How Modern Quick Change Lathe's Change/Convert internally from Inch to Metric Threads

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Blair

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To convert an old Southbend Lathe to cut Metric TPI you use 127/100, 80/63, 47/37 Transposing Compound Gears, Ideally 127/100
My question is:
In a modern QC Lathe with metric or Imperial thread choices, Is the conversion exactly 1.27 or do they use a close approximation to make it easier to accomplish, do to the size of the gears?
Is it accomplished using a 127/100 transposing compound gearing internally or do they use a multiple of ratios through gears to reach a 1.27 ratio if it is exact conversion?

Just wondering if when we choose a Metric thread pitch by moving the appropriate leavers, are we getting an exact pitch or an a close approximation, and in 99.9 % of the time it is never noticed

I would assume it is exact, but is it really exact. You know what they say! If you Assume It makes an ASS out of U & ME

Thanks for your comments
 
Yes.

It has now (since 1959) been defined as exactly 25.4mm/inch:
"In 1959 the inch was set to be exactly 2.54 centimeters. Prior to that, it was set at 39.37 inches per meter. The difference is only 0.0002%, or 2 parts per million. The new definition is slightly smaller than the old definition. See "US Survey" unit description below." as per http://www.elivermore.com/conversions.htm

127/100 x 2 is 2.54. In gearing the x2 is very easy. The 127/100 is tougher - unless you actually use a 127T gear together with the 100T gear - then it is exact.
 
You can cut most common metric pitches close enough by simply changing the stud gear to 19 or 26 teeth on a 9A or 10k
 
So I guess the answer is that it's a very close approximation, unless the lathe actually has 100/127 gears. A lathe with a metric leadscrew would inherently cut perfect metric threads and very close imperial ones. The opposite being true for an imperial leadscrew equipped machine.
Right?
-Mark
 
A universal gearbox does both with no change gears. Assume a series on gears, it is exact.
 
It's often an approximation used for translation, such as 80/63. Anything that gives close to 1.27 ratio. 127 tooth gears are large!!

I'm not sure which absolute clown decided to go for that number when the inch essentially got adjusted anyway. It's most inconvenient. 2.56 would have made so much more sense. 256 not being prime, and all...
 
Thanks for all the replies
To clear something up, I used the Southbend lathe as an example of how they use to have to do the conversion, what i really want to know is how the manufacturers accomplish it in the QC gearbox in a modern lathe? Is it done inside the gear train of the QC with a 127/100 compound gear in there somewhere or do they use another method inside the QC, and is it an exact conversion or do they use a close conversion like 80/63 and we never ever notice because 99.9% of the time we don't cut a thread long enough for it to matter

thanks
 
I have another Question, Don't know if it is appropriate to post it in this thread, but you'll let me know if it aint
If you have a 40T 8DP Idler gear driving a 48 tooth gear and you want to change the gears before the 40T idler to a 10 or a 14DP gear set!
If you attach, say a 40T 14DP gear to the side of the 40T 8DP, and you keep all the gears before the 40T 8DP gear the same number of teeth but now they are 14DP gears. Will this change the ratio between them because the 40T 14DP gear is smaller than the 40T 8DP gear it is attached to the side of it.
I think it will not because its the same number of teeth on the same axis, but I'm not sure
I want to change this to a smaller DP because i am limited for room to put in my metric transposing gears
 
There may be many schemes employed, but there's only once accurate way to do it and that's with a 127 tooth gear somewhere. 127 being a prime number... anything else is an approximation, though there are some very close ones.
Yes, it's quite possible to use transposing gears of smaller pitch than the rest of the change gears to keep their physical size down. The pitch makes no difference at all; only the ratio is important.
 
Lo-Fi said:
There may be many schemes employed, but there's only once accurate way to do it and that's with a 127 tooth gear somewhere. 127 being a prime number... anything else is an approximation, though there are some very close ones.
Yes, it's quite possible to use transposing gears of smaller pitch than the rest of the change gears to keep their physical size down. The pitch makes no difference at all; only the ratio is important.
Click to expand...
I have a 40t x 8dp, I bolt a 40t x 14dp to the side of the 40t x 8dp, does this change the ratio? The 14dp has a smaller diameter than the 8dp, but it has the same 40 tooth count so the ratio shouldn't change. Am I correct?
Thanks
 
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