The micro-machining of various (hk0) silicon plates in an aqueous KOH solution was studied. Orientation effects, the formation of limiting facets and concave or convex undercuttings have been analysed for (hk0) membranes and mesa obtained starting with circular masks. Procedures have been derived to predict 3D etching shapes using a tensorial model for the chemical etching. Predicted shapes for membranes have been found to closely agree with experimental shapes because of the important role played by limiting {111} facets. Theoretical general shapes for etched mesa were also in crude accord with experiments. Consequently some attempts have been made to identify limiting (hhl) planes which can participate to convex undercutting and to discuss the role of the dissolution slowness surface.