On Monday, February 13, 2012 10:21:08 PM Jon Elson did opine: > gene heskett wrote: > > Guy's, maybe I don't understand cutting alu as well as I thought. > > > > All along, I have believed that it was more important to keep the > > oxygen in the air away from the cutting surface in order to slow the > > formation of alu oxide on the surface, which in normal air, not > > blown, can get a good start in 0.001 seconds > > I have some doubts about this. The oxide will form unless you run under > Argon shielding, > which may not be real practical.
Nitrogen should work equally well since the idea is to flood away the oxygen. And its 99% cheaper. > > This oxide is also the 2nd > > > > hardest substance known to man and can take the edge off a carbide > > tool that has to cut thru it in seconds under the right set of wrong > > cutting params, which my slow feed made worse. > > Any slow cutting so that the tool is barely getting below the surface > increases wear. > Taking the biggest cuts the tool can survive reduces tool wear by > removing more > workpiece material with each cut. > > > Sealing the cut surface against the air and its oxygen, blown or > > otherwise, that causes this instant alu oxide film with its > > subsequent wear on the cutting tool has always been the reason for my > > use of a cutting oil, deep enough to flood and seal the surface, or > > misted, particularly when I don't have the spindle rpms to throw it > > away from the cut. > > Our shop at work does ALL aluminum dry, and usually use HSS cutters > (although possibly > they may be M42 or such cobalt cutters.) So, I think you are going WAY > overboard > with this oxide thing. > > > The majority of the heat you are referring to is not the heat of the > > tools cutting action, but is the result of the chemical reaction that > > forms this alu oxide film so rapidly. > > OH, COME ON! Where do you GET this stuff? Yes, oxidation is > exothermic, but really. This effect was discussed at length when we made the first 'test' cases for our tv cameras out of alu we had cad plated back in 1960 and discovered that no amount of cad plating could protect them from 8 hours over the side of an LST 50 miles west of San Diego. 3/4" thick cases came back to shore with 1/2" deep corrosion pits from the salt water exposure. In 8 hours. None of those fancy, khaki colored Amphenol connectors survived either. So we wound up buying a $25,000 (in 1959 dollars too!) Clausen lathe and making all that stuff that was going on the Trieste for its dive into the mohole in Feb '60, out of a bronze alloy the navy said would work. The lathe wasn't straight, and it took Clausen techs about 3 weeks to get it to both turn and bore that bronze, solid rod 8" in diameter to within a thou of taper in the 20" we needed to contain the camera, which was itself only 2.5" in diameter. I think in the end they were even holding seances over it. All that leads up to the machinist, trying to get that last half a thou to a perfect fit, had the alu marching right along in the smaller 10 foot lathe, and had to stop suddenly as the chip string coming off his tool was actually burning about 4 to 6" away from the tool. All this took place about 20 feet from the bench where I was busy assembling serial number 3. Something about the smoke made us open the windows and clear the building for a smoke of our own for about 10 minutes. > Shave some aluminum with an X-acto knife and see if you can detect this > heating! > I seriously doubt you can detect it. Nope, not near enough mass to measure. > Rubbing of the tool when it is > having trouble > digging below the material is the largest source of heat, next is the > heating of the > chips as they are curled up. That heat should not get to the remaining > workpiece > material when things are done at the right speed, but we both have that > problem > of limited spindle RPM. > > > That is my take on it. How right or wrong am I? > > Sorry, I think your theory is full of holes. Many shops cut aluminum > dry, some at > insane rates. I read a book on high-speed machining, they were cutting > aluminum > at 640 cubic inches a minute removal rate, putting 80 HP into a 1/2" > end mill at 75,000 RPM. That is 0.37037037037 cubic feet of material removed in a minute? I can see why it took an 80HP spindle. > This was done dry, as no coolant could reach > the cutting are due to the bullet-like spray of chips coming out. > Also, the thermal > shock was harder on the carbide than running dry. I have seen discussions of that source of edge cracking in carbide. If its allowed to run hot, that helps its life. > I cut a fair amount of it dry, and get excellent tool life either with > M42 Cobalt > cutters in the larger sizes, and solid carbide in the 1/8" size. I do > use water-based > coolant when I am doing a lot of cutting in a small area to prevent > heating of > the workpiece, or when there is a lot of material to remove. I can > often run > for days on one tool. So can I, if the mister is on & the work is kept swept clean by it. > And, don't ignore climb milling, it makes a HUGE improvement in tool > life. No doubt about that at all. > > Jon Its a bit hard to climb when you are going in a circle which will eventually become where the part is 'parted off' :) However, this does lead to the question: Is there a calculator, online or downloadable that will tell one the ball park correct feedrate for a given bit and depth of cut at x rpms? I would have far less problems with my un-SWAG methods of doing this if I had a good, known to be safe, starting point. And, needing an air sweep or mister supply, I might be able to justify one of those little pancake air compressors that I could actually run inside my 12x16 building. Thanks Jon. Cheers, Gene -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) My web page: <http://coyoteden.dyndns-free.com:85/gene> I am a traffic light, and Alan Ginzberg kidnapped my laundry in 1927! ------------------------------------------------------------------------------ Keep Your Developer Skills Current with LearnDevNow! The most comprehensive online learning library for Microsoft developers is just $99.99! Visual Studio, SharePoint, SQL - plus HTML5, CSS3, MVC3, Metro Style Apps, more. Free future releases when you subscribe now! http://p.sf.net/sfu/learndevnow-d2d _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
