Creating perfectly aligned negative (female) only features to align cut parts

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1

Just can’t seem to figure out what to do here.

I have a completely irregularly shaped object to print. There is not a single flat plane anywhere on it. Think about an oddly shaped rock.

I need to cut it into two halves and I need those two halves to be oriented down toward the print bed when I print, because I need those large flat surfaces to ensure that it adheres to the plate. This print has failed multiple times because even using supports, brims, etc, because of the shape there’s really no large area to put onto the bed. It’s large and those small points of contact keep failing even with large brims.

I want built in features to align the two halves after the print. They cannot be a “plug and socket” because the raised plugs will not allow that flat cut plane to fit down on the plate. It will require the whole face have support under it which I’m trying to avoid.

What I want is sockets in both halves, aligned to each other, so that I can print separate dowels to go into both halves after the print. The problem:

Orca seems to have no way to do this.

Doing it in CAD is problematic. The file is an stl only. Tying to convert it to step has been a complete debacle. It’s just too complex a shape for that. Being a mesh and not solid I don’t know of a way to add the features I want in Shapr3D to then export for the slicer.

Any ideas?

2

Use Dowel in slicer.

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3

Well that was easy. Thanks. Didn’t realize that what dowel did. I assume the fact that it even created a plug for me means it worked, but it doesn’t look like there’s actually any holes in the halves.

Screenshot 2026-05-20 175941
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4

I’m not sure why this would be hard in Shapr3d. You can split an stl (a mesh) using a plane. Then you can create your plugs using sketch and extrude. Orient the plugs exactly where you want them in the mesh. Use the plane for alignment since that is already aligned to the split. Then you can use subtract to remove the plug shape from the mesh. You’d have to work out the tolerances (e.g. oversize the plug before using subtract.

If you need to lay the part flat and the plane is not at a simple angle, just measure the angle(s) of the plane used for splitting to the destination plane and then rotate the mesh accordingly.

5

Here showing what both @shaprvision and @Xdrakosha are suggesting.

Note to scale down the plug to take into account 3D printing tolerances.

6

In my personal experience, on a well calibrated printer you can go as low as 0.05mm clearance for a press-fit. 0.1mm to be on the safe side, you can always use a drop of superglue.

The main thing I do for pegs that need to be press-fitted is slightly chamfer all the edges of the peg but not the holes. The corners are usually the place where precise fitting fails with FDM 3D prints.