COMPOSITE FRAME AND CONCENTRICITY 1

[SMCC]

I'M TRYING TO UNDERSTAND THE MEANING OF THIS COMPOSITE FRAME. TO ME IT LOOKS LIKE THE 2ND FRAMES CONTENT IS TO TOLERANCE FOR CONCENTRICITY.
I DO NOT FULLY UNDERSTAND HOW THIS FRAME MIGHT BE DOING THAT. DOES THIS MEAN THE HOLES HAVE TO BE 1 THOU WITHIN TORANCE OF EACH OTHER? THE BOTTOM OF THE PART IS DATUM A. DATUM C IS ORTHOGANOL TO B AND ON THE SURFACE OF THE FACE CONTAINED BY THE RECTANGLE. THANK YOU.

 

[3DDave]

First: The lower segment of a composite frame is required to duplicate the upper segment datum feature references from left to right until all the necessary ones are selected.

A|B|C -> A or A|B, for example; it cannot be A|B|C -> C as shown.

Second: it isn't only controlling concentricity, it is controlling mutual location relative to the lower segment datum references. In this case, that happens to control concentricity for the pairs of holes.

Third: the diameter 0.002 tolerance applies at the smallest hole diameter and may apply depending on hole shape at larger manufactured holes. You can gain up to an additional diameter of 2* 0.0005 of location tolerance, for what that may be worth, if the hole is cylindrical and at the maximum material size.

The matching gauge for the lower segment is two pins of diameter .8745 - diameter 0.002, or diameter .8725 that are perfectly parallel, separated by the basic dimension between the holes, and oriented to whatever the correct datum feature that should be in the lower segment.

(If you have a spelling checker it likely skipped the misspelling of orthogonal due to all-caps. )

 

[SMCC]

Thank you. This does clarify the second frames intent. Does your first statement come from the most recent Y14.5 standard. The frame comes from a reputable source unlikely to make mistakes like this, but it is possible. From my new understanding, I believe the holes can translate within the tolerance of the first frame, but it has to be perpendicular to datum C within the second frames tolerance. This would make sense if a shaft was going to run through the coaxial holes. It would be great if you have any objective evidence to support your first statement otherwise, I will try to find it in Y14.5. FYI this is not the part I'm working on only a simple example.

 

[3DDave]

A composite tolerance reflects a relaxation of the upper segment pattern location requirements while refining the orientation requirement. If a different datum reference order is in the lower, then that establishes a new and different orientation for the datum reference frame, which undoes the point of using a composite tolerance.

You could have a separate single segment oriented to C alone.

Per -2018

(3) When datum feature references are specified,
one or more of the datum feature references specified
in the upper segment of the frame are repeated, as applicable,
and in the same order of precedence, to constrain
rotation of the FRTZF. In some instances, the repeated
datum feature references may not constrain any
degrees of freedom; however, they are necessary to maintain
the identical datum reference frame, such as datum
feature B in the lower segment in Figure 10-43.