Cutting imperial and metric threads

There’s plenty of detailed information online about the way to cut threads on a lathe, which I’ll not repeat here – a quick search will get you any number of guides and multiple youtube videos of the process in action. There’s a basic thread cutting guide in the Clarke CL300M user manual. The essential principle is that the leadscrew is geared to the main drive spindle at a much higher ratio than is ordinarily used for power feed, and the ratio is chosen so that the cutting tool moves the right distance to cut a single turn of thread for each turn of the main spindle.

Many versions of the mini-lathe come as standard with a 16tpi imperial leadscrew,  including the one I have. This means that for every 16 turns of the leadscrew, the saddle advances an inch. If the leadscrew is geared to run at the same speed as the spindle, then you get a 16tpi thread. If it’s geared to run at half the speed, then you get a 32tpi thread, and if it’s geared to run at twice the speed, you’d get an 8tpi thread. (For power feed, mine is geared to run at 1/16 of spindle speed, equivalent to 256tpi, just by the way.)

Obviously, trying to cut threads with a very coarse pitch on a small lathe is going to be problematic, as there comes a point when the main cutting effort is being done not via the drive spindle, but by the leadscrew. Trying to cut threads with a pitch much coarser than the leadscrew’s native 16tpi on these lathes is going to place great strain on the (plastic) gears which drive the leadscrew. I personally wouldn’t attempt anything coarser than about 8tpi on this machine without giving some serious thought to minimising strain, and I have a feeling that 8tpi may well be pushing it. People have speculated elsewhere about driving the lathe via the leadscrew instead of the spindle, when trying to cut very coarse threads, and using some kind of toolpost grinder rather than a regular single point tool, so the lathe isn’t exerting the actual cutting effort.

“Officially,” if you have an imperial leadscrew, then you can cut only imperial threads, though everyone knows it ain’t necessarily so. There’s a metric leadscrew conversion kit, and some of these mini-lathes ship as standard with a metric leadscrew. Whichever you have, to cut the “wrong” sort of threads on these lathes involves a couple of potential problems:

  1. You have to somehow come up with a gear ratio which gets the thread pitch you want.
  2. Cutting pitches inappropriate for the leadscrew and dial indicator means you can’t disengage and re-engage the halfnuts during threading.

The second is more or less a non-problem on the mini-lathe, because it runs just as happily backwards as forwards, and the chuck is bolted to a flange (so it’s not going to unscrew and fall off when backing up). (It is sort-of possible to use the indicator, but with so many caveats that it’s not worth the hassle, frankly.)

As regards getting an appropriate gear ratio, my lathe at least came with  a fairly wide selection of change gears – ten of them – and if you throw in the two 20-tooth and two 80-tooth gears normally used for power feed, the number of achievable ratios is staggering. It’s usually just a question of finding a combination which works.

With the imperial leadscrew, then, one can find gearing which will give a result near enough to various metric threads (easily within practical tolerances). Indeed, there are some imperial threads, outside of the table printed on the change gear cover, which are acheivable only very approximately, and some which can’t be done without extra gears. There are also some perfectly achievable imperial threads which are not included in the standard table, for some reason, most notably the 27tpi used on many microphone stands.

Here, then, is my own gear table for the 16tpi imperial leadscrew, arrived at after writing a little computer program to go through every possible combination of the gears I have available, and winnowing the output down to pitches I reckon might be useful. The table also includes the percentage error as well as the “amount out per inch” and “amount out per centimetre” (in theory – obviously you can expect some unavoidable and unaccounted error in the leadscrew itself.) Table entries which are the same as the table printed on the change gear cover are shown in bold. I’ve not included any indicator table information – generally, regardless of pitch, if it’s an imperial thread with an integer number of turns per inch, then you can always use the ‘1’ mark on the indicator dial, regardless. You may be able to use others as well – f’rexample for multiples of 8 you can use any of the 8 positions, and for multiples of 16 you can just ignore the dial and engage the halfnuts at any time. But it’s hardly inconvenient just to stick to ‘1’ if you’re not sure. (This does NOT apply to non-integer numbers of threads per inch, though – for these, which includes all metric threads, you should leave the halfnuts engaged the whole time.)

Remember – this table is for a 16TPI imperial leadscrew.

IMPORTANT: The obvious disclaimers apply – I don’t guarantee the accuracy of this table, nor do I make any claim for the advisability of any particular gear combination. This information is offered as-is and in good faith, with no warranty nor claim to fitness for purpose. It is entirely your own responsibility to ensure that a particular gear combination runs smoothly and without undue strain on your lathe, and that it gives the results you want. It is up to YOU to check, and it’s up to YOU to judge the likely stresses involved. I accept no responsibility whatsoever for any damage to your projects, lathe, person or anything else arising from your use of this information.

error
TPI mm Pitch A B C D % tpi per in per cm
8 3.175 60 50* - 30 0 n/a n/a
12 2.1167 40 65* - 30 0 n/a n/a
13 1.9538 40 65 60 30 0 n/a n/a
14 1.8143 40 65* - 35 0 n/a n/a
14.5091
(14.5143)
1.7506
(1.75mm)
40 35 55 57 -0.036% 0.00034″ 0.0034mm
16 1.8143 40 65* - 40 0 n/a n/a
16.9231
(16.9333)
1.5009
(1.5mm)
65 55 40 50 -0.06% 0.0006″ 0.006mm
17.0182
(17)
1.4925 55 65 50 45 +0.107% 0.00016″ 0.0016mm
18 1.4111 40 65* - 45 0 n/a n/a
19 1.3368 40 65* - 45 0 n/a n/a
20 1.27 40 65* - 50 0 n/a n/a
21 1.2095 20 35 60 45 0 n/a n/a
22 1.1545 40 65* - 55 0 n/a n/a
22.9744
(23)
1.1056 30 35 65 80 -0.1115% 0.0011″ 0.0111mm
24 1.0583 40 65* - 60 0 n/a n/a
25 1.016 40 55 50 57 0 n/a n/a
25.4222
(25.4)
0.9991
(1.0)
50 55 45 65 0.09% -0.0009″ -0.009mm
26 0.9769 40 60* - 65 0 n/a n/a
27 0.9407 40 45 40 60 0 n/a n/a
28 0.9071 20 65* - 35 0 n/a n/a
32 0.7938 20 65* - 40 0 n/a n/a
36 0.7056 20 65* - 40 0 n/a n/a
38 0.6684 20 50 60 57 0 n/a n/a
40 0.635 20 65* - 50 0 n/a n/a
44 0.5773 20 65* - 55 0 n/a n/a
48 0.5292 20 65* - 60 0 n/a n/a
52 0.4885 20 65* - 65 0 n/a n/a

* For any combination with no ‘C’ gear, the ‘B’ gear is just an idler and can be whatever size is to hand and will fit. In all but one of the setups in the manual, the 65 tooth gear is suggested in this role.

Note that many theoretical gearings are variously impractical. For the 16tpi ratio, for example, in theory any gear can be used for A and D, so long as they’re the same, and any gear can be used for B. However, the only gears which are duplicated in the usual set  are the 20 tooth, the 40 tooth and the 80 tooth. In many cases, the limitations are caused by the geometry of the spindles – some combinations of gears simply can’t be made to span the gap between the spindle-driven layshaft on which gear A is mounted and the leadscrew on which you place gear D (without adding an additional intermediate shaft, at any rate, and/or modifying the banjo) and some combinations result in a gear contacting either a shaft or the banjo adjusting nut. ALWAYS check for clearance when assembling a gear train! Remember, there’s nothing in particular to stop you assembling the gear train with the ‘B’ gear the other side of the line between the A and D spindles, except that to do so may involve some improvisation with (or replacement of) the banjo.

If you’re prepared to replace the banjo and introduce an extra gear position, then the range of ratios possible is significantly increased, likewise if you add gears beyond the normal set. (To support metric threads, people with imperial leadscrews, especially if they have TPI selector gearboxes, often use a 100/127 tooth gear pair to get the 1:1.27 ratio required to “convert” the leadscrew (hence all derived pitches) from tpi to “turns per 2cm”.)

Some screw threads you might want to know about

Most small camera tripod mounts use 1/4″ Whitworth (BSW) thread. This is a quarter inch diameter outside with twenty threads per inch, and a 55 degree thread profile with rounded crests. The pitch and diameter are the same as 1/4″ UNC, and the thread profile “near enough” for most purposes; you’ll find that 1/4″ by 20 UNC bolts will mate well enough with tripod sockets, and 1/4″ by 20 UNC nuts will fit well enough on most tripod screws.

Larger camera tripod mounts – most professional video cameras, medium format cameras and many stills tripod heads themselves, also many microphone holders outside of the USA, all use the next size up BSW thread, a 3/8″ BSW, at 16tpi. Again, this is a close enough match for a 3/8″ by 16 UNC.

Many microphone holders, goosnecks, etc. (most in the USA) use a 5/8″ imperial thread with 27 threads per inch. The thread form is (I think) UNS, which is basically the same thread shape as ISO metric threads, a 60 degree tooth with flattened crests.

The mini-lathe’s drive spindle nuts have a metric thread, M27 x 1.5mm pitch, according to this chap (you may want to know this if considering making a spindle extension).

Leica camera mount threads are a metric diameter with an imperial pitch – 39mm by 26tpi. I’m not sure of the thread form. (Early soviet Leica copies used metric M39x1.0 threads, which don’t quite mate with Leicas, ditto early Canon screw mounts. 26tpi is around 0.977mm pitch. Confusingly, Leica’s mount is referred to as M39.)

Practica/Pentax 42mm lens mounts are a metric M42x1.0.

T-mount threads, as found on many industrial lenses etc., are metric M42x0.75 (T for Tamron. There was an earlier version which was M37x0.75. Tokina did an M47x0.75 version.)

CCTV lenses and so on tend to be either 5/8″ or 1″ diameter 32tpi threads.

Most round photographic filters come in metric sizes with either 0.5mm, 0.75mm or 1.0mm pitch threads, depending on size.

Many popular DSLR “rigs” (for improving camera handling during video use) use rods threaded together with regular M12x1.75 metric threads.

Certain Libec video tripod heads use M6x1.0 twin-start (!) screws for locking the movements. That is, they have a pitch of 1.0mm but a lead of 2.0mm. You can make these, but it’s a slight pain – the easiest is probably to cut a 2.0mm lead thread to the right depth for a 1.0mm pitch, then turn the workpiece exactly 180 degrees in the chuck and cut another one. Or buy a replacement screw from Libec.

Many focusing mechanisms and larger diameter fine-pitch threads are multi-start.

American “garden hose thread” (GHT) is 27.0mm diameter and 11.5tpi. British garden hoses, and pretty much everyone else’s, on the other hand, tend to use BSP threads, typically 3/4 BSP, with 14tpi. Note that 3/4 BSP isn’t 3/4″ diameter, that would be too simple, rather it’s the thread deemed suitable for a standard pipe of 3/4″ internal diameter – 1.059″ or 26.90mm (which is close enough to the American GHT to jam horribly, I guess.) There’s also a smaller size usually found on taps in the UK, 1/2 BSP, which is 0.839″ (21.31mm) diameter, also at 14tpi.

6 Responses to Cutting imperial and metric threads

  1. isitjustme80 says:

    Good write up. One question. Which gear combinations matter? I am running a gears vb from http://varmintal.com/alath.htm program from another site, and it gives the same gears but in a different combination for 25.4 tpi (45,55,50,65) vs 50,55,45,65 as written here. Which is better for the lathe? Best Regards.

  2. lathenovice says:

    I don’t think there’s much to choose between those, to be honest, unless one or other combination comes out with too little clearance somewhere, or snagging. In picking among options I try to avoid combinations with much step-up anywhere, but usually it’s just a question of looking at the various possibilities for one which is do-able. My own lathe gears calculator, linked in the menu at the top ( direct link http://www.imagesalad.com/lathenovice/lathegears/lathegears.html ) lists first the one you cited, too

  3. isitjustme80 says:

    Thanks for the response. As a newbie, I was wondering if there was a consensus on this. Im going to download your calc right now. 4 big questions.

    1. Do I need to swivel the compound to 29 1/2 degrees when cutting metric threads?
    2. I see that some in the uk have a sticker that says a 60,45,30,60 change gear combo with a 16 tpi leadscrew will work for 1mm thread pitch. Does this mean I could order another 60 tooth change gear and it would be spot on?
    3. One person Ive seen cutting metric on a mini lathe had the compound set at 59 degrees vs 29 1/2. Which should I think about for 1mm pitch M6 threads?
    4. Am I to only move the compound when cutting threads, no movement of the cross slide is needed?

    I think these are the biggest questions I have, and would think that a lot of newbies reading this are prob going to have the same. Any help would be greatly appreciated.

    Best Regards

  4. lathenovice says:

    60, 45, 30, 60 will, if I’m not mistaken, get you 1.5 * 16tpi, so that’s rather a long way out – 1.05mm – there are at least 17 ways to get 24tpi with the standard gears, but you can also get way closer to 1mm, again with no extra gears.

    These things are simple ratios, so if you see 30 driving 60, you could use instead 20 driving 40 or 40 driving 80. The way the above works is – if the leadscrew goes slower, you get more turns per inch (because the tool moves fewer inches per turn) If it goes faster, you get fewer turns per inch. The leadscrew itself moves the tool an inch every 16 turns, so if it’s 1:1 with the drive shaft, you get 16tpi. For 24tpi as above, then, you want the leadscrew to go 1.5 times slower (because 24 is 1.5 times 16). For your 60, 45, 30, 60 set, 60 driving 45 means the 45 gear will go 1.33 times faster than the 60, and then the 30 driving 60 (or 20 driving 40, whatever) will result in the final gear turning at half the speed of the 45, i.e. 0.6667 (two thirds) times the speed of the first 60. 1/0.6667 = 1.5, the leadscrew will be turning 1.5x slower (i.e. 0.6667x, two thirds the speed of) the spindle.

    You can achieve the same with any gear combo which has the same ratio end to end, so, from the standard set, the obvious pair to give a two thirds ratio would be 20 and 30. Put the 30 on the leadscrew, the 20 on the spindle, and bridge the gap with any gear which will fit – e.g. the 80.

    Except, you don’t want 24tpi, you want 25.4tpi… 45, 55, 50, 65 is close, at 25.4222 tpi or 0.9991 mm – it’s within 0.0009 mm of the desired pitch, it will be out by about a thousandth of a mm every cm. No extra gears needed.

    As to the compound angle and so on, the angle you set depends on the thread profile you want to cut, and whether to use the compound at an angle or leave it straight and use the cross slide seems to be a preference thing in many cases, as far as I can tell, though I should point out that I am a novice too – there’ll be an old hand along shortly to tell you the proper way to do it, I feel sure! (Case in point, the chap who wrote this guide had a “learning experience” trying to use the cross-slide alone.)

    With the compound angled, and feeding with that, you’re cutting on just one edge, and a similar amount per pass, while feeding in with the cross-slide you’re using both edges of the tool, relying on the tool having the desired thread profile angle, and taking bigger and bigger cuts on every pass (because you’re not just going a few thou deeper, you’re widening out the developing thread valley). There’s no particular reason why that shouldn’t work as such for small threads in easy materials though, as far as I can tell. My impression is you get a less chattery cut and a cleaner thread feeding with the angled compound, and it seemed an easy enough way to work to me, so I don’t think there’s much advantage in “cheating” with the cross-slide.

    In most practical terms, so long as you err on the side of too steep (or narrow) for bolts and too shallow (or wide) for nuts, the exact angle probably isn’t as highly critical as folk might have you believe. Whitworth and UNC parts seem to inter-work, for example, despite nominally different profile angles.

  5. isitjustme80 says:

    That makes complete sense. Thank you for taking the time to include the information I was looking for.

    I do have two last questions and then I will leave you alone. When you say that it will be approx 1 thousandth off of a mm every cm, does this mean long spans of thread, say 20cm will be unusable? Or just 20 thousandths of over the entire length?

    And The other question is for me, just so that its all in one place and others reading this might find the information in this one location… What is the alternative to changing the gears to get exact? Would that just mean a new leadscrew and half nut? A new leadscrew halfnut and indicator?

    Here is a video of a guy in the UK I believe, with a similar lathe to me, using a 16tpi leadscrew (I think), yet with stock stickers indicating the gears needed for cutting metric threads. This video is what has me confused. I thought I had it mentally figured out until I saw this.

    • lathenovice says:

      0.001mm out per cm is a fairly tiny error; *if* you had a very long nut at genuine 1mm pitch then yeah, it would jam on a 0.9991mm pitch bolt – it would have to be pretty long, though. *if* you were using a 0.9991mm pitch thread to move or measure something, e.g. for a screw to drive a lathe cross-slide, or a leadscrew, then it would be out by the expected amount. Nuts of standard sizes would work without hassle, though.

      The “official” way to get metric threads is to use the metric leadscrew conversion kit – this comprises metric leadscrew, half-nuts and dial indicator.

      The guy in the video says his lathe is set up for metric. He cites a 16 tooth drive gear for his dial indicator – dial indicator arrangements are more complicated for metric.

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