Biomedical Engineering Reference
In-Depth Information
speed is lower than 60,000 rpm for longer drills and even 120,000 rpm for
short drills. Authors have also explored the effect of drill geometry - coolant
holes, web taper, and number of flutes - showing that increasing the web taper
causes a slight decrease in bending frequency, the increase in the number of
flutes increases the stiffness of the drill bit and the increase in the coolant hole
size slightly increases the bending frequency of the drill. The authors also
investigated the effect of material properties along the drill bit, showing that
the natural frequency of a steel tool increases by 26% if steel is replaced by
carbide and 20% if the drill was made as a combination of steel and carbide.
2.4. Chatter Suppression
Chatter suppression has been one of the main directions of research in the
area of vibrations in machining. Chatter control in turning and milling has
been extensively covered in [28-37]. Spindle speed variation methods have
been successfully applied for chatter suppression in turning and milling
operations. Reference [38] presents a spindle speed variation method for
chatter suppression in drilling. It has been shown that based on the stability
charts chatter can be suppressed by changing the spindle speed to a certain
range. Using thrust force as the basis for chatter detection, authors have
presented both theoretical and experimental results. The predicted stable
spindle speeds are considered to be:
30
c
f
n
s
N
where N = 1, 2, 3, … , and f
c
is the chatter frequency.
Further in [39], the relationship between the spindle speed and the phase
shift has been studied. It has been shown that phase shift can be modified by
changing spindle speed. Therefore, the cut width limit can be increased or
decreased by the change of the spindle speed as well. Chatter occurs when the
width of cut is greater than the limiting value and is suppressed if the spindle
speed is changed so that the width of cut is smaller than the limiting value.
Two strategies are investigated: the first one that states that the regulation of
the spindle speed to phase shift will be optimal at 2π, and the second one at
π/2. It has been experimentally determined that the idea of the optimal shift
being π/2 is impossible while the 2π phase shift has been confirmed.
The value of the width of cut at the stability limit has been defined as:
