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
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) .
To learn more about GD&T have a look at this course
Categories: GD & Tolerancing