Rotary shear machines are becoming a popular tool for rock friction experiments. Such machines are of particular interest to study conditions approaching those of natural earthquakes (slip rates of one meter per second or more, normal stress of several tens of MPa, total slip up to several meters). One key issue for a successful rotary shear experiment is the use of a very carefully and precisely shaped sample. Imperfect samples induce misalignment and spurious vibrations which alter the mechanical data or induce sample failure in the early experimental stages. Traditionally, the cylindrical or annular sample is shaped from a block of the rock which is the object of the test and then introduced in the machine sample holder, either with or without a metal jacket slid around its...