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