Re: [CAD_CAM_EDM_DRO] mirror finish on aluminum?

george@... wrote:

> I have a retrofited RF 30 and I'm trying to get a mirror finish on a
> 4"x4" piece of 6061T Aluminum. I'm using a 3" fly cutter with various
> tooling but I just cant to get a mirror finish. Is it posible? Or is
> there another step after milling to get a mirror finish. I know the
> head needs to be trammed but figured that would'nt effect this.

You need a very sharp cutter, with either no rake or a slight positive
rake. It needs to be a really good tool material. I am getting good results
with a cobalt HSS lathe blank. I cut just a little radius on the cutting
point, maybe .030" or so, and ground back relief to it. I set the feed
rate for a very slow feed, coming to about .001 to .002" per rev of
the cutter. I set the spindle to run at about 300-600 SFPM, so for
a 1.5" radius, that would come to about 400 to 800 RPM on the
spindle. You should cut just a few thousandths on the last pass to
get a smooth finish. You may actually want the machine a hair out of
tram, so the cutter doesn't skim over the work again and mar the finish.
I spread cutting oil on the surface with a brush before starting the cut.

Now then, 6061T should be followed by a number to indicate the
heat treating that was applied. T3 through T6 cut very well, T0 through
T2 can be cut with the proper tooling and coolant, but is more finicky,
as they are softer. If the metal gets hot, don't try to cut it, wait for it
to cool. If it gets hot, it will melt onto the tool, rather than cut cleanly,
and you will NEVER get a good finish, and it may severely mar the
work or wreck the tool.

What is the problem you are having? Does the surface have waves
in it, not parallel to the sweep of the cutter? That would indicate
vibration (chatter) of the tool, or possibly the workpiece. Make sure
the tip of the tool is extended only a short distance from the flycutter
body.

If you have a series of circular grooves, you may be feeding the table
too fast. You need a very slow feed to allow the successive cuts to
merge together. If you are feeding at .001" per rev or so, then there
may be aluminum sticking to the cutting edge in varying amounts
each pass. A coolant/lubricant will help with this. Everybody has their
favorite, but thread cutting oil seems to work OK for me.

If the surface doesn't have waves either with or across the tool
cutting direction, then you may have a heat treat condition that is not
conducive to smooth cutting, your machine is not rigid enough, there
is a high frequency chatter of the tool tip, or the shape of your cutting
edge is not right. A broad cutting edge requires a VERY rigid machine.
For a flycutting final pass, you should be making hair-like chips,
literally hair thin. If you are scraping a thin but wide chip, that could cause
chatter that ruins the finish.

These are the things to check and think about. Try different settings
and see what it takes to get better surface finish. Once you find what
makes it go on your machine, you should be able to pretty much reproduce
those results again.

Anyway, a flycutter will not achieve a truly mirror finish, but it will make
a piece shine when you hold it up to the light. It is really more of a diffraction
grating with circular grooves than a mirror.

If you truly need a mirror, you will need to lap it with finer and finer grades
of micro abrasives, finishing with cerium oxide and india ink.

Jon