Milling generated an increase in sound pressure level (Lp) of
13 dBA, which corresponds to 4.5 times higher pressure. Moreover,
by comparing the idling sound pressure of the machine in the
operator’s zone (71 dBA) and the sound pressure level in milling
(92 dBA), the sound pressure was 10 times higher. Such a
difference indicates that the noise of the machine in milling is
dominant.
Fig. 5 also shows that milling sound pressure level increased
from 84 dBA to 94 dBA when cutting speed vc was varied from
500 to 2500 m/min for the 125 mm surface mill (rotation speed:
1273–6329 rpm). In Fig. 3, it can be observed that part of this
increase is due to aerodynamic noise. Moreover, by observing the
frequency domain (Fig. 5) for the reference milling condition an
important peak can be observed. This corresponds to the tooth
passing frequency (4000 12/60 = 800 Hz) together with its
corresponding harmonics. So the increase of the sound pressure
level in parallel with increased cutting speed can also be attributed
to the increase in impact energy provided by the spindle. In fact,
the cutting power in milling can be expressed by the following
equation