# imperial screwcutting on metric lathe

Newbie here Hello ,
Been a lurker for a while and enjoyed the posts ,My shop activities have lead me into a problem that i am sure there is an easy ish answer
too ..............
Need to cut a Imperial thread on my metric lathe ,is it as simple as finding the pitch of the imperial thred and setting the machine to that I have a Harrison 11" with the 3 lever gearbox ,and also a pile of changewheels ,i still dont fully understand how to read the chart of pitch rates yet but i am on the case !!!
Any help greatly appreciated .
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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 = 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 127/100 tooth gear to allow metric threads to be cut on it. If it were a metric lathe, it would use the same gear but as 100/127 to cut imperial threads. Other lathes often use smaller "conversion gear" combinations but, because 127 is a prime number, any other combination will only be approximately correct.
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.
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In article

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
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David Littlewood

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

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

4 * arctan(1)
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Amateur Machinist wrote:

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

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?
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On Thu, 18 Dec 2008 18:36:29 -0000, "Amateur Machinist"

which gives pi as 180
.....I think you forgot to say that the "1" is in radians
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That should always be assumed unless degrees or grads are specified, since radians are unit-less :-)
Mark Rand RTFM
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It would be more accurate to say that radians are dimensionless.
David
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David Littlewood

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wrote:

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
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Mark Rand wrote:

But then isn't 60� the internal angle of a triangle where each side is the same length giving the same unit of length/length?
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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
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JG wrote:

What unit does pi have then?
If pi x angle in radians = angle in degrees, don't both sides of the equation need to have the same units, which would mean pi and angle in radians can't both be unit-less?
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Cliff Ray wrote:

But it doesn't!
Angle in radians x 180 degrees/pi= angle in degrees
so the angle in Radians and Pi have the same dimension and both are "dimensionless"
Bob