Flatness & Surface Finish

[Smile9999]

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?

 

[KENAT]

Except in extreme cases flatness and roughness are on a different scale of surface deviation.

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?p=59777

In your example you probably need to specify both flatness and roughness.

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[BillPSU]

There may be reasons you want a flat surface with some roughness. An example is a sliding surface such as a way of a machine. A scraped surface holds lubricant to improve sliding. A scraped surface can be extremely flat. This same situation occurs on clyindrical surfaces. Some roughness will hold lubricants.

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

 

[unclesyd]

I’ll bet that the above references to pumps were Diamond. Lapped. Aat one time we used thousands of metering pumps for polymer that carried some very tight specs on flatness of the end plates and gears. the finish on was less than 0.050 RMS, a rather nebulous number but it worked. We would lap our spinnerets flat to less than a quarter light band over an 8" span.
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.

 

[geesamand]

It's been said already, but flatness and roughness are of entirely different orders of magnitude.

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