Position Tolerance - Flat datum, and two circular datums.. What is this control frame saying ?

[Radius1]

This is just a concept drawing I created to show the example. I have a hole with a position tolerance of 1 ref to datum a, b and c. and I am confused.

Does this mean that the hole can vary 1mm from datum a (easy to see) , and the same for datum b and c ?

I am struggling to see how for b and c.. Are the datums just to fix the part on the CMM table so the hole can be measured ? Or does the control frame mean the hole can vary 1mm from a, b and c



Thank you.


Position Tolerance - Flat datum, and two circular datums.. How is this measured?

 

[3DDave]

Picturing the fixture for this:

A flat plate that represents datum feature A.

Perpendicular to that plate a pin that represents the Maximum Material Boundary (smallest hole) for datum feature B
Perpendicular to that plate a pin that represents the Maximum Material Boundary (smallest hole) for datum feature C

The pins are located as exactly as possible to their relative dimensions.

Then a pin that fits in the hole in question and is located and oriented exactly to [A|B(M)|C(M)] and is exactly 1 mm Smaller in diameter than the smallest (Maximum Material Condition) hole.

If the hole isn't oversize, undersize, and the pin fits in it the part is acceptable.

There is no movement of the center of that inspection pin allowed relative to [A|B(M)|C(M)], though because the datum features for B and C might have clearance with their respective pins, the part might be wiggled about to get the inspected feature to fit its pin. This wiggling about is called "datum shift." It's really shifting the part relative to the datum in the case of a gauge.

 

[Belanger]

The 1 mm tolerance can be gobbled up in any combination of directions. So it might still be the correct distance from datum A, but move left/right within the 1 mm (thus using the tolerance, but relative to B and C).
It can't use 1 mm relative to datum A and at the same time use 1 mm relative to datum B; that would place the location outside of the cylindrical zone of 1 mm. In reality, it will be a combination of errors in different directions, but it's only documented as one cylindrical tolerance.
Caveat: the 1 mm I'm referring to could grow because of the "M" modifier, but for now I'm just calling it 1 mm.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[Burunduk]

I am struggling to see how for b and c.. Are the datums just to fix the part on the CMM table so the hole can be measured ? Or does the control frame mean the hole can vary 1mm from a, b and c

They (datum features B and C) are used to fix the part on the CMM table indeed, but it's not just that - there is much more to it. You need all three datum references to define the required orientation and location of the hole in 3 dimensional space. Let's assume you removed datum references B and C from the position feature control frame (FCF) and only referenced datum A. The interaction between datum feature A (the part surface) and datum feature simulator A (a surface plate) constrains 3 degrees of freedom - the translation in Z direction, the rotation about X and the rotation about Y. This means that when inspecting the position tolerance with reference to A only, three degrees of freedom remain open - you can rotate the part about Z, and move it in the X and Y directions. The only requirement from the axis of the hole is to fit into a 1 mm diameter (or larger per the "bonus") tolerance zone which is located at basic 10 mm above datum plane A. This is an incomplete definition of the hole location and orientation - it can be anywhere on the part as long as it is parallel to and located at the basic height from datum A, within the error limit allowed by the tolerance zone. Now consider that the positional FCF referenced only A primary and B(M) secondary. In addition to the degrees of freedom already constrained by A as described above, the interaction between datum feature hole B and its datum feature simulator (a fixed size pin perpendicular to the surface plate) would constrain additional two degrees of freedom - the translations in the X and Y directions. Now the part also can't move in directions X and Y (except for the limited amount of movement allowed by datum shift related to the "M" modifier on datum reference B), therefore the hole axis is required to fit a tolerance zone which is at a fixed basic height above datum A and parallel to it like in the previous case, but also with a particular offset from datum B as set by a basic dimension. But you could still rotate the part 360° around datum axis B while inspecting it, therefore the actual hole can also be anywhere around datum B as long as the basic distance is maintained within tolerance. To lock the remaining degree of freedom and fully define the hole's orientation and location in space, the tertiary datum reference C is added to the FCF. The simulator of datum feature C (another fixed size pin perpendicular to the surface plate and at a fixed distance from the pin used as datum feature B simulator) will lock the rotation about Z (with some limited amount of rotation remaining available to the "M" modifier on datum reference C).