The Viewer may question why back cutting is evident in both directions as with the
trailing edge being higher it should be clear of the workpiece. However, with home
workshop machines being rather light weight, vibration can exist causing the cutter
to rise and fall.
On these lighter machines therefore, maybe there is a case for the error in the spindle
to table angle to be rather more than the 0.025mm over 300mm, thereby increasing
the gap between the trailing edge and the workpiece. In any case, I would suggest
that when making adjustments, rather than seeking perfection, that is zero error,
at least accept some error so that a better finish can be achieve in one direction,
rather than a slightly poorer finish in both.
I realise that the content of this article does not set hard and fast rules for using
your milling machine only some factors that should be born in mind when carrying
out milling operations. For many, dare I say, its content will be new ground in terms
of their knowledge regarding the machine's use. Do though go further than just reading
the article but also get out the dial indicator and test where your machine stands
in terms of its accuracy and make a record of these. Then, by observing the results
you achieve whilst considering how your machine is set up, your understanding of
the milling operation will be increased.
Having encouraged you to check your own machine it occurs to me that I have no experience
as to how close our home workshop machines come to the standards laid down. Realising
that these are predominantly budget machines perhaps it would be expecting rather
a lot for them to conform fully. However, they should come close Having removed
the pile of shims from my machine, as mentioned earlier, I found my machine was within
the quoted requirement in the left/right direction and only just outside back to
front. Why then were those pieces of copper there I wonder?
The test pieces were 230M07 (En1A) steel and the cutters running at about 500 rpm.
Photograph 7 shows the same tests being carried out using a 20mm end mill that is
very blunt. Evidence of back cutting is very apparent on the right hand cut but also
confirms my earlier comments that a blunt cutter will not cut at every turn but only
when the depth is sufficient for the cutter to get below the surface. This is seen
by the back cut circles as they occur randomly. Whilst the left hand test is appreciably
better it is still poor, a case for keeping your cutters sharp.
Having explained the theory and checked my machine I decided to machine a test piece.
Photograph 6 shows a test piece that has been machined both ways using a 14mm cutter
and at a depth of 0.2mm. On the right the direction is where the trailing edge is
lowest and back cutting has taken place. When machined in the opposite direction
with the trailing edge highest, seen on the left, back cutting is almost absent and
the finish is improved. However, having been machined with a very sharp cutter both
surfaces would be acceptable in many cases.
Do click on the pictures to see a clearer view of the results