Model Forum / General / Models / November 2008

In article
<d4756a61-b029-489e-ba21-cdde3d23dfad@d23g2000yqc.googlegroups.com>,
mikecb1 <mike.crossfield@virgin.net> writes

Mike,

I believe you would get that pattern if the spindle was slightly out of
alignment with the column, with the tip of the cutter being slightly
further away than it should be. It may not be as serious as it looks;
such tracks can look very pronounced even if they are only a fraction of
a thou. If it needs correcting, you may be able to shim the head (not
familiar with that particular machine).

There may be other explanations involving the dynamics of the cutter,
but you may be able to test this by making a similar cut in the other
direction. If it is misalignment of the spindle, the pattern will be the
same; if it is cutter dynamics, you should get the saw-tooth on the
opposite side of the slot.

David

Mike -

I've never had great success cutting slots to the width of the cutter
- quite possibly because both of my machines are small (a Taig and an
X3) - in your situation I would probably choose an undersized cutter
(say, 1.4") and cut the slot in 2 passes.

Regards,
Tony

Not really surprising, in fact pretty much predictable. You'll always get
some patterning on one side of the cut because the cutting forces are
different. You reduce that with rigidity, high speeds, low feeds and smaller
depth of cut. In your case you had everything working against you. A small
diameter long series cutter which will be very flexible, a small machine,
low speed, high feed per tooth and trying to cut to final width with one
cutter.

Also when you're climb milling the tooth meets the workpiece at the top of
the cut and vice versa for conventional milling. This alters how the cutter
pushes away and whether top or bottom of the cut gets more metal removal.
That's probably where your sawtooth came from but it might also be partly
due to misalignment of the quill and bed. If you altered the cutting depth
on the quill that shouldn't have mattered but if you altered it on the knee
it might have done.

At 500 rpm and 3000 to 4000 thou per minute you're feeding at between 6 and
8 thou per revolution or 3 to 4 thou per tooth. That's a roughing feed. For
finishing you'd want 1 to 2 thou per tooth.

I'd also say you can't ideally cut a smooth slot in several passes of depth
with the final sized cutter, especially with a flexible cutter. You need a
final cut with a decent amount of metal removal with the whole depth of the
slot being machined in one go. In your case the cutter would have been
bending towards one side on each pass and when you reached the final depth
you'd already have removed more metal somewhere than the final pass cleaned
up. If finish is important I always machine a slot by making a smaller one
and then cleaning up each side in the same direction of cut. A half-way
house to that is make a smaller slot and then clean up in one pass at full
depth with a higher speed and lower feed, ideally with a short rigid multi
tooth cutter rather than a long series slot drill.

Try making a smaller slot first and then using 1000 rpm and the same or
slightly lower feed for a finishing cut at full depth and see how you go. If
that still doesn't cure it you'll have to machine each side separately.

A twist drill removes metal a lot faster than a milling cutter and only
costs pennies if you break or blunt it. I'd probably have chain drilled at
7mm to get the bulk out, run one fast roughing pass by hand with a smaller
milling cutter to make the small slot and then one pass under feed with the
finishing cutter. I never peck away at a slot in several passes of different
depth with the final size cutter. That's a guaranteed recipe for a poor
finish on one side and in fact the wrong final width. It'll always be a bit
oversize on the ragged side.

When you've got a rigid machine, rigid cutter and no backlash in the feed
you can get away with almost anything. When you have one or more of those
issues present you adapt your technique round the problem.