According to my copy of Machinery's Handbook 25th edition, Line weight
and lettering is controlled by ANSI Y14.2M-1979
Line weights are as follows:
Approximate width (not reason for drawing rejection)
Thick lines .7mm
Thin Lines .35mm
Outlines, section lines, cutting/viewing planes, break lines, and chain
lines are thick.
All others are thin.
Christopher F. Gosnell
FPD Company
124 Hidden Valley Road
McMurray, PA 15317