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Aluminium vs steel


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Hi all

I have a pillar mount which is a little short. I could buy a 16" pillar extension but this would still not be quite enough for my long frac. I am therefore thinking that I should buy a length of 4" ali pipe and make my own.

the current steel pipe is 4" OD and has walls 3mm (approx) thick. The ali pipe would be 4" OD and have walls 4.8mm thick. It will be 1300mm long. I am convinced in my own mind that this will be as stable and solid as the steel pipe but am I deluded? I am only likely to use my 6" f5 newt, 80mm f11 frac or Lunt 50 (or any combination of two from three) so the overall weight will be quite low.

Eventually I'll be likely to apply a white finish to match the feet but might grow to love industrial aluminium.

One advantage of doing this is I can buy a length of 1500mm pipe and cut at 1300mm leaving me with a 200mm short piece to make a similar extension for my EQ5 tripod too. All for £41 delivered.

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A quick chat to someone at work suggests that the thicker aluminium should be ok. Whether you're deluded or not is another matter

I have been using an alloy extension in the obs for a fair few years now, no problem what so ever. It has 3mm side walls..  Originally it didn't have side vanes, I added them after I upped the weight

Motors? How dare you?! :0)

I see no problem with your idea. It would certainly be strong enough. I would warn you about putting steel and aluminium together - they are dissimilar metals. What that means is that over time and with the addition of moisture the steel and the ali with fuse together. You can solve this by initially using some etch primer:- http://www.amazon.co.uk/250ml-Primer-Galvanised-Steel-Aluminium/dp/B00AC1Z78W/ref=sr_1_5?ie=UTF8&qid=1406031212&sr=8-5&keywords=aluminium+etch+primer

and then finishing off with a coat of smoothrite (hammerite without the dimples)

Ian

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When using aluminium for cars they say aluminium is half the strength of steel but a third the weight so they can double the panel thickness, maintaining the strength, but have 2/3 the weight. So I suppose that really you would need 6mm walls.

However panels are flat not round and I suspect that 3mm steel is overkill anyway.

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That aluminium section will be ok as long as the use is kept to the smaller scopes / focal lengths.

I think the choice between steel / aluminium comes down to what materials you have available and equipment you have to put the job together.

Dave.

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thanks all - I am relieved to see I am (hopefully) not deluded!  :grin:

The feet from the pillar mount only touch in two thin areas so rust welding should be OK. In fact might actually help the sturdiness of the whole thing. That said, I'll probably paint it (or get it powder coated) in any case.

I am confident enough to proceed I reckon.

Cheers

Shane

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  • 3 weeks later...

I have been using an alloy extension in the obs for a fair few years now, no problem what so ever.

It has 3mm side walls.. 

Originally it didn't have side vanes, I added them after I upped the weight to take a 10" newtonian & guide scope etc..

It would be fine for a 100mm refractor or 6" newt without.. 

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Aluminium is notoriously difficult to paint Shane so this is something to bear in mind. Also aluminium requires some skill to weld should you wish to make any such modifications in the future. I made my extension out of a length of steel pipe similar to the dimensions you have mentioned but this is quite weighty. It adds stability when used on my tripod but whether this stability would be wasted and only go to make a pillar mount cumbersomely heavy ??

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Cheers Nick

I am pretty happy with this as is and for the intended scopes. It does retain about the dame weight as originally despite the height increase

really happy how it worked out

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@Zak, it's a technique (the foam core sandwich) used throughout the aircraft industry. Although carbon fiber has replaced the aluminum skin in places. There is a manufacturer of carbon fiber telescope tubes which uses a fiber-foam sandwich.

@Moonshane, the amount of stiffening is proportional to the weight per volume of the foam chosen. Boat builders foam is available in several grades. I believe builders foam would be the lightest grade, but may work ok. Foam has it's own resistance to bending, and being closed cells constrained by the tube, it is prevented from changing shape in cross section. It performs the same function for the hollow tube. In addition it helps dampen vibrations, as it is nearly non-resonant in contrast to metals--Jack

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@Zak, it's a technique (the foam core sandwich) used throughout the aircraft industry. Although carbon fiber has replaced the aluminum skin in places. There is a manufacturer of carbon fiber telescope tubes which uses a fiber-foam sandwich.

Correct.

But there's a substantial difference between a thin outer and inner skin bonded to an aluminium foam, and an aluminium pipe filled with builders foam.

I spent years fabricating ally and steel structures and never once filled an ally section with builders foam....

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@Zak, so you haven't tried it. I didn't recommend builders foam specifically. I do have 13 years with Boeing building airplanes. If you look at new products online, foaming tubes is pretty standard with the companies using composites, fishing rods especially. I am talking about a poured in, expanding product. One property of which is to bond to surfaces. Boat builders have been reinforcing fiberglass for decades as well as plywood. --Jack

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@Zak, so you haven't tried it. I didn't recommend builders foam specifically. I do have 13 years with Boeing building airplanes. If you look at new products online, foaming tubes is pretty standard with the companies using composites, fishing rods especially. I am talking about a poured in, expanding product. One property of which is to bond to surfaces. Boat builders have been reinforcing fiberglass for decades as well as plywood. --Jack

Jack,

I understand, but you're not talking of the same application. A foamed cored composite gives high stiffness and strength to a relatively this section. A tube section gets it's strength from the cross sectional area and wall thickness. Adding a foam core to a tube section with a wall thickness of 3+mm will add nothing to the strength.

What would be interesting is a thin-walled section, with a bonded ally honey-comb. But it'd be easier and cheaper to just get a piece of thick-walled tubing.

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Motors? How dare you?! :0)

Shane you may find you need motors when you start solar imaging.........................................................just teasing

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