Imperial threadcutting on a metric lathe.

[Lisa]

I was poking about in the workshop earlier, seeing as the weather's not too bad for a change, and was looking at some future threadcutting.

Naturally, things can't be too easy, so I'd need to cut a 26tpi thread on a metric lathe, which I'm not convinced is possible.

The leadscrew pitch is 1.5mm, which is about 16.9tpi unless I've messed up my maths, and I can get a 63 tooth gear to bring things somewhat closer to possible. However the best I've come up with is this:

40 20 20 63, which gives 26.61tpi, this seems wildly far out to my eyes. The maths:

TPI = L / ((A/B) * (C/D))

26.6175 = 16.9 / ((40/20) * (20/63))

I can always just use a die in the tailstock, but the job is a fairly simple one, and in brass, so seemed a good place to try out threadcutting for the first time on this machine.
Is that inaccuracy generally deemed acceptable? Or is there a better setup I'm not seeing? Or should I just stick to metric threads and use a die for this?

Had another look around while writing this, and managed to find a few more 'odd size' change gears available, and with a couple of them I was able to come up with:

21 20 35 57, which gives 26.21tpi. This is closer, and expensive from what I've seen so far to get all the odd sizes; but I guess it comes down to what error is acceptable, especially considering it'll have to go in a tapped hole.

Also, no idea if the above gear combinations will actually fit, still just playing with ratios.

[Cro]

Lisa,

I've never done screw cutting but as you are interested in the process rather than the final thread size why not cut a metric thread and use a metric tap for the mating part? Or even internal screw cutting???

Good luck!

Adam

[Lisa]

Hi Adam, I've already done the internal part, so can't really change it at this point. I did consider using a metric thread, but settled on the imperial thread as it's what the plan says, and I already had the tap and die; whereas I don't have an M13 anything, which is probably what would have been needed in metric.

Anyway, just toying about, will likely end up using a die.

[jon38r80]

I'm not sure what help it will be but there is an article on gear cutting dealing with your question on the Arceuro website here www.arceurotrade.co.uk/Catalogue/Machines-Accessories/Lathes/SIEG-C3-SC2-SC3-Mini-Lathes/C3-SC2-SC3-Mini-Lathe-Upgrades/Mini-Lathe-63-Tooth-Metal-Gearwheel that may help. The article is a PDF accessible from this page.

[Jim]

I'll check my lathe Lisa as it allows for both metric and TPI thread cutting. Not sure it will help though

Jim

[Roger]

I've done it the other way round ie Metric threads on an Imperial lathe using a pair of reduced pitch gears that I made. As long as you can introduce a 127 tooth gear you can do it either way. The problem is usually that a 127 tooth gear is necessarily large and difficult to fit into the available space.

The excellent book 'Gears and Gear cutting' by Ivan Law showed the way to do this by using a pair of gears with finer teeth so the diameter of the 127 tooth gear was a more manageable size. I added a pair of dowels to each gear so they could be connected on the same idler as the fine pitched gears. It's a bit of work, but gives complete flexibility so I found it worth the effort. I made the large gear from Aluminium since it wasn't going to be used enough to wear out and it was easier to make with the equipment I had.

[GWR 101]

Hi Lisa, as Roger points out a 127 tooth gear wheel is the usual solution (well used to be back in the late 50's) Another thing to bear in mind is that when cutting imperial pitch threads using a metric lead screw or visa versa it is advisable to keep the lead shaft and nut engaged until the thread is finished. This obviously needs the lathe to be able to be reversed and ideally a run out groove so that the machine can be stopped without the tool crashing into the work piece. I always found that successfully cutting a screw thread, particularly one that had some unusual element gave a sense of achievement (perhaps I should get out more often). Multi-start and unusual thread shapes in particular fall into this category, hope it works out well. Regards Paul

[Roger]

Feb 1, 2018 15:56:01 GMT GWR 101 said:

Hi Lisa, as Roger points out a 127 tooth gear wheel is the usual solution (well used to be back in the late 50's) Another thing to bear in mind is that when cutting imperial pitch threads using a metric lead screw or visa versa it is advisable to keep the lead shaft and nut engaged until the thread is finished. This obviously needs the lathe to be able to be reversed and ideally a run out groove so that the machine can be stopped without the tool crashing into the work piece. I always found that successfully cutting a screw thread, particularly one that had some unusual element gave a sense of achievement (perhaps I should get out more often). Multi-start and unusual thread shapes in particular fall into this category, hope it works out well. Regards Paul

Hi Paul,
The reason I mentioned the 127 gear was because not everyone is aware that you can use a smaller one with a finer pitch.

I never release the half nuts, regardless of the situation. A variable speed drive allows me to slow it to a near stop at the end, then wind the carriage in reverse quickly without releasing the nut. That way it's one method for all threads, at least, all the single start threads.
If I'm running a thread out at the end instead of into a groove, as it often necessary, I stop the lathe and pull the last bit over by hand to zero on the DRO and then wind out the cross slide, giving the chuck a twist at the same time to run it out from a consistent place. You can do something similar by marking the chuck position at the top.

[Lisa]

Thanks Roger, I'll have to look into that. It's interesting that 127 is a multiple of 25.4, which is of course 1" in millimetres; it makes sense that that would be the 'trick' to it.

[GWR 101]

Hi Lisa, that's what I was led to understand but that was a long time ago and I now get to play the Grey Hair card when in doubt. I was also led to believe that 63 being nearly half 127 was a good substitute and as Roger states only half the OD size for any given profile. We used to cheat to get a good metric thread on an Imperial lathe, after cutting the nearest pitch we could get we used to leave the thread a bit over size. Then we had a set of metric pitch hand chasers which we used to finish with, this gave us an almost perfect profile and superb finish. Obviously only works on relatively short thread length. Regards Paul

[uuu]

Can you fit 63 35 20 55, which gives 25.87, if you correct your 16.9 to 16.93333...?

Wilf

[Lisa]

Thanks Paul, 63 is readily available (listed as for "for cutting imperial threads on a metric lathe"), so I'll be adding one of those to my set. I might look into the possibilities of a 127 tooth gear though, probably with a finer pitch as Roger says; otherwise I think I'd have to cut a slot in the bench to use it.

With a 127 tooth gear I was able to come up with 60 40 55 127 which gives 26.06tpi; which is probably as close as one would get, I'd imagine.

Wilf, ta, if I can hunt down a 55 tooth gear then that'd be close enough to give it a go (can always bin the job if it's no good).

Sieg's manual suggests buying an imperial leadscrew, which I suppose would be the most accurate option, though not the simplest thing to swap out.

Also, I found this online calculator, which reduces the headache considerably: www.gizmology.net/changegears.htm (also lists the gear combinations needed for a given tpi with an imperial leadscrew on a similar lathe, just to spite me no doubt).

[GWR 101]

Hi Lisa, found this most interesting and far more pleasant for me sitting in a nice warm lounge rather than a cold workshop. Your final range 60 40 55 127, is slightly improved by using 55 30 45 127 and if my calculations are correct this gives a thread lead error of .001" per rev. Not sure what type of lathe you have but is it possible to use another set of gears in the chain making three sets which gives you more permutations. As you rightly say you need to weigh up the usefulness against the cost. As Metric is the way things are heading an Imperial lead screw would seem the least appropriate method. Good luck with which ever method you decide upon and thanks for the chance to get the brain in action over a few calculations in the warm. Regards Paul

[Lisa]

Just messing about with random numbers to amuse myself: 68 47 27 60 gives 26.008tpi. Getting such gears could be rather difficult though.

Actually, this place sells pretty much any number of teeth, could probably be made to work: www.mechanicdrive.com/buy-spur-gears/module-1.html

[chrisb]

My old imperial Boxford had a 100/127 compound gear for metric screw cutting

[Roger]

Feb 1, 2018 23:52:43 GMT Lisa said:

Just messing about with random numbers to amuse myself: 68 47 27 60 gives 26.008tpi. Getting such gears could be rather difficult though.

Actually, this place sells pretty much any number of teeth, could probably be made to work: www.mechanicdrive.com/buy-spur-gears/module-1.html

Hi Lisa,
Those gears are so cheap it barely covers the material cost. Maybe you could buy a pair that includes a 127 gear, I see they do make those. That would give you a universal solution.

[Lisa]

Feb 2, 2018 8:25:08 GMT Roger said:

Hi Lisa,
Those gears are so cheap it barely covers the material cost. Maybe you could buy a pair that includes a 127 gear, I see they do make those. That would give you a universal solution.

Looking at the site, it seems the base price may not include a bore/axle hole, just a toothed disk. I'll have to look into it, but even so, I'll probably grab a few from them.

[Roger]

Feb 2, 2018 10:27:51 GMT Lisa said:

Feb 2, 2018 8:25:08 GMT Roger said:

Hi Lisa,
Those gears are so cheap it barely covers the material cost. Maybe you could buy a pair that includes a 127 gear, I see they do make those. That would give you a universal solution.

Looking at the site, it seems the base price may not include a bore/axle hole, just a toothed disk. I'll have to look into it, but even so, I'll probably grab a few from them.

Fair enough, it's easy enough for you to add a boss or hole anyway. I'm amazed that they can do it for the price.

[andyhigham]

Standard gears are usually supplied with a pilot bore, normally 8mm. It is easy to clock the pilot and bore to the required size

[GWR 101]

Hi Lisa, as Roger states the prices certainly look very reasonable and 11.2 Euros for a 63 tooth certainly looks a good option. Also increments of 1 tooth up from 12 to 70 teeth would surely offer a very good chance of getting close to cutting a reasonable Imperial pitch. Obviously carriage charges may make a difference, am I correct that they are based in the Netherlands ?. This has prompted me to go to the workshop to look at which gear combinations I have for my metric lathe and see if I can perhaps justify buying a couple of useful sizes. Regards Paul