True Position (Bolt Hole Circle)


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I am seeking some help on the forum's understanding on how to check this. Inspection department is checking the true position from hole to hole as it looks to be called out on the print since the FCF is located at the diameter tolerance. However, the part is slightly warping during manufacturing and the manufacturing manager is asking if we could check this based on the bolt circle rather than the 3x .4380 holes individually. What is Elsmar Coves thoughts?

True Position (Bolt Hole Circle)


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As I am reading the print, the .438 has a true position of .015 to Datums B and A AND all three holes are required to be within .001 TP of each other. Both cases includes across the entirety of the .25 thickness of the part (unless the part is heavily warped, the thickness should not affect the part too much).

If I understood your question correctly, you have to measure it both ways (individually and as a group).


Involved In Discussions
Assuming that by slightly warping you mean that Datum B does not meet the .005 flatness requirement, then the part doesn't meet specifications anyway. Is there a restrained condition note on the print?

Also, if you are going by the bolt circle diameter the angle between the holes is not being taken into account.

Paul F. Jackson

Quite Involved in Discussions
Assumptions: The print is to the ANSI Y14.5 1982 Standard ( (S) is specified rather than assumed by ASME Y14.5M 1994 and subsequent)
Surface B is the Primary datum feature (literally for the composite feature control frame and practically for the other two FCFs)
The 3X DIA .438 hole pattern is the Secondary datum feature (removing all remaining degrees of freedom)
Note: Composite feature control frames were redefined in the ASME Y14.5M 1994 standard by limiting the control of degrees of freedom in the lower segments to (translation and rotation among the pattern features) but (only rotation to specified datum features). However since this drawing appears to be prior to 1994 the composite control can be interpreted same as two single segments.

Two datum registry setups on the CMM are required for inspection according to the "existing specification"
1) Level plane (stop 2 rotations, 1 translation) on surface -B-, Move reference (stop 2 translations) on center of diameter -A-, probe all three diameters -C- and best fit rotation among the pattern (stop 1 remaining rotation) and check the axis of each hole to its 0.015 cylindrical tolerance.

2) Level plane (stop 2 rotations, 1 translation) on surface -B-, Probe all three diameters -C- and best fit rotation and translation among the pattern (stopping 1 remaining rotation and 2 remaining rotations) Then check the axis of each hole to its 0.001 cylindrical tolerance and check all remaining feature control frames (but make a note on the inspection report for the two FCFs that the datum features are specified [C(S)|B] were inspected [B|C(S)].


No expert, but this is how I see it...

1. It is called out as 3x meaning each individual hole must satisfy the composite feature control frame.

2. Each hole must be a distance of 2.2677 from datum A center hole.

3. The bottom FCF is actually perpendicularity. Each hole must be perpendicular to B.

4. The drawing does not call it out as a bolt circle. See point 1.


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No expert, but this is how I see it...
2. Each hole must be a distance of 2.2677 from datum A center hole.

Two minor things to change this statement: Each hole is 1/2 the distance of 2.2677 from the center hole as the 2.2677 is the defined bolt circle AND two of the holes must be within the geometric tolerance of 120° as well. Note, one of the holes is defining datum -C- for all other angles.
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