Composite position: locating a pattern and relating its features

A bolt pattern usually has two requirements that are not equally tight. The holes have to agree with one another closely enough for the fasteners to go in, and the group as a whole has to sit on the part accurately enough for the joint to work. Those are different numbers, and a composite position frame states both at once. Splitting the frame into two ordinary frames looks like the same drawing and is not.

Two tiers, one symbol

A composite frame shows the position symbol once in a tall compartment shared by both rows. The upper tier is the pattern locating tolerance and uses the full datum reference frame. The lower tier is the feature relating tolerance: a tighter set of zones, fixed to each other by the basic dimensions of the pattern.

That single shared symbol is the only visual difference from two stacked frames, and it is the difference that matters.

What the lower tier is free to do

The tighter group of zones may translate as a unit within the upper tier zones, while staying oriented to whichever datums the lower tier repeats. Repeating a datum keeps the group square or parallel to it; repeating every datum would pin the group in place and turn the frame into an ordinary tighter position tolerance.

Choosing which datums to repeat is a functional decision about what has to stay aligned, not a drafting convention.

Both tiers apply

The tiers are not alternatives. Every hole must lie inside its upper tier zone, located in the full frame, and inside its lower tier zone within the group wherever the group has been placed. A part that passes one and fails the other is rejected.

An inspection report that quotes a single position value per hole cannot answer a composite frame. It needs both verdicts.

Bonus, and the split frame trap

When both tiers are stated at MMC, each hole earns one bonus from its own produced size and that bonus widens both of its zones. Nothing about the composite structure changes how bonus is computed.

Replacing the composite frame with two separate single segment frames of the same values tightens the requirement, because the lower frame then locates the holes within the datums it names instead of only orienting them. Good parts become scrap with nothing visibly changed on the drawing except one shared compartment being split in two.

A hole in a composite pattern at MMC

The frame states 0.80 mm to A, B, C above and 0.25 mm to A below, both at MMC. A hole toleranced diameter 6.00 to 6.20 mm is produced at 6.09 mm.

Departure from MMC: 0.090 mm
Upper tier allowance for this hole: 0.890 mm
Lower tier allowance for this hole: 0.340 mm
Both must be satisfied: Yes

One bonus, computed once from the produced size, applied to both zones. A report should show the hole against both allowances, because the tighter one is usually the one that fails.

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Frequently asked questions

What does each tier of a composite position frame control?

The upper tier locates the pattern within the full datum reference frame. The lower tier controls the features relative to one another, with the tighter group of zones free to translate within the upper zones while staying oriented to the datums it names.

How is a composite frame distinguished from two stacked frames?

By the position symbol appearing once in a tall compartment shared by both rows. Two separate frames each carry their own symbol and are two independent requirements.

Must a hole satisfy both tiers?

Yes. The tiers are not alternatives, and a hole that passes one and fails the other is rejected. An inspection report needs a verdict against each tier.

Why does the lower tier repeat only some of the datums?

So the tighter zones stay oriented to those datums while remaining free to translate as a group. Repeating every datum would remove that freedom and defeat the purpose of the composite frame.

How does bonus tolerance work across the two tiers?

Each hole earns a single bonus from its own produced size, and that bonus widens both of its zones. The composite structure changes how many zones the bonus applies to, not how it is computed.