So the cut-off disk that I mentioned in the previous post stood up to cutting through several inches of 1/4" angle iron. The angle iron that I bought from Home Depot was "weldable steel". There were lots of sparks of red-hot bits of metal flying, and eye-protection was a must.
The next step after cutting a piece of angle iron was to cut one side down to about an inch, thus forming an L. This would be the side resting against the bed of the X-Y table, while the other, longer side would hook into the vise to clamp it down.
After the cut, I had to make sure the end was straight, so I clamped the piece into the vise, straightened it using a dial test indicator until it was within about 0.002 inches of straight, and then ran a 1/8" end mill along the side, removing 0.005 inches each time using "conventional milling", which means that the direction that the bit spins is in the opposite direction which the material is being fed into the bit. Here's a quick diagram:
Apparently, with small mills like mine, you're not supposed to feed the material in the other way, which would be "climb milling".
Anyway, when I was done planing the edge, I saw something funny with my bit:
What happened? Dan Reetz posited that my feed rate was too high, which caused the bit to chip. And how does one determine what feed rate is correct? Here's a wikipedia article which helps. Let's see if I violated the feed rate rules.
I had set my spindle rate to 1700 RPM, which is a guess. This is the reported fastest speed in first gear for a belt-converted X2. What material in the table was the "weldable steel"? Who the hell knows; this was Home Depot where that's the most specific description you're going to get. I'm going to guess it's "steel (tough)".
Using the approximate formula RPM = (4 x speed) / diameter, where diameter is in inches and speed is in feet per minute, and using a diameter of 1/8 inch, which is the diameter of my bit, we get a speed of 1700 / 32 = 53 feet per minute. This is the speed at which the surface of the bit is whipping past the workpiece. This speed seems correct based on the table given in the article.
There is another formula further down, for determining feed rate, which is the speed at which the cutter moves into the workpiece. This is feed rate = RPM x T x CL, where T is the number of teeth in the cutter, CL is the "chip load", which is the maximum amount of material which should be removed per tooth pass. The chip load is dependent on the material, and here's the kicker: apparently nobody is in agreement as to what the correct chip load is.
Do a web search for chip load or feed rate for steel, and you'll find all sorts of ranges. Ranges so large as to be useless. On once site I found a range of 0.002 to 0.006 inches. Plugging this in to the feed rate formula, and using T = 4 since my cutter has four flutes, I end up with a feed rate of between 13.6 and 40.8 inches per minute (or 0.23 to 0.68 inches per second). Considering that I feed my material into the cutter and 0.1 inches per second, my chip load should be even less than the maximum load.
So why did the cutter chip?
I think I'm going to have to cry on the shoulder of a CNC forum to see what's going on.
Another possibility: perhaps the cutter I used wasn't meant to cut metal from the side, but only from the end.