The failure problem of the thread gauge used in CNC lathe was introduced. The concept of deviation ratio was introduced and the relationship between the deviation diameter ratio and the diameter tolerance and the pitch was deduced. The conclusion is that the deviation diameter ratio causes tooth type halfangle diffusion directly.The main reason for the problem is that the thread tool tip is not just on the rotation centre plane of the workpiece.Some advice on how to judge it and how to deal with it were provided.