The problem of transforming the format of SoildWorks

[Philippe Hurbain]

1. After opening, the part defaults to down instead of up;

As suggested by Owen, it's because of y axis orientation in Ldraw system. You can compensate for that either by modeling upside down, or by using a 180° rotation matrix in stl2dat (-m 0 0 0 1 0 0 0 -1 0 0 0 -1)

2. Strange lines appear;

This comes from a slightly buggy edge lines / conditonal lines generator of stl2dat. To avoid this, I use the raw mode of stl2dat (-raw) then I generate condlines/edgelins with Edger2 (https://www.philohome.com/isecalc/edger2.htm)