# Explain tolerances in technical drawing

I'm not a mechanical engineer and am not used to looking at mechanical drawings. I'm trying to understand what these tolerance features mean.

There is a positional tolerance feature, which references datum A. But, what exactly is it specifying? That the middle point of the flat must be within 0.1 mm of the rotational axis? Or the thickness of the disc?

• surface flatness or runout? Nov 14 '17 at 12:59
• @SolarMike As far as I'm aware, total runout is noted by two parallel arrows, joined at the bottom by a line. This is a positional tolerance, i.e. a circle with a cross in it. Nov 14 '17 at 13:50
• The image provided does not give me that detail - all I get is some amorphous blob Nov 14 '17 at 13:52
• @SolarMike Apologies for that, Mike. The copy I have is a little dodgy, but on my version, I can see it's a positional tolerance. What I'm trying to understand is what datum A specifically refers to? It seems to me like datum A is the direction perpendicular to the perimeter of the circle. Nov 14 '17 at 14:02
• There are tons of websites which list mech drawing symbology and their meanings. Nov 14 '17 at 15:32