Confusion about the lead screws sold on the store -> 8mm Metric Acme Lead Screw The page claims that they are: - ACME - Metric - Trapezoidal ACME is the imperial standard and not generally referred to as trapezoidal. And has a 29 degree thread angle. Metric *are* trapezoidal and have a 30 degree thread angle... So, which is it really? ACME or Metric Trapezoidal?
Yes, you could call it trapezoidal as well but ACME is not metric. The listing does say it's a Tr8*8-2p which is definitely a metric standard, so I guess I'll go with that. It's just confusing to have it also labeled as ACME It'd be like selling a bolt as M3 / 1/8" - close but not the same thing.
The ACME trapezoidal thread came first in 1894 (yes imperial, 29 degree thread angle), the metric trapezoidal thread came later (30 degree thread angle). In the same way as "Hoover" became synonymous with vacuum cleaners and "Biro" with ballpoint pens "ACME" became the term everyone used for trapezoidal threads (it's a lot less of a mouthfull than "metric trapezoidal thread"). For most of the fifty-odd years I've been tinkering with machine tools the imperial Acme thread was the commonest in use throughout the world - although that may be changing now - and everyone I met referred to all trapezoidal threads as "Acme" (and they still do - my son recently bought himself a vice which proudly proclaimed on the box that it had an acme thread - we measured it and it's metric!) Openbuilds are being a little bit sloppy in describing a metric thread as "Acme" but most engineers (at least those over 50) will know what they mean. Alex. PS The first (very old) lathe I managed to get my hands on had a leadscrew with a square thread. It was constantly binding and the wear on it was awful. At least with a trapezoidal thread the binding issue is (almost) eliminated and if it wears you can adjust the half nuts to compensate.
