No matter how tight I tun down the locking screw , my tool post rotates very slightly when turning between centers! The post is an "A2Z" qr post with an "A2Z" tool holder using a carbide (removable)tool and the work is a steel 3/4 in. dia. bicycle pedal crank. I have tried a short extension on the 3/16 in. locking screw Allen key with no success. The lathe is an EMCO 8 in..X 18 in.