Summary

Some objects here, notably a few models of women here, do not have a flat bottom. Now I will concede that I prefer women who don't have a flat bottom, but if you want to 3d print them this is difficult.

Instructions

The slanted bottom happens on the "pink panther woman" http://www.thingiverse.com/thing:1216 and the "venus bust" http://www.thingiverse.com/thing:24188 .

I find that loading such models into cad programs and then intersecting with a cube or something like that is not possible. (I stopped trying after freecad errored out with an hour and a half of CPU time already spent on the conversions). It should be possible with more advanced CAD programs.

Compile the program. I use gcc -Wall -O2 clipstl.c -o clipstl .

Run the program with the STL file as an argument. It will tell you the ranges of the X, Y and Z coordinates.

Now you can clip off a small portion of your model by adding to your commandline minz and then a number slightly larger than the minz in your model. The program will spit out the new STL on stdout. Redirect to a file.

Bugs and restrictions:

ASCII stl is not supported.

The program is quite simplistic: It simply clips off the coordinates of vertices that lie outside the wanted region.

If your object is not convex in the area you're clipping the clipping will cause overlapping triangles. This could confuse slicers.

The program doesn't touch the normal vector. As it changes one coordinate, it should adjust the normal vector. The normal vector is used (if at all) to determine what is IN and what is OUTside the object. This will still work, even though the vector is not normal to the triangle.

I don't know why Thiniverse shows the download "clipstl.c" as "lipstl".