GD&T: Flatness - Where does it start from?
GD&T: Flatness - Where does it start from?
(OP)
Hi,
I've got a question, say you have the flatness surface control applied to a surface:
http://creativewebart.net/GDT/images/Flatness1.gif
Now, by definition, this means
"The surface must lie between two parallel planes 0.25 apart"
My question is, where do you position the 2 parallel planes? That block area could be taller or shorter, so where do you start to apply the 2 parallel planes?
Do you apply them .125 each side of the top surface in the example avbove?
I've got a question, say you have the flatness surface control applied to a surface:
http://creativewebart.net/GDT/images/Flatness1.gif
Now, by definition, this means
"The surface must lie between two parallel planes 0.25 apart"
My question is, where do you position the 2 parallel planes? That block area could be taller or shorter, so where do you start to apply the 2 parallel planes?
Do you apply them .125 each side of the top surface in the example avbove?





RE: GD&T: Flatness - Where does it start from?
http://www.tec-ease.com/gdt-tips-view.php?q=214
RE: GD&T: Flatness - Where does it start from?
Still confused.
RE: GD&T: Flatness - Where does it start from?
"Art without engineering is dreaming; Engineering without art is calculating."
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RE: GD&T: Flatness - Where does it start from?
RE: GD&T: Flatness - Where does it start from?
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: GD&T: Flatness - Where does it start from?
Thanks for your help guys.
RE: GD&T: Flatness - Where does it start from?
In other words, the two parallel planes that are .25 apart floats within the limits of size. That is why the standard states that the flatness tolerance must be less than the size tolerance.
RE: GD&T: Flatness - Where does it start from?
Mark 3 points about 120 degrees apart near the perimeter but not on the edge of the flat surface. Turn the part over so that the flat surface is facing up and have the adjustable jacks under each of the 3 points.
Adjust or zero off on surface above each jack (adjusting each screw jack up or down) so that you will create a plain. Once all three points are zeroed, sweep the full surface with your dial indicator and record your highest and lowest readings combined. The range of these reading is your actual flatness on the surface.
It does get fuzzy on whether or not an edge burr is considered part of this surface or is it a separate entity.
Dave D.
www.qmsi.ca
RE: GD&T: Flatness - Where does it start from?
You don't position the planes.
If I can find two planes 0.25 apart that contain your entire surface, the part conforms.
RE: GD&T: Flatness - Where does it start from?
In your original question, you may be thinking of Profile FCF, which would place the range relative to a datum.
Matt Lorono, CSWP
Lorono's SolidWorks Resources & SolidWorks Legion
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RE: GD&T: Flatness - Where does it start from?
The fact is the feature may be within the flatness tolerance if a more optimal alignment to the surface is found. In the attached file I've attempted to simplify the alignment of a flatness tolerance zone to a considered feature by making it a 2D illustration that shows two possible alignments. When measuring flatness it may be optimal to align a plane of the tolerance zone to the high points, but it may instead be optimal to align to three low points on the surface and measure to the high point relative to that plane. It may be that neither of those two alignments are optimal. What if the surface is convex?
Zeroing three points by using two adjustable feet and the third that's fixed won't produce a proper flatness value either. The alignment that is needed is probably only going to be obtained using a CMM and either its software of offline software to provide the mathematical fitting that is needed to minimize the measured flatness value. A scanning CMM touch probe can gather essentially as much data as a dial indicator and a scanning line laser will make even more points practical. If the form and roughness of the surface isn't too bad relative to the specified flatness value then a normal touch probe may be completely sufficient.
RE: GD&T: Flatness - Where does it start from?
Dean
www.d3w-engineering.com
RE: GD&T: Flatness - Where does it start from?
Hope my file uploads.
Dave D.
www.qmsi.ca
RE: GD&T: Flatness - Where does it start from?
Peter Truitt
Minnesota
RE: GD&T: Flatness - Where does it start from?
SeasonLee
RE: GD&T: Flatness - Where does it start from?
I tried to upload a pdf file but it did not show. Any suggestions?
Dave D.
www.qmsi.ca
RE: GD&T: Flatness - Where does it start from?
SeasonLee
RE: GD&T: Flatness - Where does it start from?
Anyway, the page should be self-explanatory.
Dave D.
www.qmsi.ca
RE: GD&T: Flatness - Where does it start from?
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: GD&T: Flatness - Where does it start from?
The most important process to measuring flatness is to set the part up towards the perimeter but never on it. If I rotated the set up, say 90 degrees, would I get the same result? No, but pretty close to it. I would say that I could have a confidence level of about 90% or more.
I am creating a plane on the surface in the best location possible and it is repeatable (to a certain degree). From the created plane, the indicator could go down to -0.23 and then up to +0.12 while sweeping the surface giving one an actual flatness of 0.35. Let's change the location of the points but maintaining the 120 degree difference in their location but keeping the set up points towards the perimeter. Let's see. We could now get 0.14 and +0.23. We now have an actual flatness of 0.37. Pretty close but not exactly the same.
If we set up towards the middle of the part, our results would be dramatically different especially sweeping towards perimeter.
If we use a CMM and tell the machine we want 20 points, would we get the 0.35 - 0.37 result? Absolutely not. We will, most likely, get a smaller value. A CMM in a scan mode will somewhat simulate our hand result but it would take a long time to perform contacting the part every 4 mm, as an example. We still have to maintain the part thickness as shown in my example.
By the way, I have actually measure flatness as shown when I was relatively young measuring parts for GM. I was probably one of the first people in Canada to start up a measuring company in the early 70's but a bit to premature for the time.
Dave D.
www.qmsi.ca
RE: GD&T: Flatness - Where does it start from?
My original explanation above was simply meant to convey the essential idea to the OP. I suspect that we've all stirred the pot beyond what he needed to know. :)
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems