Doesn't have to hit the melting point to release hazardous particles, and any sort of scraping/scratches/tears will increase this. There's a reason they dont sell aluminum / plastic pipes. If you have access to metals you should be able to make a very simple steel or copper bat or pipe. Even someone without your knowledge of metals could easily buy a metal tube with a large opening at one side, and put a screen in it to make it a bat, or put a bend in it and make it a pipe. With your skills I'm sure you could could make a bong out of aluminum, and just make the stem out of a steel tube.
Oh no, I would never use anything from my shop to smoke out of at all, period. 99% of that stuff was cut with while coolants were being used on them, and coolants are PROVEN to cause kidney failure just by absorption through the skin (by small cuts, ect ect). Hazardous vapors in kitchen grade aluminum are far to low to cause any damage at all, the grade of alloy they use on cans is much to refined for anything long term effects. the only things that could cause any kind of "damage" would be the ink on the outer part of the can.
When it comes to making pipes, most people usually only have house hold equipment, so i doubt they are going to run out to home depot or forest city and purchase metal piping just to smoke.
and they DO sell aluminum pips for kitchen wear for businesses, I know, because I've been renovating a coffee house in down town cleveland, pretty much EVERYTHING from table tops, to tubing is aluminum or alloyed with aluminum. IT is honestly, quite harmless. Nothing hazardous, like lead, would even make it near the market now-a-days, do to law suits, smokers could sue companies saying that the can never had a "DO NOT SMOKE FROM THIS" printed on them, thus cause a frivolous law suit.
Another point is you use Aluminum foil to cook baked potatoes in the oven, on some aluminum foil containers it actually has instructions printed on the box. If the product was harmful, you would still be ingesting the same shit into your body, this time through your digestive system, instead of your lungs.
So honestly, stop freaking out about the home made pipes, seriously though stop being cheap bastards and go buy a real one, you only should use home made ones (using tin foil, cans, ect) if you are in a serious bind.
But, that is also why I always keep rolling papers on me as well.