Mirror cell for 20 inch

[dark star]

I will hopefully start grinding my 20 inch mirror blank this weekend, this has been delayed by the need to make a very heavy duty grinding table and the fact that I am still finishing my 14 inch truss dobsonian (nearly there at last!).

I therefore need to start thinking about the mirror cell. I was planning on making the triangles in the cell from 1/4 inch thick aluminium (will this be strong enough?).

However, I have just seen on line the 30 inch TROK truss dobsonian. Here the triangles for the cell were made of 3/4 inch thick birch ply. I have lots of this, and it would save me quite a bit of money buying 1/4 inch thick alumium, which is not cheap. Also, I would not need to buy a metal cutting bandsaw now (although I will almost certainly end up buying one in the long run anyway).

On the TROK site they stated that they used the cell for a few years without problems. Is there any way in which aluminium is superior to birch plywood for this application?

David

[JRM]

The only real advantage for using aluminium is the weight really, you can use most anything so long as it is strong enough to 

support the job at hand.

Rick M

[RAC]

If you haven't seen this write up then I recommend you have a read. http://www.loptics.com/articles/mirrorsupport/mirrorsupport.html 

The good thing with aluminium is you can make the cell lower profile. I have 5mm aluminium for my triangles for my 18" cell.

[mapstar]

Good to see someone else taking up the challenge of making a mirror and I will watch with interest.  I wish you luck and take a look at my thread for an idea on the grinding table. I can recommend the flag route I ended up with if doing it by hand.

A cell made from birch would be at least different and wood doesn't suffer much from thermal distortion at least if treated for the outdoor environment.  1/4" (6mm) thick ally for the triangles is what I used last time and no problems 

Damian