Re: ATM - density of plywood ??

["Vegos" <astro@rvi.net>]

Bill:

Light & rigid is better!  But it depends...

Using the info below 3/4" white fir core/birch faced plywood is nearly the
same weight and 2 or 3 times as stiff as 12mm Baltic birch.

According to Kreige and Berry's book alder (ApplePly core) is the least
stiff of the three core materials i.e..: birch, white fir or alder.  But is
~ the same density as white fir.

The most rigid and light traditional sonotube scope I've made so far used 1"
white-fir core/birch faced plywood for the rocker sides and 1" Douglas-fir
shop for the base.  The extra weight of the 1" was reduced by removing
material from the   sides.  Avoided the front wooden cross brace by using 2
pieces of Al tubing to anchor the sides together.    Guts feathers and all
the 8" dob weighed 30lbs.  Never have built a tube Dob with 12mm birch
plywood.  But I have seen a few and they're not as rigid as that scope with
the 1" sides.  Especially if material is removed to lighten the scope!

BUT - 12mm Baltic Birch went into a 10" truss where the total amount of wood
used is significantly less than on the 8" tube Dob.  Total weight of the 10"
is 30lbs despite the mirror weighing 2x as much.  The short mirror box and
rocker sides plus corner gussets negate the need for the extra bulk and
stiffness of  thicker material and the scope actually feels more rigid that
the tube scope.

BUT - Baltic Birch and ApplePly are more void free than what I've seen in
even the best white fir core hardwood faced furniture plywood.  The also
seem to splinter less on the saw or router.  And the edges are more
attractive if left exposed than white fir core plywood.  In addition one of
my pet peeves with white fir core plywood is that molecule thick hardwood
facing!  If you even let the wood SEE a piece of sandpaper the facing is
GONE!

Personally I like to use hardwood plywood better, especially in a truss
scope with real short mirror and rocker sides.  But in traditional tube Dobs
with the tall rockers I think I'll continue to try to scrounge up 1" white
fir core plywood.  The extra width with less dense white fir makes for a
stiff, light scope.

$.02

Steve
http://www.rvi.net/~yovw


>
> Doug Tanaka wrote:
>
> >1/2" appleply... 0.0095 lbs./sq inch.
>
> 7/16 Russian Baltic Birch: 0.011 lbs/sq. inch.
>
> 1/2" Furniture grade Birch plywood from Home Depot: 0.0085 lbs/sq. inch.
>
> Does this mean the Home Depot stuff is an exotic, lightweight material?
> Isn't lighter always better? ;^)
>
> Bill T.