gdt-explainer

AI skill that explains a GD&T feature control frame in plain English — what it controls, what the datum reference frame means physically, the tolerance zone shape and size (including bonus tolerance for MMC/LMC features), how to measure it, and the common gotchas. Uses ASME Y14.5-2018 vocabulary.

Use case

Younger machinists, programmers, and CMM operators see an FCF like ⌖ Ø.005 Ⓜ | A | B | C and freeze. The textbook explanation is in a 200-page standard nobody reads. This skill gives them the practical answer in one paragraph, with the specific math for the tolerance they're holding and the inspection method that proves it.

Also useful for: - QE writing inspection plans, who wants a sanity check on their interpretation - Sales people responding to customers asking about drawing requirements - Senior estimators flagging "this is non-trivial" callouts to the quote

Invocation

• Messaging: @gdt-explainer position 0.005 M to A B C on a Ø.250 hole
• WebChat: same @ prefix
• REST: POST /api/skill/run
• Drawing image: paste a screenshot of the FCF if the AI assistant supports vision — gives a much better answer for complex composite tolerances

What it explains well

• Position (with MMC/LMC bonus tolerance math)
• Profile of a surface (bilateral vs. unilateral)
• Orientation tolerances (perpendicularity, parallelism, angularity)
• Form tolerances (flatness, straightness, circularity, cylindricity)
• Runout (circular and total)
• Datum reference frame physical setup — what surface plate, fixture, or pins are needed
• Material modifier semantics (Ⓜ, Ⓛ, Ⓟ, Ⓕ)
• ASME Y14.5-2018 changes vs older revisions (e.g., concentricity deprecation)

Sample output

User: @gdt-explainer ⌖ Ø.005 M to A B C on a Ø.250 hole at MMC .249

Skill:

What it controls
  The position of the hole's axis relative to the datum reference frame
  A|B|C. The hole's center must lie within a Ø.005 cylindrical tolerance
  zone whose axis is perpendicular to datum A and located by basic dims
  to B and C.

Datum reference frame
  Primary A — typically the largest planar surface; constrains 3 DOF
  (one rotation, two translations into the plane). Physically: part sits
  on a surface plate.
  Secondary B — a planar surface perpendicular to A; constrains 2 more DOF.
  Physically: against a precision angle plate or fixed stop.
  Tertiary C — a single feature locating the third axis; constrains 1 DOF.
  Physically: against a single locator pin.

Tolerance zone
  Cylindrical, Ø.005 at MMC. The hole MMC is .249, so when the hole is
  produced at the MMC size (smallest hole), the available zone is .005.
  When the hole departs from MMC toward LMC (larger), bonus tolerance is
  earned 1:1:
       bonus = actual_size − .249
       effective zone = .005 + bonus
  Example: if the hole is measured at .251, bonus = .002, effective zone
  = .007. The hole's axis only has to be within Ø.007 of the basic
  position to pass.

How to measure
  CMM with a touch probe in the hole, after establishing the |A|B|C|
  datum frame. The CMM measures the hole's center coordinates and the
  hole's actual size. Software computes the cylindrical zone diameter
  the actual axis consumes and applies bonus from the measured size.
  Report value: the cylindrical zone diameter (not the X/Y deviation
  alone).

Common gotchas
  - Bonus tolerance is often forgotten. A hole that fails position at
    MMC may pass with bonus when the actual size is computed.
  - The basic dimensions defining the hole's nominal position are NOT
    measured — their job is to define where the zone is centered.
  - Datum order matters. |A|C|B| is different from |A|B|C|; the C-then-B
    order constrains rotation differently.

If this fails inspection
  Most common: hole drifted from nominal during the second-op fixturing.
  Re-verify fixture pickup before re-machining.

Strengths and weaknesses

Customization

• House conventions — if your shop has a "rules of thumb" doc on common inspection setups, add a section to SKILL.md so the skill defaults to your specific surface plates / fixtures / CMM probe sizes.
• ISO vs ASME — duplicate the skill as gdt-explainer-iso and update the vocabulary (e.g., "datum system" → "datum system" but with ISO 1101-2017 nuances) for shops doing international aerospace.
• Drawing image input — when used through the AI assistant with a vision-capable model (Claude Sonnet/Opus, GPT-4o, Gemini 2.5), pasting a screenshot of the FCF gives much better interpretation than typing it out.

Pairs with

• quote-estimator — flags tight GD&T as a complexity item; this skill explains what makes it tight
• spec-summary — for the standards referenced by the FCF (e.g., AMS, MIL-STD)
• coc-generator job — when the Form 3 row has a position tolerance with bonus, you may want to add this skill's output to the inspection plan as context

Source

See SKILL.md.