Aluminium tube and threaded inserts for truss Dobsonian

[Stevo69]

Originally Posted by Rod

...I am experimenting with keeping my struts in pairs, held together with a captive bolt. I haven't fully de-bugged it yet but happy to send a photo if you are interested...

Rod, I'm intrigued! can you post any pictures to explain further. It sounds interesting!

[Rod]

Hi Steve,

The first photo shows the two joined trusses. On the side you can't see, I drilled a small hole through the bolt and placed a pin in it to hold everything together. The aluminium block is tapped to accept the bolt. The first few millimetres are drilled with a wider hole to accommodate the pin. It's a very crude attempt to do what is shown here:

http://www.wcc.net/~myastronomy/ATM/B3/Upper%20Cage_truss%20tubes.htm

As I have built it at this stage, collimation does not hold between setups so the idea needs some work. It is however very rigid.

Hope this gives you some ideas.

Rod.

[CoombellKid]

I thought about the loose wing nut in the wedge clamps I made, so what
I did to was to save the extra weight of a safety chain was smack the
thread ends with a flathead screwdriver, there is no way the wing nuts
can come free now without the use of a couple of pairs of pliers and or
damage to the clamps themselves. Besides I have to make it kid proof

just a thought

However wedge clamps are easy to make if you have a router and
a drill press, hell if I can make them anyone can. I mean the last time I
built something in wood was neary thirty years ago in high school.

regards,CS

[Stevo69]

Hi Rob, I checked out your web page and I see you successfully bent the 3-ply after soaking it in water overnight. Any issues in doing this? After it dried out, did it crack or split? I've never used this process to bend plywood in this fashion. I'm seriously thinking of doing this with my secondary cage.

[CoombellKid]

Hi Steve,

It was pretty easy doing a 734mm x 55mm strip. If I was doing a full size
cage I would think about making a mold and use veneer. You could make
it just as strong but lighter. I did try to use a clean bit with no voids but
one did sneak in. You can pretty much hold up in bright sunlight and see
them. I'm surprised it didn't snap to tell you the truth. It dried within about
5-6hours pretty much on a rather warm day. It pretty well kept it shape
or had no problem. No cracking just as you would of seen on that page
I wrap it around an old 8" newt tube and gave it a sand. Sticking it onto
the inside of the rings was a lil tricky. I used the inner circle from cutting
the rings and a bunch of lil wooden wedges I made and glued it. Tacking it
with tacks or nails seem like it might be a lil hairy as the ply was still fairly
rigid and there isn't much meat in 3mm ply.

regards,CS

[Rod]

Originally Posted by Stevo69

Hi Rob, I checked out your web page and I see you successfully bent the 3-ply after soaking it in water overnight. Any issues in doing this? After it dried out, did it crack or split? I've never used this process to bend plywood in this fashion. I'm seriously thinking of doing this with my secondary cage.

Hi Steve,

If you want to use ply to line the secondary cage, I used 1.5 mm thick ply I bought from marine timbers in seaford. It was about $32 for a sheet of 8x4 gaboon. You don't have to do anything special to bend it and it is very strong.

Rod.

[Satchmo]

That would probably be a door skin ply ? In Sydney we have Mr Plywood which sells specialty plies including thin marine/aircraft ply. The 1.5mm is a great choice for cage lining. Kydex if it gets subjected to high heat in Summer as you might find in a locked car, will go dimply like a bad case of cellulite . I've seen a few Obsession style scopes suffering from the `fat thie' syndrome.

Rolled 1.5mm ply has a lot of inherent strength too. Monoquoque construction of rolled plywood panels is how they used to build aircraft.

[Stevo69]

Thanks Mark and Rod. I'm warming to the idea of a very thin plywood, such as this as a substitute for Kydex. Worst case scenario will be 3-ply soaked and bent, but I'll try to find a supplier for the 1.5mm plywood this week. I'm gathering all the secondary cage parts and materials for a monster working bee in 3 weekends.

[Rod]

Originally Posted by Satchmo

That would probably be a door skin ply ?

Hi Mark,

I think it could be used for door skins. I bought it from a shop that supplies boat building materials so I assumed that it was some form of marine ply.

Rod

[Stevo69]

Yesterday I did some experimenting...

Experiment 1.

I bent some 3mm MDF in a 19" cylinder to prove it could be done. It is held in place nicely and there was no "creaking" or "splitting" as I was bending it. I'm now going to leave it for a week or so and see if it breaks. No soaking in water.

Experiment 2.

I bent another piece in the same way except I lightly sprayed it with a fine mist of water on on side before bending. Then I bent it into a cylinder, clamped it and immediately put it in a warm, well-heated room to dry. Two points I noticed: firstly, it bent easier when sprayed with a fine mist. Secondly, after a few hours, I removed the clamps and it retained more of its shape than the first experiment (no water).

I'm going to keep an eye on these two pieces and check their condition in a week, then I will seal them with primer, then semi-gloss black on the outside, and matt black on the inside. Then check them in another week.

A cylinder of 3mm MDF is very light in weight and I know it may be "heavy" and unacceptable to some, but I'm prepared to wear that.

Kydex is definately "out" in my books and a timber material is now a preference.

My only issue is that if I use this type of thickness, it will add approx 3-4mm to the ID of the secondary rings. Now, with Kriege's calculations, my secondary cage ID will be under size by approximately 1/8 - 3/16 smaller than the calculations. In the scheme of things, probably not noticeable.

Originally Posted by Rod

If you want to use ply to line the secondary cage, I used 1.5 mm thick ply I bought from marine timbers in seaford. It was about $32 for a sheet of 8x4 gaboon. You don't have to do anything special to bend it and it is very strong.

Rod, You wouldn't have the name of the shop in Seaford? And the name of the material you purchased? I might also consider it for the secondary cage lining. Can you let me know?

hoo roo,
Steve

[Rod]

Originally Posted by Stevo69

Rod, You wouldn't have the name of the shop in Seaford? And the name of the material you purchased? I might also consider it for the secondary cage lining. Can you let me know?

Hi Steve,

It is called gaboon and the shop addrss is:

Factory 3,
10 Rutherford Road, Seaford 3198 (facing Keppler Circuit)
Ph: 03 9775 0006 Fax: 03 9775 1776

Are you a member of the ASV and going to the meeting on Wednesday night? If so I can bring what I have left over for you. I'm not sure there is enough for your project. You would definitely have to lay it as two strips side by side. PM me if you are interested.

Rod.

[Stevo69]

Thanks Rod, sorry I didn't get back to you sooner, but I couldn't get to the Wednesday night meeting anyway. I'll give them a call first thing on Monday and might go for a day trip next Friday.