You may wonder why I have not mentioned the contributor by name, this is because
I cannot remember, I do though remember that it was someone well thought of at the
time. I also remember it was being referred to in relation to the Dore Westbury milling
machine which had the facility for tilting the head,the workshop owner therefore
had control of which way the head tilted.
Unfortunately, for most vertical milling machine owners, you are faced with whatever
situation you find you have. That is of course unless you are prepared to make adjustments
to the machine in some way. In the case of a typical mill/drill,shims between the
column's base casting and the machine's base.
When I purchased my mill/drill I had to dismantle it to enable me initially to transport
it and ultimately to install it as it was too heavy if remaining intact. I found
that there was a lot of copper pieces between the column's base casting and the base
of the machine. Rather like pieces of flattened water pipe. I could only assume that
these allowed some adjustment if the nuts on one side were tightened more than the
other. I did not like this and they were left out when reassembled. In any case,
I could not remember which pieces went where, having dismantled the machine quickly
at the supplier's. It was because of this that I checked the spindle's relation to
the table at the time and decided that it was acceptable without them. I do hope
that this approach is not being used on later mill/drills but see no reason why a
very thin shim cannot be used to provide the accuracy required.
Workshop owners who have one of the milling machines with a tilting head will have
a ready way of setting the spindle to the required position in one axis. If though
it is calibrated for setting the head to varying angles, Photograph 2, under no circumstance
use solely the zero mark for setting this position, this will not be sufficiently
accurate, see later comments.
What the standards say
What then is the mathematical extent of the situation? Well British Standards give
us some values for the permitted error of the spindle in relation to the table's
surface. Photograph 3 shows the set up for testing the requirements which are that
there should be no more than 0.025mm variation in the dial indicator reading over
a distance of 300mm, that is with the dial indicator set at 150mm from the machines
spindle and the spindle rotated through 180°. The error can be either way, that is
with the table higher on the left, or higher on the right. For my suggested preference
though, higher on the right would be the situation to aim at . The tests should be
carried out with all three axis locked. To express this another way the angle of
the spindle to the table should be less than 90° on the right and more than 90° on
To quantify the effect at the end mill the above maximum error would cause one edge
of a 10mm diameter cutter to be 0.0008mm deep compared to the other. Whilst this
is very little its main effect would be in the quality of the surface finish if fed
in the least preferred direction, most important when the work entails light cuts
to provide a good finish.
When the test is carried out in the back to front direction the same level of error
is acceptable but only in one direction, that is the front of the table high, that
is the table to spindle angle in front of the spindle to be less than 90°.