Problem with step file

Good morning

I have an “old” problem showing up every once in a while , it happens sometime when i export 3d step files to be used for laser cutting purposes from rhino to other softwares.

PROBLEM : Holes in 3D are looking perfectly circular in Rhino viewport just as shown below


problem is that same hole is looking all fragmented and irregular when you open it for instance in catia or others.

if the error is not detected i receive parts looking like these


QUESTION : is there a way within reference to 3D export file to avoid such a problem ? something like “_SimplifyCrv” that apply to 3D solids or surfaces ? the error is hardly detectable cause i cannot crosscheck the export result and the supplier cannot know how original hole should looks like.

thanks in advance for your support

We would need an example file to test. This can happen if you have incorrect export settings with something like DWG or DXF, but I haven’t seen this happen with STEP exports.

1 Like

I was going to write the exact same. Looks like a DXF export

This is one of the step files that are giving me problems.

20210526 STEP EXPORT PROBLEM.stp (509.6 KB)


Looks good in the autodesk step viewer

Looks fine re-imported into Rhino as well.

  1. What if you explode surfaces and use ShrinkTrimmedSrf command on all surfaces and rejoin all surfaces again into solid?
  2. What if you explode surfaces and use ToNURBS command on all surfaces and rejoin all surfaces again into solid?
  3. What if you change precision settings before joining and exporting?
  4. What if you will export STEP without splitting surfaces (export option)?

Hi Guys and thanks for your suggestions

problem fixed , just a quick update in the hope this could be somehow useful to others:

1 ) important to say that the holes affected by problem were obtained by boolean difference with entities generated by curves which were not circular based ( arc or circles )
2 ) fragmented looking holes were just the result of visualization setting in catia , with finer settings holes were looking circular
3) MOST IMPORTANT : within reference to point 1 , when exporting/importing the dwg/dxf from any cad software to laser cam software it is very important to export curves as polilines and NOT AS SPLINE.
when exporting as spline the problems shows up and the path is deformed accoridng to control point numbers and distribution. I think amd rather than approximating original curve ( as poliline does ) the software draw a brand new spline by using underliening control points



Very helpful, I’ve ran into this issue a year ago when I started ordering lasercut parts drawn in Rhino. The Splines/NURBS discrepancies can be very hard to detect especially working with exported STEP files. Most machines & suppliers are unable to read Spline/NURBS curves and will simply generate a polyline approximating the control points .

The way I work around this issue is by using “_Convert” command on all curves used to create the lasercut before they are joined and or used to change the geometry of the resulting step file.

The simplest way I’ve found to check my exported DWG & DXF files is by first e-mailing them to myself in Gmail. The DWG/DXF Preview in Gmail can’t read Splines and will convert them to polylines as wel.

I have not found a way to change the DWG/DXF export settings that automates this because curves export settings can only be changed to Lines, poylines, 3D polylines or Splines

Autodesk DWG viewer, or any other 3D editing software that I have used to check my DWG/DXF files are capable of reading splines and will not show these export errors. So usually I add a PDF with all of the cutting contours so that the supplier can “doublecheck” for any importing/ exporting errors in the toolpaths. For many lasercut suppliers are very much unaware of Splines/NURBS export/import problems.

I tool that is able to highlight Splines/NURBS in Rhino during or before exporting DWG/DXF would be very much appreciated.

When it comes to step files, I’ve not found a single way to detect and or convert NURBS Surfaces in my polysurfaces/Breps before and or after exporting them to a step file other than making sure all of the curves involved creating the geometry are converted before applying them in operations. This is very much prone to error and can create very costly errors when receiving the finished product.

All of these variables differ when it comes to suppliers so I’d recommend doublechecking with your lasercut supplier if they can import Splines/Nurbs to their toolpaths.

Hello - I am not sure I follow - to that what would these be converted?


HI Pascal ,
I think “robbiebruinz” is referring to the possibilities to detect and convert nurbs3D based geometry in to approximated finite element objects …as seen the only way to avoid rough approximations/errors with nurbs based curves when using them for cnc purposes is to convert them in to polilines.

same concept is valid for nurbs based srf or polisrf , apparently. As tested the 3d entities based on lines or arcs are OK for cnc , nurbs based are not .

my undestanding on “robbiebruinz” point is that maybe having a 3D command that works same way as “convert” command used for curves could be very useful for fabrications purposes .