Bone drilling, despite being a very common procedure in hospitals around the world, becomes very challenging when performed close to organs such as the cochlea or when depth control is critical for avoiding damage to surrounding tissue. To date, several mechatronic prototypes have been proposed to assist surgeons by automatically detecting bone layer transitions and breakthroughs. However, none of them is currently accurate enough to be part of the surgeon's standard equipment. The present paper shows a test bench specially designed to evaluate prior methodologies and analyze their drawbacks. Afterward, a new layer detection methodology with improved performance is described and tested. Finally, the prototype of a portable mechatronic bone drill that takes advantage of the proposed detection algorithm is presented.