I am happy now, I KNOW I can cut imperial threads just as accurately as metric ones!
I think it is because the 12x36 lathes are big enough to accommodate a 127 tooth change gear. 127 is an exact mulitple of 25.4 which is the exact number of millimetres in an inch, exact and no approximations there.
By using the 127 tooth change gear the conversion is exact which is possibly not the case on smaller lathes where the designers must either use much smaller pitch change gears or some other less than exact conversion ratio.
I always sort of assumed that there was no easy and exact conversion from metric (based on the diameter of the earth?) and the foot (based on the size King Arthur's boot?), so I was nicely suprised when I came into the world of lathes and things where that mattered and I found this:
Interestingly, standardisation of the inch for worldwide use occurred only in 1958. (The inch was standardised worldwide as 25.4 millimetres exactly.) Prior to that the United States, Great Britain, and Canada each had their own definition of the inch, and in each case the inch was defined in terms of metric units, the only set of internationally-accepted measurement standards. A problem still exists for the foot, where the international foot (based on the 25.4 mm inch) and the survey foot (based on the 25.40005 inch) are both in use. Over 100 miles they differ by 32 cm, or over one foot.
at
http://www.didyouknow.org/decimal.htm So I dont feel so bad about my ignorance as I can actually remember 1958 and it is quite likely that we were told at school that there was, then, no convenient exact equivalent.
