Fighting chatter with nonproportional spacing

Chatter is an expensive and persistent problem when milling. Chatter vibrations can be large enough to damage a tool, cause scrap and even damage a machine. Worse, the risk of chatter may cause machine tool operators to be conservative in their selection of machining parameters, severely underutilizing machine capacity. Machine tools are commonly underused by a factor of two or more.

Chatter is a self-excited vibration, which means a steady input of energy from the spindle motor is transformed through some mechanism into vibration. The primary mechanism in machine tool chatter is “regeneration of waviness.” Essentially, the machining system, including the cutting tool and workpiece, is not dynamically stiff enough. When a tool tooth encounters the workpiece, the contact causes vibration, and the vibrating tooth imparts a wavy surface. The next tooth encounters that wavy surface, and the wavy surface causes a variable chip thickness. The variable chip thickness then causes a variable cutting force, and the variable cutting force causes vibration.

One way to stop this mechanism is to measure the dynamic characteristics of the machining system, use those measurements to calculate the stability lobe diagram and select cutting conditions in the stable region. This previously covered strategy relies on aligning tool vibration with the wavy surface. When the waves are aligned, the chip thickness is no longer variable, and the vibration stops. The stable zones on the stability lobe diagram appear when there is exactly one, two or any integer number of vibration waves between the subsequent teeth. This strategy requires knowing the stable speed, maintaining a stable speed within the acceptable spindle speed range, having evenly spaced teeth and accurately controlling the spindle speed.

An alternative strategy is to disturb the waviness-regeneration mechanism by changing the spacing between the cutter’s teeth. If the teeth have nonproportional, or uneven, spacing, then each tooth encounters the wavy surface left by the preceding teeth with a different alignment, and the vibration is suppressed. A tool with nonproportional spacing can often achieve a higher stable axial DOC than a tool with proportional spacing.

However, the gain needs careful evaluation. Because the feed is constant, a variable tooth pitch leads to a variable feed per tooth. That typically means only one tooth can take the full chip load and the remaining teeth are underutilized. The effective feed per revolution of the tool must be reduced, and the feed reduction must be matched by an increased axial DOC just to break even.