This question line video is in response to a student’s question regarding surface finish and flatness. This student wanted to know if it is permissible to call out surface finish and flatness on the same surface, as they are both limiting the amount of variation in a surface – one at the micro level and one at the macro level.
To answer this question, let’s first look at surface finish and flatness individually.
Surface finish is called out on a drawing to indicate the required level of roughness for a surface. When surface finish is specified, it is indicated using the symbols shown below in Figure 1. The value included with the symbol tells us the degree of roughness required for the surface.
Figure 1: Surface Finish symbols
Different manufacturing processes result in varying surface finishes. For example, sand casting produces a rougher surface finish, and polishing produces a smoother finish. The surface finish symbol on our drawing helps us determine which manufacturing process to select to achieve the required level of finish.
Flatness 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. It is called out using the GD&T flatness symbol and a tolerance. The flatness tolerance defines a zone between two parallel planes where the entire surface must lie.
Figure 2: GD&T Flatness
Surface Finish vs Flatness
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. Surface finish controls the surface at a micro-level, defining the required roughness/smoothness of a surface. Flatness controls the surface at a macro-level, defining the form of the surface. Therefore, there is no issue with calling out a surface finish and a flatness control on the same surface.
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