GD&T Flatness vs Parallelism
Mufaddal Rasheed 
What is the difference between Flatness control and Parallelism control on a GD&T drawing ? How to differentiate them ?
Flatness is a geometric tolerance that ensures all points on a surface lie within two parallel planes (no reference to another surface)
Parallelism ensures that a surface or axis is equidistant (within tolerance) from a datum surface
Lets take this example into consideration :
Tolerance zone for flatness :
Tolerance zone for Parallelism :
The shape of tolerance zone is similar but the definition of tolerance zone changes.
Key difference is that the tolerance zone is located at basic 35 from a datum in parallelism whereas flatness tolerance zone is irrespective to any datum.
Categories: : GD & Tolerancing