From: Mark Holm 
To: atm{at}shore.net, tylosaurus{at}sisna.com
Reply-To: Mark Holm 


I sent off my recent message on this topic before I checked to see if there were
other replies already in the digest.  (I was in a rush because I needed to
make a milk run to the grocery store before it closed.)  The S&T
article describing how to make the plywood cell is the one referenced by
Gary Seronik.  He ought to
know.  He wrote it!  Only Gary wasn't quite precise enough about about the
radius of the circle to put the silicone blobs on.  Make it 1.6 inch like I
said.  There is good engineering behind that number.

Actually, after I wrote the first message I though about the thickness of
plywood.  1/2 inch is plenty for the structural requirements, but 3/4 is
probably better for a couple of reasons.

1. When threading bolts or threaded rod into plywood, getting a good hold in 1/2
inch could be tricky with ordinary plywood.  If you have some really high
quality stuff then 1/2 might be good enough.  Threading the holes will be
even easier if you can get or borrow a machinist's tap and tap wrench for
the size threaded rod (or bolt) you will be using.  Taps are a little
tricky in metal, but in wood, they are dead simple.  Just turn it into the
hole, and instant threads.  You have to drill the hole smaller in diameter
than the O.D. of the threads.  In metal, this is a problem requiring some
precision, but in wood, it is again dead simple, just use a drill about the
same diameter as the root diameter of the threads.  Just eyeball it by
holding drill bits up to the threads until you see one that is close.

If you can't find a tap, just thread your bolt or threaded rod into the
hole. It will form the threads, though they will be tight, and you will
have to turn it pretty hard.  (lubricate with a bit of dry soap if needed.)
 The tight fit isn't all bad.  In this design, you want the rod to stick in
the hole. (The hole
in the front plate.  The rear plate hole has to have clearance for the
threads to slip through.)  If you tap the threads, they may be loose enough
that you will need to glue them in with a little super glue or epoxy.

2. When you mount the rear plywood plate into your tube, you will likely want to
drill some small holes for screws into the edge of the plywood.  It is less
exacting to hit 3/4 inch than 1/2.  the screw holes need to avoid the edges
of the plywood to avoid splitting out.  Much easier with 3/4 than with 1/2.

Gary is definitely right about that counterweight.  Do what it takes to get
it out of tube.  If you are going the fan and vent route, you may not need
a ventilated cell.  Leave the rear plate of the cell a solid circle so it
acts as a plug in the tube.  Then mount the counterweight on the rear of
the rear plate.
    Use 3/4 inch plywood for sure if you do this since the extra weight might
flex 1/2 inch too much.  This would keep the counterweight nearly in the
same position, so balance would not change much, but the heat from the
counterweight would not be inside the tube.

BTW, the alignment bolts/screws/threaded rod segments do not need to be at
the same 1.6 inch radius.  In fact it is better to put them as far out as
you can. The farther out they are, the less sensitive the collimation
adjustments will be, and the more stable the mirror mounting.

Mark Holm
mdholm{at}telerama.com

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