Guide
Composite position tolerance or two single-segment frames: which should a hole pattern get?
Composite position tolerance is a two-tier feature control frame, with one position symbol spanning both rows, in which the upper row locates a hole pattern as a group to its datums and the tighter lower row controls the holes to each other and, through any datums it repeats, only the orientation of the pattern. Two single-segment frames look almost the same on paper, but each row is a complete requirement, so a datum repeated in the lower row controls location as well as orientation.
By Saola Manufacturing engineering team · Published · Updated
This guide is about the decision, not the symbol. It takes the most common case on a housing, a bolt pattern that must match a mating cover or motor flange, and shows what each callout lets the part do, where the two diverge on a measured part, how each is gauged, and what to write on the drawing and the RFQ so a supplier inspects the requirement you meant. The rules quoted follow ASME Y14.5-2018; the ISO route is covered near the end.
On this page
What problem does a composite position tolerance solve on a bolt pattern?
It separates two requirements that a single position frame forces to be equally tight: where the pattern sits on the part, and how accurately the holes sit relative to each other. Most bolt patterns need the second far more than the first.
Think of a four-hole pattern on a gearbox housing that takes a bolted cover. The cover only cares that its four clearance holes line up with the four tapped holes; if the whole pattern lands a little toward one edge of the housing, the cover simply lands there too. A single frame such as ⌀0.2(M) | A | B | C applied to all four holes would demand that tight accuracy for both questions at once, which means the supplier must hold the pattern to the housing edges as tightly as it holds hole spacing. That is the expensive half of the requirement, and on a die-cast housing whose edges are as-cast it may be the half that is hardest to hold.
A composite frame splits it. The upper row, the pattern-locating tolerance zone framework, carries the larger value and all the datums needed to fix the pattern on the part. The lower row, the feature-relating tolerance zone framework, carries the smaller value and controls spacing. Under ASME Y14.5 the lower row may repeat datums from the upper row, in the same order, but those repeated datums only control orientation; the lower framework is free to translate. The MMC bonus tolerance guide covers how each tier earns bonus as hole size departs from MMC, and the threaded-hole position guide covers how to size the spacing tolerance from the fastener.
When do composite position tolerance and two single-segment frames give different answers?
They give different answers only when the lower row repeats a secondary or tertiary datum. With only the primary datum repeated, the two callouts mean the same thing; once datum B appears in the lower row, the composite version controls only rotation to B, while the single-segment version also controls distance from B.
The table applies three upper-and-lower combinations to the same four-hole pattern and three simple measured errors, with every hole produced at its MMC size so that no bonus tolerance confuses the picture. A shift is a rigid move of the whole pattern; the rotation is about the pattern center, so all four corner holes sit the same distance from it and move by the same amount. The upper row passes in every case, so the verdict comes down to the lower row and how it is read.
Read the middle row carefully, because it is the one that catches buyers. A pattern that is perfectly spaced but sits slightly far from datum B passes the composite lower row, because translation is free, and fails the single-segment lower row, because a repeated datum in a single-segment frame locates. A drafting package that draws a symbol in each row when you meant one symbol spanning both has silently tightened the pattern's location to datum B, and to any later datum the lower row repeats, from the upper value to the spacing value.
| Measured error | Composite, lower ⌀0.2(M) | A | Composite, lower ⌀0.2(M) | A | B | Two single-segment, lower ⌀0.2(M) | A | B |
|---|---|---|---|
| Pattern shifted 0.3 mm parallel to datum B (toward C) | Pass | Pass | Pass |
| Pattern shifted 0.3 mm away from datum B | Pass | Pass | Fail (⌀0.6 mm deviation) |
| Pattern rotated 0.25° about its center | Pass | Fail (about ⌀0.44 mm at each hole) | Fail (about ⌀0.44 mm at each hole) |
Which callout should a housing bolt pattern get?
Start from what the mating part references. If the mating part only bolts to the pattern, use composite; if the pattern must also be centered on or set back from a functional feature, use two single-segment frames that reference that feature in the lower row.
Four design intents cover most housing bolt patterns:
- Cover bolts to the pattern, nothing else aligns. Composite, lower row referencing the primary datum only:
⌀1.0(M) | A | B | Cover⌀0.2(M) | A. The lower row keeps the holes perpendicular to the mounting face and spaced to each other; pattern location and clocking stay loose. - Pattern must also stay square to a machined edge, for example because the cover carries a label, connector or rib that must line up with a housing wall. Composite, lower row
| A | B |with B on that edge. Rotation is controlled, location is not. - Pattern must be centered on a pilot bore, as with a motor flange or bearing cap whose spigot locates in a bore while its bolts only clamp. Two single-segment frames, lower row
| A | B(M) |with B the bore. The pattern is located to the bore tightly and clocking is left to the upper row. A composite frame here would control only orientation to an axis that is already perpendicular to A, which controls nothing useful. - Pattern location and spacing matter equally, as with a pattern that lines up with a second part through a fixture or dowels elsewhere. Skip both and use one single frame at the tight value; a second row adds inspection work without loosening anything.
The rule of thumb from the table above: composite loosens location and keeps spacing; two single-segment frames keep location to the features you name in the lower row and loosen only what you leave out.
What are the common mistakes with composite position tolerance on drawings?
Most errors are structural rather than numerical: a frame drawn with the wrong number of symbols, a lower row that references datums the upper row does not, or a lower value that is not smaller than the upper. Each one changes what the supplier is allowed to accept.
- Two symbols where one was meant. A stacked frame is easy to build in CAD annotation as two independent frames rather than one composite frame. Check that one position symbol spans both rows if you intend composite.
- Lower-row datums out of order or new. Per ASME Y14.5, datums in a composite lower row must repeat those of the upper row in the same order of precedence, starting from the primary, so
| A | C |under| A | B | C |is not valid. A datum that appears only in the lower row is not valid either, and suppliers will either query it or guess. - Lower value not tighter than upper. The feature-relating zone must be smaller than the pattern-locating zone, otherwise the upper row is the only one doing work.
- Expecting datum C in the lower row to locate. In a composite lower row it can only orient, and with A and B already repeated it adds nothing. If you need location to C, you need single-segment frames.
- Forgetting multiple patterns. Upper rows of patterns that share datums, in the same order and with the same modifiers, form one simultaneous requirement by default, while composite lower rows are separate requirements unless the note SIM REQT is added. Two hole patterns that must both match one mating plate need that note.
- Leaving basic dimensions off. Every hole needs basic dimensions to the datums or to the other holes; without them, neither row has a true position to measure from. The GD&T symbols chart shows the frame layout, and the datums on cast parts guide covers picking A, B and C on a housing.
How are the two tiers inspected, and what should the inspection report show?
Each row is a separate check with its own alignment, so the report should show two results per hole, one against the pattern-locating zone and one against the feature-relating zone. A single positional value per hole means only one row was evaluated.
With fixed functional gauges, a composite frame takes two gauges, both designed on the principles of ASME Y14.43: one with datum simulators for A, B and C and pins at the upper-row virtual condition, and one with pins at the lower-row virtual condition that is oriented, but not located, by the datums the lower row repeats. On a coordinate measuring machine the same logic becomes two evaluations: the upper row fixed to the full datum reference frame, and the lower row best-fitted with the translations, and for a primary-only lower row the rotation, left free. Ask the supplier to state which degrees of freedom were released for the lower row; it is a common place for a measurement program to drift from the drawing.
The report format matters as much as the method. The ballooned drawing guide shows how to balloon each row separately so that a first article report cannot merge them, and the first article inspection guide covers what to require before releasing production.
How do you write the same intent on an ISO drawing?
ISO GPS has no composite frame. The equivalent is two stacked frames, with the lower one using a combined zone for the pattern and the orientation-constraint modifier on its datum so it orients without locating.
Under ISO 1101:2017 and the pattern rules of ISO 5458, the upper frame locates each hole to the datum system, and the lower frame adds CZ after the tolerance value so the holes are treated as one combined zone that may move as a unit. Putting the orientation-only modifier after the datum letter in that lower frame reproduces what a composite lower row does in ASME. If a drawing mixes the two systems, for instance an ASME-style composite frame on a print whose title block invokes ISO GPS, ask which standard governs before quoting; the ASME Y14.5 vs ISO GPS guide lists the other defaults that change with the standard.
What should the drawing and RFQ say when you send a composite position tolerance to a supplier?
Name the standard and edition, mark each row as its own inspection characteristic, and say which feature the mating part locates on. That last sentence is what lets a supplier question a callout that does not match the assembly.
A practical RFQ note for a housing pattern reads: “Dimensioning and tolerancing per ASME Y14.5-2018. Composite position on the 4X tapped holes: report upper and lower rows separately for each hole, with actual hole size, on the first article and on each lot record. Mating cover locates on the bolts only.” Send the cover drawing or a stack-up sketch if you have one, so the supplier can see why the lower value is what it is.
On our side, CNC milling is a second operation on castings: datum faces, bores, pockets and tapped holes held to geometric tolerance, and the milling page asks for one datum scheme used on both the casting drawing and the machining drawing so no second fixture has to be built just to reconcile them. Our first-article report covers every toleranced dimension on your drawing, a dimensional record ships with each lot, and a dimension you mark as a process-capability control point is watched through the run, as described on the quality page. We do not offer simultaneous five-axis machining. The drawing review returns a marked-up copy of your own drawing, so a lower-row value that looks out of reach is raised before tooling rather than at first article; send the drawing and the assembly note together.
Frequently asked
Can the lower row of a composite frame have no datums at all?
Yes. A lower row with no datum references controls only the spacing of the holes to each other, with the pattern free to translate and rotate in every direction, including tilting relative to the mounting face. On a bolt pattern that is rarely what you want, because perpendicularity to the face is what keeps a screw square; repeat datum A unless the holes are genuinely free to tilt.
Does bonus tolerance apply to both rows?
Each row earns bonus independently when it carries the MMC modifier, so a hole drilled toward its largest size gets the same added tolerance in both rows. The two rows also have different virtual conditions, which is why a functional gauge set needs two pin sizes.
Is composite profile the same idea?
The structure is the same, with an upper row locating a pattern of surfaces and a lower row controlling their form, size and relationship, and the same caution applies to repeated datums. It is common on cast outlines and is covered in the profile tolerance guide for castings.
Why not just tighten one single frame and stop worrying?
Because it pays for accuracy nobody uses. Holding a pattern tightly to as-cast or loosely machined edges usually forces an extra setup or a dedicated fixture, and it raises the rejection rate on parts that would have assembled. See the cost of tight tolerances guide for where that money goes.