How to use this calculator
- Enter the deviations. Enter the measured ΔX and ΔY of the feature axis from its true (basic) location.
- Enter the upper PLTZF tolerance. Enter the larger upper-segment tolerance that locates the pattern to the datums.
- Enter the lower FRTZF tolerance. Enter the tighter lower-segment tolerance that controls feature-to-feature spacing and orientation.
- Read both verdicts. Read the true position and the PLTZF and FRTZF margins. The feature is acceptable only when both margins are non-negative.
How it works
A composite position callout stacks two tolerance zone
frameworks under one position symbol. The upper
PLTZF (pattern-locating tolerance zone framework) locates the
whole pattern to the datums with a larger tolerance; the lower
FRTZF (feature-relating tolerance zone framework) controls
feature-to-feature spacing and orientation with a tighter tolerance. The same
true position is checked against both:
TP = 2 · √(ΔX² + ΔY²)
where ΔX and ΔY are the deviations of the measured
axis from its basic location. Because position tolerance is a cylindrical
(diametral) zone, the radial deviation is doubled.
Each tier reports a margin of tolerance − true position:
a non-negative margin passes that tier. The feature is acceptable only when it
passes both — within the upper PLTZF and within the
lower FRTZF.
Worked example
Verified against the live calculator
A feature is measured 0.1 mm off in X and 0.1 mm off in Y, so its true position is
2 × √(0.1² + 0.1²) ≈ 0.283 mm. Against a 0.5 mm PLTZF the margin is
+0.217 mm — the pattern is comfortably located to the datums. But
against the tighter 0.2 mm FRTZF the margin is −0.083 mm, so the
feature fails the feature-relating tier: it sits where the pattern
is allowed to be, but not as tightly spaced to its neighbours as the lower segment
requires. The calculator shows exactly this.
Frequently asked questions
What is composite position tolerance?
Composite position tolerance is a single position callout with two stacked tiers: an upper pattern-locating tolerance zone framework (PLTZF) that locates the whole feature pattern to the datums with a larger tolerance, and a lower feature-relating tolerance zone framework (FRTZF) that controls how the features sit relative to each other with a tighter tolerance. A feature must satisfy both tiers.
What is the difference between PLTZF and FRTZF?
The PLTZF (upper segment) is the pattern-locating tolerance zone framework — it locates the entire pattern to the datums, and carries the larger tolerance. The FRTZF (lower segment) is the feature-relating tolerance zone framework — it is tighter and controls only the feature-to-feature spacing and orientation within the pattern.
Why does a composite callout use two tiers?
It lets you locate the pattern loosely to the datums while holding the features tightly to one another. Mating parts often care more about how the holes line up with each other than about exactly where the group sits, so the upper PLTZF can be generous while the lower FRTZF stays tight.
Does the lower (FRTZF) segment control location to the datums?
No. The lower segment never controls location to the datums — only orientation and feature-to-feature spacing. Any datums repeated in the lower frame refine orientation (parallelism, perpendicularity) of the zone framework only; the upper PLTZF remains the sole control of where the pattern is located.
How do I read a composite feature control frame?
A composite frame shares one position symbol for both rows. The top row gives the PLTZF: the larger tolerance plus the primary, secondary and tertiary datums that locate the pattern. The bottom row gives the FRTZF: the tighter tolerance plus, at most, the datums that orient the feature-relating zones — never enough to relocate them.
Does this work in metric and imperial?
Yes — toggle SI/imperial anytime. Deviations and tolerances convert; the math runs the same per ASME Y14.5 composite positional tolerancing.
Method & assumptions
- Two-dimensional (X-Y) position from the measured deviation, per ASME Y14.5 composite positional tolerancing.
- The lower (FRTZF) segment controls feature-to-feature orientation and spacing only — never location to the datums; datums in the lower frame refine orientation alone.
- One feature is evaluated against both tiers; the upper PLTZF and lower FRTZF tolerances come straight from the two rows of the composite feature control frame.