Returning to the situation in Sk. 4. In this case, any one who has attempted to machine
a workpiece in this way with a substantial error in the spindle's orientation will
without doubt have been aware that the traverse of the worktable requires considerably
more effort than normal. This therefore is not an acceptable way of using a milling
Having established above the basic difference between traversing left to right and
right to left, how does this effect machining when the error is much less than in
the illustration. Well, let us consider a common operation of skimming a very small
amount from a surface just to improve its appearance, say 0.1 to 0.2mm. Sk. 6 shows
that the cutter, as sharpened, will cut on its outer diameter (x) but also on its
end cutting edges at (y) and (z)
If we were to put a value on the depth of cut per tooth on the end cutting edges
it would be found that this would be minute requiring a razor sharp edge for this
to be possible. Actually, it would just rub the surface until after a number of revolutions
the depth of cut was sufficient for the cutter to get under the surface.
End mill end cutting edges
This now leads nicely, I think, into a subject that has raised questions on occasions,
that is, should the end of an end mill be sharpened flat or with a minute concave
form. Sk. 6 showed how with a flat cutter each tooth will be called upon to remove
a very thin slither whilst Sk. 7 shows that with the cutter having a concave end
the situation is quite different taking relatively narrower but deeper cuts per tooth.
This shows that the concave end enables the cutter to cope with errors in the spindles
vertical position much easier and is therefore absolutely essential.
With that situation having been understood and conformed to it would appear that
milling can take
place equally well in both directions. Certainly the increase in the load on the
leadscrew will be infinitesimal. Reference to Sk. 8 though shows clearly that in
one direction (A) the cutter only cuts on its outer diameter (preferred) whilst in
the other direction (B) it cuts both on its outer diameter and its trailing tip,
“back cutting” is the term for this. This understanding has now brought us to a
situation, where, if knowing the direction of lean of the machine spindle, and if
an option is available, then choosing the direction where back cutting is eliminated
is to be preferred.
There is though another factor to be considered, that is the direction of movement
with increasing numbers on the leadscrew micrometer dials. Machines mostly apply
the same configuration, that is numbers increase with clockwise rotation of the hand
wheel resulting in the table moving away from the operator. Whilst this seem to be
predominantly the standard I have machines in my workshop that work differently,
most confusing! Another feature on my mill drill table is that the leadscrew runs
in bearings at both ends but with a thrust race at one end only. A result of this
is that when taking a relatively heavy cut the table traverses much easier one way
than the other, the easiest being with increasing numbers on the leadscrew dials.
No doubt, superior machines will have thrust races both ways. Even so, increasing
numbers, and the position of the thrust race on my machine at least, still appear
to dictate that the table moving to the left is the preferred direction.
Having chosen therefore movement to the left being the preferred, orientation of
the cutter spindle should ideally be in accordance with Sk. 8A rather than Sk. 8B.
This I believe was where the knowledgeable contributor based his suggestion, rather
than what the standards (if any in those days) had to say regarding the situation.