Archive of the former Yahoo!Groups mailing list: Homebrew PCBs

Well, I can't say as I am thrilled or dissappointed. I tried running
a small PCB with fairly large traces. There are a number of problems
that I will need to resolve before expecting a good PCB from the
machine.

1) I have to fix the problem with the Z axis saddle - this is
something one of the newer list members has been helping me with
(Ron), but I screwed up my end of the fix so I was running the test on
the old hardware. This is a mechanical problem that creates some
inaccuracy in positioning in the Z axis. With a samll 'V' cutter, you
don't have much room for variations before your trace gets milled away.

2) A glitch somewhere caused the machine to lose position in X axis by
about .200". This didn't happen on the first attempt, but did on the
second attempt. Not sure at all what the cause was or could be so
this one will be a long term bug hunt I think.

As one would expect from cutting copper with a V shaped tool, the cuts
are not the greatest. Copper itself is not the best machining metal.
Couple that with the reality that the cutting SFM is so low because of
the small cutter diameter. The most RPM I can currently get out of
the machine is only 7500 RPM, ideally it should be running at 100K or
even faster but that can't happen till I can figure out a new spindle
and afford some new hardware to do it.

The traces that were not mucked up, were a tad smaller than they
should have been. They should have been .020" width but as best as I
can measure it, they appear to be .012" wide. I am using a small
adapter chuck that is normally used as a quick change chuck for Dremel
type tools. Having a few thousandths run-out on that is expected so I
am not too concerned yet.

The cutter I have is a V cutter, 60 degrees "D" shaped or in other
words a 1/2 round tool bit (or another way of putting it is a single
flute cutter). It is solid carbide and seems to have held up good
during these cuts so far.

Had the X axis not lost position on the second attempt, I would
probably have a good PCB to continue onto the second side machining.

Also not overly thrilled with the Gerber tranlation software I have.
It's a trial version so I can't really complain till I learn a bit
more about it. The isolation milling seems to work well, but I can't
figure out what it needs/wants for the drilling operations. Luckily,
that is pretty easy to program manually so if I can't get that to
work, it still may be okay.

I can see I will have to get going on the closed loop concept of the
software and hardware. Even though I am running stepper motors, I do
plan to have a "near closed loop" operation at some point along the
way.

As for the speed of the process, I can see where it will be slow.
However, I cannot yet determine how much longer it will take to do a
PCB using isolation milling versus etching. A higher speed spindle
would certainly reduce the machining time so that is a primary
drawback with my machine at present.

But, looking at what is involved, it may not be much slower overall if
you have a good spindle. By comparison, the layout software I use can
be used to print the layout for toner transfer then etching, or it can
output the gerber files. The translation software I am playing with
takes about 1~2 minutes to translate the gerber into G-Code. Once I
have that, I load the G-code into the machine, set the origin on the
workpiece and can (once I fix a few problems) hit start and let it
run. Once the first side is done, flip it over, re-set the origin,
and let it run the second side. Using stops on the table will reduce
the amount of time there. The machine has a 38 tool, automatic tool
changer so once it get's going, there isn't much to do except watch it
do the work. It will do the isolation milling, drill the holes, and
then mill the second side (although that requires manual intervention
to flip the part).

Compared to etching, I would eliminate the toner transfer which is
mostly "Hit" for me rather than "Miss" so that doesn't take too long.
Still have to print the layout, cut away the excess, align the two
sides, transfer it, soak off the paper, scrub off the residue, touch
up any spots that are not good. Then it's into the etchant for about
20 ~ 30 minutes (it's cold in the shop so it etches pretty slow this
time of year). Once done there, it's manual drilling. That goes
pretty fast for me, but it is SOOOOOOOOOOO boring to me that the CNC
machine is a welcome sight just for that alone.

So, that's my first attempts at isolation milling. I have to fix some
software and hardware bugs before I can really do much else with the
machine.

Chris