Re: [CAD_CAM_EDM_DRO] Using a porter cable router to cut aluminum

camfambmw540 wrote:

>Hi all, new to the group...seems like a very excellent group :-)
>
>My name is John; I live in the Chicago area. I have a nice compound
>milling table that I would like to use in a very simple cnc setup.
>My plan is to fabricate a very sturdy z axis for a porter cable
>router (1.75 hp), use the compound milling table as my x-y axis. I
>have 2 nema34 173 oz in stepper motors for the xy, still have yet to
>get the z motors. I also have yet to get the controllers or power
>supply. I have a 1 GHz pc dedicated to my shop needs.
>
>I plan on cutting odd shaped holes (1.5"x2.5") into 1/4 and 1/2 inch
>6061 aluminum. I also would like this setup to be able to assist me
>in making a much better cnc sysem (when wife approved funds become
>available).
>
>

I think you are dreaming! I do a fair amount of this sort of work on
a Bridgeport, and really would not want to do it on a much lighter machine.
We have a 3-in-1 machine at work, and it is an absolute joke! Not an
awful lathe, but for milling, it is actually WORSE that awful. It has
a Morse #3 taper spindle, too.

>Question: Does anyone see flaws in my idea? Will I be able to cut
>holes of this size in aluminum (if I go slow and keep lubricant
>constantly sprayed on the metal)? Any suggestions on how to
>fabricate a z axis that will hold a porter cable router (simple
>plans??)
>
>
>

Is the router designed to handle coolant getting into the bearings and
motor? Assuming it has a brush-type motor, you probably don't want
to be feeding oil mist into that motor, so you'd better use a water-
based coolant! Can the bearings really handle the side loads of
cutting aluminum? What diameter cutters do you plan to use?
For a 1/2" thick piece, you can't use 1/8" end mills (which would be a
good match for the router's spindle speed). I think you'd need at
least a 1/4" end mill (maybe 3/16" would do) to get enough
projection past the collet.

You say "go slow". Well, that's the problem. You CAN'T go slow
at these spindle speeds (~20,000 RPM). You have to keep the chip
load per tooth within a fairly narrow range. If you go too thin (slow), the
tool skates along the previously cut edge, and wears out very quickly.
If you go too thick (fast) then either the tool breaks or the spindle
stalls. With these size tools, the range is somewhere around .0002"
to .002" per tooth. A 2-flute cutter allows you to go at half the feedrate
of a 4-flute, so that would help. But, to put a number on it, 20,000
RPM on a 2-flute cutter is 40,000 teeth/minute. At .001" per tooth,
that requires a feedrate of 40 IPM, which is certainly moving right
along.

Now, you see that we have the tool cutting (approximately) .001" for
every cutter tooth that passes into the workpiece. So, if the rigidity
of the ENTIRE machine, from the X-Y table, through the supports,
up to the head, and including the router mounting and its spindle,
flexes more than .001" under the cutting loads, then the machine is
going to vibrate wildly and break tools or just make a lousy cut.
To some extent, you can make roughing cuts that leave stock to
be cleaned up by a finishing pass. But, on 1/2" plate, that finishing
pass will have to clean up a 1/2" wide face of the material, and that
will subject the machine to a torture test. If the vibration builds up,
it will leave nasty patterns on the surface. Maybe you won't mind
that.

Jon