Re[4]: [ATM] Glueing plate glass

[Richard <cnc@cncservo.co.uk>]

Hi Ken,

Wednesday, February 23, 2005, 5:45:06 PM, you wrote:


KL> My question for some time has been:

KL> If you grind the two pieces against each other for good contact, do you
KL> really need to glue them?  Just edge supports to keep them from
KL> slipping...

Yes.

Imagine a stack of two or more thin pieces, and imagine a force
applied so that they visibly bend. What do you see at the edge?
The answer is a staircase - slipping of one layer over another.
The reason is that for each piece, the top surface is in
compression and has decreased in length, the lower surface is in
tension and has increased in length. Your stack therefore tries,
at each bond, to stop one surface from compressing and the other
layer from tensing.

Unless you can bond the layers with very strong resistance to
shear then you will have a weak mirror. The strength of a single
layer mirror will go with the cube of its thickness. Unless you
resist shear the individual layers will each have a strength
proportional to the cube of their individual thickness. Without
that bond, a mirror of given thickness has relative strength
proportional to 1/N^2 where N is the number of layers it is
laminated from. (N layers each contributing 1/N^3 to the bending
strength).

Fuse it or forget it!. Better still, if you have the furnace to
fuse it then you may as well start with cheap broken glass and cast
it.

-- 
Best regards,
 Richard                            mailto:cnc@cncservo.co.uk

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