Thickness, Parallelism and Flatness. How do they relate?

    Thickness, parallelism and flatness are related terms that are especially important with regards to plate dimensions. But how do they relate? How are they different? We hope to offer here some standardized definitions and explanations for thickness, parallelism and flatness.

Below is an example of a plate description:

2.50″ x 24.00” x 36.00”, A36, Rectangle, Flame cut and cleaned, Stress Relieved, Blanchard Ground to 2.304” (+/-0.010”), flat within 0.005” per foot, parallel within 0.005”

Thickness description: 2.50″ is the nominal plate thickness that the part will be cut from. The dimension 2.304” (+/-0.010”) is the finish thickness with the maximum allowable thickness being 2.314″ and the minimum allowable thickness being 2.294″

Flatness description: flat within 0.005” per foot

Parallelism description: parallel within 0.005"

Thickness,parallelism and flatness definitions

Plate thickness: the distance between two surfaces of a part (commonly the top and bottom) with a deviation tolerance specified (ex. 2.307” +/-0.0010”). This distance is most commonly measured by using precision micrometers.

Plate parallelism: parallelism is related to thickness. The difference between the thickest and thinnest points on a plate (ex. 2.307″ thickest measurement, 2.302″ thinnest measurement = parallel within 0.005″)

Plate flatness: Flatness is not related to thickness or parallelism! Plate flatness is measured relative to a reference surface such as a grinder table or granite table. Flatness is most commonly measured as a deviation per linear foot from the reference plane.