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Possible illegal gd & t callout

Shane-o
I believe I've stumbled across an incorrect use of Runout to a Flat Datum-A-Plane.
Imagine a bolt with a flat head: this is called out as Dat -A-. It also has a Flatness callout to itself, which is fine. The threaded shaft has a Runout callout to Datum -A-. This I believe is incorrect. If the pitch diameter of the threaded shaft was described as Datum -A- with the flatened side of the bolt head having the Runout callout, this would be correct. Am I correct? I just want some verification.
  • I have. Threads pitch can be a datum, but this has nothing to do with your part since your datum is the screw head.
  • I agree, the callout, as described, is invalid, or quite unorthodox at the very least. Run-out requires an axis of revolution. A plane, by itself, does not have that.

    9.4.1 Control of Circular Elements
    Circular runout provides control of circular elements
    of a surface. The tolerance is applied independently
    at each circular measuring position as the part is
    rotated the full angular extent of the surface about
    the simulated datum axis.     See Fig. 9-2. Where applied
    to surfaces constructed around a datum axis, circular
    runout may be used to control the cumulative variations
    of circularity and coaxiality. Where applied to
    surfaces constructed at right angles to the datum axis,
    circular runout controls circular elements of a plane
    surface (wobble).     When verifying circular runout, the
    indicator is fixed in a position normal to the toleranced
    surface.

    Also, earlier in section 9:

    9.3.1 Datum Features for Runout Tolerances
    The datum axis for a runout tolerance may be
    established by a cylindrical datum feature of sufficient
    length    , two or more cylindrical datum features having
    sufficient axial separation    , or a cylindrical datum feature
    and a face at right angles to it    . Features used as datum
    features for establishing axes should be functional, such
    as mounting features that establish an axis of rotation    .
  • Awesome thank you! I couldnt even use to cmm to check it :it said the primary datum needed to be co-axial
  • To be fair, what Nano said also has truth to it. The problem isn't that the screw head is the A datum, it just can't be the primary AND sole datum. If the flat head was Datum A and the OD of the head was B and the FCF was run-out of the pitch to A|B, it would be valid. I really hate ever having to deal with the pitch though, because it is almost impossible to actually use the pitch. The pitch almost always ends up being used indirectly, and run-out of a pitch is physically impossible, because the pitch is not a surface. So, I guess I would debate ever calling run-out of the pitch, for that reason. But ignoring that part of it, oohh, squirrel!

    Unless! Maybe it could be construed as a helical path along the thread at the pitch diameter?! But, then it wouldn't technically be a circular element anymore... Maybe that would be more appropriate for TIR? Interesting idea. Hmm.
  • I knew it wasnt correct ven before putting it on the c.m.m.
    Just wanted som verifaction. Thank you
  • You could also use the minor or major as well instead of pitch but it must be stated on the print
  • You could also use the minor or major as well instead of pitch but it must be stated on the print

    Yeah, but neither the minor nor the major can be guaranteed to be coaxial with the pitch.
  • I believe I've stumbled across an incorrect use of Runout to a Flat Datum-A-Plane.
    Imagine a bolt with a flat head: this is called out as Dat -A-. It also has a Flatness callout to itself, which is fine. The threaded shaft has a Runout callout to Datum -A-. This I believe is incorrect. If the pitch diameter of the threaded shaft was described as Datum -A- with the flatened side of the bolt head having the Runout callout, this would be correct. Am I correct? I just want some verification.


    Shane: please sketch the print in Paint, or sketch it with pen/pencil and scan it in, so we can see exactly what you are talking about.
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