From: atm{at}misterg.fsnet.co.uk (Andy Gray)
To: "atmlist" 
Reply-To: atm{at}misterg.fsnet.co.uk (Andy Gray)


Thanks to all for the suggestions received, both on and off list, but
events have overtaken the situation - I went to press the lap again today
to do some more work and it was
*miles* out of flat (almost literally - 0.7mm high spots at
various points around the periphery) after sitting face up overnight.
Against my better judgement, I made this lap on a painted MDF base - looks
like some water has got in, so I need to find some good ole' reliable glass
to cut a blank from and rebuild the lap and see if I still have the same
problem. I've got plenty of 6mm float, but only the one piece of 10mm which
was just large enough for the secondary blank and a "pressing
plate" for the oversize lap. My 4" flat is actually 3 5/8"
square, so it tantalisingly too small to press the lap against! Will
probably end up gluing a couple of 6mm pieces together.

My intention is to cut the finished flat to an octagonal shape, rather than
try and trepan out a "sausage slice". If I chamfer the
"bottom" 3 edges, the obstruction is only 7% larger by area than
a "sausage slice" (45 degree cylindrical section ??), all of
which is actually useful area. I trepanned out the 7" blanks for the
primary mirror and tool, and the 3.5" blank for the secondary, but I
still think my chances of success in shaping the secondary are better with
an octagon - If nothing else, I could mount it on a mandrel and smooth the
corners off. (main concerns with trepanning are the uncertainty of the
final diameter, and the risk of the tool wandering or the work moving and
giving a wonky cut and/or undersized secondary - it's a long way to drill)

Also of note is that I believe Pilkington have stopped making 25mm float in
the UK. Some stock is still available, but get in quickly if you're
thinking about it (I can suggest a sensible UK supplier if anyone wishes).
I believe St Gobain are still making it in France.

Andy

Andy Gray, Dyserth, N. Wales.

P.S. My apologies for mixing on/off list replies - everything ATM ends up
in the same folder at the moment.

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