PRODUCTIVITY IS OFTEN LIMITED BY CHATTER RATHER THAN BY OTHER PROCESS CONSTRAINTS WHEN MACHINING WITH HELICAL END MILLS, ESPECIALLY LONG AND EXTRA-LONG MILLS. MANY PREVIOUS MODELS BASED ON PREDICTION OF CHATTER HAVE BEEN DEVOTED TO TURNING. THE FEW MODELS THAT HAVE ADDRESSED THE PROBLEM OF MILLING HAVE USUALLY BEEN CONCERNED WITH STRAIGHT TEETH CUTTERS AND VERY SMALL AXIAL DEPTH, SUCH AS FACE MILLING. NEW THEORETIC MODELS AND SOFTWARE HAVE BEEN DEVELOPED TO PREDICT THE OCCURRENCE OF REGENERATIVE TYPE CHATTER, AND ALSO OF A PREVIOUSLY UNREPORTED TYPE CALLED AVALANCHE CHATTER. THE LATTER IS THE MOST SEVERE TYPE OF CHATTER IN END MILLS, AND IS A STATIC PHENOMENON, CAUSING THE EDGE TO DIG IN WHEN A CERTAIN FEED PER TOOTH IS EXCEEDED. THE SOFTWARE GENERATES THE BOUNDARIES OF CHATTER- FREE MACHINING IN THE SPINDLE SPEED-FEED TOOTH PLANE FOR COMBINATIONS OF CUTTER, TOOL HOLDER, AND CUT CROSS