imperial screwcutting on metric lathe

Yes. In general terms that's correct. Metric threads are specified as pitch in mm whereas imperial threads are specified as Threads per Inch (TPI). 1/pitch in mm =3D threads per mm (TPmm), so you need a factor of 22.4 to convert TPmm to TPI

In practice, you need a compound gear that gives a ratio of 22.4:1 (or some factor very near that). My Taiwanese lathe (imperial) has a

If you have the "conversion gear" offered by your lathe manufacturer - often an optional extra, then the calculations are (relatively) easy. Remember, however, that you cannot release the leadscrew from the carriage once the thread is started and that the thread indicator is of no use. You have to reverse the lathe to the beginning of the cut each time (not with the tool cutting) before putting on more cut and taking another pass.

You may ask how come an approximate "conversion set" ratio works. The answer is that, for the short lengths that are usually needed yo be cut, the error in imperial pitch created is maybe only significant after a few 100s of pitches. So, unless you're cutting a very long screw or want "leadscrew accuracy" you'll never notice it.

I think you mean 25.4.

Ditto

Note for OP: 127 = 25.4 x 5, it's the lowest whole number of teeth that gives an exact conversion.

David

thanks for that ...will get on and make a plan

If you look at Ivan Laws book Gears and Gearcutting, he tells you how to screwcut odd pitches by selecting the right changewheels, This will give you the theory you need. Peter

All

Sorry. Must have had a CRAFT moment there. Can't understand why I was thinking 25.4 and typing 22.4 - - - AND did it twice as well ??!!??

Andy

Probably been watching Jose Rodriguez's crap DVD on gear cutting. Not only does he get Pi wrong all the way thru the DVD but so are all the calculations.

John S.

Which raises the question is there a better/simpler way than 355/113?

Hmm! I seem to have mislaid my arctan(1) changewheels right now

Multiples of 22/7 are pretty close to Pi. By definition Pi is irrational so not exactly equal to any ratio.

Don Young (USA)

Have I missed something?

Why would you wish for there to be a factor of PI in the changewheel train itself, rather than just in the calculations for that train?

Of more interest to mathmeticians than engineers, here's

which gives pi as 180

.....I think you forgot to say that the "1" is in radians

That should always be assumed unless degrees or grads are specified, since radians are unit-less :-)

Mark Rand RTFM

Why are radians unit-less? Surely radians are a unit.

In article , Cliff Ray writes

It would be more accurate to say that radians are dimensionless.

David

Because one radian is an angle that gives a circular arc length the same as its radius. This would give units of length/length. Which is a dimensionless number, not a unit.

Honest, that's what they teach in higher mathematics education. :-0

Mark Rand RTFM

But then isn't 60=B0 the internal angle of a triangle where each side is =

the same length giving the same unit of length/length?

from Cliff Ray

No. With radians the figure is the _ratio_ between the distance along the circumference between the two radii which delineate the angle, divided by the radius but when using degrees to dimention an angle you are counting the absolute number of 360ths of a circle between the two radii - not a ratio at all, an absolute measure which has to have a unit.

Similarly, when using sine, cosine, tangent etc. you are refering to a ratio (p/h, b/h & p/b respectively) which doesn't have a 'unit'.

JG