Mesh to Mesh Report and alignment structure

[[Jo...]]

Hi all,

I'm having some trouble establishing a reliable and repeatable workflow for our before-and-after scan comparisons in GOM Software 2022.

Currently, we compare two independently acquired 3D meshes to generate a color-coded heat map showing surface deviations, typically to evaluate material added or removed.

Our current workflow is:

1. Import both meshes (previous and current scan).

2. Convert the previous scan into nominal CAD to serve as the reference.

3. Recalculate our alignment using Local Best Fit.

4. Recalculate the surface comparisons and deviation labels.

5. Export the inspection report.

The problem I'm encountering is that when I import the meshes and convert the reference scan to CAD, the converted CAD sometimes appears offset from its original position in space.

When Local Best Fit recalculates, it aligns the current scan to this offset reference. As a result, some deviation labels no longer correspond to their intended measurement locations, while others fail to compute entirely.

Because each part has its own unique reference scan, maintaining a single fixed CAD model isn't an option.

My questions are:

• Is there a way to preserve the original mesh coordinates when converting an actual mesh to CAD?

• Can the two meshes be aligned to one another before converting the reference mesh into CAD?

• Would a reference-stage comparison or another mesh-to-mesh workflow be more suitable for this application?

• What would be the recommended approach for automating this process so that surface comparisons, deviation labels, and report pages update reliably for each new pair of scans?

Ideally, we'd like to establish a project template or automated workflow where we import two meshes, calculate their differences, and generate a report without manually correcting their positions each time.

Any guidance specific to GOM Software 2022 would be greatly appreciated.

Thank you!

[[DW...]]

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 I am not sure I understand everything correctly. You bring two STL files into a project presumably that has no CAD file, designate one of the mesh as a reference by turning it into the "CAD", and then a localized best fit recalculates (from what?) the other STL file location. What is the initial alignment?

When you transform the STL into the "CAD", the alignment coordinates do not change. 

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This is going to need more explanation. 

If you want the parts to align correctly to an actual CAD (normal to the global axes) without having the CAD present in the project file with the surface comparison, here is my suggestion - 

1. Create new project.

2. Import actual 3d CAD file (STP etc) into project.

3. Align the CAD file to whatever orientation your surface to surface comparison project needs by using the menu Operations>Alignment>SingleElementTransformation>SetMatrix. 

4. Save this as your CAD alignment template.

5. Import your reference STL file. Menu Operations>Alignment>InitialAlignment>Pre-alignment. This snaps your reference STL to the CAD.

6. Import any other STL files you want, and do recalculate the project. All files are now aligned to the CAD and to each other.

7. Select the entire aligned reference STL file.

8. Menu File>Export>Mesh>STL.

9. Select the radio button "Export in multiple files". Under Export stages, select "All stages." Click OK.

10. Your exported files will retain their new aligned coordinates.

 

Now when you import them into your surface to surface comparison project, they are already best fit aligned to each other. You can set the Operations>Alignment>InitialAlignment>DefineOriginalAlignmentAsInitialAlignment.

Also for automation, use the Python editor and record whatever sequence you are doing and then tweak it as needed. Once finished, you can just fire the macro and the magic happens.