DatumLab
ASME Y14.5-2018, Inspection

From a drawing requirement to a measurement that answers it

A tolerance is only as good as the measurement that answers it, and the same drawing can be checked in two ways that give different numbers if the setup is wrong. This guide covers the practical bridge: how a gauge is sized, what a coordinate measuring machine has to establish before it measures anything, and which numbers a report needs so that a verdict can be re-examined later.

Sizing a functional gauge

A functional gauge realises the virtual condition as a physical boundary. The gauge pin for a hole is made at the virtual condition size and held at true position in the datum reference frame, so a hole that receives the pin has satisfied size and position together.

Gauge tolerance and wear allowance are then taken out of the part tolerance, so the gauge accepts only good parts and may reject marginal ones. The cost is a few unnecessary re-inspections, which is the cheap failure mode.

Why MMC gauges and RFS does not

A gauge is a physical boundary, so the boundary has to be the same for every part. The MMC modifier provides exactly that. Under RFS the acceptance limit is the stated tolerance at every produced size, with no constant boundary to embody.

An RFS requirement is therefore normally verified by measuring the produced size and the location and computing the result, which is a variable measurement rather than a go and no go one.

Setting up before measuring

On a coordinate measuring machine the datum reference frame comes first, built from the datum features in the precedence order the drawing states. Every coordinate reported afterwards depends on where that frame was placed.

Best fitting to the part instead of following the drawing produces numbers that look precise and answer a different question. So does building the frame in the wrong order, which on a part with out of square surfaces moves every value.

What the report has to carry

For any feature toleranced at a material condition, the report needs the produced size as well as the location, because the allowance depends on the size. Without it the verdict cannot be re-judged and the failure cannot be attributed to location or to size.

Unless the drawing says otherwise, dimensions apply at a standard reference temperature of twenty degrees Celsius with the part free of applied forces other than gravity. On a long aluminium part a few degrees is worth more than the tolerance.

Sizing a pin gauge from the frame

A hole with MMC 6.00 mm carries position diameter 0.15 mm at MMC to A, B, C.

MMC size
6.000 mm
Stated position tolerance
0.150 mm
Virtual condition, and the nominal gauge pin
5.850 mm
Direction of gauge tolerance
Into the part tolerance

A hole that admits this pin, with the gauge seated on the datum features in the stated order, has passed size and position in one operation. What the gauge cannot say is how much of the zone the hole actually used.

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Frequently asked questions

How is a functional gauge pin sized?

At the virtual condition of the feature, and located at true position in the datum reference frame. For a hole with MMC 6.00 mm and position diameter 0.15 mm at MMC that is 5.85 mm, before gauge tolerance is applied.

Why can a fixed gauge not check an RFS requirement?

Because there is no constant boundary to embody. Under regardless of feature size the stated tolerance applies at every produced size, so the check has to be computed from a measured size and a measured location.

What should a position inspection report contain?

The measured true position, the produced size, and the total allowance the feature was judged against. Without the size, a result under a material condition modifier cannot be re-judged later.

What is a paper gauge analysis for?

For finding a single displacement of the pattern that brings every feature inside its zone at once, which is what a functional gauge does mechanically. It is the correct way to apply available datum shift to a pattern.

At what temperature do dimensions apply?

At a standard reference temperature of twenty degrees Celsius unless the drawing states otherwise, with the part free of applied forces other than gravity. A warm part measured against a cold gauge produces disagreements that re-measuring does not settle.