Fig 4-48 and origin of dimensions / measurements 3

[aniiben]

Fig 4-48 from Y14.5-2009

I am trying to find the origin of dimensions, origin of measurement or Datum Point 0,0,0 for the part shown in fig 4-48 page 86.
Four datum planes are shown and therefore creates a little bit of confusion on where the origin of dimensions might be.

For reference see
fig 4.1: Datum Point 0,0,0
fig 4-45: Origin of dimensions
fig 4-46: Origin of measurement

 

[Belanger]

Datum B is a single datum, but it is comprised of the 2 offset or broken planes shown in that picture. So for 0,0,0 you can take your pick from two choices: the intersection of the 3 planes as shown near the top right of the "means this" picture (which uses the B2 level), or the intersection of the 3 planes formed by using datum target B1's level, although that intersection isn't shown (it would be further away from the fixture).

So it doesn't matter. As long as we know that the offset within datum B's broken planes (which happens to be 11.4), then there is no effect on the subsequent measurements. If you wanted to sketch an x,y,z coordinate system, flip a coin as to which of the two intersections it should be placed at.

 

[axym]

aniiben,

I agree with J-P's explanation of how Y14.5 addresses this. The details are in Section 4.24.8 on Datum Planes Established by Datum Targets.

This example also shows, however, how confusing and arbitrary Y14.5's concept of datum planes can be. It was built from simple datum feature combinations, where a three-plane XYZ coordinate system can be placed in a reasonably obvious way. In more complicated examples such as in Figure 4-48, part of that arbitrary nature is acknowledged (for stepped surfaces, the datum plane should contain at least one of the datum targets). In even more complicated examples such as Fig. 4-26 with the hole pattern datum feature, the origin of the DRF can be put at the center of the pattern or at any other location defined by basic dimensions. In other words, the placement of the "datum planes" and DRF origin are completely arbitrary. The only thing that really matters is the mutual relationship between the datum feature simulators.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca