Hi Ken,

Nice drawing. The focal length of your lense is important and will drive the radius of the end. It will want to be slightly past the end of your chunk of acrylic so you get your magnification and can still see (in focus more or less). Lets say 2mm past the end should be close enough, or start a little longer, and face a little off at a time until it looks right. For acrylic the focal length would be roughly twice the radius of the curved end, so for a 40mm long lense like you have drawn, your radius wants to be 21mm or thereabouts.

I imagine (haven't tried it so all theory) it would be pretty forgiving as long as it is concentric, so in the lathe making sure the radiused end is absolutely concentric with the bit that slides in the holder. I'd turn it all in one go and part it off. The ball turning tool would improve the view, but doing it by hand would get you pretty close as well.

Good luck with it ... I'll be interested to see how well acrylic polishes up with the average Joes equipment.

Chris