roughness in flatness
roughness in flatness
(OP)
Hello,
I assign a flatness error 0.1 to a surface and I get a part having surface profile shown in attached draft. From the definition of flatness in Y14.5-2009:
Flatness is the condition of a surface or derived median plane having all elements in one plane.
It means to me that the flatness error is A-C. But during inspection, I think the measurement equipment gives A-B. The difference B-C is from the roughness of the surface.
Any comments?
I assign a flatness error 0.1 to a surface and I get a part having surface profile shown in attached draft. From the definition of flatness in Y14.5-2009:
Flatness is the condition of a surface or derived median plane having all elements in one plane.
It means to me that the flatness error is A-C. But during inspection, I think the measurement equipment gives A-B. The difference B-C is from the roughness of the surface.
Any comments?





RE: roughness in flatness
You'd have to look at the settings on the measurement equipment -- there are algorithms such as Ra and Rz for roughness, but those shouldn't be used if your only specification is flatness of 0.1.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: roughness in flatness
RE: roughness in flatness
Surface roughness is typically considered to be the high-frequency, short-wavelength component of a measured surface (that would be A-B). The larger-scale, general up-and-down pattern that you see (A-C) is usually considered as the waviness.
How were you looking at it?
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: roughness in flatness
I want to confirm my understanding, that is:
By definition, the flatness error is distance A-C
In reality, the measured flatness is distance A-B
Thanks.
RE: roughness in flatness
waviness high, Wt - the peak-to-valley height of the modified profile from which roughness and part form have been removed by filtering, smoothing, or other means.
so, if B-C is the peak to valley roughness, then A-B would be the closest to what the profile would look like with the roughness removed, assuming that the part form has been removed as well.
TTFN

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RE: roughness in flatness
If the letter do show up for you and I mistakenly assumed the wrong sequence, then that would be the source of the confusion!
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: roughness in flatness
TTFN

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RE: roughness in flatness
There is no confusion. Based on you answer I assumed that the letters did not show up to you, and that is why I used terms "middle" and "bottom" line in order not to muddy further discussion.
But coming back to the main topic and bxbzq's graphics, I would say it is rather unlikely to assess form of surface and its roughness based on one picture. So if what we see is real picture, we can look at different surface characteristics depending on filtering frequency and stylus tip used, but not at all of them at once. In other words, if this plot is coming from flatness check, we probably can say that A-C is actual flatness error but do not see roughness parameters well and conclusions like "roughness is B-C distance" may be wrong.
RE: roughness in flatness
Besides, the flatness tolerance is specified in millimeters and surface texture is specified in micrometers. Therefore, these are different unit and surface texture should not applicable to flatness control.
I'm just wondering, please let me know if I am wrong.
Season
RE: roughness in flatness
Season,
As far as I know, there are applications where the roughness and form tolerances are at same precision level. Critical components in servo valve, for example, have form tolerances at micrometers while roughness is about Ra1 micrometer or less. In cases like this, form tolerance measurement methods need to be clearly defined.
RE: roughness in flatness
I agree with your thoughts. We check roughness with a profilometer and flatness with a indicator.
Dave D.
www.qmsi.ca
RE: roughness in flatness
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: roughness in flatness
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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.
RE: roughness in flatness
Look at the sketch attached in my post at 24 Aug 13 11:52,
By flatness definition, the flatness error is distance A-C. Yes or no?
Using indicator, the flatness error is close to A-B, right?
Using profilometer(this is something I'm not familiar), the flatness error is close to what, A-C?
RE: roughness in flatness
It depends on your sampling length.
The idea is to keep sampling length short enough to keep profilometer reading close to B-C.
(You can find some additional info here: http://www.bcmac.com/pdf_files/surface%20finish%20...)
That allows to filter (separate) your surface texture from your flatness.
RE: roughness in flatness
----------------------------------------
The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.
RE: roughness in flatness