From: "Frank Q" 
To: "Andreas Derwahl" ,
        "'Ray Desmarais'" ,
        "Atm-US \(E-mail\)" 
Reply-To: "Frank Q" 


Hi All

Please see comments below
Cheers
Frank Q

From: "Andreas Derwahl" 


> > carried out for years,
> > not weeks or months.You still need to sag a faceplate for a
> > mirror to get a
> > flat back.
>

### Is sagging the same as slumping (partial "melting" / softening) or do
you mean "grinding"??

> To reduce the thermal stress and increase the strenght of the bond it
seems
> a good strategy to make the glue layer as thin as possible. In order to

### This implies grinding 2 surfaces together to get mating surfaces for
extra good contact.

> achieve that, matching the surfaces to be bonded as good as possible is
> desirable and also to apply pressure while gluing. In the paper they bond
> the primed glass surfaces just by pressing without any glue at elevated
> temperature (100 - 200 deg C) and high pressure (up to 50 Mpa). Their
second

## At these pressures/temps, wouldn't it be easier to cut out the
appropriately shaped
pieces of glass, grind and mate them, assemble and slump the lot ??? The only
uncertainty here is that slumping temps may not be high enough to fuse (ie glue)
the pieces together.

> method uses epoxy glue at 90 deg C and 1 Mpa pressure. This does not seem
> feasable (20 tonnes on a 10" mirror), but maybe one can get away with
lower
> pressure?

### Do not underestimate epoxy. It is amazing stuff. Back in 1972 (Yep, 30
years ago)
my father, in his enthusiam, epoxied the diagonal to its mount. To this
day, the two remain
firmly bonded. I have not seriously tried to separate them - boiling them
in a pot of water
is the extent of my efforts.

>
> Also important seems to be the joint design. Because in a ribbed mirror
> there's no sufficient heat bridges between front and back plate, the joint
> desing should be able to mediate the stress of the two plates of different
> size. I might be wrong here, but to me that implies avoiding radial
> arrangement of the ribs and prefering concentric. Also the thickness of
the
> ribs might be important. Thinner might be better?

## What are the consequences of not having a back-plate at all ???
## Re Different ribbing patterns, does the 200" have a hexagonal ribbing
pattern ??


>
>
> Andreas
>
>
>

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