Drill Bit Angle
The spiral (or rate of twist) in the drill bit controls the
rate of chip removal. A fast spiral drill bit is used in high feed rate
applications under low spindle speeds, where removal of a large volume
of
is required. Low spiral drill bits are used in cutting applications
where high cutting speeds are traditionally used, and where the material
has a tendency to gall on the bit or otherwise clog the hole, such as
aluminum or copper.The point angle, or the angle formed at the tip of the bit,
is determined by the material the bit will be operating in. Harder
materials require a larger point angle, and softer materials require a
sharper angle. The correct point angle for the hardness of the material
controls wandering, chatter, hole shape, wear rate, and other
characteristics.The lip angle determines the amount of support provided to
the cutting edge. A greater lip angle will cause the bit to cut more
aggressively under the same amount of point pressure as a bit with a
smaller lip angle. Both conditions can cause binding, wear, and eventual
catastrophic failure of the tool. The proper amount of lip clearance is
determined by the point angle. A very acute point angle has more web
surface area presented to the work at any one time, requiring an
aggressive lip angle, where a flat bit is extremely sensitive to small
changes in lip angle due to the small surface area supporting the
cutting edges.The length of a bit determines how long a hole can be
drilled, and also determines the stiffness of the bit and accuracy of
the resultant hole. Twist drill bits are available in standard lengths,
referred to as Stub-length or Screw-Machine-length (short), the
extremely common (medium), and Taper-length or Long-Series (long).
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