Tilted ball mills are disclosed in u.s. pat. re. nos. 12,778 issued apr. 7, 1908, to bonvillain for apparatus for grinding or crushing foundry sand and 927,516 issued july 13, 1909, to emerick for tube mill and such mills in which the angle of inclination can be changed are disclosed in u.s. pat. nos. 1,772,737 issued aug. 12, 1930, to wise.