Flatness & Surface Finish
Flatness & Surface Finish
(OP)
for example, if I have a plate that needs to have a finished and flat surface, what would make me choose surface finish over flatness or otherwise?
I suppose my question is really about why would I ask the machining to make me a flat surface or a finished surface when the flat surface would do for both flatness and surface finish?
btw I am talking from Geometrical Tolerance perspective, where I can just put a flatness tolerance instead of a surface finish symbol
I suppose my question is really about why would I ask the machining to make me a flat surface or a finished surface when the flat surface would do for both flatness and surface finish?
btw I am talking from Geometrical Tolerance perspective, where I can just put a flatness tolerance instead of a surface finish symbol





RE: Flatness & Surface Finish
John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
Siemens PLM:
UG/NX Museum:
To an Engineer, the glass is twice as big as it needs to be.
RE: Flatness & Surface Finish
In fact, when measuring roughness, you first have to "read" the surface geometry with the proper instrumentation, then you must filter the data so that any macroscopic deviation from the ideal shape (i.e. geometrical tolerance) is eliminated. What is left is the micro-geometry of the surface, which we call roughness. Then, though tolerances and roughness may not be generated by the same phenomena, from a geometrical perspective they are the same thing, but of different order of magnitude.
Leaving behind academia, geometrical tolerance controls how much a particular shape is allowed to deviate from an ideal one (e.g. a pin must be cylindrical enough to rotate well inside a hole), while roughness measures the finish of a surface (e.g. specular, as-welded, as-cast etc.), which is, for example, important in seals (different types of gaskets require different types of finish).
Hope it helps.
Stefano
RE: Flatness & Surface Finish
roughness even thou measures max hollows & peaks & valleys, it is at a smaller measuring area.
It is standard for a machinist & inspector to verify each separately.
roughness symbols are generally not used to control flatness.
Mfgenggear
RE: Flatness & Surface Finish
RE: Flatness & Surface Finish
Therefore, while there might a few applications where flatness and roughness have the same numerical values, in general, flatness is rarely as small as the roughness requirement. In the case of optical surfaces, such as mirrors, flatness and finish might be comparable requirements, as any deviation from the ideal might create optical errors. Even then, since flatness is derived from wavefront error, while finish is derived from scatter, they still might be different, since they have different reasons for existence.
One of the downsides of not doing systems engineering on derived requirements is that the recipient engineer will often not know why a requirement is the way it is, since there are often disparate requirements that dictate the flatness and finish.
TTFN
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RE: Flatness & Surface Finish
thread281-326455: Flatness & Surface Finish
Or if you absolutely feel you need the input of folk in more than one forum then have one 'main' thread and in any duplicates just put a link to the main one and ask respondents to post there - though even this sometimes upsets site management.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?