Rolled Al tube option for newtonian

[Ian Robinson]

Been sourcing materials to produce a DIYS carbon fibre tube for V3 of my 10" astrograph.

0.8mm x 1.2m x 2.4m Al sheet from Capral (Cardiff) is $69 and to get a 1.5m long x 0.306m OD tube rolled from an offcut from the sheet will cost $60 (labour only) by Harveys Sheetmetal (Carrington).

Harveys estimator reckons the tube will hold the shape OK once rolled , but I'll probably get a couple of spot welds to make sure.

Set me to thinking , since it's going to cost me the better part of $400 to make my carbon fibre tube , I may settle for an aluminium tube for V3 of my 10" OTA.

Wwill give a tube 1.27m x 0.31m OD of 0.9kg.
I'm pretty sure with a few reinforcing disks it'll do for mandrel , but as a stand alone unreinforced OTA tube , I'm not so sure.

Aiming at an OD of 308mm to fit my spider and cell.
2mm thick sheet will make a tube of 2.1kg.
3mm thick sheet will make a tube of 3.2kg.
4mm thick sheet will make a tube of 4.3kg.

All of these are considerably lighter than my V2 tube , the PVC tube weighs 11kg.

Must be some aluminium tubed OTAs out there ( as 10" or 12" or 12.5" newtonians ).

What's the wall thickness of the tube ?

How does it perform as far as stiffness and strength is concerned ?

Is 0.8mm thick aluminium too thin ?

[mick pinner]

l am making a new tube for an astrograph out of aluminium using 1mm sheet, l've made a couple in the past and 1mm is perfect for weight and rigidity.

[gbeal]

I did one a few years back Ian, for an 8" f4 newt. IIRC it was about 1.2mm alloy. Worked a treat, used contact adhesive on the overlap butt joint, but then wimped out and slipped some pop rivets along the seam as well.
Very stiff, so yes, I reckon you could go lighter and possibly stiffen that the key areas, ends, and where the focuser/spider fits, as well as where you figure the tube rings will be.
Gary

[Ian Robinson]

Thanks Mick and Gary,

What's the best way of reinforcing an aluminium tube :
at the ends (to protect from dings when moving it about off the GEM) ?
is a strip of aluminium sheet the same thickness as the tube under compression on the inside sufficient
under the focusor tube plate ?
see above
at the spider legs ?
see above
at the cell mounting location ?
see above
at the ring clamp locations (note the rings will move up and down depending on whether I'm imaging or just viewing) ?

See attachment for proposed structure of structural strengthening rings , these look suitable ?

Any chance Anthony Wesley from Murrumbateman might chime in ? - I hear he's done several aluminium tubed scopes .

Images of who others have solved thes reinforcement issues would be appreciated .... a picture is worth a 1000 words.

I guess easiest way to make reinforcing rings is to have the sheet metal company roll a tube for me that's a bit longer than I need , and to ask them to cut some rings off the end ( or if they charge too much , use the mighty Dremel and the mini cutting disk to do the deed ) and then take a bit out (enough to allow the compressed ring to be slid/ pushed into the tube0 , the compression of the ring trying to spring back will give a bit of extra reinforcement , because the spring force will be acting outwards).

Looks like I might need to buy a Demel circle cutting jig for my Dremel to make fabrication of the annular ribs easier than using my big electric jig saw.

[DavidU (Dave)]

1.2mm rolled into a tube would be quite stiff. Fabricate 4 internal baffles, one top one bottom and 2 where the focuser is fitted.

[gbeal]

Ian,
while a strong tube is a must, I wonder if you are making it too strong. The rolled tube will be a good start.
Then stiffen the ends (mainly as that is where I "handle" it from), and the areas like the tube ring areas, and possibly the focuser. For the focuser I would just get another section of the sheet and roll it to the same radius, and then contact adhesive glue it inside. Tube ends I would use a thin section something like 25x1mm or similar, and glue there as well.
All of my recent projects have had a selection of interior baffles as well, allowing the baffles to be glued all the way around must add some stiffness, as well as making me feel better about reflections.
Remember to take plenty of piccies.
Gary