1st off, I have diamond core drills at work to acomplish this, with water swivel, coolant, etc.
Even though some people seem to get away with drilling partially through the back, then boring through the front (with cover) after polishing, but (?) before final figuring??, this seems contrary to my optical experience, in terms of stress relief, and then trying to figure with that hole there.
More logical seems the method of boring most of the way thru the back, filling with plaster, and when rough grinding is done, the front end would be exposed, the plaster scrubbed between grit sizes, and varnished over before polishing. We use plaster blocking with prisms and brewster tubes, and plaster dosn't add stress to the parts And, at least with flat work, it really helps control overall test with as much support in the center as possible. The core drills we use generally have a .035" wall thickness, or the ring of plaster would be fairly thin.
As usuall, those of you who have been through this and would like to comment...please do!
BTW, the Bk-7 slab arrived today!!! (Actually Ohara's Japanese version is called SBL-7, but is basically the same animal) It looks like its thick enough that with carefull mounting and cutting in the big slab saw, maybe I can get two plates out of it (just in case the unmentionable occurs) ..... or maybe build two scopes.... stop stop stop ..enough is enough...
Bill Marriott Forest Knolls, Ca. USA btk@ix.netcom.com