Reduce chatter marks

Milling cutter on a machine tool

Problem

For certain operating parameters in machining centers, unwanted, recurring patterns (e.g. rings) occur on the material surface: so-called chatter marks.

Solution

Measurement of the vibrations occurring at the spindle head to determine the main frequency of the chatter vibrations. A subsequent natural frequency analysis at the machining centre determines the resonant component.

Product: Measuring kit for machining centers

Beispiel

On a CNC travelling column milling machine, chatter marks occurred at a spindle speed of 1300 rpm (21.7 Hz). The measurement was carried out directly on the main spindle using an accelerometer which was coupled to the main spindle by a magnet. In order to get an overview of the occurring vibrations, the time signal of the vibration velocity was displayed in real time using the measuring kit for machine protection/machine diagnosis. The picture shows in high resolution the individual revolutions of the milling cutter and at the same time the material contact of the 5 cutting plates of the milling cutter. At a milling speed of 1300 rpm, the time interval between the material cuts is 9.3 ms or 108 Hz.

Chatter marks on a machining center

The frequency analysis carried out in parallel also clearly shows the rotational speed (21.7 Hz) and the material contact at 5 times the rotational speed (108 Hz).

By examining the natural frequencies of the spindle drive, it was checked whether an unacceptable resonance case caused the excessive vibrations and chatter marks. After excitation by hammer impact, the linear guide was found to have a natural frequency at 104 Hz. Thus, the linear guide is excited to strong vibrations by the material cut of the individual cutting edges of the milling cutter, which acted on the material surface as chatter marks. Due to a design change in the linear guide (stiffening), its natural frequency was shifted accordingly. The result was a clean milling pattern.