Difference between nominal and major diameter in threads

hihiip201

Hi guys:


I am having trouble with differentiating nominal and major diameter, from what I get on google as well as my textbook, the nominal diameter is the largest diameter in the thread and major diameter is the distance between crest to crest. but aren't the two the same?




thanks

Travis_King

Nominal diameter is the diameter from crest to crest that the fastener is listed as. i.e. a 1/8" bolt has a nominal diameter of .125 inches. However, if you measured the outside diameter of the fastener from crest to crest you would find it slighly different (positive or negative depending on the material, grade, purpose, etc.) depending on the manufactured tolerances. The Major Diameter measures the distance between external crests for male threads and internal roots for female threads, and describes an actually measured dimension, rather than a theoretical, design dimension.

hihiip201

√
so if tolerance is zero (in theory), the major and nominal should be the same?

Travis_King

From what I understand, the Nominal diameter is the diameter of the cylindrical piece that they cut the threads from. So if they were able to cut the threads so that the crest-to-crest diameter was the same as the original cylinder, then yea. But Nominal means it's what they are calling it. A .5" bolt may have no points on it which measure .5", it's the way they label them.

hihiip201

ya I remember what I learned from technical drawing class is that, nominal is just what you will order at home depot...