GD&T Projected tolerance zone

What is the concept of projected tolerance zone in GD&T ? How to interpret it on the drawing?

The projected tolerance zone (PTZ) is a special modifier used in GD&T to control the orientation and location of features—typically holes—based on how they are used in assembly. It accounts for how features like pins or fasteners extend out of a part and interact with mating parts

Consider this example of a plate being assembled on a block with a threaded hole

Part A is the block with a threaded hole M10

Part B is the plate with a clearance hole

Part A Drawing

Tolerance zone for the M10 threaded hole:

The problem of misalignment:

There has to be a relation between tolerance zone of threaded part and the part with clearance hole

Modified drawing of the block with threaded hole.

The change: in the feature control frame a P symbol and a number is added indicated projected tolerance zone and the length of projection.

What it means :

Now this projected tolerance says that Part B – the mating part also has to be located such that it satisfies the projected tolerance zone

Why projected tolerance zone is important

Without the projected zone:

The hole may pass inspection but cause misalignment in the assembled product.
Functional failures may occur when the mating part doesn't fit due to angular variation of the pin or bolt.

It helps ensure fit, alignment, and stress-free assemblies

Another example of application of projected tolerance zone :

Categories: : GD & Tolerancing