On Thursday 21 May 2020 21:46:50 John Dammeyer wrote:

> So I popped over to a friends to borrow a carbide 3 cutter face mill. 
> Alas the MORSE Taper draw bar fitting wouldn't work with my R8 to MT3
> adaptor.  But the more rounded cutters gave me the idea to use my
> green wheel on the carbide brazed cutter I was usingin the mid size
> fly cutter holder.
> No difference really other than the cast aluminium tended to melt onto
> the tip more easily.  I had clamped a failed casting into a large
> vise.
> Having seen my friends fly cutter bits (modified carbide inserts) I
> started thinking about the amount the 1/4" shank cutter protruded.  I
> found a 5/16" HSS tool bit and touched up the tip.  Then screwed it in
> place.  Ultimately I ended up using a set screw (long story) but all I
> can say is Wow!
> http://www.autoartisans.com/mill/TestFlycut_1.jpg
> http://www.autoartisans.com/mill/TestFlycut_2.jpg
> Silky smooth.   No light past a straight edge laid across it.  With my
> eyes closed dragging fingernail across it I can't feel the pass
> transitions.  Milling done by moving X axis.  Last pass was 0.003"
> DOC, 390 RPM, 2.1 IPM.  About 2"diameter fly cutter.  Overlap about
> 0.2" between passes.
> So I suspect the 1/4" tool bit vibrated like heck as it grabbed and
> cut the work and that's what did much of the scratching on the
> retreating edge.
> John

More than likely the 1/4" tool was borderline dull, and flexing sideways, 
tipping its face.

I have also found, when doing alu, that an HSS bit can be made much 
sharper than carbide on the powered rouge waterstone I sharpen wood 
plane blades with giving a much smoother finishing cut than carbide 
inserts ever thought of.  Carbide inserts can be sharpened but take a 
huge toll on the rouge if not using a feather touch, taking a long time 
to get the mirror edge that HSS can be done to in just a few minutes.

But you absolutely must have active coolant flow, mist working better 
than a liquid dribble, to keep the air away from your just cut surface 
to reduce the alox formation behind the cutters passage, which in turn 
eats the edge off your tool as it comes back around on thenext 
revolution.  Its that quick if left dry, even with carbide as the alox 
from burning alu is harder than your carbide. That alox formation behind 
the cutting edge in less than a thousandth of a second is also the 
majority of the heat generated when cutting alu, far more than the 
friction of the cut, that heat is carried away by the chip being 
removed.  Alu, in the presence of the 21% airborn oxygen is a very 
active metal, keep it sealed with a mist directed at the back face of 
your cutting tool so its wetted and sealed as quickly as possible behind 
the edge of your cutting tools passage.

So I do roughing passes with carbide, but finishing passes with HSS.

Another alu cooling method might be done by boxing the machine up in an 
air tight enclosure, and flooding the enclosure with dry nitrogen.

That of course has its problems with operator safety, and doesn't work as 
well for steel though, due to nitrogen embrittlement and the flooding 
gas costs lots more. I probably should not have mentioned it except I've 
been around it since the '60's as its used to pressurize transmission 
lines on broadcast towers.  Dried air is cheaper, but 50 yo copper still 
looks brand new when nitrogen is used in it, and the connection failure 
rates are vanishingly smaller.
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Cheers, Gene Heskett
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 soap, ballot, jury, and ammo. Please use in that order."
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