So, up until now, when I place a piece of Aluminium onto the Carvey workspace, I look across the edge of the metal, to see which way it wants to bend. Then if the top-face bends up, I'll place it top-face-down, to minimise the deflection in the center - since the metal is only clamped at the outsides. This way, the natural stress is to force the metal into the waste-board which gives a relatively flatter surface on which to carve. It has its own problems when you're taking off the last 0.1mm, because the springy metal then wants to jump upwards as the last of it is released, but placing a sufficient number of tabs generally gets around that issue.
Somehow, I got this wrong, last night, and placed it the wrong way up. Drilling holes is ok, because the pressure is vertically downwards, and the drill will force the metal onto the waste-board surface, but when the CNC starts to move sideways to carve out channels, things can go very awry, very quickly...
This is the start of the first pass. The CNC machine is supposed to cut 0.1mm of metal per pass to protect my bits and stop from overloading the stepper motors. Even the first plunge went further in than I would want, and then it got progressively worse, to the point where the machine was routing through the full 0.8mm of Aluminium sheet. It was also complaining, bitterly, about being forced to do so. I hit the E-stop pretty quickly! Fortunately, the bit is fine, and the machine seems to be no worse for the wear.
It doesn't help that (because Aluminium is prone to melting if placed under too much stress), I'm using an up-cut single-flute bit. A "flute" is basically a cutting-edge combined with a spiral escape-chute for the material that's been cut by the cutting-edge. Typically I'd use a 2- or 4-flute bit for wood, because the number of flutes multiplies your productivity - a 4-bit flute will, all things being equal, cut 4x as fast as a 1-flute bit, and so you can dial-up the speed of the machine as it makes passes over the material. However, for my machine, it's important to use an up-cut 1-flute bit for Aluminium...
- The up-cut bit forces the fragments of metal upwards and away as they are removed, this is important because otherwise a tiny fraction of that fragment might melt under a down-cut bit, and then again a little later, etc. etc. The result is that your end-mll bit becomes blunt and eventually useless.
- The single-flute bit is important because Aluminium produces (relatively) larger flakes of material as it is machined. If there are too many flutes, the "escape routes" for the removed Aluminium aren't large enough, they get clogged, and you're back to the problems in the previous point.
On some (read: more expensive than I'm willing to pay for) machines, they can mix it up even more - there are machines that cut underneath oil or water, ones that spray water/oil on to cool the material, some that have air-feeds to help blow away (and cool) the flakes of material removed etc. All this is to do with preventing the cutting edges of the bit from heating up too much, and melting the Aluminium. Remember, the spindle on a CNC machine is rotating really fast - mine (a relatively slow one) runs at ~12000 rpm. There's a lot of energy being dumped into the contact point of bit and material...
So there's no new update with pretty pictures today, nor tomorrow either. As I've mentioned, it takes 5 hours or so to perform a carve, and I have an "All hands" comms meeting today from 3:30 to 6pm. There'll be no time for a new carve by the time I get home - my machine is in the "Man-shed"[+] at the bottom of the garden, close by the neighbours house (who don't have so large a garden as us). I like to keep the noise down at night, and carving metal isn't exactly quiet. It's not as loud as I thought it would be (presumably because I'm only taking off such small amounts per pass, *usually*) but it's still too loud for night-time carving.
[+] I used to have a room where all the toys would be put, but that quickly disappeared when the kid came along. These days I'm relegated to the shed. It's a nice shed, though