Well on Saturday I travelled an hour northwards to see grinding maestro mr John Nichol to check how im getting on with my grind.Before I went up I was getting back down to sagittal with the tile tool and making the mirror spherical before polishing and figuring.John confirmed what I originally though, that my mirror was needed edge deeping but he said it would only take a couple of hours to do this and get it to a sphere which was good news.We discussed that I will likely need about 2kg of pitch to make my lap which was a bit more than expected but no worries.
Did the first hour with the tile tool with a big overlap to deepen the edge before checking again and it was just below 10.5mm.Another half an hour later in the day had it to bang on 10mm which is the depth for an F3.5 sagittal which is 0.25 mm shallower than the F3.6 I was after but this is what John advised.I then did half an hour with a 2" overlap on the tile tool to bring it back to a sphere.I was getting good contact with the tile tool on the pencil test in all but the very centre section.I have done another half hour since which has lessened it but its still not 100% contact on the centre but tbh my tile tool is a fair bit thinner on the centre now which may be why im not getting the contact 100%.Ive emailed John for further advice to see if it should just continue with the 2" overlap until the contact is good but I think its likely as good as it will get without casting another tile tool which John said was unnecessary.He did comment that I could glue another set of tiles on if it got really bad but he said it would probably be ok.
So in anticipation ive cleaned off the mirror and the tool of all 80 grit in readiness for 220grit!!!took me a while to clean all the tools,table,floor to avoid contamination and scratches with the finer grit.Just waiting for a bevelling tool to arrive tomorrow to re-bevel the mirror before carrying on through 220, 400 and 600 grit then I need to make my ronchi tester.Ive ordered some parts for the variable led and I have an old acrylic pc case that im going to cut up so should just need wood for the mirror holder and a macro focusing rail slider for the tester end.
Really hoping when I test it for the first time that its close to a sphere or its back to 80 grit again and a new tile tool but tbh that's part of the learning curve.
hopefully have an update next week.
clear skies Rich