Script for convert curve to arcs and lines


Is there a script if you want to convert a curve to arcs and lines at the SAME time.
Normally if you convert a curve you have to choose if you want lines or arcs.
Now I want to convert a curve to arcs but if the radius is bigger than a given number (99999 mm) I like to have lines for that part of the curve, of course within the settings: convert to lines.
This would be a very nice and helpfull tool for cnc cutting machine’s and the cam software.
I know some cam’s have there own sort of filter for this so you get a small cnc file with the accuracy you want. (not everybody owns a cam like that, me neither)
But if there is a script, your drawings will be very small as well.
I search the forum but couldn’t find it.

Thanks already,

In acad you have a plineconvertmode command but you still have big radius.

can I do it myself???

Sounds hard, to me… I do not know if any tool in Rhino or scripting that can just ‘do it’ in this case.


Maybe I’m missing something but it sounds reasonably straightforward. I would first convert the curve to arcs then go through all the segments and make line of those with a radius greater than your setpoint.

This works as a proof of concept:

import rhinoscriptsyntax as rs

curve=rs.GetObject("Select Curve to Convert",4)

#set the parameters of the convert command

#set the arc to line conversion tolerance. An arc with a radius greater than this will be converted to a line

cmd="_-Convert " + "Selid " + str(curve) + " _Enter " + "Tolerance " + str(Tol) + " " + "AngleTolerance " + str(AngTol) + " " + "_EnterEnd "


#go through all created arcs, if the radius is > the ConvertTolerance, convert it to a line
for i,Arc in enumerate(CurveArcs):
    if rad>ConvertTolerance:

#join all segments back into one polycurve

many many thanks, I am going to test it.

What does that actually save you in reality? Won’t make your file size any smaller. A line segment is a single line entry in CNC code, so is an arc segment - no matter what the radius…


Hi Helvetosaur,

This is what I sometimes get from customers (see file)
Of course many plates full of them.
With convert to arcs I get it real clean but you will end up with big radius and to big for many cnc controllers.
In my post processor I have a filter but you have to reload (import) the cnc program to see if its done correct, but you can’t check it correctly /easy because of the kerf.
I think a lot of people want to have their drawings correct, meaning as small as possible with no huge arcs before they put it into the cam.

Now I gone find out how to get the script into rhino to test it because my first attempts didn’t work
I think there will be a post.
forgive me my english

many points.3dm (80.9 KB)h


Hi Mitch,

With a customer I’ve run into the same issue: somewhere down the line, the arcs that had a too big radius would cause a error/stalling.

@E.O.Stam I believe my customer found a setting when importing that would clean up the arcs with radii that are too large. Maybe it’s an option for your customers as well? If I’m not mistaken I exported splines into a dwg and the customer’s software did the conversion.


The script above is written in python. To access Rhino’s Python editor, type in “_-EditPythonScript” into the command line. Copy and paste that script into the window then click on the green play button at the top.

You can adjust the parameters I entered to suit your needs but it should be noted that the script is only intended as a proof of concept. It has not been tested against any of your typical curves. Having said that though, once you have it working well, you can create a button on Rhino’s UI to run the script rather than from the python editor. Have a look in the Scripting category, you will find lots of examples.

For those finding this topic in the future, @E.O.Stam and I worked out a solution in private.
If anyone is in need of similar functionality and the solution provide by @wattzie is not sufficient, let me know.


Thanks wattzie and Willem this is perfect for cnc cutting my file sizes are never been so small and I have no more errors in my cam software or cnc controller.
After cutting the products looks great as well, and converting is superfast.


Hi Willem,

Would you mind sharing your script. I was just about to write my own, but thought I should make a search here before I put my mind to it.


You can now use Curve.ToArcsAndLines, see: