1. The aluminum in hardware stores is probably 1100 (99% pure aluminum.)
2. It was probably in the annealed temper.
3. Regardless, you followed an appropriate practice to anneal it. If you
got it red hot, it would have melted. Quenching in water is not required.
4. Because you got fair results with shallow bowls. I conclude that you
should do an intermediate anneal after partially spinning the deep bowls.
Hold for 2 hours at the highest temperature that your oven will attain.
5. I have never spun metal on a wood lathe. Here's what Dave Hout did on
the Woodturning show on DIY network to spin aluminum:
a) made a male form and attached to the headstock.
b) made a small "tail block" and attached to tailstock.
c) centered an aluminum disc between the two.
d) used a vegetable oil lubricant.
e) buffed to a shine.
6. I recorded the show. I will E-mail a copy to you, if you like.
7. You may wish to try aluminum closer to 1 mm rather than 0.3
mm. The appeared to be about the thickness that Dave Hout used, but I
couldn't really tell.
p.s.
I have been an aluminum metallurgist for 40 years.
"Nick" wrote in message
...
Hi all,
I'm having a problem spinning some mirrors for some experiments. The
mirrors are parabolic in shape, and I will place a light globe in the
centre. The trick is for my report I need to compare different curvatures,
eg, deep parabolic bowls and shallow bowls.
I have a Record woodlathe which I have been using in an 'outboard'
arrangement. The bowls themselves are 12 inches in diameter. I turned the
mandrels from hardwood that I laminated in a criss-cross fashion like
plywood mounted using a faceplate. To spin the metal bowls I have been
using 0.3mm Aluminium from the local hardware shop. I'm not sure of its
hardness so I annealed it by baking it in the oven at the highest setting
and turned off the oven and let it cool slowly as the oven cooled down.
Maybe this was my first mistake but I have noticed in other threads that
some recommend quenching the metal in water etc after heating. I would of
thought this would have hardened it but maybe I am mistaken.
The aluminium disks are attached to the mandrel by a centerbolt at the
back of the faceplate which protrudes through the mandrel and the
aluminium. A 2'' washer and nut are then used to tighten the aluminium
against the mandrel.
I have turned 3 shallow mirrors so far with fair results, however now that
I am starting to spin the deeper bowls I am finding the aluminium is
cupping back towards me (I stand end on off the lathe so the job wont
slice me in half incase everything breaks down). Cupping did occur with
the shallower bowls but I was able to apply enough pressure to correct it
and finish the job. Now however if I apply too much pressure I notice
small creases in the aluminium when I stop the job midway.
I would like some advice firstly regarding the metal. I find even though I
*thought* I softened the aluminium that it is quite stiff nonetheless and
am wondering if it is pure aluminium. I am hoping someone could offer some
advice on the type of metal I could use for best results, particularly the
thickness. I am wondering if I went a bit thicker whether the job would
spring back at me as I apply pressure. Also I was considering whether
something other than aluminium might be the go.. maybe copper? I dont
have alot of experience at the metal stuff. I am using a spoon shaped
tool, I whittled and sanded out of some eucalypt. I have been using a
liquid wax polish for lubricant whilst the job is underway.
Thanks in advance
Nick
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