Frequently Asked Question

Sometimes, the stability lobes do not match with cutting tests. The stability pocket with the high depth of cut chatters, while the speed at the lowest depth of cut does not. Why?
Last Updated 8 years ago

This can happen if the machine dynamics change at different speeds. When the machine is tap tested, it is assumed that the natural frequencies and stiffness of the machine remain the same at all speeds. However, the natural frequencies and stiffness of some spindles (but not all) change as the speed increases (especially after 10,000 rev/min). The variation may be due to:

- Automatic changes in the bearing preload mechanism;

- Thermal expansion of the spindle;

- Geared spindles (low speed – high torque, high speed – low torque stages).

It is impossible to predict the changes in the spindle dynamics with tap testing at zero speed. One has to measure the spindle at different speeds, which is not practical in the shop. The practical method is as follows:

- If this event occurs, measure the noise with MALDAQ. Obtain chatter frequency using with FFT or automated MALDAQ function.

- Shift the lobes manually to coincide with the speeds recommended by MALDAQ.

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