Machine tool vibrations and violin sound fields studied using laser vibrometry

Abstract: The knowledge of the dynamic behaviour of a milling process is very important for finding an optimum process window. In today's manufacturing industry the machining parameters are often predicted using experimental data from non-rotating spindles. Many times the predicted machining parameters prove to be ineffective and inaccurate which lead to reduced quality of the machined surface, tool wear, noise or at worst spindle failure. The best way to study the dynamics of the milling spindle is of course to measure the spindle response under actual operating conditions. Laser vibrometry is a non-contact, non-disturbing method commonly used for measurements of vibrations on static objects. The technique offers the possibility to measure vibrations on thin-walled (light), and rotating objects as well as sound fields. However, two major problems occur when measuring on rotating spindles: (1) speckle noise and (2) crosstalk between the vibration components. These two drawbacks make vibration measurements on rotating spindles difficult to interpret. In this Licentiate thesis the principles of laser vibrometry is introduced and the speckle noise and the crosstalk between the velocity components of a rotating spindle is studied experimentally. The rotating spindle is excited by an adaptive magnetic bearing and the response is measured by laser vibrometry and non-contact inductive displacement sensors simultaneously. The work shows that by polishing the measurement surface optically smooth we are able to avoid the speckle noise and the crosstalk problem. By using this approach, the vibrations as well as the roundness of the measured target can be resolved. Hence, the laser vibrometry technique can be used for measuring the spindle dynamics under operating conditions. Measurements on a bowed violin are performed. The chain of interacting parts of the played violin is studied: the string, the bridge and the plates as well as the generated sound field. The string is excited using a rotating bow apparatus and the vibrations from the string transmits to the violin body via the bridge and produces the sound. The measurements on the string shows stick-slip behaviour and the bridge measurements show that the string vibrations transmit to the bridge both in the horizontal and the vertical direction. Measurements on the plates show complex deflection shapes which are a combination of different eigenmodes. The sound fields emitted from the violin were measured and visualized for different harmonic partials of the played tone. However, the visualized sound field obtained by the laser vibrometer is a projection of the sound field along the laser light and the image obtained is a 2D map of the real 3D sound field. This effect is illustrated by measurements of a sound field emitted from three ultrasound transducers.