How to make relief cuts for 3d part removal

Hi, I am working with complex 3D shapes made by pairing opposing irregular closed curves in parallel planes and using sweep2 to define a surface between them and cap for the planar surfaces. This all works very well. The problem now is CNC cutting such 3D shapes from flat sheets of material. I am only interested in the material left behind after the cutting (ie. the negative), and not the material that is being cut out. Here is the core issue: how can I create relief cuts in Rhino, that will allow my 3D parts to fall out from the flat material? These relief cuts would be in the parts being removed. Any help would be greatly appreciated. Thanks!

Do you have a picture? I’m not sure I follow exactly.

If you need to cut curved shapes from flat sheet (metal plate, fabric etc.) then unrollsrf is what you’re after. This will unroll/flatten developable shapes.

I’m not sure why you need to define cuts in Rhino, unless you plan to hand-code the G-code. Normally one would model the part desired, export in an appropriate format (usually STL for a 3D part) to a CAM program, and let that package decide where to cut to leave the part. The CAM program will figure cutter offsets and allow you to choose machining strategies (2D or 3D milling, one or two sided, etc.) If you have overhanging areas the mill may not be able to reach all the surfaces to be cut.

It sounds like you have modeled the negative space, but the CAM package wants to know what the positive looks like, so I would model the sheet and then boolean difference out the parts you have already modeled.
I could be totally missing your question, if so than you should post a picture as Ncik suggested.

Hi, thanks for your reply. My fabricator’s waterjet CAM program apparently cannot deal with my files; so they are manually going in and defining a rough relief cut 3D shape, along with and a whole bunch of radial cuts between
that relief cut and my artwork. This seems crude and inaccurate to me, but it appears this is their only option (short of their buying some other CAM program). I have uploaded a simplified model; I will be doing hundreds of these and they are all highly irregular shapes far more complicated than this. Despite the fact the machine can only cut in straight lines through the substrate, there are numerous areas there the cross section of the cut is hourglass shaped, with overhangs in either direction. I am only interested in preserving the negative volume after the final cuts are made. I would like to provide them with a generated relief cut that is very snug to the final cut and that passes only vertically through the material.
For example, with the uploaded file, if I take the Silhoutte of the 3D surface, flatten that silhouette, take the CurveBoolean of the large interior region of the flattened curves, and extrude that result to the desired thickness,
then compare the resulting solid with the original, it appears to fit snugly, with a possible area or two that are questionable. I am wondering if there is a better way.

3d example for relief cuts.3dm(161.6 KB)


I’m still confused with what you are trying to achieve. Having looked at your file I will say you have an unnecessary number of control points defining your curves. I rebuilt your curves reducing the number from 365 to 90. As you can see it barely altered the shape. I would reduce the number even further and then manually reposition some of them to achieve the desired shapes.

Hi, please reread the thread. If this is too technical for this forum, just let me know.
The issue has nothing to do with reducing control point counts, but thanks for the beginners tip.
I do not want to change ANY shape. I AM looking to cut these preexisting EXACT shapes from solid panels, but I need to define relief cuts first, since these shapes will not fall out from their panels after cutting…ie. they are trapped.

If I understand correctly you are trying to cut shapes from a flat sheet with “draft” (for want of a better word) on the profile edges. Hence the need for relief cuts.

I’ve never seen a 5 axis waterjet cutter, which would be the only way to cut this directly. If your fabricator has such a marvellous beast, then supplying them with good closed meshes of your parts will allow them to calculate the toolpaths.
Are you going to manually add the z axis bevels?
Are you trying to work out an average cut that will allow you to fill the gaps with adhesive/weld on assembly?

How about creating a relief cut in x-ray display mode that will allow the bulk of the material to drop out. Then just break out the remaining trapped material by hand.

Indeed; 5-axis waterjet is not just scifi anymore; it is an extraordinary invention! :slight_smile:
I have provided 3D solid models to the fabricators at their request.
Still they require a huge amount of man hours to accurately lay out the relief cut paths which will be cut inside parts such as that which I uploaded, and before the final 3D cuts for the artwork. I was hoping to help them along by providing
an intelligently laid out cut plan for these relief cuts.
There will be no bevels. I would like to provide a simplistic “average” cut with vertical side walls, ie. straight through the substrate (instead of the twisty turny side walls shown in the uploaded file).
The idea being that I give them a snug fitting vertical walled interior cut path which they cut first and quickly.
Then they simply start cutting the final cut path; at each point in the cut path where the cutting head articulates through the 90 degree mark (vertical to the substrate), the final cut path will pair up with the former relief cut path, and a sub-part will fall out (be freed from the substrate). The idea being one single “intelligent” relief cut for a very complicated part.

It looks to me that what you plan to do will work. You may want to .make a macro that does all the steps at
one time… I assume you are setting the cplane and view angle to one of the planar faces before running silhouette. You may want to offset the final curve outward by the thickness of the waterjet kerf… With a macro you should be able to do each object with a couple clicks of the mouse.


Won’t the relief cuts free the whole part and risk it shifting before or during the final cut?

Ok, thanks, I will make a macro.

I am only interested in the material that remains, not the material that is cut out. Thus, yes, the part that will be freed and risk shifting will be discarded.