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 -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) If we desire respect for the law, we must first make the law respectable. - Louis D. Brandeis Genes Web page <http://geneslinuxbox.net:6309/gene> _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
