In some workshops though, the level of the error is in the hands of the user as their
machine is of the type that are fitted with heads that can tilt, Photograph 1, a
situation where greater errors then those found on fixed head machines can easily
occur. The content of this article is therefore of considerable importance to readers
with such machines.
Another factor is of course that the spindle should ideally be perpendicular to both
the X and Y axis. For the reader who is not conversant with this terminology Sk.
1 should make this clear. Beyond that, the three axis are allocated positive and
negative directions but using these will for most overcomplicate the issue and so
I will refer to left and right and towards and away throughout the article.
Returning to the spindle there will be errors as viewed from the front and from the
side but we will consider primarily that from the front, where an error exists from
the side the considerations will be the same. In practice there will be a combination
of the two errors but this having only a marginal effect on the findings.
Before going into the level of the permitted error as detailed in the specifications,
and in which direction, lets consider the theory. It is though much easier to visualise
the effects if they are magnified beyond what will occur in practice. Sk 2 shows
an end mill that is not at 90° to the workpiece, what then if the workpiece is traversed
to the left? In this case, Sk. 3 shows that the cutter will produce a concave surface.
However, if we consider the workpiece approaching from the left and feeding right
the cutter will still produce the same result, Sk. 4, but there the similarity ceases.
Closer examination of the two sketches shows that in Sk. 3 the cutter is cutting
on its outer diameter whilst in Sk. 4 it is cutting on its end, a way in which the
cutter is not intended to be used.
Having therefore decided that an error in the orientation of the spindle
is inevitable, does this mean that machining should always be carried out in the
direction that gives the result in SK. 3? If there are no factors that prevent an
operation being carried out in the preferred direction then it should be used, some
workpieces will though make this impractical. Consider the part in Sk. 5 that requires
the two recesses to be exactly parallel and the depth of one precisely the correct
depth in relation to the other. If the part had to be turned end on end to machine
the second side these requirements would be more difficult to satisfy. Because of
this, we have to accept that machining in both directions is a necessity. However,
this does not eliminate the fact that there may be a preferred direction.
I maybe wrong, but I do not think I will be the only one that gives this situation
less consideration than I should. I did when I purchased my milling machine measure
what the error was and marked this for reference on the front of the belt guard.
The guard being very heavy I replaced it and the information was lost so my machining
has subsequently been done largely without considering the consequences. Memory does
not allow me to take note of the facts without the situation being written up in
front of me.
All pictures can be clicked on to provide a larger view