Impact Testing

Let’s talk about impact (or tap) testing, where we use an instrumented hammer to excite a structure, such as the free end of a milling tool, and measure the vibration response with a transducer, such as an accelerometer. The purpose of this test for milling is to measure the vibration response of the tool-holder-spindle so that we can use that information to select optimum cutting conditions.

This video shows the hammer force input (red) in the top panel and the vibration response (blue) in the bottom panel. We see that the impact causes the structure to vibrate at its natural frequency and that the vibration decays over time. This decay is due to damping and the more damping that exists, the quicker the vibration attenuates. The moving dots let us see that the vibration continues after the hammer impact ends. This also occurs for your cutting tool as it exits the cut in partial radial immersion milling. The force ends, but the vibration continues.

The reality is that every tool-holder-spindle has a dynamic response that is waiting to be revealed by the impact test. This offers a tremendous opportunity to improve machining productivity and reduce the trail and error that’s often required to select cutting conditions in the CAM program.