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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.

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enter image description here 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?

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  • $\begingroup$ surface flatness or runout? $\endgroup$ – Solar Mike Nov 14 '17 at 12:59
  • $\begingroup$ @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. $\endgroup$ – smollma Nov 14 '17 at 13:50
  • $\begingroup$ The image provided does not give me that detail - all I get is some amorphous blob $\endgroup$ – Solar Mike Nov 14 '17 at 13:52
  • $\begingroup$ @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. $\endgroup$ – smollma Nov 14 '17 at 14:02
  • $\begingroup$ There are tons of websites which list mech drawing symbology and their meanings. $\endgroup$ – Carl Witthoft Nov 14 '17 at 15:32
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It says the flats shall be within 0.100 units of radial dimension positioned with reference to 'A'.

i.e. if you were to make the rim which is a perfect circle, then mill the flats at the three positions, what you should see is that the flats are exactly the same sectional shape and size if the machinist was perfectly precise.

Some minor deviation from that precision (+/- 0.1 units) is permitted which limits the variability of size and position of the cut.

Why? Perhaps a precision tool is needed or the mating piece has a similar tolerance, to ensure a good fit.

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I believe this is a related mating envelope related to datum A for the three plates, per ASME Y14.5-2009.

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