GD&T — validation & limitations

What it calculates

• True position (ASME Y14.5) with cylindrical tolerance zone — instant PASS/FAIL against the drawing callout
• MMC and LMC bonus tolerance: automatic bonus calculation for holes and external features (shafts/pins)
• Flatness check from up to 20 CMM or height-gauge points, with per-point deviation bar chart
• Circularity from radii and cylindricity screening from multi-section two-point diameters, with radar chart — FAIL only when proven, otherwise NOT PROVEN
• Circular and total runout (TIR) — FAIL when a reading exceeds the tolerance; within it, NOT PROVEN (one section, or per-station readings)
• Perpendicularity and parallelism deviation with remaining tolerance readout
• Bonus tolerance panel with virtual condition (VC) and feature size range check

Standards this tool applies

• ASME Y14.5 — ASME Y14.5 evaluation of true position with MMC bonus, an MMC / LMC bonus panel with virtual condition, plus flatness, circularity, cylindricity, circular and total runout, perpendicularity and parallelism — eight of the twelve geometric characteristics. Straightness, angularity and both profile controls are not implemented, and the ISO 14253-1 guardband decision lives in the API, not on the page.
• ISO 14253-1 — ASME Y14.5 / ISO 1101 geometric tolerance evaluation on the calculator page. The ISO 14253-1 U95 guardband decision (Accept / Inconclusive / Reject) is NOT a panel on that page — it is implemented in the MechanixCalc API at /api/v1/gdt, request type "measurement_uncertainty".

Validation evidence — independent reference cases (2)

Each case runs the tool's engine on a worked example whose values come from a published source or a hand derivation from the cited equations, and an automated regression test asserts the engine against those values within the stated tolerance. "Conservative" means the engine is known to sit on the safe side of the reference and the test asserts that side. Sources are cited; their text is not reproduced.

• Conformance decision with measurement uncertainty

  Agrees

  Source: ISO 14253-1:2017 §5 default decision rule (conformity inside spec zone reduced by U; nonconformity outside spec zone expanded by U)

  Inputs: nominal 25.000 mm, tolerance band 0.100 mm (±0.050), U95 0.008 mm; readings 24.945 to 25.060 mm, bias 0.002 mm case

  Quantity	Reference value	Tolerance
  Conformance zone	24.958 to 25.042 mm	1e-9 mm absolute
  Rejection limits	below 24.942 / above 25.058 mm	1e-9 mm absolute
  Decisions at 25.040 / 25.050 / 25.055 / 25.060 mm	Accept / Inconclusive / Inconclusive / Reject	exact
  Bias-corrected reading (25.043, bias +0.002)	25.041 mm (Accept)	1e-9 mm absolute

  tests/golden/REF-gdt-iso14253-decision.golden.test.ts

• True position with MMC bonus, virtual condition, LMC variant

  Agrees

  Source: ASME Y14.5-2018 (identical in Y14.5M-1994 / -2009): diametral position zone, MMC/LMC bonus tolerance, virtual condition VC = MMC ∓ t, Rule #1

  Inputs: hole Ø14.00 +0.25/0, position Ø0.20 at MMC, centre offset dx 0.12, dy 0.09 mm; pin Ø13.75 0/-0.25; LMC variant at actual 14.10

  Quantity	Reference value	Tolerance
  Position deviation Ø	0.30 mm	1e-9 mm absolute
  Zone at actual 14.00 / 14.05 / 14.10 / 14.25	0.20 / 0.25 / 0.30 / 0.45 mm	1e-9 mm absolute
  Virtual condition (hole MMC / pin MMC)	13.80 / 13.95 mm	1e-9 mm absolute
  Verdicts at 14.00 / 14.05 / 14.10 / 14.25	FAIL / FAIL / PASS / PASS	exact

  tests/golden/REF-gdt-position-mmc-bonus.golden.test.ts

Limitations

Not checked by this tool

• Datum feature shift / mobility and datum establishment — Datums are assumed perfect; evaluate datum shift per ASME Y14.5 / ISO 5459 or on a CMM
• Profile of a line or a surface — No profile evaluation; use a CMM report against the CAD model
• Angularity, straightness, symmetry and coaxiality — Listed in the reference only; evaluate them with a CMM or a dedicated gauge
• Composite position tolerancing (pattern-locating and feature-relating zones) — Single-segment position only; evaluate each segment separately per ASME Y14.5
• Minimum-zone fitting of form from coordinate data — Form uses the range of the entered readings (no plane or axis fit); use a CMM minimum-zone evaluation