I think I want to try to use a script to make a negative

General CNC CNC Routers
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I think I want to try to use a script to make a negative (mold) of an STL file and then use the mold STL after manual modification to run my CNC machine. Does this sound crazy? I imagine I could use Cura for the slicing and that thing is damned fast. My 3D printer profile is 0.2mm layer height, so I would want to change that to 1mm or 2mm layer height, I think.

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Have you tried any of the 3D CAM packages? 3D printer paths use a weird infill approach that is very different from cutting paths.

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So are you wanting to use cura to generate a tool path for your mill? If so this won’t work for many reasons. Cutting paths use very different strategies, retraction and engage moves, tool profile compensation and cutter comp, fixture avoidance etc. Not to mention the g-code is formated differently.

I’m sure you could mod cura to do this but by the time your done you will have rewritten massive amounts of it. Your better off using something like the new CAM that’s been added to fusion 360 or something similar

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I looked at doing something similar. I recall I find a openSCAD model on thingiverse that did this. You loaded a model and it created the negative inside two halves of a mold. Had to fiddle a little to get my model to fit and be centred.

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@Daniel_Would So, did you mill the “mold” (really a negative)?

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@NathanielStenzel I’m afraid I didn’t get as far as trying to mill it. Was having trouble with my machine and subsequently have moved house. It’ll be a while before I’m in a position to try it for real, my only question remaining was how to tell the to path generator enough about the router to avoid it hitting the sides

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@Daniel_Would make sure you put it in an xyz location that is all positive and near 0,0,0 and then set the 0,0,0 in a good location, I guess.

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@Daniel_Would the mold would be flipped too.

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Ok. I know this much so far:

  1. Not all commands written by Cura are valid on gShield (runs Grbl)
  2. The movement from Cura should still be as efficient as usual, but you need to make your infill 100%
  3. Do an air cut with the gShield freshly reset after putting the end of the bit where you want it.
  4. There seems to be some scripts for OpenSCAD and others for making molds and something similar to that is what you need for your cutting.
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As far as negative molds go I made one for a bracelet but used a 3D printer to print it, but I used a boolean function in freecad and subtracted the model from a block and then cut it to make the mold i can post pic if you’re interested.

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You could just model a box around the 3D model, disable “fix horrible type A” and Cura would invert it.
However, for CNC milling paths, I’m not sure if Cura will do that properly. You might want to set the infill-overlap to 100%, as that will help. And you most likely have to post-process the GCode so it works properly on a mill.

I do have access to two big mills now. So I might do some milling GCode generation application soonish. But I care more for 2.5D milling then 3D.

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@Michael_Scholtz I would like to see those pics.

@Kyle_Kerr my mill bit is an end cut bit, so 0% infill would not be appropriate. My bit would snap.

I think Kyle is right, the flipping of the mold would have to be done via a post process script or positive Z would have to be down. If you use a post process script should probably tell a person if the part will reach the bottom of work material and if it will fit within the work piece.