Method of controlling chatter in a machine tool

A method of controlling chatter in a chip cutting machine tool cutting at a predetermined depth on a workpiece and in which the speed of rotation of the cutting tool relative to the workpiece and the rate of feed of the cutting tool relative to the workpiece are both adjustably controllable between predetermined minimum and maximum rates for the particular combination of machine tool and workpiece material, said method comprising the steps of

• a) using a vibration sensor to detect signal levels representative of relative vibrations between said machine tool and said workpiece and generated by a preliminary operation of said machine tool at predetermined conditions of cutting depth, rotational speed and rate of feed;
• b) processing said generated signals to obtain measurements of said signal levels generated by said preliminary operation as a function of signal frequency;
• c) obtaining a signal indicative of said rotational speed during said preliminary operation and determining the rotational frequency therefrom;
• d) determining a threshold of said signal levels such that said signal levels above said threshold for predetermined frequencies that exclude harmonics of said predetermined rotational frequency are indicative of chatter vibrations between said cutting tool and said workpiece;
• e) operating said machine tool at a predetermined said speed of rotation and a predetermined said feed rate with respect to said workpiece and detecting said signal levels operationally generated by operational cutting engagement between said cutting tool and said workpiece;
• f) if said operationally generated chatter signal levels exceed said threshold, then
• (1) interrupting said feed of said cutting tool relative to said workpiece and
• (2) changing said speed of rotation to an adjusted said speed of rotation so that said rotational frequency becomes generally equal to one of a predetermined set of subharmonics of the frequency of the highest said generated chatter signal level unless said speed of rotation is found to be substantially below said maximum rotational speed in which case said speed of rotation is changed to said maximum rate of speed of rotation, and then
• (3) resuming said feed and said cutting engagement with said workpiece and operating said machine tool at said adjusted speed of rotation;
• g) repeating steps e) and f) until operation of said machine tool is obtained without said operationally generated chatter signal levels exceeding said threshold, up to a predetermined maximum number of said repetitions, and, if said operationally generated chatter signal levels exceed said threshold after said predetermined maximum number of repetitions has been made, then reducing said speed of rotation and said rate of feed to predetermined maximum levels known to avoid said generation of said chatter signal levels in excess of said threshold, whereby chatter-free cutting of the workpiece is sought at cutting rates greater than the speed known to avoid chatter but with reduction to that known speed if higher cutting speeds cannot be obtained without chatter in the predetermined maximum number of attempts.