CAD Hackathon: Answering Your Questions!

Intrepid GD&T practitioners sent in some excellent questions following our recent CAD Hackathon! Take a look and see what you think – do you agree with our approach, or do you have questions of your own?

Missed the live events? Watch now!

How practical is the position callout of secondary datum feature B back to A? Would a parallel callout have been more practical since there are not enough reference(s) for an origin of measurement? (I can see position for a tertiary datum feature in this case.)

For the benefit of the rest of our viewers and readers, I’m going to provide some additional details for context.

Using the position tolerance to qualify Datum Feature B relative to Datum Feature A is not only practical but necessary due to the relationship of Datum Feature/Axis B to Datum Feature/Plane A.

Datum Plane A, established from the Datum Feature A surface, starts to build the foundation of our Datum Reference Frame (DRF), AKA our theoretically perfect Cartesian coordinate system (CCS), which establishes our origin point. When identifying the .192-inch diameter continuous feature hinge hole as our secondary datum feature, we derive the Datum B Axis from Datum Feature B’s theoretically perfect inverse shape, called the true geometric counterpart in the ASME Y14.5-2018 standard (the largest inscribed cylinder that will fit in both halves of the continuous hinge hole feature).

We know from the definition of a CCS that our origin needs to lie at the convergence of three mutually perpendicular axes, which are the result of three mutually perpendicular planes. Our primary Datum Feature A establishes the first plane of our DRF, laying the foundational plane that our origin must lie on. When qualifying Datum Feature B relative to A, it must not only be oriented with a parallelism tolerance but also located with a position tolerance relative to the plane the origin must lie on, Datum Plane A. The relationship of Datum Axis B to Datum Plane A establishes the second plane of our CCS, perpendicular to the first. Since our cylindrical position tolerance zone is also controlling parallelism, we are still controlling the orientation.

For the tertiary Datum Feature C width of .840-inch, the true geometric counterpart of the feature (two parallel planes at maximum separation) establishes our Datum Center Plane C. This datum feature is oriented with a perpendicular tolerance relative to A and B.

One of the proposed models had a radius with tolerances directly called out. Since a radius is not a feature of size, having no opposing ends to reliably generate an axis, I would have expected a basic radius dimension and profile tolerance back to the DRF to control its variation.

The decision to treat a radius as a feature of size and apply plus/minus tolerancing (versus establishing it as a basic dimension and applying a profile tolerance) is one of those “areas of conversation” that Stephen and Ryan mentioned during the Hackathon. At times, as in the case of an inside bend radius on a sheet metal part, the choice might be more obvious than our decision was for the Hackathon. In our Hackathon models, we treated radii in both manners. We profile toleranced “larger” radial features/tolerances (basic dim R .030 with a profile tolerance of .050; basic dim R .030 with a profile tolerance of .020) and size toleranced for “smaller” radial features/tolerances (R .010 +/- .002).

There is definitely room to improve on this topic to achieve a more consistent application.

Thanks to our viewers & readers for sending in questions, as there are likely many GD&T practitioners that were wondering the same thing!

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