Jim Moscheck
Carpal Tunnel
Reged: 01/08/07
Posts: 1624
Loc: Under the SE Michigan lightdom...
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Hi Everyone,
I'm rebuilding the RB on my dob to lighten it up a bit. Still have 1" thick side bearings which are probably too thick, but only .75" plywood for the bottom as I made the first one 1.5" thick, and 3/8" thick for the front and back supports as the original has .50".
I glued and screwed it together and filled in the screw holes with wood putty. Before I put on the finish it looked great, but after the spar urethane the filled holes were very noticeable(see the pic).
My question is will it be strong enough with just glue(tightbond III) and clamps? Would using a finish nailer be a better idea with smaller holes to fill? Suggestions welcomed.
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Jim Moscheck
Ford Amateur Astronomy Club
ATM truss dob
Stevens 14.5" f4.3
"A mind is like a parachute. It doesn't work if it's not open." - Frank Zappa
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Achernar
Postmaster
Reged: 02/25/06
Posts: 5029
Loc: Mobile, Alabama, USA
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Yes, it would be strong enough if you glued it together correctly. Screws help but they don't hold as well when driven edgewise into plywood as they do through the other direction. Brad nails would leave much smaller holes to fill, but all they'll do is keep the panels aligned while the glue sets. I built my rocker and mirror box using wood glue and biscuits, and it's plenty strong with no nail or screw holes to fill. The only place where I did use screws was to attach the round bottom of the rocker box to the rest of it, and those holes are hidden under the Formica.
Taras
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15-inch F/4.5 Dob under construction
10-inch F/4.5 Discovery Dob
6-inch F/8 Homebuilt Dob
4 1/4-inch F/4 Homebuilt reflector
A whole bunch of eyepieces, filters and other accessories....
Two curious cats
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dave b
Post Laureate
Reged: 05/10/05
Posts: 3548
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on a 14" dob glue might well be enough.
but...i would get yourself a "plug cutter"
http://www.hammerzone.com/archives/flooring/hardwood/plugs/sanded.htm
under $10, it cuts plugs from the same scrap material you make you RB out of.
it can look really good if you make each plug run opposite of the grain.
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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dave b
Post Laureate
Reged: 05/10/05
Posts: 3548
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also, not that you asked, but i see your cell is recessed in your MB.
you could cut off that extra inch on a table saw (watch the exit side) and make your RB one inch shorter
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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DavidinFL
member
Reged: 08/28/09
Posts: 52
Loc: Navarre, FL
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Yes, glue would hold together fine. If you're really worried, look for a urethane glue like Gorrilla Glue. When properly glued, joints break by a failure in the wood near the joint not the glue itself.
Other options include using biscuits, dowels, pocket screws, finger joints, or dovetails. Each has thier ups and downs of course.
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Orion XT8 - Orion StarMax 102 EQ
Edited by DavidinFL (10/29/09 11:32 AM)
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NHRob
Post Laureate
Reged: 08/27/04
Posts: 3159
Loc: New Hampshire
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Dave,
with the box being birch ply, using scrap pieces of the ply would not work well, since the lower layers are rough grade. I think you'd need to make the plugs from solid birch, correct?
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Rob
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Canon 12x36 IS binos
TEC-140: #433 on CGEM
Faworski orthos (7,10,16.7mm)
stuff ...
no free time ... lots of clouds !
" Life is what happens to you while you're busy making other plans" !!
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Jeff Morgan
Carpal Tunnel
Reged: 09/28/03
Posts: 1987
Loc: Prescott, AZ
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As Dave mentioned, a plug cutter is the answer. I never thought of switching the grain orientation. Interesting. I suppose if you want it to be "decorative" you could even use a different wood type. Here is my new "mirror box" before finishing. I use cherry dowels in baltic birch.
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Jeff Morgan
Prescott, AZ
Wile E. Coyote School of Telescope Making
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Jeff Morgan
Carpal Tunnel
Reged: 09/28/03
Posts: 1987
Loc: Prescott, AZ
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On your rocker arms, thicker is definitely better. That does not mean they have to weigh a lot however. I make mine hollow by constructing a frame of 3/4 inch baltic birch and putting a skin over it. The skin could be any wood type you like, I used 1/4 baltic birch. I build two pieces like this and then join them.
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Jeff Morgan
Prescott, AZ
Wile E. Coyote School of Telescope Making
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Jeff Morgan
Carpal Tunnel
Reged: 09/28/03
Posts: 1987
Loc: Prescott, AZ
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You end up with two pieces like this. Glue them together and you get a piece that is nearly as stiff as a 1-3/4" slab, but weighs somewhat less than a 3/4 single piece. The actual measurements on mine before sanding and finishing: 20" x 19" x 1-3/4" with a weight of 7.9 pounds.
On the base I still used double thickness solid. I haven't worked up the courage yet to try hollow core construction - it just doesn't feel right.
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Jeff Morgan
Prescott, AZ
Wile E. Coyote School of Telescope Making
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Jim Moscheck
Carpal Tunnel
Reged: 01/08/07
Posts: 1624
Loc: Under the SE Michigan lightdom...
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Quote:
also, not that you asked, but i see your cell is recessed in your MB.
you could cut off that extra inch on a table saw (watch the exit side) and make your RB one inch shorter
Not a bad idea as it would add stability, but the reason I moved the mirror cell up was that the collimating knob wouldn't clear the az. encoder. I took the knob off and locked the threaded rod down and it now clears the encoder by about 1/2" inch. As it is the EP height at zenith is perfect for me. BTW-Eric W. made the cell for me.
As it is I barely have enough room under the mirror/cell as I keep the Servocat electronics there. You don't normally need to have access to the SC, but when I've had to do some trouble shooting it's a very tight workspace.
I like the idea of the plug cutter. I've used one before that belonged to a friend but I'll pick one up on the way home from work. I have some scrap solid birch that I'll practice with. Or maybe zebrawood. 
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Jim Moscheck
Ford Amateur Astronomy Club
ATM truss dob
Stevens 14.5" f4.3
"A mind is like a parachute. It doesn't work if it's not open." - Frank Zappa
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Clay Kessler
super member
Reged: 04/16/07
Posts: 160
Loc: Manchester MI
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Jim,
I have several plug cutters that you are welcome to borrow. I would suggest using a contrasting wood, like Walnut, to make the plugs from. If the area is going to look different might as well make it a "design element".
Best Regards
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Best Regards
Clay Kessler
www.ssoastro.org
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dave b
Post Laureate
Reged: 05/10/05
Posts: 3548
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Quote:
Dave,
with the box being birch ply, using scrap pieces of the ply would not work well, since the lower layers are rough grade. I think you'd need to make the plugs from solid birch, correct?
you can make plugs from Balticbirch, Appleply, or Europly, but you are right, lightweight "pine core" birch ply will just crumble.
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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Jim Moscheck
Carpal Tunnel
Reged: 01/08/07
Posts: 1624
Loc: Under the SE Michigan lightdom...
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Hi Clay,
Thanks, I appreciate that, although I should get some plug cutters of my own to add to the tool chest anyways. Dang, I was at Woodcraft yesterday. 
You're right, walnut would look very nice contrasted with the baltic birch and I think I have some scrap somewhere in the garage.
If the Moon was out of the way a nice drive out to your place could kill too birds with one stone as I could do a little observing while out there.
Cheers,
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Jim Moscheck
Ford Amateur Astronomy Club
ATM truss dob
Stevens 14.5" f4.3
"A mind is like a parachute. It doesn't work if it's not open." - Frank Zappa
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