The death knell of 1" / 25.4mm aluminium tube?

[robgreaves]

Hi All

I'm building an 18" f/4.5 Obsession style Dobsonian. I have the bulk of the woodwork completed, and am using the Moonlite truss system.

However, I simply can not find 1" aluminium tubing in Australia. Capral have discontinued it, and every other place from national company to backstreet metal man I've phoned can only offer me 25mm tubing. This would result in having to re-machine the tube inserts, clamps that the trusses would wobble in, and the star nuts would be too big to fit.

Does anyone know of anywhere that might have old stock of 1" tubing?

Ironically, I've moved here from the UK where buying 1" tube is easy, but 25mm is hard to get hold of.

Regards,
Rob, Penrith NSW.

[rally]

Just a thought if you can't locate something suitable.

1" is 25.4mm so you only need to flare the end of 25mm tubing by 0.4mm Thats barely barely visible and you would have the problem solved ?

If you flare it into a steel block with a 1" hole you wont be able to overdo it (or use a dedicated flaring tool).

[GrahamL]

I'll check next week ..I'm sure noticed something around (25.4) locally after I went and bought some 25mm..I'd have to go look though :)

[Omaroo]

I just bought some at my local Bunnings a couple of weeks ago... 25mm. $9/m

[Darkplague]

I had the same problem when building my 16" dob.
Ended up having to machine all the blocks down about 1mm.
I made a jig up so I could hold them in the lathe and turned them down.

Another option is to bore out the ends of the tubes on a lathe.

Good luck!

[AndrewJ]

I bought true imperial tubing ( about 3yrs ago ) from a mob in Melb called
"Light Spars of Australia"
Its a small shop that does yacht masts, flagpoles, hangglider tube etc
I got thinwall 2" tubing from them, but IIRC he had smaller sizes too
Its contact details are on the net ( 03-9555-4622 )
but they don't appear to have a web site

Andrew

[Satchmo]

Hi Rob
The Moonlite aluminium split blocks seem to have a good clamping action on 25mm tubes. On the 12" bino I have on loan here you can clamp the tubes nice and tight. The slight amount of slop when loose assists in getting the tubes out in pairs if they are tied at the top end in a "V" .From what I 've seen th erepeaeatbility of collimation is excellent.

[robgreaves]

Thanks all for the replies.

Flaring the tube isn't a bad idea at all, as long as I can find someone or equipment capable of going that big.

Thanks for the Bunnings tip, but thats still 25mm, not inch tubing that I'm after!

Will try the place in Melbourne and see what they say.

And as Mark mentions, if the lower clamps work at 25mm - great. I can machine the top ball plugs down to fit, but the only issue then is fitting the star nuts inside (maybe grind them down?) - plus - my rings have already been drilled for 1" diameter seats so it'll look a mess. D'oh! :rolleyes:

Regards
Rob.

[GrahamL]

hi rob
this was the company
http://www.ballinaaluminiumandsecuritysupp lies.com/

I noticed the other tube after I sent a sample to moonlite to machine to fit
so didn't pay to much attention to it ..I do recall the wall was thinner than the standard 25mm .. and as such would be a fair bit lighter at least.'

[Titan]

Rob,

I work in the aviation industry and we specify a lot of imperial sized Aluminium in our designs. Many of our customers purchase from Airport Metals Australia:
http://www.airportmetals.com/index.htm (http://www.airportmetals.com/index.htm)

I not sure about the cost though but it may pay to see what the have. Hope this helps.

[Omaroo]

1 inch = 25.4mm, so the 0.4mm is the problem?

[Dennis]

Never, ever, ever say that in a room full of mechanical engineers or machine shop people – I guarantee you will not get out alive!:lol::lol:

Cheers

Dennis

[Omaroo]

The Moonlite clamps are well within their limits to clamp down the extra. We're not talking abut an interference fit here..... ;)

[robgreaves]

Granted, the Moonlite clamps will just about clamp down onto 25mm (even though they're not designed to), but my problems begin here:

1. The ball ends won't fit into the tube. They'd need machining down to fit. The 25mm tube I've seen in metal merchants tends to be thicker wall too, which is correspondingly heavier on the moment arm, thus affects the balance of the whole scope since the CofG is at the top edge of the mirror box.
2. The star nuts I have for 1" OD tube with 1.24mm wall simply won't go in. They'd need grinding down. More hassle.
3. The wooden secondary cage rings have already been drill recessed with a 1" Forstner bit. 25mm tube would leave an untidy edge making it look like I can't drill a hole accurately. More problems when tensioning the spider and the tube wants to shuffle in its recessed base.

I'm glad 25mm tube worked for you, but all my above issues would be solved with 25.4mm tube.

Titan - Thanks for the link for Airport Metals - they have it on their list. I will investigate!

Regards,
Rob.

[Darkplague]

I machined the ball ends so they would be a nice press fit into the tubes.
Then I put in 2 self tapping screw in each side through the wall of the pipe to secure them. Much better than star nuts!

Anyway without access to a lathe this info is useless to you :(

You could easily do it with a linisher/belt sander also, the plastic is easily ground, however take your time and continually measure them before taking more material off than you need to.