10inch Newtonian Part 2 - The Tube
I have been considering different material for tube and
actually purchased a length of PVC drain pipe and a length of cardboard
sonotube. As soon as I got the PVC home I realized it was far too
heavy and it now resides under the house waiting to be used for formwork
for a pier. The cardboard sonotube seems ok but I thought of a better
solution.
What I have done is to fabricate a plywood tube. In Sydney there is a company called Brims Plywood that makes plywood sheets down to 1.5mm thick which is a 3 ply. I purchased 3, 1.5mm thick 8'x4' sheets.
I made up a former using 4 disks of 19mm MDF and ripped 16
longitudinals out of some scrap timber. Here is Tilly Dog supervising and advising on construction details.
The longitudinal were nailed to the 4 disks and this provided a former to wrap the ply around. The whole lot was planed into the round by using a hand plane and template to ensure a nice round profile is maintained. The 25mm aluminium tupe through the center makes supporting and access to the tube easy.
To enable firm clamping of each layer I modified some large hose clamps by
inserting and spotwelding in a length of scrap steel. The first layer
was lightly nailed to one longitudinal at intervals of 25mm.
The next layer was applied using plenty of PVA and ensuring the seam was offset by 120 degrees and was then was left overnight to cure. I only made 4 extended hose clamps and I had to apply tape to keep the seam down. Pieces of scrap timber applied pressure to the right spot. Masking tape prevented the timber pieces sticking to the tube The next day another layer was added, again offsetting the seam 120 degrees and was then was left for about a week to fully cure.
The next weekend I split out the former after a lot of swearing and cursing
which didn't seem to help but it made me feel better at the time.
After all this I now have a VERY light, stiff and beautiful tube with
all that lovely wood grain on the outside. Here is my daughter Tara inspecting the tube.
Final dimensions......ID=285mm, OD=294mm, Wall thickness=4.5mm, Length=1285mm, total number of layers of wood per side= 3x3=9
The weight difference is amazing...... My plywood-3.2kg, Sonotube-5.4kg
and PVC....... wait for it....... 11.6kg.
I cut some 22mm wide strips of ply and laminated up some end rings to dress up the tube ends and to ensure the tube doesn't go out of round.
Twelve wraps of ply strips gave me rings 20mm wide. Each layer was clamped while the glue dried using the modified hose clamps.
Some sand paper glued to a board allowed sanding of the sides of the rings.
I mounted the ring to the faceplate and used the router to machine a rebate to fit the end of the tube.