Difference between Surface Finish (Ra) and Flatness (GD&T)

stm08007

Starting to get Involved
#1
These both seem like they would be very similar measures.. Can anyone shed some light- preferably with a drawing example on the difference between them?

Is it an obvious difference or are these terms used somewhat incorrectly in practice?
 
#2
There's a big difference. Surface finish can have a number of parameters applied to it since the finish may be cosmetic (for example) or functional - a sealing surface comes to mind. Therefore while an Ra value maybe what's called up, the "lay" of the finish may be important. The flatness feature may or may not have a surface finish characteristic or anything more than being a datum, for example.

As for what's practiced, they are all subject to "misuse" if the designer/detail draughter doesn't understand the use.
 

Miner

Forum Moderator
Staff member
Admin
#3
It is also a matter of scale. Flatness is a macro-level measurement while surface finish is a micro-level measurement. Flatness may have a large impact on fit and function, while surface finish is more typically function than fit. For example, if two surfaces must move in relation to each other, certain surface finishes are better at holding the lubricant between the surfaces.
 
#4
Surface finish can be defined by the texture of the part, running a fingernail across different surfaces (say your desk and a wall) will give you different feeling depending on the surface finish.

Flatness can be defined by looking at a flat plane (think of your desktop) Your desktop is flat. Now image lifting one side giving the desk a curve. The difference between the highest point and the lowest point is flatness.

Continue using the desk, the surface finish probably will not change although the flatness really has.
 

stm08007

Starting to get Involved
#5
Surface finish can be defined by the texture of the part, running a fingernail across different surfaces (say your desk and a wall) will give you different feeling depending on the surface finish.

Flatness can be defined by looking at a flat plane (think of your desktop) Your desktop is flat. Now image lifting one side giving the desk a curve. The difference between the highest point and the lowest point is flatness.

Continue using the desk, the surface finish probably will not change although the flatness really has.
Makes sense, but flatness would also be impacted by surface finish right? i.e. a very rough surface would cause flatness to decrease (and surface finish as well)
 


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