Wednesday, November 20, 2013

Metal Heuristics

I cast an aluminum needle for my eldest student, who is 90. Actually, no, my eldest student is 91, but she hasn't cast any metal with me this year, and probably never again. She's getting old.

Anyway, my eldest student has been making needles and pins and other tailoring instrumentation for about, well, as long as I've been at the college. His father was a tailor and his mother was seamstress. I know that like I'm starting in on the Dozens, but that's what they were. The needle he fashioned out of wax was turned around so that the point goes back through the eye.

In any case, I documented his needle because I failed to rig it properly. I'm a big fan of public admissions of mistakes, not only because you learn may something from my mistake, but I learn something by properly formulating what went wrong. This is the very essence of what Open Source is about. So here we have it:

As you can see, the aluminum froze out in two spots. One spot actually joined, but then was torn apart when the metal and investment cooled, producing what is known as a heat tear (tear as in torn, not tear drop). The as the metal cooled, it condensed faster than the silica investment (the white stuff). The unyielding investment, under compression, won out over the metal under tension. In the other spot, the aluminum flow simply did not meet. My guess is insufficient hydraulic pressure to drive out the air. Here's another view angle, and as you can see, the runner feeding the needle is almost level with the pouring cup.

The solution to all this? There should have been two additional runners at right angles to the two existing feed runners. Unlike bronze, the weight of the aluminum was insufficient to force enough metal through the feed runners. Also, a vent would have helped to eliminated trapped air, which also was a problem. So, metals are different animals, and go figure.

My youngest student managed to cast her Greek helmet, or faux Greek helmet.

It weighs 18 lbs. She looks pretty cute in it.

Guess I may be working my way up to an armorer's position after all.

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