[jola]

We are trying to develop a new and quicker method to handle complex white-light/GOM based STL geometries. To mesh these geometries we have so far been forced to either spend a lot of CAD time to manually create CAD geometries, or use fairly crude approximations in our design system.

Our application is turbomachinery components. In the design phase we mesh these nominal geometries in ICEM-Hexa, using low-Re boundary conforming multi-block meshes. The white-light/GOM based STL geometries are measurements of how these designs look after they have been manufactured. The STL geometries are fairly complex and contain small details like welds, fillets, weld-repairs, holes for bosses etc. and do not only contain gas-washed surfaces but also external surfaces. We have tried to just import the STL geometry into ICEM and use it there, but the STL's are too complex to use directly, or at least we have not got it to work smoothly yet.

As a turbomachinery company we are very good at creating very good and resolved meshes for fairly simple nominal blade geometries, but we are not very experienced in handling very complex geometries. I'm sure that many of you have other applications where this type of complex geometries are common.

What would you recommend us to look at? We could start looking at some of the "surface-wrappers" that have emerged over the last years. If so, which ones? We could also try to find a tool that can morph/adjust our nominal design meshes (created in ICEM-Hexa) to the STL geometries, if so what morph tool would you recommend? We could also give it a second try an try to get ICEM-Hexa to use these STL geometries directly if you think that it should be no problem to use complex white-light/GOM based STL geometries in ICEM-Hexa.

I appreciate any advice or comments you might have.

Cheers'

Jonas

[Rami]

Hi Jonas,

I am not suggesting help here - this is way beyond my expertise. But out of curiosity (and maybe then I can find an expert...) what is GOM?

BR, Rami

[azt]

Yes, we've had the same problem here. We got round it by using the converter (follow the link) to create a dxf file reading it into autodesk inventor and then exporting it as a stp file which was far easier to handle.

http://meshlab.sourceforge.net/

allan

[jola]

azt, we will give meshlab a try, thanks.

Rami, GOM means Gesellschaft für Optische Messtechnik (see http://www.gom.com) and this is the name of the company that sells the white-light measurement systems.

[andyj]

Using Meshlab to Clean and Assemble Laser Scan Data
http://www.instructables.com/id/Usin...aser-Scan-Dat/


http://meshlab.sourceforge.net/


http://www.stlfix.com/stlfix_ENG.html


if you use Phonics:
http://www.cham.co.uk/phoenics/d_pol...x/facetfix.htm


See below, software for dealing with STL files:
http://reprap.org/wiki
http://reprap.org/wiki/Useful_Softwa...with_STL_files

http://netfabb.com/stl_repair_fixing.php


Magics RP.
This software is good but very expensive.
http://www.totalcsteam.com/magics.htm

[andyj]

http://www.meshrepair.org/

[jola]

andyj, thanks for your suggestions. Which one of the different tools you mentioned would you recommend?

I assume that these suggestions mean that most of you recommend us to try to clean the STL geometry in a separate tool first and then use the clean geometry for meshing in ICEM. Anyone with a different opinion?

Personally I wish that we could find a morphing tool that is stable enough to just morph an existing design-mesh from a nominal geometry to a measured STL geometry of a manufactured part with non-conformances. But this might still just be a dream.

[Martin Hegedus]

OK, I don't know the ins and outs of this, but could you take results from an adjoint method on your nomial geometry to determine sensitivities to real world deviations? Wouldn't that tell you the areas you need to focus on?

Sorry, it's not an explicit answer to your question since I don't have one.

Edit: Maybe due to nonlinearities, the results from an adjoint method could be questionable. Don't know much about the methods.

[jola]

Martin, we want to mesh a white-light/GOM measured STL geometry in order to get a better prediction of how the real manufacturing non-conformances affect performance and losses. When we design these components we use DOE simulations and build response surfaces of our designs in order to evaluate how robust they are. We could perhaps also use adjoint methods to design a more robust component that is less sensitive to manufacturing deviations. However, running CFD simulations using the measured STL geometries gives us the answer if our designs are really as robust in reality as we have designed them to be.

[andyj]

Cad files (STL in this case) can have a lot of problems.
Someone could be simply using
cut and paste from the windows clipboard rather than using copy
from the cad program.

You could have a corrupt cad library somewhere.
Cad files usually have layers. A layer could be
designated as hidden.There could be
incompatibility between cad programs. Name clashing is possible.
You could have construction lines in the cad file which were supposed
to be deleted, but were not.
Cad troubleshooting is difficult.

There may not be a way to stop minor errors if you are scanning an object
and creating an stl file from the scan.

Once you have a complete cad file in STL format,
meshing should be easy. There should be numerous controls over
meshing.
There may not be any way to prevent the errors from occuring.

Y ou may want to fix errors in the STL format before meshing.
I am guessing, but most likely you would want
to fix the cad file before meshing, while it is still an STL file.
The errors may not be coming from your software.
The errors could be coming from a cad library that
someone else working on the project used.

Academic cad programs do not allow printing and are watermarked.
There could be an academic cad library somewhere
someone is trying to use. Find and remove all academic software
and any academic cad libraries. Same applies to Demo software.

Ansys has a cad file troubleshooting section in the manual,
But again , Cad troubleshooting is a body of knowledge all by
itself. You might want to find books and collect articles
on cad/stl troubleshooting.

You would want to benchmark your hardware to confirm correct
operation. Cpu overheating can cause problems.
Is your hardware powerful eneough to do this? Add more ram.
Most likely it is not your hardware.
Check your power supply voltages.

More cad/stl repair software:
http://www.additive3d.com/sw10_lks.htm
Forum response on stl repair and software:
http://jetset.freeforums.org/which-s...u-use-t15.html
Common stl problems:
Common issues are:
· Reversed normal’s
· Bad edges
· Holes in the mesh
· Noise shells
Guide to Quality STL files:
http://www.redeyeondemand.com/Downlo...tySTLFiles.pdf

What would I reccomend? Meshlab and either an open source repair program or Netfabb.
If money is no object, them Magics. So meshlab and an open source repair program.

A lot of people are using wrappers to fix some problems.
I am no expert, just a few thoughts.

[andyj]

Cad files require large amounts of ram on the video card. Its a case where the video card is important, not neccessarily fast, but lots of ram.
At least 1 Gb of ram on the video card, otherwise you can blue screen the machine if you try to load a gigantic stl file.

[andyj]

http://www.qcgroup.com/3d-scanning-a...nned-Files.php
Quality Controll::
By creating a virtual model of an object scanned, any measurement on that object can
be accomplished. Graphical analysis in the form of colored meshes or simple dimensions can
all be obtained from the STL data. By overlaying the STL file with the original CAD data,
deviations in piece parts can be viewed.
Point Cloud software:
http://pointclouds.org/downloads/
point cloud tools:
http://sourceforge.net/projects/simeia/
Cloud compare Software (open source):
http://www.danielgm.net/cc/
CloudCompare is a 3D point cloud (and triangular mesh) processing
software. It has been originally designed to perform comparison
between two 3D points clouds (such as the ones obtained with a laser
scanner) or between a point cloud and a triangular mesh.
It relies on a specific octree structure that enables great
performances1 in this particular function. It was also meant to
deal with huge point clouds (typically more than 10 millions points,
and up to 120 millions2 with 2 Gb of memory

On video cards I saw a 2Gb radeon 5450 for $53 USD. Not fast but fairly big.

[Martin Hegedus]

How does one keep sharp edges crisp (concave and convex) with STL data? Years ago, that was an issue. Haven't dealt with stereolithographic data recently.

Sorry, probably off topic.

[andyj]

How does one keep sharp edges in STL?
Well when you mesh it you would increase the number of vertices or triangles/polygons in the mesh.
You could do that with the entire mesh model. However, in some cases, if you did that you could make the mesh so big that it is difficult for a desktop computer to work on. In that case, you would segment the mesh and pick the part of the mesh that you want to increase the resolution of the model (just like in Paraview, increasing resolution), and similar to parallel computing, where you disassemble the task into smaller tasks and each computer works on one small task, then you reassemble it into a finished product. there may be other ways, but I like this one. Elmer Finite Elemnts software uses this method.


In CAD programs, Bezier curves have improved over the years, along with computing power to draw them.

In a CAD system , you must use "snaps" or computerized precision joining of lines. Almost all CAD programs allow a cad file to be exported as a STL file.