Full Version: CURVE TANGENCY - CONTINUITY RESOLUTION

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Any advice on curve tangency and is there a resolution level within OPENNURBS ? if I draw a series of arcs all tangent to each other whether in MOI or Rhino and try to import the part into PIPEBEND software it doesnt read it.
If I import the same data as a DXF into Solidworks and snap each tangent with tangent lock and re export the same file the PIPEBEND software reads the file without problems. If I take the same arc set of curves and insert small straight sections that are all tangent between each arc and import that into PIPEBEND out of MOI/Rhino what imports AOK.

When drawing with the MOI continuity arc tool what level of continuity does it uphold?

Are these just natural limitations with the OPEN NURBS structure?

I lifted this on a quick web search.

Tangency continuity (G1) ensures that two curves or surfaces meet smoothly with the same tangent direction at the shared edge, while curvature continuity (G2) adds the requirement that they also have the same curvature at that point.

Image Attachments:
arcs.jpg 

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Hi Alex,

re:
> if I draw a series of arcs all tangent to each other whether in MOI or Rhino and try to import the part into PIPEBEND software it doesnt read it.

Can you please post the .3dm file that has this series of arcs in it?


> If I import the same data as a DXF into Solidworks and snap each tangent with tangent lock and
> re export the same file the PIPEBEND software reads the file without problems.

If you can also post the DXF output from SolidWorks then I might be able to compare them to see what the pipe bend software is sensitive to.

There is a relatively common bug with DXF importers that they don't handle circles and arcs with a plane normal pointing in the negative z axis direction.

In moi.ini there is a setting under
[DXF Export]
FlipNegativeZAxisCirclesAndArcs=n

You might try turning that on and see if that works better.

- Michael

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Some previous discussion here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=11231.7

- Michael

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Hi Michael,
Many thanks for speedy reply - Here is master DXF from moi and half modified from within solidworks the curve data is made into a solid and the step file is the import format into PIPEBEND.

I have locked the tans in the bottom half of the solidworks export file see if you can see a difference. Ill go over the old conversation.

Attachments:
LEG01 EXPORT SLD.DXF
LEG01.dxf

Image Attachments:
LOCKED TAN.jpg 

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Hi Alex, so it is the negative z axis thing.

The difference between these files is that "LEG01 EXPORT SLD.DXF" has the normal for each arc's plane pointing in the +z world axis direction.

The file LEG01.dxf has some plane normals in the +z world axis direction and some in the -z world axis direction.

It's a fairly common bug for DXF reading code to not do the AutoCAD "arbitrary axis algorithm" properly on a normal that is pointing in the negative z world axis direction. It's likely that the pipe bending software has this bug in it.

It's possible to flip any arc with a -z plane normal to go in +z instead but this will have a side effect of reversing the direction of the arc after it's flipped.

Some CAM workflows need the arc direction to stay undisturbed so that's why MOI doesn't flip them by default.

But you can go to Options > General > "Edit .ini file" button and inside moi.ini in the "[DXF Export]" section set:
FlipNegativeZAxisCirclesAndArcs=y

That should then flip any arcs that have a plane normal in the -z axis direction to the +z axis direction instead and avoid the bug in the bend software.


Does it work ok once that has been set?


For Rhino there's an export option "Flip arc normals to +Z" that you can turn on.

- Michael

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Hi Michael,
Just tried that change but still no go its like it cant read the center line for profile. Strange as if I import the same DXF profile center line into solidworks and just lock tangents and re export it all imports correctly. It would be great to have the complete workflow coming from MOI as it wins hands down in modelling time. At this stage I am doing all the prep in MOI then export the individual master profiles into solidworks for export from there which works.

If not an easy fix all good Ill keep doing the same workflow just made me wonder if tangency did have a finite point in the transition from arc to arc.

Image Attachments:
ERROR.jpg 

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Hi Alex, can you please post the new .dxf file you generated from moI with the FlipNegativeZAxisCirclesAndArcs=y set so I can double check it?

> just made me wonder if tangency did have a finite point in the transition from arc to arc.

It doesn't seem like that's the problem, the arcs seem to be tangent to a very tight tolerance. It's more likely to be some difference in ordering or directions.

- Michael

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Thanks for the exploration here is the test file - I have marked the start curves with the MOI script when I draw i start at one end and manually trace it out from the same direction.

Attachments:
TEST ARCS.3dm

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Hi Alex, I'm kind of confused about the different file formats being used.

I thought the recent thing I was checking was the difference between a DXF file exported out from SolidWorks vs a DXF file exported out from MOI.

There was a difference between them, the SolidWorks one had all arc plane normals pointing upwards in +z. After setting that moi.ini flag then the DXF file exported from MOI should also have all arc plane normals in +z axis direction.

But then your last message has a .3dm file not a .dxf file?

- Michael

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Hi Michael,

So many programs so many workflows sorry it gets confusing - here is how the parts get into Pipebend.

With the pipebend software it imports 3d STP files - so I start with the governing center line I create the path curve and shell that to import as a STP. ( No Splines - All arcs and polylines only)
To move between MOI and Solidworks I export the DXF file only which I then lock the arc nodes in SW's and then shell followed by exporting the 3d STEP to import into Pipebend.
Somehow Pipe must read the outside surface radius and then construct a new Centerline from which to CNC control the motions. No dxfs are imported directly into pipebend not sure why they chose this workflow but the PIPEBEND software is made in China I have tried to contact them but no avail.

Here are their only 2 videos on Youtube - not alot of traffic but from what i gather all the tube bending machine makers use their software.

https://www.youtube.com/@yishansoft1510/videos

Many thanks for your continued assistance - appreciated.

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Hi Alex, thanks. It seems like it will be difficult to figure out what is going on.

I don't understand why they would want to take a 3D model and attempt to reverse engineer the centerline from that instead of just taking in the centerline directly.

I think that you're lucky that there is any method that works reliably.

- Michael

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Yep I agree I would have thought a DXF whether 2d or 3d imported would be the way to go but that is how these machines work - all good MOI exported STP files work 80% of the time I its just when there are multiple arcs back to back that it tends to fail and that is where the SW lock tangent gets them over the line.

Thanks for looking into it for me.

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Does that error message come upon initial import of the STP file or is it generated by the post processor engine generating GCODE in pipebender?

The files to compare are the stp files not dxf.

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Hi Burrman

The error comes up with the STP import before getting to Gcode stage so its definitely something in the tangent to tangent structure as the only change I make to the same underlying curve pathway in SW is to lock the tangents - which must mean there is a further degree of tangency that perhaps the PARASOLID engine enables for the PIPEBEND software to read the 3d part.

Thats just my current thinking.

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Ok. I was trying to apply my CAM softwares thinking when it is parsing arc and such. The post processor has several setting for various machines and software outputs.

Break arcs into quadrants. Combine more than 180 degrees. Etc… etc… something about signed arcs and the processing into xyz coords that the machines controller reads. There are tons of controllers. Not all created equal.

Was taking a guess at what “lock tangents” is actually doing or setting in the step file with regard to arcs. Was trying to figure out if yyspipebender is actually trying to post process the geometry and needed one of these specific settings

Maybe if we can see the 2 stp files we can figure that out…

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Thanks Burrman,
Here is my workflow - file created in MOI
Export dxf into SW
Lock Tangents in SW
Export Step
Import STP into PIPEBEND

Attachments:
1TEST ARCS.3dm
2 TEST ARCS.dxf
3 TEST ARC SW.STEP
4 TEST ARC SW.DXF

Image Attachments:
20250320_104353 (1).jpg  LOCKED TANGENTS.jpg 

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Thanks for follow up Alex. I will certainly look over you outpit and locked tan files a bit later.

With my last post and just “looking at the screenshot” in your last post, it appears that last arc is greater than 180? (Havent verified this yet)

If it is, try adding a couple trim points to make it 2 or 3 smaller arcs and try it without the locked tan process to see if yyspipebender will accept that….

I’ll respond again after i get a look at the stp file output

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Hi Alex,

I took a look at your file "3 TEST ARC SW.STEP"

Pipebend likes this file, correct?

Would you also post a STEP that Pipebend rejects?

- Peer

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""""""Would you also post a STEP that Pipebend rejects?""""""

Take the MoI 3dm file and save it as a STP...

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Thanks here are files

Attachments:
TEST ARCS MOI EXP.stp

Image Attachments:
IMPORT FAIL.jpg 

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