3/1/2024 0 Comments Flatness gd&t symbolAll the points on the specified surface must lie between these two planes for part approval.Īs flatness refines a surface, we can also use it in a tolerance stack without interfering with other requirements. It defines two parallel planes on either side of the flat surface as the tolerance zone for the surface. The flatness callout controls the uniformity of a surface or a median plane as needed. No surface is perfectly flat but using GD&T, we can develop parts with a surface that is flat enough for our application. Many applications need parts with a flat surface. Therefore, there is no issue with calling out a surface finish and a flatness control on the same surface.VI Benefits of Using Flatness Tolerance What is Flatness? Flatness controls the surface at a macro-level, defining the form of the surface. Surface finish controls the surface at a micro-level, defining the required roughness/smoothness of a surface. Though both surface finish and flatness callouts are limiting the amount of variation in a surface, they are doing it at different levels, so one does not override the other. The flatness tolerance defines a zone between two parallel planes where the entire surface must lie. It is called out using the GD&T flatness symbol and a tolerance. The surface finish symbol on our drawing helps us determine which manufacturing process to select to achieve the required level of finish.įlatness is called out on a drawing when you want to limit the amount of waviness in a surface without tightening the dimensional tolerance of that surface. For example, sand casting produces a rougher surface finish, and polishing produces a smoother finish. The value included with the symbol tells us the degree of roughness required for the surface.ĭifferent manufacturing processes result in varying surface finishes. When surface finish is specified, it is indicated using the symbols shown below in Figure 1. Surface finish is called out on a drawing to indicate the required level of roughness for a surface. To answer this question, let’s first look at surface finish and flatness individually.
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