Spherical part, not really spherical


So I am working as a biomedical engineer at a hospital in Sweden. We are printing everything from the most complex fetal hearts with issues I can’t even fathom how the surgeon supposed to fix, to regular office gadgets.

Now we have an interesting project where we want to design a tool that can help measure out the size of a hip transplant from CT scans. The idea is fairly simple, create a spherical object that is 30mm in diameter and a marker in the sphere that shows where the center is. We designed a hollow ball that had a smaller ball in the center with a small rod that connected the two. We also added a hole so that the sphere could be emptied of resin.

Now, after that long intro, we realized that the ball was not 30mm and did not look spherical at all. The best way to describe it I guess is that it was pointy in the z-axis on both top and bottom.

I understand that something that is sliced could not be a complete sphere, but this was not the result I was expecting.

So is it possible to mod the CAD file to fix this or is it a lost cause? The error of the model was about ± 2-3% and was not accepted by our clinicans.

Divide the sphere into hemispheres so that the flat surface can be on print bed. The location of largest curvature will be the tip of the hemisphere. The layer resolution of your printer will determine how round the sphere will be. To join the hemispheres you could print a cylindrical sleeve around the centerpoint and press-fit or glue the 3 pcs together. Leave a void at center point, fill it with a higher density plastic or steel ball for contrast in the CT scan.