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:
- You have to somehow come up with a gear ratio which gets the thread pitch you want.
- 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.)
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.
|TPI||mm Pitch||A||B||C||D||% tpi||per in||per cm|
* 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.
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.