Problem with circle holes when exporting for CAM

Hello everyone
I’m having issues with some models when opened in Alphacam and I can’t figure out if it’s a geometry, compatibility, tolerance problem or what else. Here the facts: The model is composed by various parts to be carved out from a wooden panel. Every hole in the parts is obtained with MakeHole command from a circle curve.
The model is then exported from Rhino7 in 3dm for Rhino5, then converted in IGES ( from a rhino 5 user ) and then loaded in AlphaCam for the CAM part, but the issue occours even when converted directly in IGES from Rhino7.
Alphacam fails to recognize any of the circle holes.

Strangest part is when measuring the diameter on AlphaCam I get different values depending on where I take the measure.

Any idea on what could cause this issue ?
And maybe how to solve or work around this ( in Rhino ) ?

Thank you !

Here’s one of the actual models:

laz_int3.3dm (4.4 MB)

Hmm, the holes look nice and circular here in the Rhino file you uploaded. What IGES export scheme are you using? If you are having success with Rhino V5 iges exports but not with V7, can you post a sample .3dm file along with an iges export of the same part from both V5 and V7 so that we can analyze the difference(s)?

Also, have you tried STEP instead of IGES for your exports?

Hi @Helvetosaur,
When the model is generated and exported from Rhino7, the only way to automatically get the circles in CAM is to extract the curves and import them as curves ( it’s not a 100% accuracy rate, though ).
When is generated in Rhino5 it recognizes the holes as circles also in a 3d model.

Both use the default IGS settings with 0.001mm tolerance.

Here two models generated with the two different versions that behave like above mentioned

Rhino.3dm (25.9 KB)
Rhino5.igs (9.2 KB)
laz_int3.3dm (4.4 MB)
Rhino7.igs (472.4 KB)

With STP format it’s the same

OK, I think I see the problem.

If I use Open and open your two iges files in Rhino V7, I get this:



i.e. they are the same structures - NURBS circles. Why are they NURBS circles ? Because they have been extracted from 3D geometry edges.

If I do the same thing in Rhino V5, I get this:



So, what looks like is happening is that V5 is automatically simplifying the NURBS circles to “real” circle objects - which is what your CAM program looks for to tell it that this is indeed a circle (hole).

I don’t know why V7 exports the IGES differently than V5. As you are exporting 3D objects and not 2D curves, that makes a difference. I tried messing around with some of the iges export settings, but it still always exported as rational NURBS surfaces. So the answer is, I don’t know… @chuck ?

If it’s just curves your exporting, running SimplifyCrv before exporting will convert NURBS circles to real circles, which should help.

Another trick is that drilled holes in CAM programs may only need a center point for the bit.

So, just sending centerpoints also may work.

@Helvetosaur Mitch, since you pinged me… I don’t know much about our IGES IO. I’ll find someone who does. Meanwhile, I looked into your observation of NURBS Circle in V7 vs Circle in V5. It looks like V5 did not distinguish between the two. If I open Rhino.3dm in V5, duplicate one of the circular edges, and call What, It tells me I have a circle. If I call List, instead, which is much more detailed, geeky, and often useless for anyone besides a developer, I see that I really have a NURBS. Somewhere between V5 and V7 we must have started distinguishing between the two.

As far as we can tell, nothing has changed in the way the edges are written to IGES files. They’ve always been written out as NURBS. This is most likely what is causing your issue. IGES is a fairly outdated file format and as such we haven’t put much effort into it in quite a while. Our STEP exporter has the same issue with circles, but it would not be difficult to fix that. I can do it for one of the next V8 WIP releases. Are you able to work with STEP files? I made this YT for STEP. RH-72940 STEP export should use circle entities

We have made a YT issue for IGES, but I would not expect it to be solved in the near term. RH-72939 Support CIRCULAR ARC ENTITY (TYPE 100) in Iges Export…

@Lightafuncool Here’s a STEP version of your Rhino.3dm file that has the circular edges represented by circle entities. I know Rhino will read them back in as NURBS, but I’d like to know if your CAM application detects the holes. If you can test, please let me know.
RhinoCircleTest.stp (16.9 KB)

Hi @chuck, yes with that STEP version it detects all the holes

Great. The next V8 WIP, which may be available later today if there are no major issues, will have this change to STEP export.

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This is in today’s WIP. The change was too recent to make the What’s New post.

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Awesome, thank you !