Many thanks Geof, you came up trumps yet again. I found some stuff called "Shapal" which is a machinable ceramic as you suggested. It's got a low co- effiecient of expansion and looks perfect (I can get round not using M5 screw holes).
My only concern (as you rightly pointed out) was out- gassing which is a nightmare for optical coating plants, but they say that this material is used for vacuum deposition crucibles (which we use in the coating chambers), so if it's good enough for that it'll be good enough for my jigs.
We've coated using the stainless jigs and it seemed to be ok, apart from a couple of lenses chipping due to the difference in expansion co- efficients between glass and SS, but we're still at the prototype stage.
I'll ask the company to send me a sample to see how brittle it is and will redesign for the production batches if it's acceptable.
Many thanks again Geof.
Iain.
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