Flatness around a hole

[admsteck]

What would be the best way to measure the flatness around a hole?  We have a 13mm hole in a stamped sheet metal part with a 26mm diameter plane around the hole called out with a 0.3mm flatness tolerance.  I have tried several different methods and get different results.  I am just wondering what the correct/most accurate method would be.  Thanks.

[GoBeavs]

My first thought would be to create a plane using the pick, then fit option.  This would snap the plan to the circular area.  Then I would just use the flatness GD&T option in the Feature Controls window to determine the flattness. 

Are you familiar with these procedures? Or was the question a bit more complicated than this?

[admsteck]

Here is the process I was using:
- Select the hole I would like to measure flatness around.
- From the menu, select / data points / compared to primative
- Enter (diameter of flatness area - diameter of hole) / 2 for the max distance (for example, if the hole diameter is 13 and the desired flatness area is 26, enter 6.5)
- This should highlight all the points within the flatness area.
- Create a plane using the fit / fit to elements method using the best fit option.

The problem is that I get differing results depending on if "Reject Outliers" is selected or not.  Should I be using reject outliers?  Should I use max or min instead of best fit?  Or would it be more accurate to compare data to reference for the selected area and view the min and max errors? 

[Firefly]

I agree with GoBeavs, the only way to get the correct measurements would be GD&T, cause normally you need the Tschebicheff fitting methods (not sure if I got the name right...). If you use GD&T in PolyWorks the original points for creating the plane are used for a complete refitting of the parallel min/max planes.

Do you need another idea to get the right points for starting GD&T?

[admsteck]

I am using GD&T to get the flatness, but I get different values when I use different options to create the plane (reject outliers, best fit, min, max, etc....)  What are the best settings to use to create the plane after I select all the points I wish to use?

[Firefly]

The Best Fit, Min or Max are using exactly the same points, so the GD&T measurement is also the same. Best-Fit with reject outliers turned on reduces the points - so you get a 'better' result  ;D

I would suggest to take a look at the smoothness of the scan data - if there are no outliers (from reflections or something) I would not use it, if the data is very rough I would turn it on. And I would turn it on if the normal result is out if tolerance  ;D

[Admin]

I am using GD&T to get the flatness, but I get different values when I use different options to create the plane (reject outliers, best fit, min, max, etc....)  What are the best settings to use to create the plane after I select all the points I wish to use?

admsteck, did you come to any conclusion or is it just going to remain a judgment call on the situation?

Can I change the icon for this topic from a ? to  :)

[PW User]

The Best Fit, Min or Max are using exactly the same points, so the GD&T measurement is also the same. Best-Fit with reject outliers turned on reduces the points - so you get a 'better' result  ;D

I would suggest to take a look at the smoothness of the scan data - if there are no outliers (from reflections or something) I would not use it, if the data is very rough I would turn it on. And I would turn it on if the normal result is out if tolerance  ;D

The flatness (and all GD&T controls) will be computed on all points kept for the fit of the feature. So if you check on the "Reject Outliers", it will change the GD&T result.

In my opinion, it is best to do GD&T on a Polygonal model instead of a point cloud. When you use the latter, you add to the real result the noise of the scan. with a polygonal model, you remove that scanner noise. Furthermore, if you restrain the max angle to a smaller value like 20 degrees, it will also remove the edged of the area.

Good luck!