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?

 

[lifttrucks]

This Tec-Ease tip should help:

http://www.tec-ease.com/gdt-tips-view.php?q=214

 

[km4512]

lifttrucks, thanks for your reply. However, even after reviewing that link, I'm still not sure. In regards to that example you gave in that link, it seems like all that matters is the any point on the surface should not exceed 0.2 (outwards). But then the definition of flatness doesn't make sense to me.

Still confused.

 

[MadMango]

3 points make a plane. If your tolerance zone for Flatness is .250", then your parallel planes will be .250" apart. As long as the part fits within this boundary your part meets the Flatness. You do not split the tolerance.

"Art without engineering is dreaming; Engineering without art is calculating."

Have you read faq731-376 to make the best use of these Forums?

 

[km4512]

MadMango, I know the planes need to be .25" apart but where do you position these 2 parallel planes relative to the surface it is applied on?

 

[Belanger]

Think of it this way: Set a flat gage plate/block on top of the part. That naturally contacts 3 points (minimum). That plane formed by the gage plate is now the upper side of the tolerance zone. All other points on the surface may be no more than .025 mm below that gage plate.


John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[km4512]

Belanger, that makes sense to me, thanks a lot. Reading your response in combination with looking into liftruck's link I get it now.

Thanks for your help guys.

 

[lifttrucks]

In the example you show, there will need to be a specified size limit. The size must be inspected first. If the part is within the limits of size, then you must inspect the flatness.

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.

 

[dingy2]

I believe that the best way to confirm flatness is with a dial indicator, stand, 3 adjustable jacks on a granite table found in the toolroom. A CMM will only have a limited number of contacts while a dial indicator would have an infinite number.

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.

http://www.qmsi.ca

 

[drawoh]

...
"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?

You don't position the planes.

If I can find two planes 0.25 apart that contain your entire surface, the part conforms.

JHG

 

[fcsuper]

The parallel plans are not "placed" anywhere. The flatness FCF doesn't define some absolute points. It only says that the surface must full within two parallels (which are related to the surface itself).

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
Follow me on Twitter

 

[DeanD3W]

The statement Jean-Paul made is completely correct, but I would like to mention that he chose not to point out that the distance from that tangent plane to the low point is not a measured flatness value... Yes, as long as the low point is within 0.25 mm of the plane he found then the feature is within tolerance, but what if the measured distance is 0.257 mm? The Tec-Ease tip referred to above points out the shortcomings of its first two methods shown.

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.

 

[DeanD3W]

OK - One more attempt to attach that file (for my post above).

Dean

http://www.d3w-engineering.com

 

[dingy2]

Here is a example of flatness of a surface. In the ASME Y14.5-2009, there is also flatness of a plane but we are talking about surface here.

Hope my file uploads.

Dave D.

http://www.qmsi.ca

 

[ptruitt]

Although I am going beyond what was asked, I would suggest that a composite profile feature control frame might be an efficient and concise way to define the feature in the illustration.

Peter Truitt
Minnesota

 

[SeasonLee]

Dave, there is no upload.

SeasonLee

 

[dingy2]

SeasonLee:

I tried to upload a pdf file but it did not show. Any suggestions?


Dave D.

http://www.qmsi.ca

 

[SeasonLee]

Dave : Try to upload again, it works now.

SeasonLee

 

[dingy2]

Will try to upload again. Looks like it uploaded.

Anyway, the page should be self-explanatory.

Dave D.
www.qmsi.ca

 

[Belanger]

Interesting, Dave. But I wonder: how do we know where the set-up spots are? Isn't there still a need for a tangent plane to be created when leveling out the part?


John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems