What does it mean to rotate with respect to a datum plane (in your example, datums A and/or B)? In math/physics, rotation is defined with respect to an axis -- not a plane. So when the FRTZF tolerance refers to datum A, for example, what rotation is being controlled? Is it rotation with respect to an axis *normal* to datum plane A? And in your first composite tolerance example, the FRTZF tolerance refers to datums A & B. What rotation is being controlled here?
If you think about a plane having a normal vector, then you can rotate around that vector. Notice in the example that the "A plane" is the surface behind the piece so rotating the way he showed here is rotating on that plane
I believe the wording used around rotating about a datum here is a consequence of what the datum reference actually does. Referencing datum A controls the perpendicularity of the axes of the holes w/respect to A. Referencing datum B controls the parallelism of the pattern of axes with datum B. When deviating from those controls you can think of the axes rotating.
The location of Datum C on the drawing actually needs to be shifted over to be in line with the dimension. Otherwise, it's implying the lower surface of the slot is actually Datum C. But these videos are very helpful nonetheless!!
thank you “Composit Position” appears to be meaningless when the part is completely fixed in the assembled state and cannot move or rotate at all. So, is “composit position” used when parts can move or rotate slightly in the assembled state? I am curious in what cases “composit position” is applied to part drawings.
The lower tier of the composite tolerance is a refinement of the tolerance on the upper tier. This idea of refinement appears often in GD&T. A diameter tolerance controls size and form. But you might need to control form more tightly independent of the size, so a circularity or cylindricity tolerance is applied. A position tolerance will control orientation. But you might need to control orientation more tightly independent of the position, so a perpendicularity/parallelism/angularity tolerance is applied. A position tolerance applied to a pattern of holes will control their location with respect to a datum reference frame and consequently, the position of the holes with respect to each other. But you may need to control the position interrelationships among the holes more tightly independent of the position of the pattern, so a composite tolerance is used. For example, the holes in a piece of paper need to be located precisely enough that the paper can be placed in a 3-ring binder. The placement of that pattern of holes on the page can be much looser and the binder will still function.
We are using CheckMate for SOLIDWORKS Geometric Tolerancing Manager. It can be used as an ADDIN to SOLIDWORKS or as a standalone application for some CMM’s reports. info.originintl.com/resources/video-library-rptg
1) If B & C are the secondary and tertiary datums, there have to be some co- ordinate basic dimensions indicating the desired locations. 2) If any part is under inspection and if every coordinate location is found to have some departure, then how to divide the observed error into two divisions, one allocated to the upper part of the feature control frame and the other allocated to the lower ? Please clarify. Thanks.
I also have a problem with this. If a smaller pt( positional tolerance) is applied same features ( holes) referencing datums A and B ( not C) then to me, it's the same as references to ABC but just less the datum C - I can't see how by not referencing the C datum, it gives you more freedom in the B datum direction. Referencing datum B still allows a rotation within the zones of pt tolerance and because it's a square pattern, it's permitted rotation is the same if also or not, referencing datum C. I understand it with only datum A referenced - provided there is no basic dim from an edge positions in X, Y directions with PT's only between the 4 holes only .
There are 2 types of framework in composite tolerance. The upper one is called Pattern-Locating Tolerance Zone Framework (PLTZF), it constrained both translational and rotational degrees of freedom. Just like the 1.5 tolerance in this video. The lower frameworks are called Feature-Relating Tolerance Zone Framework (FRTZF), it only constrained rotational DOF relative to any referenced datum features. Therefore, the tolerance zone of 0.5 can be translate away from the true position. The definition of both PLTZF and FRTZF are different, so we don't need to divide the error into two divisions.
