Re: ATM Ultra-lite Dobs using foam core components

["at making" <atmaking@hotmail.com>]

Hi,


>I am in the process of re-designing my 24" F4.5 traditional (and heavy) Dob
>into an ultra-lite design.  I am currently investigating building materials
>an am looking at 1/4 inch birch ply with a foam core for the bearings and
>secondary ring.  I am making some test foam board using self expanding
>insulation foam sprayed between plywood sheets spaces with wooden spacers.

1/4 is thick enough for all parts of a dobson scope. In fact,this thickness 
don't have a big influence, the thickness of the foam is more important 
because the rigidity come from this thickness. I haven't use expansion foam 
because I'm always afraid that the foam don't fill all space and is hard to 
control is overpressure that can change the shape of the pieces you fill 
with it. However some TM seems to have done a very good job using it, like 
Statis for is 24".



>I have not worked with foam for structural components and I'm looking for
>experienced advice.  There are a few questions I have been pondering:
>
>1.  Is there a particular type or brands of foam I should use?  Is foam
>spray insulation adequate or should I look to boating or aircraft building
>materials?

For the 22" and the 32", I haven't succeed in finding the same brand but 
that don't effect the structure a lot. The important point is to take 
extruted foam cause is stronger, dow chemical, basf for example. Their is 
different brand, color and structure and some are more rigid but they can be 
hard to find, so if you find a good extruted, take it. One of the thing to 
check is the thickness regularity, it's often more work to use unheaven 
thickness panel cause that mean sanding and holes to fill with glue/foam 
sanding powder.

>
>2.  Foam board works well for compression strength but how strong is it for
>shearing or rotational twisting as I would expect on the secondary ring and
>bearings?

No special problem for bearing as the foam seems to be quiet regular in 
properties. For the diag cage, take a thick panel and if it's not enough, 
glue two panel together. As the weight is very low, that don't move the 
center of gravity a lot and give a much stronger cage.


>
>3.  Is it better to make a sheet of foam board and cut out the components 
>or
>build the components then fill them with foam?

I always cut wood panel, cut the foam often slightly larger and I sand to 
remove all the unnecessary foam. The only "hard" problem is to keep the wood 
panel in the good position as with the glue, it can easily slip. If you 
like, you can use a "hot wire" system (in fact a small special metal wire 
that receive electrical tension to cut the foam). It's not hard to make and 
for thick panel, it's much easier than use a sharp knife. In some hobby 
shop, their is ready made system but they can be expensive compare to a home 
made system. They are not hard to make.

It's possible to use some "nails" (I don't know if it's the good word) to 
keep all part in the good position. Don't hesitate to use a lot of clamps or 
weight. The yellow exterior grade wood glue works well but the brand I use 
is probably not available everywhere.



>4.  Any suggestions on how to finish off exposed foam ends?

For the 32" bearing, I have use stainless steel glue with the same yellow 
glue cause it's a strong glue and can do a good job on metal parts, for the 
22" I have use formica, only one thickness cause the plywood was 9 mm thick. 
The foam is very strong in compression. For diag cage, I have use a very 
thin wood, less than 1 mm thick but some people like Steven Oerholt orJohn 
Lightholder use something like "poster cardboard", I'm not sure of what is 
is exactly. Whatever you use, if it's thin, light, can take a round shape, 
and easy to glue is good, the important point is to protect foam. For other 
parts like
mirror box, the same plywood you use to cover panel is good.

I have use much thicker plywood for the mirror box than the classical 1/4" 
because my collimation screw that carry the load of the mirror cell and 
mirror are throw this the side of the scope, otherwise, it's possible to use 
a doorskin plywood, something arond 1/8" thick.

Try to use thick foam panel, around 1,5" to 2" if you are not looking for 
high compacity scope. The weight difference is minimal but the rigidity is 
much better. On all parts you plan to put screw, like for example, a 
wheelbarrow system, use pieces of plywood inside in order to have a hard 
spot.

Take a look at our web site, the 3 top right scope are foam core. The blue 
one, mid left is also foam core but with graphit and fiber glass.

The pages are not all translated but some pics can help :

http://www.astrosurf.com/altaz/materiel_e.htm
http://www.astrosurf.com/altaz/gluing.htm
http://www.astrosurf.com/altaz/pate_e.htm
http://www.astrosurf.com/altaz/dobpolyst_e.htm

Happy new year to all and of course clear sky !!!

Frédéric Géa
http://www.astrosurf.com/altaz



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