(Figure 6) and then rough-cut 0.2”
in a single pass with a big 3/4” end
mill from one end of the blade. I
then moved the blade in the vise
and repeated this on the other end
(Figure 7). Ideally, one would not
move the part but my vise is too
small to adequately support the
whole cut at once. However, with
the knee of the table locked (the Z
axis), the part can be moved and the
cut depth remains constant. Note I
also left about 0.050” around the
boss, as well. I repeated this again
for the remaining three blades.
The 0.050” finish cut is made with
a 1/2” end mill that has a 0.060” radius
on the cutter (Figure 8). This tool
leaves a nice 0.060” chamfer on any
internal corner and thus reduces stress
risers that might allow a crack later.
The blades are then trimmed to
length after carefully measuring and
scribing the correct distance from
the center hole and machining off
the excess (Figure 9).
To reduce weight and improve
the aerodynamics of the blades, I
beveled the edges using a 1/2” 45°
chamfer mill (Figure 10). I could
have tipped the head of my mill to
do the same thing, but this method
is quicker and easier.
The teeth fit into pockets in the
end of the blade and these were
carefully measured, scribed, and
then machined using the same
0.060” radius mill used earlier
(Figure 11). The teeth had arrived
by this time and after ensuring they
fit okay, I machined the pockets on
the other blades. Next, the two
holes for the screws were added and
countersunk (Figure 12).
I recommend transfer punching
and then drilling one hole first,
fitting a tooth, and then using it as a
guide for positioning the second
hole to ensure that they line up
correctly. If you try to just transfer
punch and drill both at the same
time, it is too easy for the drill to
wander, and you’ll end up with the
holes incorrectly spaced. I didn’t fully
countersink the holes so I could
leave more metal around them
FIGURE 12
FIGURE 13
which will make the mounting
stronger. The tooth mounting screw
heads remain slightly proud of the
surrounding metal (Figure 13). You
can see that the teeth
also have 0.060” radii
on all internal corners
and are symmetrical
(allowing them to be
reversed to give a
fresh sharp edge if
required).
A completed
blade can be seen in
Figure 14. I need to
order slightly longer
screws for securing
the teeth and a 12-24
tap to clean out a few
of the threads them, FIGURE 14
but the new blades will be ready for
their first competition later this
summer. Pure Dead Brilliant will
hitting harder than ever! SV
SERVO 08.2010 31