Line Profile Calculation Problem

[[ca...]]

Hello everyone,

In the first figure that i shared below, i try to calculate the line profile for a section. Since black line refers to nominal element, I could not understand why it shows plus deviation inside. I check the normal vectors of surfaces and i wasn't able to understand why it happened.

I need to check the line profile without using a datum system. When I try it once again with referring the  global coordinate system this time , it shows a much normal deviation map which is more representative of the real condition (referring the second  figure below.) When it is not required to use a datum system, should i need to use 3D deviation command to make it work in a more robust way?

Thanks,

Canset

 

 

[[Ch...]]

"Problem" is that GD&T computation are calculated into its own "alignment" as it is defined from the datum system. That means the distance computation for the deviation plot is not done inside the global alignment but inside a different one. In fact a line profile computation is the computation of a nominal to actual transformation matrix in a way that the distance from actual to nominal is minimal. This allowed transformation can be restricted from the datum system and if as in your own case no datum system is defined the no restriction has to apply.

 

In short the following happens

1) Computes a transformation for the actual data (or nominal one it doesn't really matter) so that the difference between actual and nominal data is minimal

2) Computes deviation between invisible transformed actual data and nominal data and display this at untransformed data.

Yes we can discuss about a better visualisation of this transformation but the result seems to be correct.

 

Hope this helps

Christoph Schult

 

Note: In your case it can happen that the software will rotate the actual data about 180° switching inside and outside direction. But although this no computation error at all because it can be forbidden using the right uncomplete datum system.

[[ca...]]

Hi Chris,

Thanks for the detailed explanation. As far as I understand, it is basically a visualization issue not a computation error.

Since I did not use any datum system according to my technical drawing requirements, software computes the  line profile  in a way that no restriction applied when searching for minimum distance between nominal and actual data.

I have one more question. With no datum system applied, only form control is required to inspected .What is the difference between using global coordinate system and no datum system? If we use global coordinate system, we apply restrictions but it always much understandable in terms of visualization. However, it makes the results out of tolerance, probably it should not be used interchangeably.

Thanks,

Canset 

[[Ch...]]

No datum system = no restriction, i.e. you can rotate and translate as you will to find the minimum distance

Global coordinate system = all restricted, i.e. no rotation and translation is possible. The software simply computes the deviation and have not to compute any transformation.

 

See at your example your actual tolerance value is in the second case almost 17 times higher causing from the rotation and translation restrictions.

[[ca...]]

Hi Chris,

Everything is clear to me now. Thanks for the explanation.

Canset