Abstract: In a general aspect, a machining tool includes a diamond crystal having a working section configured to machine a workpiece. The machining tool includes a body and a shank extending from the body. The shank defines a cylindrical slot outside the body. The machining tool includes an assembly carried by the shank. The assembly includes a cylindrical pin that resides in the cylindrical slot. The assembly also includes a diamond crystal secured to an end of the cylindrical pin. The diamond crystal has a curved perimeter that defines a working section, which contacts a workpiece during operation of the machining tool. The shank is adapted to allow rotation of the cylindrical pin within the cylindrical slot to modify the working section.