Multiple Datum Reference Frames
Our block below is going to be located to another part by a net hole and slot in the bottom of this part.
A sensor will pilot into a hole in a side of our block, and it will be held on with two small bolts.
A sensor will pilot into a hole in a side of our block, and it will be held on with two small bolts.
We see the datum scheme below. Since the block will sit on the mating part, we make the mating surface datum [A].
We have constrained 3 degrees of freedom (1 translation and 2 rotations) with datum [A]. Now we create a net hole that will be barely larger than the dowel pin that goes into it. We control the perpendicularity of this net hole relative to datum [A]. We label this net hole datum feature [B]. Datum [B] constrains two more translations.
We constrain the final rotation with a slot. The slot is the same width as datum feature [B]. The axis of the slot must go through the center of datum feature [B]. The tolerance of position on the width of the slot controls both the alignment to datum [B] and the perpendicularity to datum [A]. The width of the slot is designated as datum feature [C].
The length of the slot has a tolerance of position that controls the location of the slot relative to datum [B] and the orientation of the slot relative to datums [A] and [C].
We need to control the surface where the sensor will mount. We control this surface with a profile of a surface that will orient it to [A] and locate and orient it to [B] and [C]. We then label this surface datum feature [D].
Now we specify our large hole for our sensor. We control the perpendicularity to the surface by making [D] our primary datum. We control the height from the bottom surface by making [A] our secondary datum. We control its location relative to a line from [B] to [C] by making [B-C] our tertiary datum. We label this hole datum feature [E].
Finally we control the M3 threads with a tolerance of position. We control the perpendicularity to the surface by making [D] our primary datum. We control the hole pattern's orientation to the bottom surface by making [A] the secondary datum. We control the location of the threaded holes relative to the sensor hole by specifying [E] as our tertiary datum.