GD&T applied to patterns of features
Mufaddal Rasheed 
How to interpret the meaning of a GD&T callout on a pattern of features? What are the tolerance zones like?
When there are multiple features in a pattern then the meaning of the GD&T scheme changes
Here is an example:
- The feature control frame says that there 7 features which are in a pattern and have to be located with respect to datum A, B C as per the basic dimensions and with respect to the basic distance in between them
- Controls perpendicularity orientation of each hole with datum A
- Locates the pattern within the geometric tolerance zone with respect to datum B and C
- Points of the pattern can only vary within the blue zone.
- The tolerance zone is called as Pattern locating tolerance zone.
- If there was no “4X’ then the position control would only apply to a single hole which is the first and not others.
Simplifies the GD&T drawing avoids repetitive annotations
Example no 2 shows a rectangular pattern of holes
A manufactured part with exaggerated form is shown here . the four plus signs indicate the centres of holes and the datum reference frame is show as the co ordinate system at top right edge of part .
Analytical definition of tolerance zones :
Each tolerance zone is a circle and those circles are locating a distance with respect to the origin which is the origin of datum reference frame.
Hence by finding the equations for each circle we can arrive the condition to satisfy each tolerance zone requirement.
Assumption:
- Only verifying position of hole in 2D
- Considering negligible variation in perpendicularity of axis
These are the equations of each tolerance zone with the condition for acceptance.
If we have a measured manufactured actual part and replace x and y values then the equations should be satisfied for acceptance.
Learn more about how to Interpret GD&T callouts from this Free e-book on GD&T interpretations
Explore more topics in GD&T :
What is virtual condition in GD&T
Factors in Specifying tolerance
GD&T Regardless of feature size
GD&T maximum material condition
What is datum feature modifier
What is actual mating envelope
What is 3-2-1 principle in tolerancing
Applying GD&T scheme to a bracket
GD&T applied to patterns of features
Tolerance stack up analysis of a simple part
What are material conditions in GD&T
Composite position tolerance in GD&T
What are datum targets in GD&T
Verification of manufactured GD&T drawing
What are simultaneous requirements in GD&T
Calculating the geometric tolerance of a part
Developing GD&T scheme for a part
Traditional tolerancing vs GD&T
Categories: : GD & Tolerancing