GD&T True Position

What is the meaning of Position control in GD&T? how to interpret the drawing and how to define the tolerance zones

One of the most commonly used symbols and geometric control is the true position control . an example is shown here

Position control is used on Feature of size and cannot be applied to any surface

Features of size are features which have opposing elements which can be measured with a vernier caliper or micrometer like holes, widths, slots, pins

Here is an example of its application:

A block with a central hole

The primary secondary and tertiary datum features are three mutually perpendicular surfaces of the block

A feature control frame for position of the hole is applied to the Size dimension of dia 20 with a size tolerance of +/- 0.2

The feature control frame means

  1. The position of the hole has to be controlled within a cylindrical tolerance zone of diameter 0.1
  2. With respect to primary secondary and tertiary datums
  3. The basic dimensions for the positional deviation are given as boxed dimensions of 20 and 25 originating from datum feature B and C .

Positional tolerance can be used with material condition modifiers as shown in this example :

The M symbol beside the positional tolerance value of 0.1 indicates that this value of positional tolerance applies to only the Maximum material condition . Which means it only applies when size of the hole is smallest that is 9.8 .

If L symbol is used it means the other way around 0.1 is positional tolerance for Least material condition (largest hole) .

When the M symbol is used , position can be inspected using functional gauging

This is a setup of inspection for position of hole for an as manufactured part in pink .

The blue blocks are gauge blocks and there is a red pin which is inserted once the workpiece or part is located with respected to the gauge blocks

If the Pin goes through then the part is acceptable as per position control applied

The gauge size of the pin is nothing but the virtual condition .

Difference between perpendicularity and position of hole:

The tolerance zone in perpendicularity is not fixed in its location. It moves with respect to the location of the center pin . But tolerance zone in position is fixed in its place with respect to the basic dimensions (22.5,22.5) .

Learn more about position control in GD&T from this e-book on GD&T interpretations  

Categories: : GD & Tolerancing