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Re: [atm_free] Re: [ATM] Polishing /Figuring a Slumped Blank

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  • Jarvis Krumbein
    Ulhas, I don t think that you will be able to control the depth that accurately. I would go as close as possible but leaving it slightly on the shallow side.
    Message 1 of 18 , Apr 1, 2008
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      Ulhas, I don't think that you will be able to control the depth that
      accurately.  I would go as close as possible but leaving it slightly on
      the shallow side.  You will be working tool on top but with undersize
      tools so you will be able to control the final depth.  I used a 16" tool
      for almost all of the grinding and polishing on the 20".  One thing is of
      great importance and that is to check the glass for strain after slumping
      and before starting any grinding.  A very good anneal is all important.
      Check using crossed polarizers and unless there are no strains visible
      you might need to re anneal the glass.  Once the glass is OK, I would
      also recommend lightly grinding the back side of the blank to break up
      any reflections from the rear surface while testing later on.  I did not
      do this with the 20" and sorting out the reflections while testing some
      times proved troublesome.  I have ground the back of other slumped
      mirrors and found testing much easier.  The problem with reflections is
      caused by both the front and rear surfaces having almost the same ROC.  I
      would also edge the blank before starting to grind but after slumping.
      When you make the dental plaster support for the blank, use a cushioning
      material (very thin) between the blank and the casting and mark the edge
      of both the blank and the support so that you don't change there relative
      position while working the mirror.  This can be important because the
      rear surface may not be truly spherical and changing the position may
      change the support with the possibility of introducing astigmatism.  As
      one of the others on the list mentioned, the rubber impregnated fabric
      such as used for kitchen shelf liners works well.  When testing, leave
      the mirror in its back support.  My test stand is setup so that the
      mirror is always pointing slightly up with the stand close to the floor
      while the tester is about five feet above the floor.  This approximates
      an actual position that the mirror may be used at when finished and is
      very comfortable while actually testing.
       
      Jarvis Krumbein

      Ulhas Deshpande <dulhas2001@...> wrote:
      Jarvis Thanks
      Does that mean that the glass should be slumped slightly less?. I need a sagitta of 6.944 mm so I should slump the glass to 6 mm depth allowing a mm for fine grinding and polishing. Right
      Thanks
      Ulhas


      Jarvis Krumbein <jkoptic@juno. com> wrote:
      Ulhas, I've been working on a slumped 20" f3 (1" thick) on and off for a
      number of years. This is entirely doable and your use of a dental
      plaster support is probably the way to go. Working tool on top with sub
      diameter tools also works well. You will definitely need to grind as the
      slumped glass will not have a good or useable spherical surface.

      Jarvis Krumbein

      On Sat, 29 Mar 2008 16:48:35 +0000 (GMT) Ulhas Deshpande
      writes:
      > Hello All
      > I have been toying with the idea of a 20 inch mirror F 4.5. As the
      > only glass available easily is 19m mm th. Float glass, I am
      > constained to think of either fusing i.e ribbed mirror or slumping
      > .
      > While the local suppliers are read to paste /glue two pieces of 19
      > mm glass they are unable to fuse .I have found one person who is
      > ready to slump the glass disk.
      >
      > When I accessed the archives to find out how a slumped disk is
      > ground and polished, I found that everyone is advising MOT. Has
      > anyone tried a machine and TOT with a subdiameter tool ? I have a
      > Weino/M-O-M type machine on which I have done 8 inch mirror and
      > propose to scale this up to take 20 inches.I have to use a machine
      > as physically I am unable to otherwise.
      > I plan to make a mould of Dental plaster to hold the slumped glass
      > and work on it TOT with a subdiameter tool. Is this a sensible idea?
      > Any obvious pitfalls?
      > One last question. Since float glass is already well polished, can
      > I go straight to polishing/figuring? I would of course carry out a
      > redout/laser test to check the quality of polish and if possible a
      > Ronchi test to check the surface. Will the slumping process affect
      > the polish of the glass plate?
      > Thanks for advice/comments
      > Ulhas
      >
      >
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