Composite Tolerance

this right?

to the book example:
the upper tolerance zone can't rotate or translate.
the lower tolerance zone can't rotate, but is allowed to translate (within the confines of the upper)
the actual feature can rotate and translate as long as it is within the confines of both tolerances

if the lower was to AB the smaller tolerance could rotate with respect to C (but needs to stay within confines of upper)

Book example says Rotate only for the lower segment. Book is 2009 version
I like to imagine a hard gage when I'm trying to understand a GD&T Position callout. Having a tough time with this one. Lack of imagination, maybe?

rotation constrained, can't rotate.

Composite tolerances are used to give leeway to manufacturing.

We could just have one segment with the tighter tolerance, but if the location and orientation relative to the mating part isn't so critical, we're tying manufacturing up to tighter tols than is actually necessary for the part to function thus adding cost to the design.

By having a composite tolerance we can say, the mating part must assemble with a particular type of fit (think limits and fits here, we don't want a loose fit on the mating part), however once it's assembled we don't mind if it's orientation (angle) to the mating part isn't quite as good.

Sorry for the fairly non-engineering scenario below but hopefully it will demonstrate the principal (I've used three segments to illustrate the options)


Imagine we were designing a floating shelf (say 1000mm long), so we have a bracket which is a steel strip, with two rods protruding from it near each end, this will be fixed to the wall (using a level to ensure it's horizontal).

The shelf is a thick plank of wood with two holes bored in it which locate on the rods.

Let's call the back edge A, the top face B, and one end C.

We need to ensure a decent fit on the rods, so the pitch, perpendicularity and diameter of the holes must be controlled fairly tightly. (This will be our lower segment)

However, if the location of the holes to the top face of the shelf isn't perfect it's not a major issue, we might have a bit of a slope on the shelf (left to right) but 2mm over the length of the shelf it will have a negligible effect. (This will be our middle segment)

Now the shelf is much thicker that the bracket and the holes aren't right at the end (so it will be hidden from view), so if the holes are 5mm up/down or left/right it's not an issue. (This is our top segment)

We might have a three segment control frame like this:

Ø5|A|B|C
Ø2|A|B
Ø0.5|A​

NinjaBadger

The OP's example has the same datums for each segment, yours does not (and as such, easier to understand/grasp).

True, but I was simply trying to explain the idea behind composite tolerances.

In the OP example it's simply saying that the orientation of the hole pattern to datum B us more important than the location.

So, our shelf has 10 mm holes in it and we've improved our drawing to include MMC for the features. We want hard gages for this because, while business is good, we can't yet afford a CMM

For the top segment, the gage would be 3 mutually perpendicular planes. In datum A there would be 2 pins with a diameter of 5 mm. Pins would be located as close as the toolmaker can place them to the holes basic dimensions.

Middle segment gage would have 2 perpendicular planes at -A- and -B-. Datum A would still have 2 pins at their Basic location, but now the diameter would be 8 mm.

Bottom segment gage is a single plane, again with two pins, but now the Pin diameter is 9.5 mm.

This setup makes sense to me.

However, I'm having trouble trying to imagine a hard gage that fulfills both segments of the callout in the original post. Gage would have to make contact with both -A- and -B- with a chuck to restrain -A- while holding the part against -B-. Assuming that the position should be at MMC, the Top segment pin size would be .001 smaller than the smallest allowable hole size. Bottom segment pin should be .0005 smaller than the smallest allowable hole size.

I don't see how the lower segment is refining the relationship of the holes.

Not trying to be difficult here, just trying to understand the logic.​

imagine the part in the OP is going into a "D" shaped slot. if the bolt pattern moves in X or Y no big deal as long as the slot has enough offset from the part, but once the bolt pattern starts to rotate the flat on your part will not be parallel to the flat on the slot and cause interference.

The datums don't have a Maximum Material Boundry so there is no datum shift. D shaped slot has to collapse around the part eliminating all movement except along the Z axis.