Gear Cutting Tools

left-hand start machines a gear
with right-handed teeth. This computational
consideration of chip
forming geometry confirmed what
Thämer (1) had already found in
his studies. Flank wear takes place
precisely at those transitions from
tool tooth tip to tool flank which
are no longer actively participating
in the metal cutting process. He
states: "In this case the tool
cutting edge which just at this corner
no longer removes a chip exhibits
particularly large wear mark
widths, which in turn makes it
clear that no direct connection exists
between chip thickness and
tool wear."
The plotter images produced by
Sulzer's method (9 and 10) confirm
this assumption. Sulzer's studies
covered mainly the wear behaviour
of carbide hobs. Instead of flank
wear, he found micro-chipping in
this area. Using the scanning electron
microscope, he studied the
leaving flanks for chip traces and
found pressure welded deposits
on the flanks. He states: (11 and
12) "The different dircection of the
cutting traces and of the streaks
indicates that these streaks are
caused by the chips being removed.
They occur at those points
on the tooth flank which do not
come into engagement with the
cutter tooth concerned, i.e. there
is generally a gap between the
cutting edge and this flank area."
The collision between chip and
workpiece flank can be explained
by the chip form and the chip flow.
The cutting process commences at
the leaving flank near the cutter
tooth tip. At this stage it can still
curl freely. After that the tip area of
the cutter tooth moves into engagement.
Because of the complicated
shape and the tight space
conditions in the tooth gap the chip
can no longer curl freely. It is at
the end pushed by the entering
flank beyond the cutting face to
workpiece
acc. to Sulzer, Aachen polytechnic
hob
5 31 2
46
Fig. 17: Determination of the chip forming cross-sections
the other workpiece flank, where it
is welded on. As a result of the
cutting motion of the cutter tooth
the pressure welds are separated,
but are formed afresh by the flowing
chip. In addition, a workpiece
rotation takes place during the
cutting motion. This means that
the workpiece flank moves away
from the leaving tool flank. It is this
relative speed at which the chip is
pushed from the cutting face over
the cutting edge. This produces
tensile forces on the cutting edge
cutting planes
cutting planes
1
1 mm
3
2
4
0,2 mm
entering flank
tip
leaving flank
A B C D
6
5
1 mm
acc. to Sulzer, Aachen polytechnic
Fig. 18: Determination of the chip forming cross-sections
190