Advice on making an aluminium tube.

"Andy Dix" wrote in message ...

Although I'm fairly happy with my cardboard tube Dob I've been thinking
about replacing the tube with an aluminium one.

The question then is... What gauge aluminium would be good for a tube?

Opinions and advice welcome.

Well you did ask: Brace yourself?

I've made telescope tubes (amongst many other things) in many sizes
from many different materials over more than 4 decades. I've rolled
aluminium tubes from thin sheet and pop-rivetted the overlap. Hated
the looks and the awful diametral flexibility. Rolling your own is not
as easy as it seems. Even if you use a suitable PVC tube as former
you'll probably end up with a (creased) toy ballon cross-section and
two wavy edges where they meet. Nice! But if that's your taste in
telescope tubes who am I to argue? The thinner you go in materials.
The easier it is to make a tube and the easier it is to dent and
harder to keep truly round.

If you do pre-drill a rivetting strip to go inside the butt joint as
you suggest. The chances of it lining up (after the first couple of
rivets go in) is absolutely minimal. You could support the joint (and
rivetting strip) on a heavy section of raised timber while you drill
one hole at a time & then rivet each new hole. Still hard work unless
you know a tall thin kid who'll get inside the tube and push the
joining strip firmly up into place when it wanders. While four helpers
hold the tube straight enough for your to drill and rivet. With one
extra to poke a stick at the kid in the tube when he complains you are
drillin' holes in his fingers and droppin' swarf in his eyes! Don't
expect the tube to be very round after this palaver.

To use a joggled edge and industrial adhesive requires two flat edges
and even pressure along a considerable length of (what is probably now
rather wavy) material. Two G-cramps and two heavy steel bars? You're
joking! I'd just pop-rivet the (engineering firm pre-joggled) overlap
and pray it ended up acceptably flat.

You talk in SWG's? Let's get serious. Thin-wall aluminium tube is
very easy to dent, scratch, deform and very difficult to keep usefully
round without proper internal (or securely fixed external) support. I
have a length of 5" dia. x 2mm wall thickness, seamless aluminium
tube. That is far too easily deformed and dented for my tastes in
telescope tubes. I'd use 3mm minimum in aluminium. Once dented,
aluminium hardens locally and is difficult to straighten without
stretching and producing an ugly wave in place of the original dent. A
typical 12" diameter tube is a big lump to be carrying about without
the risk of at least some damage.

Don't use steel. It's simply too heavy for a portable scope.
Particularly in this size!

You could get a sheet metal specialist to roll and joint a tube
nicely for you in a minimum of 3mm wall aluminium. But you'll pay if
you want a cosmetically attractive finished product from
foil-protected new aluminium sheet. Which you should specify before
the price is agreed. Not when you are handed a wobbly, oval, tapered
pipe covered in scratches, dents, folds & waves and rivetted with
crooked round headed copper rivets as an exercise for the keen young
sweeper boy.

One alternative that might suit would be a self-laminated (minimum of
3 x 1mm) aircraft ply tube. Lots more work but a nice lightweight,
stiff tube. Do some heavy browsing before starting. Use brushing epoxy
not Cascamite. (Not like what I did and shouldn't have!) Still a very
nice finished tube though it could have been much stronger and stiffer
with epoxy. Make a really strong (collapsible) former and order the 3
miles of bungee rubber well ahead of time. I can't even find 1mm
aircraft ply these days. Wooden boat makers might have some though
probably not big enough for a whole 12" tube. I remember a maximum of
5' square sheets when I made mine about 15 years years ago. There's
always epoxy & sheets of veneer if you can get hold of that. I saw a
whole streamlined car body made of that material.

Forget GRP and particularly the exotics unless you've been through
the steep learning curve of lay-up & mould making to a high standard.
The finished tube would look like complete crap. Been there done that!
Despite lots of previous GRP experience re-modelling my kit car,
making water tanks, pipes etc. You could always ask a GRP mixed
products manufacturer to quote for a 12" tube? Be sure to say you want
it nice on the outside! That should get his interest. And multiply the
price dramatically!

PVC ducting would be the ideal material in this diameter if you can
find it in short enough lengths locally. Buy it from an astro dealer
and it should be scratch free and the right length. Buy it from a PVC
stockist and it may well be badly scuffed from being slid across a
concrete floor & probably 7 metres long! You pays your money and takes
your choice. I'd use 12" minimum internal diameter with a 10" mirror.

Or buy a finished aluminium tube from an astro dealer. It would save
a lot of heartache when you discover the difficulties involved and the
money that has been wasted on good materials trying to roll your own.
Just handle it carefully when it's yours. Or paint it with Hammerite
when it gets really ugly. Don't forget the proper primer first!

BTW: Using shallow U-shaped handles |__| on telescope tubes this size
makes handling g almost easy compared with the usual bear-hug
approach. Use large washers inside and outside the tube. Or preferably
strips of thicker alloy long enough to spread the loads inside. (With
the large washers just on the outside). Or the handle fixing screws
might literally tear a hole through the tube wall. The handles also
help to point the scope easily when the tube is too big to get a
proper grip on. (Yes I know! Poor Ingleesh)

Above all do not underestimate the difficulties involved. It is not
easy to make a strong, attractive, lightweight, nicely round tube in
aluminium or any other material! It just seems like it should be.

Stick with the cardboard tube? Why not? Just give it a couple of posh
ally or stainless handles to make it look sexy and easier to use. Just
don't forget the spreader washers.

With the usual apologies to those uninterested in this subject matter
or anything else I might have to say.

Chris.B

(Frey Hall of Fame Finalist 2003)

Chris.B wrote:
snip
Well you did ask: Brace yourself?
snip
Thanks Chris for a great deal of info on this.

I am more than happy with the cardboard in terms of optical performance and
only wish the whole thing wasn't quite so heavy (it's 17.8kg without the
rocker box). I'm not sure how much of this is the actual tube though.
Anyone know the density of these cardboard tubes?

The mirror and mirror cell weigh a fair amount, the cell being two peices of
18mm ply both 250mm in diameter with an 80mm hole cut in the centre. Maybe
I should just work on making the cell lighter. Also the altitude
box/bearings are made from the same 18mm ply so that's also fairly heavy.

Given that you'd not go any less than 3mm with aluminium, a tube of 1400mm
in length and I.D of 305mm would weigh approx 11kg. I'd say that that's
more than my cardboard tube weighs before adding the mirrors etc. With mild
steel being roughly 3 times denser even at 1mm thick it would still weigh
the same 11kg. Too heavy for my liking.

Maybe I'll just admit defeat before I try and purchase a PVC or aluminium
tube from Beacon Hill or the like. Or maybe I'll just throw away the whole
thing and pay £2000 for an Orion Optics factory made 'scope. Or maybe I'll
just try a few things and possibly waste some money on failing then give up.
Still I've enough money to waste so why not! Hmm... maybe rolled titanium.

On the other hand I could try, succeed then patent my methods and become a
millionaire from the resale of the method.

--
Regards
Andy

"Andy Dix" wrote in message ...

On the other hand I could try, succeed then patent my methods and become a
millionaire from the resale of the method.

If weight is your most important criterion (and you have referred to
it a number of times) then investigate a self-laminated,
epoxy/birch-ply tube. The technology is extremely simple but rather
time consuming. Google was surpisingly unhelpful on this subject
despite using a number of different search terms. I vaguely remember
reading something about it in "Telescope Making" in the early 80's.
There are American commercial tubes in various eotic woods I believe.
You could always start a UK tube making service?

The solidity and unbelievable lightness of my finished plywood tube
was incredible compared with my sagging piece of standard 6" PVC
drainpipe. Which never allowed me to hold collimation on my 5" f/15
self-made refractor doublet.
The butt joints needed extreme care to avoid the usual "toy balloon"
profile. I eventually placed two square section metal bars over the
joints on opposite sides of the tube while only the first couple of
rubber bands were in place. This applied enough local pressure to
flatten the joints. The joint edges tend to be pushed ever closer
together by the inward pressure from the rubber bands around the
circumference. So repeated trimming at the joints was required. The
chance of a longtitudinal void or hump (where unwanted butting
actually takes place) was then eliminated.
I did a couple of dry runs with a few rubber bands in place. The
small initial overlap was marked & sawn away. Then the joint gap
carefully matched with a sharp block plane. After gluing and adding
more rubber bands the joint closed even more and butted hard together
in a long hump! Arghh! (I assume the glue acted as a lubricant) So I
had to remove all the rubber bands and trim a little more. Then add
more glue and start all over again. Patience will be rewarded. Just
don't use fast setting adhesive!

If you want to lighten your Dobs components I'd put a decent hole saw
in bench drill and use that to make holes in carefully marked
positions (using a pair of compasses). It looks better than the
"random cheese effect" and actually makes things look quite "techy"
(with a little care and a bit of sandpaper to smooth the cut hole
edges). I doubt you'll save much very weight though. You could always
weigh a piece of similar material and calculate its area. Then compare
with your hole size area and multiply by the number of holes you
intend to drill. You might save the cost of a Sandvik hole saw (or
two). g

My apologies for starting a new thread. I was going to put a long
post warning in the title then changed my mind and suddenly there I
was standing naked and alone in the threads list!

Chris.B

Chris.B wrote:
If weight is your most important criterion (and you have referred to
it a number of times) then investigate a self-laminated,
epoxy/birch-ply tube.

I will indeed investigate this method further.
Did you use a former of some sort for yours?
Am I correct in assuming the material you used is 1mm thick birch?

--
Regards
Andy

"Andy Dix" wrote in message ...

I will indeed investigate this method further.
Did you use a former of some sort for yours?
Am I correct in assuming the material you used is 1mm thick birch?

Give me a couple of days to recover further from my injuries from
falling off a ladder. I will then delve through my (couple of cubic
metres & 4 decades of boxed reference material) to see if I can find
the original article in Telescope Making Magazine. Right now I can
hardly walk, lie down, stand up, or sit down without considerable
pain. My idiocy has apparently no limits.
I will then scan the article and e-mail it direct to you if that
helps.
In the meantime you could search the yellow pages for wooden
boatbuilders, canoe builders, plane builders, panel/sheet stockists
(or whatever) looking for waterproof birch faced plywood less than 2mm
thick. Try asking real model shops where they buy their smaller sheets
of the stuff for their customers.
If you can't find the materials you can't build a tube. I have seen
specially flexible (in one direction only) plywood available in
builders merchants & DIY stores. That might work if you protect it
from moisture. Though it won't offer the advantage of multiple thin
crossed plies.(Plys?) You could always sandwich glass fibre/epoxy
resin between two layers of that stuff and wrap that tight with bungee
rubber cord on a collapsible former.
Sometimes I feel my entire life has been spent searching for exotic
and unobtainable materials or 'things' for my many interests. Usually
it just takes persistence, a smiling approach and sheer good luck. I
can hardly believe the times I've been told "we don't have" something.
Then found it nearby on the shelves myself. How some DIY stores
survive (with the staff they employ) remains a modern mystery. Not
unlike trying to solve the UFO and crop circle problems
simultaneously. Perhaps that's why it's called a DIY store? g

Chris.B

(Chris.B) wrote:

"Andy Dix" wrote in message ...

I will indeed investigate this method further.
Did you use a former of some sort for yours?
Am I correct in assuming the material you used is 1mm thick birch?

Give me a couple of days to recover further from my injuries from
falling off a ladder.

Good god - these wax models really work.

In the meantime you could search the yellow pages for wooden
boatbuilders, canoe builders, plane builders, panel/sheet stockists
(or whatever) looking for waterproof birch faced plywood less than 2mm
thick. Try asking real model shops where they buy their smaller sheets
of the stuff for their customers.

Andy

I've got quite a few dozen sheets of 1/16th birch ply, 48x48 inches.
The imperial measures indicate that its been in stock for 15 plus
years a project that came to nothing. Its in mint condition and a
beautiful material to work with and to finish.

Get in touch if you're interested (snip snip for my email). A lot
would depend on where you a the stuff won't go through a letter
box, though it is amazing flexible in 1 dimension.

Cheers

Martin

--------------
Martin Frey
N 51 02 E 0 47
--------------

Martin Frey wrote:
I've got quite a few dozen sheets of 1/16th birch ply, 48x48 inches.
Get in touch if you're interested (snip snip for my email). A lot
would depend on where you are

I would be interested if only I were slightly closer to you. From your sig
I see you are near Ashford. I am up a little way in mid Suffolk. It
wouldn't be an out of the question journey but I'll have a search around my
local suppliers first. I'll contact you if I have no luck here.

Thanks for the offer.

Chris.B wrote:
Give me a couple of days to recover further from my injuries from
falling off a ladder.
Ouch. No hurry, mend first, search later.

I will then scan the article and e-mail it direct to you if that
helps.
That would be most helpful. Let me know if you can find it and I'll let you
know where to send it to.

Thanks again for the assistance.

--
Regards
Andy

Chris.B wrote:

Give me a couple of days to recover further from my injuries from
falling off a ladder.

The sinister & mysterious Martin Frey wrote:

Good god - these wax models really work.

You blighter Frey! I knew you were behind this. Keep your pins to
yourself!

I've got quite a few dozen sheets of 1/16th birch ply
a project that came to nothing.

Sounds like a dome waiting to be put up! Lazy git! 15 years? Tut,Tut!

What project Martin? Or should I ask someone else to ask you and then
they can tell me afterwards?

Hello? Anyone want to ask Martin Frey what the project was? Thankyou!

Chris.B (nobbled (in agony) and now very, very afreyed)