Re: [ATM] glued aluminium mirror cell update

["Dream - Telescopes & Accessories, Inc." <shane@dreamscopes.com>]

Andreas,

Hmm, it sounds like you've done the basics correctly.  I'd suggest you 
contact the resin company directly and tell them exactly how you used their 
product.  Tell them how you prepped the surface and how you cooked the 
part.  Some times there are chemicals within the compounds that do not mix 
well with others.  Typically Acetone is fine but maybe that epoxy "doesn't 
like" it.

As you mentioned, the repeated cook cycles might be a problem for that 
particular resin.  Again, talk to a tech rep (or a chemist!) at the epoxy 
company and give them all of this information.  Something might stand out 
to them and being the culprit to the failure of your bonds.

Another thought is that you can probably bond all joints at room 
temp.  Once set, then put the whole enchilada in the oven and cook to what 
they require.  This way it is one cure cycle and the piece is solid enough 
to hold itself together during the cook.

Yes, surface area is a key factor in bond strength.  If you could plug each 
square end with a cap that either on the outside or inside provided a large 
surface area for both the cap and onto the other piece of aluminum, that 
would be best.  On a very garage shop style bond you could take a thin 
strip of thin aluminum and bend it around the piece.  Start roughly 1-2" in 
from the joint of the two pieces you want to bond, wrap the thin strip 
around and underneath.  Then it ends the same 1-2" past the bond joint but 
underneath where it started.  Bond all of this with epoxy, gooping both 
surfaces to be bonded before putting them together.  Even a thin strip of 
aluminum would add enormous strength to the joint.

Yet another idea would have been to leave one or two of the square sides 
long.  Then when you bond that onto another piece, wrap those "tabs" around 
the other piece.  It acts like the above idea but is better because the 
aluminum is still connected.  Although the tubing might be thick enough in 
wall to make bending it precisely a real bear, unless you have proper 
equipment.

And another...  Cut a piece of flat stock aluminum into a shape that allows 
for fans, if needed, and all moving parts (collimation hardware).  Bond 
this plate to one side or make two and bond them to both sides of what you 
have now.  That will increase your surface area greatly.  Unless you have a 
great deal of intricate cuts to make on such flat panels, it is also a very 
simple fix.

I would talk with the resin company first though.  I have the feeling 
something else is going on.  I really doubt you are putting any kind of 
serious load on any of these joints.  You're not putting the mass of the 
mirror on one joint so I believe there are other issues besides load and/or 
bondable surface area.

Boats will often use big flanges on the inside of the metal 
hardware.  Spreading the load is key in most composite applications because 
of the nature of the material and how it is fabricated.  Compared to 
metals, composites just hate being drilled through...  They are extremely 
weak in those areas because of the point loading that occurs (and again, 
the nature of the material/fabrication).  Certain applications are using 
composite hardware now.  But that is specific to composites.  It would do 
nothing for your aluminum parts.

Flexure is a whole other topic....

Thank you.

Sincerely,
Shane Santi - Owner
Dream - Telescopes & Accessories, Inc.
http://www.dreamscopes.com
610 - 365 - 2833

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