Meaning of dimentional tolerances Meaning of dimentional tolerances Jess10 (Aerospace) (OP) 12 Nov 10 05:57 Hi,What does "our industry standard dimensional tolerances of +/-0.125mm per 25mm" mean? Thanks RE: Meaning of dimentional tolerances rb1957 (Aerospace) 12 Nov 10 07:27 that dimensions are plus or minus 0.125mm for each 25mm in the dimension ... so a couple examples ... 24+-0.125mm, 26+-0.25mm, 80+-0.375mm ...clear as mud ?0.125mm = 0.005" per 1" is pretty darn tight RE: Meaning of dimentional tolerances KENAT (Mechanical) 12 Nov 10 10:53 Fundamentally it means the tolerance grows with the size of a part, which isn't unreasonable especially for casting, molding and similar.What I'm not sure I agree with rb1957 on is whether it's discrete steps or proportional. I've always treated it as proportional, so (0.125/25)*Dimension, so for 80mm I'd think +/-0.4.I've usually seen this type of tolerance expressed as "+/-0.125mm for dimensions less than 25mm, +/-0.125mm per 25mm for larger dimension" or similar.Then again, I've actually seen "+-.005 per inch", I'm assuming your mm is trying to say the same thing, but maybe it is saying descrete steps, but it doesn't' ring true.Note, this type of tolerance is appropriate for a design guide - it is not generally appropriate for a drawing (or MBD), at least in US documentation systems (ISO2768 does something similar though).http://www.eng-tips.com/threadminder.cfm?pid=1103 might be better in future. Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?