SERVO 06.2015 23
which need to be removed.
I practiced this on an older
slightly different motor first
and they popped out right
away, but the ones on my
chosen motor proved
unwilling to come out so
easily. I used a small
hammer and a blunt nosed
punch. That got the bigger
one out, but I had to
(carefully) use my drill press
to get out the other one.
It’s easy to damage the soft
aluminum of the stator
housing, and I mangled
two motors before finally
succeeding (Figure 6).
The bore left between
the two bearings was
almost exactly 1/4” in the trial motor
but slightly less in the one I finally
used. I drilled it out to 1/4” in steps to
ensure it would be a nice close — but
sliding — fit on the axle. Too loose
and the stator and rotor would no
longer be concentric; too tight and
the axle would be hard to fit and
remove for servicing.
I removed the bell housing and
the old 1/8” axle from the rotor by
lightly clamping it in the jaws on my
lathe and machining it carefully
(Figure 7) until the two parts
separated, leaving the rotor as a
separate ring (Figure 8).
The drum itself is 7075 aluminum.
I turned the exterior down to a
diameter of 1.7”, drilled out the
center with a 15/32” drill, and then
reamed it out to 0.500” (the size of
the outside diameter of the ball
The next step was to machine the
recess for the rotor using a boring bar
This needs to be a close sliding fit
for the rotor. A small recess (Figure
10) for the flanges on the bearings
was then added to each end of the
1/2”. To ensure the features on the
outside of the drum are all correctly
oriented on top of each other, I
machined a flat on one side of the
drum (Figure 11), and then flipped
the part over and used that flat
to orient the drum to allow a
matching flat to be machined
opposite it. The part is then
turned 90°, and the two flats
align the part in the vise so that
flats can be added at 90° to the
A vise stop and edge finder
(Figure 12) together with the
DRO on my mill made positioning and
milling out the teeth mounting holes
and the slots to allow the rotor to be
levered out a quick and easy task. The
last major machining job was to
remove enough material from the
drum to get the weight down to
about 4 oz (Figure 13).
I chose flanged bearings (Figure
14; McMaster part# 57155K323) to
make it easy to limit how far they get
pressed into the bore. I then chose
Figure 10. Counter bore for the
Figure 7. Removing the bell housing.
Figure 8. Rotor.
Figure 9. Machining a recess for the rotor.
Figure 11. Machining flats.