Having quantified the error permitted the reader should now appreciate how the concave
nature of the end mill will ensure that any cutting that does take place on its trailing
edge will be very small (Sk. 7) rather than along the length of the cutting edge
Unfortunately, the above tests do not tell the full story, the reason being they
do not take into account the possibility that the workpiece may not be travelling
exactly parallel to the table's surface. Sk. 10 shows that even though the spindle
is at right angles to the machine table, the workpiece can still approach at an angle
if there is an error between the table's surface and the slides on which it moves.
Photograph 4 shows the method of testing this requirement with the permitted error
levels being the same as those above, that is 0.025mm in 300mm, left to right and
back to front. In this case the down feed and the traverse not being used should
be locked. Unfortunately, this is a condition that the machine user would find difficult
to improve should the error be excessive.
Setting your milling machines tilting head
I have already suggested that shims can be use to set the head of a fixed head machine
but the process with a tilting head will be quite different. The problem is that
one is trying to set the head within a very narrow band. To quantify this, 0.025
in 300 amounts to a maximum angle of 0.005°
Loosening the head, moving it manually and then re-tightening it will be very much
a case of trial and error, probably with a lot of trial and a lot of error. My method,
but makes it possible to make small adjustments relatively easily is seen in Photograph
5. In this, the head is fitted with a centre and a plate placed onto the machine
table having a drilled centre in it. The head, which must be quite close to the required
angle is lowered into the drilled centre and the plate fixed in place.
The dial assembly can then be rotated through 180° to establish the error. With the
head loosened, the table can now be traversed by a very small amount to rotate the
head and the head fixed again in that position then the setting can be checked once
more. It is of course, possible to observe the results on the dial whilst the adjustment
is being made. Even so, do not expect to arrive at the required setting at the first
attempt, or second or third but is should be quicker and easier than the unaided
trial and error method.