Is prealignment only a Starting Point?

[[Sa...]]

Hello,

could somebody explain me the use of prealignment? 

I guess it is only used to bring the data (3D and scan etc.) together? I see colleagues using this as for surface comparison and measuring some areas with labels. What could be the use of such measurements when we are not taking the assembly (datums into considerations) ? Could someone explain me this myth. Thanks in advance.

Kind Regards

Samkit

 

 

[[Be...]]

Hi,

usually the coordinate system of your measured data does not fit to your nominal CAD data. Of course this depends how you measure your data or if you have already aligned the data in a different sofware package. Due to our parametric approach we force the user to think about this first alignment step between nominal and actual data and we generate "error messages" for checks if no initial alignment step is fulfilled. You have to create an initial alignment (e.g. Prealignment or 3-Points or 3-2-1 or ...) or you have to define the existing "Original alignment" as initial alignment (for example if your imported data is already aligned).

Based from this point where we can assume that your actual data is good enough aligned to your nominal CAD data you can evaluate checks (comparison between nominal and actual) or you create further more specific alignments like 'RPS' or a 'Geometric alignment' or ... which takes into account that you can calculate 'semantically correct 'actual geometries from nominal positions (elements), e.g "Intersect mesh from a nominal point"

Many customers do not have any specification how they have to align their data and due to the fact that in the 'Prealignment' a bestfit option is included they get their best assumed alignment in one step even if the actual data is 'far away' from the nominal CAD position.

Hope this helps?!

Regards,

Bernd

[[Sa...]]

Thanks Bernd for comment but don't you think adding a alignment of the datum system in the software so that I can select the primary, seconday and tertiary datums and then quickly check the surface comparisons make more sense. The measurements can be quite accurate with this selection. 3-2-1  works for simple parts but I guess datums are very good for complex parts. If you can give your opinion on this, I will be glad to hear it. Thank you in advance. 

Kind regards

Samkit

[[Be...]]

Hi,

you can already do this: 

• Create a prealignment
• Create nominal primitives (e.g. planes, cylinders, ...) and apply the measurement principles to find these elements also on the actual mesh
• And then create a geometric alignment with the prealignment as parent alignment and the constructed nominal and actual elements

The surface deviation will be automatically recalculated in each alignment if you have Auto-Recalc enabled (e.g. in free GOMInspect this is automatically enabled) otherwise you have to trigger the recalculation.

Regards,

Bernd

[[Da...]]

Hi All, 

sorry to jump into this topic, but I have some doubts  about using of different alignment methods. For some symmetrical parts I obtained a good prealignment  but when I apply a geometrical alignment through primitive geometries created, the results get worse.

Do you have any suggestion when we have to analyze symmetrical part? 

Thanks for your help

Regards

Davide

[[Be...]]

Hi,

can you give some more explanations (with images) what you like to achieve or where you see problems? Quite not sure if we we will be able to discuss this problem without data. And if you have to supply data to explain your topics, you have to contact our professional support!

General hints:

• Symmetrical parts which are not completely symmetrical (e.g. due to one small unique feature) could usually be aligned by Prealignment with a help point in the unique area
• If this is not working you have to apply a different alignment strategy, e.g. 3-point alignment and additional Local Bestfit
  • Some customers are creating a mechanical fixture with Reference points on the fixture and are using these Reference points a definition for the correct coordinate system
• Geometric alignments are usually applied if part is already orientated to the CAD (because usually you have to derive geometries from the mesh)
  • If it's getting worser you have to check why -> are the primitives not well fitted? Or would this part looks distorted if the assembly situation which is expressed via the Geometric alignment is applied? Or...

Hope this helps?!

Regards,

Bernd

[[Lu...]]

Pre-alignment is a Bestfit Alignment basically and it will look better than any alignments because is on a least square condition, however if the drawing calls for a datum system scheme  you can not use the pre-alignment for the inspection.

For best practices you can live the pre-alignment and create a Datum system instead, i found that to be the best way for the software to create accurate geometry on the actual mesh.

So as a workflow example, i prealign (make sure the"compute additional best-fit" box is checked) then create geometry

for the datum"s features then create a Datum System (not alignment) and after that start measuring but make sure every dimension is to the datum system and not to the current alignment (which is the prealignment).

 If this is too advanced for you try using the "Align to coordinated system" on the main alignment menu. this will align

the Datum system to the actual mesh.

Regards