Sunday, February 2, 2014

CNC Corner Fix Utility

I created a Rhino Python script to quickly draw the geometry to allow the CNC router to clear out material in corners.

Download Here
(Note: This a Windows only plug-in. It will not work with Rhino for the Mac.)

To run the script load it into the Rhino Python editor. From Rhino type EditPythonScript. This launches the Python editor.

Choose the Open icon and load the script. Select Run Script from the fly-out menu shown.

The user interface is shown below.

Choose if you want to clean the corner material with a Router bit or a Drill bit. You can use a drill bit with a small diameter to make the least visible mark. Choose Router if you don't want to change tools and just cut the contour into the corner.

Choose the diameter of the bit you wish to use. Then set the Shift Cut %. For the least visible cut use the default of 99%. This will just barely cover the corner. If you change it to 0% the arc will be centered over the previous intersection point. Here are a few example of a router path to visualize the difference (the polylines intersect on the grid intersection points):

Click the Select Lines button. Select the two line segments leading into an inside corner. The arc will be drawn. Note: The program always draws to the inside corner - there is no reason to add arcs to outside corners because they can be cut without them. If you are using a drill bit a center point is drawn as well as the circle. Not trimming of the corner is done.

There is an extra 0.001" added to the width of the arc and lines drawn when routing. This allows CAM software toolpaths to enter the arc. Without this, particularly in Mastercam, the corner arc will be skipped.

Note you can fully operate Rhino as the script runs. This lets you explode, undo, join, whatever... as you fix up the corners. To quit the program just close the interface.

Important Safety Note

If you make the corner curve very small (as is shown above) you run the risk of burning the material. That's because the router bit is not moving much as it clears the corner material. This leaves the bit spinning in place (or almost so) and heats it way up. Therefore be careful when using the circle setting. It's better to drill the holes (for the smallest amount of material removed from the corner) or use a larger circle so the bit will continue to move as it clears out the corner. That is, use a diameter larger than the bit.

Here's an example of the burn that can occur - this is doing some pocketing (before the finish pass was cut):

In this case, there were so many corners cut this way, the bit go so hot that the spoil board beneath the material caught fire. That's never good.

Current Limitations:
  • The program works on degree 1 curves only (polylines). 
  • The polylines need to be open. So if you have a closed polyline explode it first, then use the tool. 
  • The polylines need to intersect at a point or cross over one anther. If they don't fillet them with a radius of 0 first. Then select the lines. 

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