easy.

The Tapping size is (Nominal size - pitch ) so your 24 x 1.5 requires the bore to be 22.5 mm

You can make the parts in which ever order you like. having produced one thread, the component can then be used as gauge with which to assess the other part..

Possibly, in this case it might be easier to turn the material to 24 mm diameter and then to cut the external thread by applying a gradually decreasing cut until full thread depth is reached.

Zeus charts state that the depth for 1.5 mm pitch is 0.9202 mm with a flat of 0.1875 mm. This would give a root of 22.5346 mm. So 22.5 mm should suffice for the bore before screwcutting.

0.0346 mm difference is only 0.0014 inch in old money, so within a fairly tight tolerance.

When you choose to cut the external. thread, you may wish to turn the diameter slightly undersize, to produce a flat on the thread, but do not forget to make allowance for that when setting the final depth of cut, so that you not plunge in too deeply. Again, work on the basis of the root diameter being 22.5 mm. So the depth of cut, once the tool touches the work, will be less than 0.920 mm

The flats on internal and external threads will prevent any interference.

If you do not have a set of Zeus Charts, buy one. You will find the booklet to be a mine of information on threads, metric / Imperial equivalents, bending allowances for sheet meta (and sizes ) as well as details of Morse tapers, and trigonometric and log tables. My set bought in 1958 are grubby but regularly used!

One the various books on Model Engineering that you will find to be an invaluable reference for guidance and information when you need it. (I have quite a library, not including, by any means, all the titles available )

Normally with plain diameters it is easier to produce the bore first of all, and measure it, because it will be easier to turn the external diameter more easily.

Boring tools tend to spring, so just below the required bore size, you can take a number of spring cuts, without altering anything, and the tool will still be cutting; but less with each spring cut

The same will probably be true for cutting the internal thread..

HTH

Howard

Minor diameter of the thread = tap drill size for the thread is where I would start

Howard

Thanks for your reply that's all very helpful, I will have a go today.

Peter

Thanks also, that seems obvious now you have said it but for some reason that never occurred to me!

As an alternative to Zeus you could download Presto Counsellor

https://www.presto-tools.co.uk/Downloads

Howard, surely the outside diameter is with the flat taken into account. You should not be making the part undersize to start with.

Martin C

For clarity … Here is the diagram from the bottom of the Zeus tabulation

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MichaelG.

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Edit: __ Dontcha just love all those fractions on a ‘Metric’ standard 

Edited By Michael Gilligan on 24/11/2021 08:52:17

The bar is always bigger than the nut hole before you start, about 1.5mm would be fine

I do the bar first because its easier and can be used to test the nut, internal ones take longer on a hobby unit because of boring bar flex

A DRO on the lathe makes the job miles easier

You can buy 24mm on various sites if you cant be bothered

Quick question about Zeus

Does a modern version (metric revision) cover all the older information too, or is it worth getting an older, say 1965 edition as well?

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Sorry, Ian … I’ve never compared the various editions

The image in my post is from the 1976 ‘Metric Revision’ priced at 65p

… which does contain a lot of Imperial data, but perhaps not all of it.

Does anyone here have a very early edition ?

MichaelG.

I gave away my 1974 edition but based on that and other editions offer this (corrections welcomed):

Edited By ega on 24/11/2021 10:38:22

There are, of course, alternatives to Zeus including the Babani Press Engineers and Machinists Reference Tables. The latter is probably out of print and is best used by those with keen eyesight (BP also do a series of computer guides for the "Older Generation" ).

Edited By ega on 24/11/2021 10:56:03

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That’s very helpful … Thanks, ega

MichaelG.

Agreed

so based on that seeing as I use a lot of BA threads (old Triang/Hornby etc) I night just invest in an older version too. I know I could get the info from elsewhere

Now you have the specs for an internal thread, you could look up the external specs and cut yourself a test gauge before doing the real job.

Hi, for what it's worth, I have three editions of the ZUES charts, one dated 1972; copyright 1970, one from 1978; copyright 1974 and one that just has copyright 1980, which are just the same as ega has said in his 1974 and 1980 columns, but with exception of my 1972 edition having the primary and secondary selection of fits (British Standard data sheet 1 & 2 B.S. 1916C August 1954) instead of the ISO ones and there are no conversion factors and my 1978 edition has an error in the data known column of the solution of the right angled triangle. ZUES 1978 error also two in the middle formulae where Cos was printed instead of Cot.

Regards Nick.

Edited By Nicholas Farr on 24/11/2021 15:24:41

Does that make it extremely valuable?

Hi ega, no idea if it makes it extremely valuable, as it's a bit grubby and tatty round the edges where I used it in my day jobs and that error caused me a lot of confusion once.

Regards Nick.

As luck would have it there is an Agricultural Engineers just across the road from me and and they have lent me an M22x1.5mm nut which is a John Deere Combine wheel nut apparently. So anyway with that as a gauge I have modified the part and made a nice thread on my bar. The depth of 0.92 that Howard mentioned worked a treat (after I doubled it for my cross slide) with a nice shake free thread. So now I have a good external and internal thread so making the nut to fit should be a doddle.

By the way my Zeus (95) has very limited metric fine data and doesn't have a 24 x 1.5mm in it even though it is a metric edition.

Now you'll set me off comparing my 1grubby 958 edition to your scan!

Howard