14.3 Functional Gaging and Verification

14.3 Functional Gaging and Verification

Quick Answer: A functional GO gage is a physical fixture built at the virtual condition (VC) of the controlled feature(s). For a hole pattern controlled at MMC, the gage has pins sized at VC = hole MMC − position tol, located at the basic true positions. If the part seats on the gage, every controlled hole is within its MMC position tolerance — assembly-worst-case is verified in one pass.

How a position VC gage works

For a hole pattern with position Øt at MMC:

1. Compute each hole's VC = MMC − t (internal feature).
2. Build gage pins at ØVC, one per hole, located at the basic true positions from the datum reference frame.
3. Add a datum simulator for each datum: planar datums as flat blocks, datum holes at their MMB (creating datum shift in the gage matching the drawing).
4. The part is offered to the gage. If it seats — all pins engage — the part passes. If any pin fails to enter its hole, the part fails.

Because the gage pin is at VC, it represents the worst-case mating pin the part will ever see. A part that assembles to the gage will assemble to its mate.

Why bonus is automatic

The gage pin is fixed at VC. As the actual hole grows larger than MMC (departs from MMC), the extra clearance lets the hole accept the pin even with more position error. The gage never computes a bonus — it just physically exploits the extra clearance. This is why MMC position and functional gaging are inseparable: the gage is the physical embodiment of the VC plus bonus logic.

NO-GO gages and the resultant side

A GO gage verifies the VC (assembly) boundary. A NO-GO gage checks the opposite side:

• For an individual hole, a NO-GO pin at LMC (largest hole) verifies the hole has not exceeded its largest allowed size — a size check, not a position check.
• For minimum wall or edge distance, a NO-GO fixture at the resultant condition (RC) verifies the LMC-side boundary has not been violated.

NO-GO checks are typically separate from the position GO gage. The position GO gage does NOT guarantee minimum wall; the wall must be checked separately with its own fixture or by CMM measurement against RC.

Gage design walkthrough

A plate has four Ø10 ± 0.2 holes positioned Ø0.1 at MMC to A|B(M)|C, where datum B is Ø8 ± 0.1 at MMB.

The part is placed on datum A, located on datum B's Ø7.9 pin, and stopped on datum C. The four Ø9.7 hole pins then enter the four holes. If all engage, the part passes assembly-worst-case.

Note the datum B simulator is at MMB (Ø7.9), not at the actual datum size — this builds datum shift into the gage. A part whose datum B hole is larger than Ø7.9 will have play on the datum pin, allowing the part to shift on the gage exactly as the drawing allows. The gage thus reproduces both the controlled feature's VC and the datum's MMB behavior.

When functional gaging is appropriate vs CMM

Limitations and traps

1. Pass/fail only. A gage does not tell you how good a part is, only whether it passes. SPC requires measurement.
2. Wear. Gage pins wear smaller over time, increasing acceptance; periodic certification and wear allowance are required (ASME Y14.43).
3. Gage tolerance direction. GO gages are toleranced to NOT accept bad parts (wear toward smaller pins for hole gages); NO-GO gages the opposite. Getting this backward is a Senior trap.
4. Cannot verify RFS. A functional gage built at VC only works for MMC design intent. RFS position has no constant boundary — the gage would have to be at actual size, which is impossible for a hard gage.
5. Cannot verify LMC-only requirements. A minimum-wall or LMC-controlled feature must be checked separately against RC, not by the VC GO gage.
6. Composite position. A gage for the lower (FRTZF) segment only checks feature-to-feature; a separate gage for the upper (PLTZF) segment checks pattern location. The two are NOT interchangeable.
7. Simultaneous requirements. Multiple patterns sharing the same DRF are a simultaneous requirement and must be gaged together on one fixture; gaging them separately rejects good parts.