GD&T Functional gauge

What is a functional gauge in measurement and verification of manufactured parts and how does it relate to GD&T

Functional gauging is an inspection method of manufactured parts which unlike variable gauges doesn’t use measurement scales but just a check of assembly.

A Functional gauge is a device which is used to verify whether a part which has been manufactured is within specifications as mentioned in the drawing.

For example, if we have a pin as shown which is designed for a function which is assembly

What condition will ensure that assembly of this pin to its counterpart(hole) will always takes place.

The condition is that pin should be below 20.5 in its size.

So if we have a gauge block with a hole at 20.5 (gauge tolerance)

If every manufactured pin passes through the gauge block with a 20.5 mm hole then we can say all manufactured pins are within the specification required for assembly.

This gauge block is called as a GO gauge.
If the Pin goes through its acceptable .
This GO gauge is called as the simplest functional gauge.

Important to note that the gauge implies that the Pin when manufactured at 20.5 dia mm should have perfect form to pass through and be verified as acceptable.

This is nothing but Taylors principle or Rule #1 in GD&T .

Functional gauges can also be used to verify position of features of size on components .

Here is an example:

The functional gauge for position control feature control frame applied here looks like this:

The workpiece/component is brought in contact with the gauge blocks which are datum feature simulators for Datum A,B,C . Then a pin is inserted , the pin should assemble with the component which verified the component to be acceptable .

Size of the pin = Virtual condition which in this case is

Maximum material condition size of hole - geometric tolerance at MMC

Which is – 9.8 (smallest hole) – 0.1(Geo tol at MMC) = 9.7 mm diameter pin

Evaluation or verification of position control using functional gauge can only be done when material condition modifier M is used along with geo. Tol in feature control frame. Not applicable for RFS (regardless feature size) and LMC modifier.

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