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Figure 4a) “Housing”, with a pattern of two holes as a secondary datum feature:
Figure 4a shows a part, called Housing, with a pattern of two parallel, 6 mm diameter, cylindrical holes as datum feature B. The feature type for this pattern is a linear extrusion.
As shown ASME Y14.5-2009 Figure 4-3, or Y14.5-2018 Figure 7-3, if a linear extrusion is a primary datum feature the datum would be a line on a plane. When a pattern of two parallel holes is a secondary datum feature, the datum is still best described as a line on a plane, but with the line and the plane both constrained in orientation to the primary datum. The plane and the line, so also the datum reference frame origin, can be placed at any location. That said, there is a logical way to locate the line on a plane to the part. The plane should include (pass through) the axis of each of the two holes, and the line should be centered between the two holes. Basic dimensions are required from the datum reference frame to each datum feature in the pattern.
In this example, datum feature B is referenced at Regardless of Material Boundary (RMB) (since no modifier follows B in a feature control frame). There would be a significant difference in the effect, if the holes were not a pattern, but instead one hole was datum feature B and the other was datum feature C.
If B was referenced at Maximum Material Boundary (MMB) instead, then there would not be a significant difference in the effect, compared to using one hole as datum feature B and the other as datum feature C. This will be clarified in the next part of this tip, #68C.
Before we discuss how this works, it is important to clarify when and why a datum feature pattern should be used. If all the features in the pattern are of equal importance in locating and orienting the part to the mating assembly, then specifying this pattern as a datum feature is the way to achieve our goal of the measurement data being able to predict whether a part will function or not. If instead we choose one feature in the pattern as the secondary datum feature and another as the tertiary, the origin used for measurement will differ from the functional origin. For this part, this is not what we want.
Next, we need to understand how the datum reference frame is established for this example.
Establishing the datum reference frame:
Datum plane A is coincident with the surface of the planar datum feature A simulator.
The datum feature B simulator is two cylindrical pins, 53.34 mm apart, and perpendicular to datum A. Since B is referenced at RMB, the pins must vary in size to reach maximum contact with the two holes. Figure 4b shows the datum feature simulators the planes of the datum reference frame, and also the coordinate axes that represent the datum reference frame [A,B]. Datum A is the XY[A,B] plane. Datum B is the ZX[A,B] plane together with the Z[A,B] axis. The baseplate of the simulator also has two very accurate reference holes, to enable easy establishment of the datum reference frame.
Figure 4b) The datum feature simulator set, and the datum reference frame:
The part is mated with the datum feature simulator set, using the order of datum feature precedence. First datum feature A is mated with its simulator. Then, as the two datum feature simulator B pins grow in diameter, the part will center itself, based on how the form, size, orientation, and relative location error of the two holes mates with the two datum feature simulator pins, all the while keeping datum feature A mated with its simulator. When the part is mated with the datum feature simulator set, the part is then related to the datum reference frame.
Note: Instead of using a set of physical datum feature simulators, an alternative is to measure the datum features directly, then establish the datum reference frame using the measurement system software or separate, point cloud software. It is very important that this process include modeling the effect of datum feature simulators. Improperly established datum reference frames are likely the most common source of measurement data that is inaccurate to an unacceptable degree in industry.
This concludes tip #68B. In tip #68C we will clarify why specifying datum feature patterns isn’t necessary if datum features are referenced at MMB or LMB.
This tip is in accordance with ASME Y14.5-2009 and ASME Y14.5-2018
This tip was originally released, in accordance with ASME Y14.5M-1994, in July 2003.