GD&T Datum feature modifier

What is Datum feature modifier in GD&T ? what is the interpretation on the drawing when called out?

Consider these two feature control frames of a GD&T callout aimed at a feature of size which is a hole of basic size 8 .

On the left side the datums are without a MMC modifier whereas on the right an MMC modifier has been applied to datum B .

When this is done the meaning of the GD&T control changes.

It now means that the positional deviation of the hole which is having a basic size of 8 and size tolerance +/- 0.1 can deviate in its position within a tolerance zone of 0.1 (at MMC) with respect to datum feature A and Datum feature of size B at MMC.

This is called as Datum feature modifier.

As the datum feature B moves away from that Material condition the Datum feature becomes free to move

This means that the part doesn’t exactly get constrained but has allowed shift called Datum Shift

When the datum feature B is manufactured at MMC , there is no datum shift or reference shift .

But when the size manufactured is towards LMC the amount of shift in reference will increase

Here is an example to elaborate.

The datum feature A is primary datum

Datum feature B is a feature of size which is centre hole with basic size of 20 and size tolerance of +/- 0.1

The Top right pin which is having basic size of 8 and size tolerance of +/- 0.1 is being controlled with respect to its position with respect datum A and Datum B within a cylindrical tolerance zone of 0.1 at MMC of pin .

The Top right pin which is having basic size of 8 and size tolerance of +/- 0.1 is being controlled with respect to its position with respect datum A and Datum feature of size B within a cylindrical tolerance zone of 0.1 at MMC of pin when datum feature B is at MMC size.

Functional gauging for the drawing with datum feature modifier would look like this in purple colour

There Is a central pin which corresponds to central hole on part and a top corner hole which corresponds to top corner pin on part.

The size of the center pin and corner hole are at their respective virtual conditions.