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