Issues Exporting Polysurface as STP for CNC


I am trying to export a model to be machined on a 3D CNC, I usually 3D print models using STL files, but recently have had to convert to STP/IGES/ACIS or Parasolid as these are the file types the company I am using requires. When I convert a Polysrf to any of these file types, it appears to work, but would seem that when it is opened on the other end, it’s a mess of un-joined surfaces that cannot be machined.

Does anyone have any experience exporting files from Rhino 5 like this? I do not own Solidworks so cannot convert them in there and have tried to use FreeCad to convert them with no success.

It’s a pain that a closed model that I can 3D print with no issues cannot be CNC’d.

Thanks in advanced,


Maybe check:

Naked edges
Tolerances on your side compared to tolerances in the importing system.
Your polysurfaces joins correctly? At what tolerance setting?
My customers ask for 0.1 mm tolerance. So I’m modelling with 0.01 mm ( I think that somewhere in the Rhino documentation it says you have to set tolerances .1 smaller than what you want to model.) Check with these settings for joining and gaps. Nomally stp works great. Some milling machines use internally meshes for cnc milling, so they may also be able to use .stl. But then you have to get the right export settings…

And check by importing back into Rhino. Check join surfaces.

Hi Joerg,

Thanks for your response,

my absolute tolerance in rhino is set to 0.001, with relative and angle tolerance at 1 percent/degrees. When I import back into rhino it still appears as 80,000 open surfaces. I can join them together in rhino to re-create the closed polysurface with 0 naked edges/manifold edges that I had before converting.

When exporting, I have the options of AP203ConfigControlDesign’, ‘AP214AutomotiveDesign’ or 'AP214 AutomotiveDesignCC2, is there a right or wrong option to select here?

I will send my file again and see if they can simply join the surfaces at their end in which ever software they use (I will ask which software this is), but Ideally I would be able to send a correct and ready file for them.

So, when you re-import the step file there’s an option not to join very large files:

Make sure that is set to Join.


80000 surfaces sounds crazy to me. What’s that? Can the cam System handle this amount of surfaces? No chance to simplify the data?

The VDA, association of german car manufacturers has released a document on cad/cam data exchange, called VDA 4955 I believe. Maybe check your data against this document. It is more aimed at class a car design data, but lists quite some reasons why something might lead to trouble. My tolerances are more for boat or ship sized structures

If the file was not originally created in Rhino, what type of file did you import?

It was 3D scan data that I imported as an STL Mesh and converted to NURBS

Yeah…It’s lots of surfaces :frowning: , I had reduced it down from a far larger amount of meshes, I don’t want to reduce anymore in fear of loosing quality. The object is a 3D scanned Heritage ceramic tile with lots of intricate detail, a 3D printer could handle it, but I’m yet to find out if the CNC’s cam system can. I will look into VDA 4955 today, thanks for the recommendation.

I will post back on here if/when I get a response form the CNC company.

Thank you,

Ah, that explains the 80K ‘surfaces’. That is definitely not a recommended procedure.

You just need a CAM system that can machine directly on meshes and then import your .stl directly. Don’t try to convert it into a polysurface, that’s mostly useless.

I had originally sent it in STL format to protolabs,

Which they were happy to 3D print, but requested I send an STP/IGES/ACIS as they did not accept STL’s for CNC, is there an alternate company you know of that excepts STL’s for CNC?


As you’re in the UK, maybe @RhinoUK (my friends at SimplyRhino) know of someone?


Thanks Mitch,

Jed, as you’ve already seen you’ll need to find a CNC house with experience of machining from meshes which will also mean they own CAM software designed to handle STL files, not all can which is part of the reason you have not found a solution so far.

Maybe knowing what you are specifically looking for will help you find the right supplier.


You might want to look at some modelmakers in the UK, generally they can machine on meshes as it’s often called for on terrain models.

Fusion can machine direct from a mesh. I’m in the UK, what do you need matching, material, size etc. There are other headaches with meshes apart from just machining, like positioning and if any mods are made picking up the same position after the modification. I think this is part of what puts people off machining meshes.


Hi Mark,

Thanks for your response,

The piece I am looking to get machined measures 130mm x 130mm x 20mm, but long term I need to find somewhere to get far larger pieces machined, as large as approx. 1000mm x 500mm x 200mm.

I need it in plastic, or any non-porous material that I can take a silicone cast from.

Thank you,

There are ways to interpolate the mesh to a nurbs surface, without the extensive time to reverse engineer the surfaces. If your object is sculptural, this could be an option.

However, this does tend to soften detail somewhat, but it still might be preferred given the situation.
Can you post an image?

Hi cdordoni,

Here is an image, it is sculptural.

I am able to convert the mesh to NURBS and produce a closed polysurface, but I am interested to hear if you have a better way to do it.

Thank you,

One tip when working with a mesh for machining is make sure you have flat shade enabled so you see the mesh without smoothing done by the display. The flat shade is what you’ll get from machining and has given some people a bad surprise!

Not really my type of work I’m afraid and pretty busy at the moment. There are plenty of people around with routers that could handle the bigger sizes but not sure how to find someone.


Probably some kind of polyurethane modeling block that is made for machining maybe something like this. You could also use a lighter one like this and just paint the surface to seal it. There are several brands out there.

Again, many modelmakers can do this sort of work (and may be able to do the silicone mold as well). Have a search for one in your area…