Sheet Metal Holes as Datum Features

The below question line video is in response to a student’s question regarding using sheet metal holes as datum features. Specifically, this student asked, “If I specify a hole in a practically thin feature, such as a piece of sheet metal, is this still considered a cylindrical datum? Is this a poor choice of datum?”

The part shown in the drawing below illustrates our student’s question. In this drawing, we have a piece of sheet metal that is bent, which includes a hole in one of the flanges. Let’s assume this part is mounted into an assembly and gets bolted onto a surface that also has a hole in it. The flange with the hole in it would first be placed against the mounting surface of the mating part, then a fastener would pass through the hole in this part and into the hole in the mating part.

Notice on our drawing that we have indicated Datum Feature A as the bottom surface of the part (the flange with the hole in it), and Datum Feature B as the hole in the flange.

Is this a Cylindrical Datum Feature?

The first part of the student’s question is whether this very thin hole is considered a cylindrical datum feature. Technically, the answer is yes, but the depth of this hole is so shallow that trying to get a datum axis to represent the cylinder would be nearly impossible. However, as this datum feature is a secondary datum feature, to orient this part we only need it to lock down two translations, as the other translations and rotations are locked down by Datum Plane A. Instead of attempting to get a datum axis from this feature, you only need to find a reliable way to get a centerpoint of the hole. Having a point that represents the center of the hole is enough to stop our translations, and as the cylinder is very shallow, we are not concerned about the perpendicularity of the hole to Datum A.

Is this hole a poor choice for a datum feature?

The student also wanted to know if having this type of hole as a datum feature is a poor choice. There is actually a good reason to have this hole as a datum feature, because this hole locates the part in the assembly. However, as we have done in this part example, we would only want to use this as a secondary or tertiary datum feature. If we were to try to use this feature as our primary datum feature, we would then be using an unreliable axis to orient other features, which is poor design.