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Can I attach my datum to a dimension as per ASME Y14.5?

Matt_FarrowMatt_Farrow Member Posts: 4
It is standard practice in ASME Y14.5 to attach datums to dimensions. This communicates that the location of the datum is at the centerline/axis of the dimension. See here for a snapshot from the standard, which communicates that Datum A is an axis down the center of the cylindrical feature. This is something I have done countless times in SW, a very standard and integrated feature.

I have been unable to perform this action in a drawing in Onshape. It seems I am forced to position the datum and the dimension separately which can make the communication less clear and the drawing more cluttered.

Am I missing something? Or is it simply not possible?

Answers

  • NeilCookeNeilCooke Moderator, Onshape Employees Posts: 5,617
    Huh? Just add the datum to the dimension extension line. Yes, you have to position it by eye to be inline with the dimension leader line, but that's ok?
    Senior Director, Technical Services, EMEAI
  • Matt_FarrowMatt_Farrow Member Posts: 4
    @NeilCooke Thanks for the reply.

    I since found this post which more accurately describes what I was attempting to do. I can attach to a feature control frame, but not a dimension on it's own.

    https://forum.onshape.com/discussion/17991/datums-on-diameter-and-radius-dimensions-and-on-hole-callouts

    In the example I gave, you correctly point out I can align by eye, but a "snap" to the correct location would be faster and smoother, and guarantee the datum is communicated correctly. Technically if the datum isn't bang in-line with the dimension then it is now positioned on the edge of the part and communication is lost. Because the datum and dimension are just freely positioned and not locked together this is not robust and is a potential source of error in my opinion. It feels weird not to be able to click the actual thing I was the datum to be attached to.

    Additionally, or the axial view of any cylindrical feature, it does become quite tricky to precisely line up the datum with the dimension.
  • Aleksey_RybalnikAleksey_Rybalnik Member Posts: 4 PRO

    To follow up on Matt's original request, there is currently no way to ensure that leads between the dimension and the datum are related. As mentioned above, per Y14.5, it is often used to indicate a virtual center line. In Matt's example, Datum A is the center axis of the 15.9-16.0 dimension. Per Y14.5, it is indicated as such, by positioning the datum A in line with the dimension. If the dimension on the drawing was moved, and the datum A was no longer aligned with it, the interpretation of the datum A is that it is now a surface of dimension 15.9-16.0 and not its axis.

    I have an exact same problem with my drawings. I have a rectangular part, whose size dimensions are datums used for GD&T. Those datums absolutely must be interpreted as center lines, as the part is positioned in the assembly based on the center lines of the part. If the datum is not in the correct position with regards to the dimension, then my datum would be interpreted as either of the sides of the part.

    The request here is very simple. Please allow for the datum tags to lock to a specified dimension and move respectively, with the said dimension.

  • PeteYodisPeteYodis Moderator, Onshape Employees Posts: 540
    edited September 26

    Width datum type attachment to dimensions is work in progress now. We know it's important. And it's certainly not just for cylinders.

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