Figure 1: Flatness of a Surface on a Drawing Flatness of a surface is merely defining how flat a surface feature must be and is the condition of being purely planar. On the other hand, using GD&T will address this by specifying tolerance using the flatness symbol as the figure below shows. Noticing the flatness symbol’s placement is critical in determining whether the flatness requirement is for a surface or a feature of size (FOS). However, the mirror could pass a thickness inspection test with measurements from several points, but it could still end up with a wavy surface. If using coordinate dimensioning, the designer can attempt to achieve this by specifying tight thickness tolerance in the drawing. The boxed symbols can be read this surface must lie between two parallel planes spaced 0.2 apart in all views. For example, in designing a mirror, ensuring that the surface is not wavy is necessary to avoid image distortion. The left figure shows the drawing callout. Basics of GD&Tĭesigners use GD&T to express intentions that coordinate dimensioning and tolerances alone cannot show. In this article, we will discuss the basics of GD&T, geometric tolerancing vs size tolerancing, and categories of GD&T symbols. Moreover, GD&T defines engineering tolerancing and relationships of parts in a design. GD&T, stands for geometric dimensioning and tolerancing, and is a system of symbols and standards that designers use to communicate to manufacturers through engineering drawings.
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