Chatter: Root Cause and Tap Testing

Chatter vibration is one of the biggest headache of production engineers, machinists, and NC programmers. Chatter degrades part quality, reduces productivity and shortens tool life. Without getting to the bottom of the problem, production engineers can only rely on trial and error to eliminate this problem. But what is it that causes chatter?

Like in any other form of vibration, the root cause of chatter vibrations is the flexibility of structures involved in cutting process. The most obvious flexible component is the cutting tool due to its relatively slenderness compared to the rest of the components like spindle, workpiece, or machine table; however, tool cannot be isolated from the rest of the structure. Cutting tool is held by a tool holder attached to the spindle shaft, which is supported by flexible bearings so it is this entire structural chain that is responsible from chatter vibrations. Each flexible connection can be represented with a bunch of springs and damping elements, and their strength affects the maximum depth of cut a machine can handle without chattering. Although it is mathematically possible to model spindle structure shown below, it is practically almost impossible to get accurate results due to poor analytical modeling of actual spring constants and damping values due to complex contact mechanics. Hence, production engineers resort to experimental methods when characterizing vibration behavior of a machine tool spindle.

Exaggerated Dynamic Deformation of a Spindle Shaft; and Most Flexible Joints of Machine Tools

"Tap Testing" is one of the most popular experimental identification methods. In a previous blog, we briefly talked about "Tap Testing" and how it fits into productivity optimization. Tap testing is simply ringing the tool installed on the machine using an instrumented hammer, and recording vibrations of the tool. Measured vibrations are then analyzed using complex algorithms, which is a subject of modal analysis, and flexible elements (springs and dampers elements) are calculated. Analysis and interpretation of these is a topic for another discussion; however, it is important to mention that these flexible elements are identified collectively. That is, they all interact with each other and affect each other. Since flexible elements are responsible from chatter, it means that we cannot blame the chatter purely on the cutting tool because tool holder, spindle shaft and bearings also play a role in flexibility of the tool as they all act with in one system. Practically speaking, the following statements can be made:

• same tool with different overhangs have different chatter performance due to varying vibration behavior (which is obvious);
• same tool set up (one tool, one overhang) on different machines have different chatter performance;
• same tool set up on same model machines may have different chatter performance due to differences in spindle installation (this is one of the most frustrating cases when the same NC program does not work on another same model machine);
• tool holder-spindle interface (HSK, CAT,..) affects chatter performance;
• imperfections such as rust or grease on tool holder-spindle interface degrades structural strength and hence increases the chance of chatter vibrations.

We will cover more about machine tool dynamics in future blogs, touch on each of these bullets listed above, and recommend ways of battling chatter more effectively and intelligently.

Tap Testing Set-Up