Flatness & Surface Finish
Flatness & Surface Finish
(OP)
for example, if I have a plate that needs to have a coherent 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 finsih?
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 finsih?





RE: Flatness & Surface Finish
Additionally, roughness is typically specified as some kind of average value, while flatness is more of a worst case measurement.
http://www.leevalley.com/en/shopping/techinfo.aspx...
In your example you probably need to specify both flatness and roughness.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Flatness & Surface Finish
There are tolerances which really defines a surface quality such as the surface of a hydraulic motor/pump end caps. The norm was to be flat within .00005 within a specific diameter(about 7"). That tolerance was relatively easy to produce when you used a lapping machine. The finish was extremely smooth but was not measured. The tightest flatness spec I have seen was a charge pump body which gave a flatness of .000035 concave only. We measured the parts with an optical flat and mono-chromatic light.
Bill
RE: Flatness & Surface Finish
One thing to remember is that a very flat surface is pretty dull and if you polish it you can compromise flatness.
Another point is that normal lapping will round the edges. To keep a square edge one has to go to diamond lapping.
Surface finish is measured on instruments by Taylor Hobson, and other using a stylus. Depending on you requirements you can get replicas' of different finishes.
another caveat is that make sure you establish the units in reporting surface finish.
RE: Flatness & Surface Finish
For example, you can buy a piece of cold rolled plate, blast, and manually sand/polish it down to 15Ra. Finish is very good, flatness is not. You could also machine a face nice and flat but deliberately keep a rough machined texture. If you blanchard grind a face you get both flat and smooth together.
David