How do I measure the flatness of 6061 T651 Aluminum Plate?

The method that is use in most cases is 2 foot short span. I will make the assumption that the 6061T651 Aluminum plate has been purchased to “Type 200® wrought tooling plate or Precision Plate®” specification. If not, standard Aluminum Association tolerance of .100” for thickness of 0.250”–0.624” and .075” for thickness of 0.625”-8.000” would apply (Table 7.18*). A certified 24” straight edge is used in conjunction with certified taper gage or feeler gages and a relatively flat inspection surface.


6061 T651 Aluminum Plate
aluminum plate

The material to be measured should be oriented with the concave surface facing up. The straight edge is now positioned to span any 2 foot segment of the plate in any horizontal direction. The straight edge must contact the surface of the material at 2 opposite points. If you cannot fit a .060” thick taper gage under the straight edge (between the 2 opposite high points) the material is within Aluminum Association Flatness tolerances.


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