Quote:
Originally Posted by Rod
Thanks for the nice comments. If you are looking for a good cheap ply for an octagonal tube, I’ve found Bunnings’ red oak 7 mm thick works well. The tube comes out a bit stronger than the birch ply I normally use. I think it may be because the individual plies are thicker. You need to check the sheets carefully. The veneer quality can vary as can sheet flatness.
I use to use polyurethane glue for these tubes but now I find Titebond 2 is just as strong and much cleaner to work with.
Rod
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Hi Rod,
Nice looking scope and grinding machine. The mirror also looks like its coming along nicely.
I just wanted to point a couple of things out re your material choices that you may or may not be aware of.
On the basis that a telescope tube is used outdoors only and can get quite wet on the exterior due to dewing, the bunnings red oak plywood, isn't a good choice. This is an interior only grade plywood and it does not use exterior grade, water resistant glue to bond the laminations together. It also uses timber species on the inner plies that are less water resistant. That's why it's "cheap". The Baltic Birch plywood uses "A-Bond" glue which is highly water resistant and it also uses Baltic Birch on all plies, internal and the faces, which is a more water resistant timber species. Baltic Birch is one of the best plywoods for telescope construction because of these properties and is the reason its used by professional telescope builders like Peter Read at SDM and Dave Kreige at Obsession. You could use a few coats of marine varnish, or polyurethane on your tube inside and out and this may help its water resistance and longevity, but one ding that damages the integrity of the varnish and the water will get in and eventually the ply laminations will bow and separate.
The other thing is that while Titebond 2 is an excellent glue and specified for external use, it is classed as "Water Resistant". Titebond 3 is classed as "Waterproof". I have both Titebond 2 and Titebond 3 on hand and use both for different purposes. They are both excellent glues, but if I was choosing one to glue a telescope tube together, I would use Titebond 3. That having been said Titebond 2 should work fine in your case. If someone is starting from scratch and planning to buy some glue for this purpose, I would be buying Titebond 3, not Titebond 2.
Cheers
John B