# Practical Estimation of Machine Tool Spindle Dynamics for Maintenance Decision Making
## 5 Conclusions  
While CBM programs for machine tools have promised reduced failures and downtime costs, the end results do not generally measure up to this ideal. One reason for this shortcoming is that standard vibration monitoring criteria, such as ISO 10816 and ISO/TR 17243, neglect important dynamic characteristics of the machine, such as natural frequency, dynamic stiffness, and damping ratios, in favor of vibration RMS magnitude values, which are easier to collect. In this article a mechanical impulse generator was designed and fabricated to assist in the collection of FRF data from machine tool spindle units. This device is small, robust, requires no power for operation, and can collect data from both static and rotating spindles without endangering the machine tool operator. For the purposes of modal analysis, the performance of this device was found to be comparable to results obtained through conventional impact hammer testing. An automated data collection protocol was implemented in a CNC machining center using this new device and G-code programs to collect FRF data from the spindle. The automated protocol enables the routine monitoring of the spindle dynamics characteristics, which is valuable for CBM programs but is often too costly to collect on a regular basis. The limitations of the impulse generator are observed through some undesirable characteristics in the input force spectrum. The device is capable of exciting frequencies up to 2000 Hz, however some stiff machine tool spindles may have natural frequencies beyond this range. Furthermore, the spectrum was observed to have harmonic components, which must be accounted for in the analysis. Higher frequencies were shown to be excited by the selection of a harder material for the impact tip. Harmonic oscillations were shown to be mitigated by selecting spring and mass parameters to push the oscillations outside of the measurement range, or the effects were suppressed by the material selection to dampen undesired oscillations. A case study was conducted over a five-year period on a fleet of CNC machine tools in an automotive engine production facility. The FRFs and modal characteristics of these spindles were periodically monitored throughout the case study using the methods outlined in this article. An early warning of spindle failure was detected through modal analysis, and cost saving maintenance decisions were able to be made based on the data available from FRF monitoring. The automatic test routine for modal analysis of machine tool spindle units presented in this article affords opportunities for enhanced data collection for condition monitoring tasks, condition-based maintenance, and dynamic assessment of machine tools. Conceptualization, P.C. ; methodology, P.C. ; software, P.C. ; validation, P.C. ; formal analysis, P.C. ; investigation, P.C. ; resources, S.C.V. ; data curation, P.C. ; writing--original draft preparation, P.C. ; writing--review and editing, J.M.D. ; visualization, P.C. ; supervision, S.C.V. ; project  
Figure 35: High torque warnings were attributed to a damaged support bearing after a complete tear-down investigation was performed: (**a**) spindle rotor removed from the housing; (**b**) damaged rear support bearing. * [PERSON] and [PERSON] (2006) [PERSON]; [PERSON] Centrifugal force induced dynamics of a motorized high-speed spindle. _Int. J. Adv. Manuf. Technol._**2006**, _30_, 10-19. [CrossRef]
* [PERSON] et al. (2011) [PERSON]; [PERSON]; [PERSON] [PERSON] A comparative study on the dynamics of high speed spindles with respect to different preload mechanisms. _Int. J. Adv. Manuf. Technol._**2011**, _57_, 871-883. [CrossRef]
* [PERSON] et al. (2013) [PERSON]; [PERSON]; [PERSON] A new proof of rule of thumb regarding clearance variation with temperature in two tapered rolling bearings in O-arrangement. _Acta Tech. Napoc._**2013**, _56_, 789-794. * [PERSON] et al. (2018) [PERSON]; [PERSON] [PERSON]; [PERSON] [PERSON]; [PERSON]; [PERSON] Influence of spindle condition on the dynamic behavior. _CIRP Ann._**2018**, _67_, 419-422.