GD&T: Flatness - Where does it start from?

[km4512]

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?

 

[dingy2]

J-P:

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.

http://www.qmsi.ca

 

[Belanger]

Agreed, Dave. And that was my point. As we move the 3 set-up locations around, we get different results. So the key is to measure as many points as possible/practical.

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