Rock drill bits factory Prodrill: R32 retrac cross bit, diameter 89 mm suitable for drifting and tunneling drilling application, easier to sharpen than X-bits, inset angles are equal on all four sides, Bits 89mm and less are easier to sharpen than button bits of the same size. In certain rock conditions, cross bits tend to produce a spiraled five sided hole (especially in diameters larger than 89 mm).