True Position call out...

[Valek]

Gentlemen,

I came across on a Diameter imension of 20,36 H7 and below is a true position call out to ABC under RFS because has no M encircle.
My question is what's the H7 means it has something to do with the true position?

Thank you.

 

[pmarc]

H7 is a symbolic description of ISO standard fit tolerance for hole-like elements. In case of diameter 20.36 it means that the allowable size tolerance is +0.021/-0 (in milimeters).

For more details regarding ISO limits and fits system see to ISO 286:2010 (parts 1 and 2)

 

[pmarc]

And since position callout is at RFS basis it has nothing to do with the size of the hole. Positional tolerance value is constant regardless of the hole's diameter.

 

[flash3780]

The H7 tolerance grade could also be from ANSI/ASME B4.2. I'm not sure that it's any different than the ISO standard, but hopefully they've listed one or the other in the com docs.

For what it's worth, the MMC modifier wouldn't add any appreciable position tolerance in this case (21 microns, as noted), but would allow for the use of hard gaging, which would make inspection much more straightforward on a production part.

If you're making more than a couple of these things, it'd be a worthwhile change to the print.

 

[Valek]

Thank you Genetlemen for the reply.