using a cutoff saw. Accuracy was not
important as I would be machining
the blades to their exact lengths later.
The blades are attached to the
drive axles using a Trantorque™
bush. These work well but require
the mounting hole to be accurate
within a few thousandths of an inch.
This can be done using a boring bar
but requires considerable patience
and careful measuring to make sure
one does not go oversize.
To avoid this problem, I decided
to use a three step process to quickly
and accurately create the holes. The
diameter required is 5/8” so after
carefully positioning the mill head
over the center of the bar, I milled a
1/2” diameter hole using a 1/2” end
mill (Figure 3), followed by a
19/32” drill (Figure 4), followed by
a 5/8” reamer (Figure 5) at low
RPM and with plenty of lubricant.
The table of the mill remains locked
in position throughout the process.
The rationale behind this
method is that the original milled
hole will be straight; the drilled hole
will be to within 1/32” of the
required diameter and still
reasonably straight; and then the
reamer removes the last 1/32” to
give a straight, very accurate hole.
(A quick rule of thumb is that a
reamer 1/2” or less requires a hole
1/64” smaller, while anything over
that requires a hole 1/32” smaller
than the reamer.)
30 SERVO 08.2010