Hi Howard,

Check your inbox

Brian's book is still recommended if you want to really explore the subject.

Neil

It is my uinderstanding that to cut a metric thread on an imperial lathe or vice versa, you need a127 x 120 tooth gear, as the gender bender gear in the train,

The other gears will then give you the pitch you require. there are other combinations that are used but they only give an approximation of the desired thread pitch.

While Bob's comment is true enough in a purely mathematical sense, the "approximations" will give a result to more accuracy than the lathe is physically capable of producing. They also have the advantage of not requiring a 127 tooth gear or, indeed, finding a way to mount it on small lathes.

Cheers,

Rod

That's the conventional wisdom Bob, but it's not rock-solid. The magic 127, or slightly less magic 63, toothed gears allow one to calculate a TPI and then convert it to metric, or vice versa. The logic is analogous to 'multiply thou by 25.4 to get mm'.

Calculating ratios with paper and pencil using the method mentioned by John MC is hard work. Having 127 or 63 makes the sums easier. However, we don't have to do it the hard way – a computer can crunch the numbers in nanoseconds and tell you how big the error is. Only if the error was too big would I get a 63 tooth gear. (I would never buy a 127 tooth gear because fitting on the banjo is likely to be problematic – it's too big for comfort!)

Whilst the conversion method might suit the way we do arithmetic, and it gets results, it may not be the only way of producing an acceptable thread. My metric lathe produces more imperial threads than the imperial version and it doesn't have either a 63 or a 127 toothed wheel. What it does have is a set of gears that can be combined in ways that produce thousands of different thread possibilities. Many of them are duplicates or non-standard, but quite a few produce reasonable approximations of imperial TPIs. A quirk of the mathematics means that the gears provided with the imperial model are not quite so productive as it's metric equivalent. Nonetheless, given a calculator like that suggested by Thor, it's well worth checking to see if your existing gear set will do the job without needing a special 63 toothed gear.

Dave

Depends on whether you are using it as a driver or driven.

**LINK**

Rod

How is this figure calculated? It would seem to me that as 63 is half of 126 that is 1 full tooth per revolution, short of 127, and given that the 63 tooth gear is run with a 60 T gear it is an attempt at approximating 127 x 120 gear setup which is, and has been, used on larger lathes for a very long time.

Therefore the error occurs to me to simply be 1/127th. Ok, not a lot, and particularly in short lengths of thread such as the average nut and bolt., where the thread engagement lengthy is typically about one diameter. But it would become a problem in any attempts to make a long running thread such as a lead screw. Sure the mating nut could be made to match, but the error would be significant in the total length travelled over many turns.

Bob.

Bob,

Our esteemed Editor's article on the subject is available for download: **LINK**

http://www.arceurotrade.co.uk/reviews/MEW204-P31_P36-Thread-Cutting.pdf

… this should reveal the magic of 63

MichaelG.

Brief example cutting 1mm pitch on a 1/16" leadscrew mini lathe:

1/16 * 25.4 = 1.5875mm pitch (exactly).

So to step this down to 1mm pitch you need to divide the gear train by 1/1.5875

1/1.5875=0.62992125984 (approximately)

So if you use the ratio 63:100 in the gear train the error is:

63-0.62992 = 0.00007874

As a percentage that's 0.008% which is close enough for rough work.

Visiting this site I see their precision metric leadscrews have an accuracy of 0.08mm per 300mm.

That's 0.026%. Which is three times the error given by using a 63 tooth gear – over a foot!

Any questions?

Neil

Hmmm, and further down on the same page they say the lead error over the length of the rod is 0.3mm per 300mm, rather different. I would expect precision leadscrews to be ground rather than rolled.

Of course that doesn't change the fact that a 63 tooth gear is perfectly satisfactory for cutting metric pitches with the correct gear train. Personally I say hooray for quick change gearboxes; imperial, metric, DP and module all at the change of some levers, and one gear.

Andrew