SC QUALITY GUILDPosition tolerance and bonus calculator
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Judge each measured hole or pin against its feature control frame: size first, then position, with the bonus tolerance the material condition modifier allows.
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Q-BotClick for an explanation of this tool.For educational use. This page is a study aid. The method behind it has far more depth and detail than one page and one tool can cover, and what you see here is not everything the method offers or everything this site offers. For full understanding and application, use the governing standards and texts, and seek further education, training or qualified advice. Results depend on the data you enter, so check them before relying on them.
How position tolerance works
Under ASME Y14.5, a position tolerance controls where the axis of a feature of size may lie. The true position is set by basic dimensions from the datum reference frame. When the tolerance is preceded by the diameter symbol, the tolerance zone is a cylinder centered on the true position, and the axis must fall inside it.
Inspection reports the deviation in X and Y from true position. Because the zone is a diameter, the position value is twice the radial distance: position = 2 × √(ΔX² + ΔY²). A hole 0.06 off in X and 0.05 off in Y has a position of 0.156, not 0.078, and not 0.11.
Material condition modifiers and bonus tolerance
- MMC (circled M): the stated tolerance applies when the feature is at maximum material condition, the smallest hole or the largest pin. As the actual size departs from MMC, the difference is added as bonus tolerance: bonus = |actual size − MMC size|.
- LMC (circled L): the stated tolerance applies at least material condition, the largest hole or smallest pin, and the bonus grows as the feature moves toward MMC. It protects wall thickness and edge distance.
- RFS (no modifier): under the 2018 standard, regardless of feature size is the default when no modifier is shown. There is no bonus.
The bonus exists only for a feature that is within its size limits. A hole below its minimum size is rejected on size, whatever its position.
Virtual condition
At MMC, the worst-case boundary the mating part sees is the virtual condition: MMC size minus the position tolerance for a hole, MMC size plus the position tolerance for a pin. A functional gauge pin made at the virtual condition size and fixed at true position accepts every hole that meets both size and position at MMC.
Reading the result
Each row is judged in order: actual size inside the limits, then position against the allowed diameter (stated tolerance plus bonus). The plot shows each axis location; the gold zone is the stated tolerance and the dashed zone is the most the bonus can give. A point between the two passes only if its own size earns enough bonus.
On the CQI exam
Expect a feature control frame, a set of actual sizes and coordinate deviations. The common traps are using the radial distance instead of the diameter, adding bonus to a feature that is out of size, measuring bonus from the wrong limit (LMC instead of MMC), and forgetting that RFS gives no bonus.