Hi
I've been using a piece of soft Al plate for setting the Z zero for
several years. I have a button in Axis that runs a Z probe and then sets
G54 Z zero based on the known thickness of the plate.
There are no contact tool marks on the surface, and I'm using carbide
wood cutters. Sharp enough that there are a few marks where I've
accidentally hit the cutter when removing the plate.
On 9/4/2020 17:40, Gene Heskett wrote:
On Friday 04 September 2020 08:09:33 andy pugh wrote:
On Fri, 4 Sep 2020 at 12:37, Gene Heskett <ghesk...@shentel.net> wrote:
Raw, freshly cut alu will oxidize to a thick enough coat of
alox than can be punched thru by 5 volts in milliseconds after the
machining tool cutting edge has passed.
Where do you get this from?
Take some aluminium from your stock and your multimeter. Test this
theory.
I have, you have to press hard enough to pierce the oxide coat. Thats is
not much, but its not at all hard to take a bare wire and lay on the alu
plate without getting a connection. This oxide coat may be less
than .00001" thick if fresh, but it is an insulator. Long term, as in
years or with chemical help such as an anodizing solution, it can reach
a 400 volt breakdown withstand. Insulating sheets for power transistors
with better thermal conductivity than mica or kapton have been made out
of it.
Something like 95% of the heating of an alu workpiece while machining it
is not the friction of the cutting tool, but the invisible burning
(oxidation) of the freshly exposed alu when its exposed to the oxygen in
our air. In the presence of airborn oxygen, its a very active metal.
That oxide, seals the surface and slows the speed of the reaction by
many orders by the time the air has had access to it in the first
millisecond after the cutting edge has passed. Most coolants are water
based, but best tool life will be obtained if the cut surface is wetted
by a deluge of coolant. Its not the coolant but the instant wetting and
sealing of the air away from that cut surface even if the coolant is
H2O, but that H2O should not be agitated to encourage its oxygenation.
Even a mist, just enough to wet it, driven by enough air pressure to get
it at the alu as the tool turns on past it is a huge help, all out of
proportion to the amount of coolant in that mist. I suspect that a major
portion of the commercial coolants sold, is about a penny's worth of
kodak photoflow per gallon, making that mix many hundreds of
times "wetter". Kodak sells it (or did then) in two strengths, with the
strongest is an ounce per gallon makes it 1200 times wetter than regular
water.
I am familiar with that product because I spent from the later 40's to
the early 80's with my own color darkroom where ever I was living. I
shot weddings etc for enough to break even and keep me in supplies, even
compounded by own color print developer, substituting sodium carbonate
for the alkaline accelerator instead of the sodium hydroxide usually
used, so it was a little slower, but I could process 8 copies of a good
shot without having to chase the effects of a fading developer sitting
mixed in a 100F bath. It was always something I could do, until digital
cameras finally replaced the SLR as the utility camera in your Aunt
Tillys hands. It took a while to fine tune it, but that has now put
good quality, nearly archival quality digital color prints on the output
tray of several million printers today.
Cheers, Gene Heskett
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