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making a hollow core cylinder wall

ATM DIY dob
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#1 FlorinAndrei

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Posted 13 January 2018 - 04:57 AM

About to begin making a medium size dobsonian. Nothing fancy, and the focus is on simplicity, familiarity, and speed of execution. I just want to get it done quickly without wandering off into experimenting with new designs, which is what I tend to do unless I stay focused on more immediate goals. I need the instrument up and running before the Mars opposition, and I have yet to polish and parabolize the primary. Loosely based on the Obsession / Kriege template, but with a few twists.

 

For the UTA (upper tube assembly), I'll make it from two rings of thick plywood joined by a cylinder. I may have a few columns joining the two rings, for strength - but one of the twists is that the cylinder will be load bearing itself, and structurally strong.

 

Basically I need to make a hollow core cylinder - two sheets of some bendable material, epoxied to some cellular core material of sorts. I need help with choosing the materials.

 

Plywood doesn't seem like a good option for the sheets, does it? If it's thin enough to bend, it's probably thin enough to crack when pushed or bumped into.

 

For the core, I could use kevlar honeycomb from the carbon fiber store (they use it to make carbon fiber hollow core sandwiches), but that's the high end choice. What would be the midrange choice in your opinion?

 

I could make the cylinder from carbon fiber and kevlar, but that's the fancy experimental design I want to avoid, for lack of time. Although the material properties of this combo are pretty sweet.



#2 howardcano

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Posted 13 January 2018 - 06:53 AM

How about filling the space between two plywood cylinders with 2-part expandable urethane foam?  It's quite strong and stiff when cured.  Ply-over-foam or fiberglass-over-foam is very popular for airplanes (real and model) because of its excellent strength/weight ratio.

 

http://www.uscomposites.com/foam.html


Edited by howardcano, 13 January 2018 - 07:07 AM.

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#3 Pinbout

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Posted 13 January 2018 - 07:21 AM

Why do you want thick rings? Especially when the cylinders is load bearing?


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#4 Oberon

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Posted 13 January 2018 - 07:26 AM

What size? 



#5 BwannaDon

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Posted 13 January 2018 - 08:33 AM

That sounds difficult, time consuming, and complex in execution. You might look into strip boat building methods. They're built on a strong back armature, laid up in narrow strips with two opposite joints fitted closely but left unglued. Then the assembly it is split at the unglued joints to remove the armature and joined back together (with epoxy or wood glue), sanded to a smooth round profile, and laminated with fiberglass cloth inside and out. You end up with a very strong composite, but the core is wood, and.... there is a ton of guidance available online.

 

Attached Thumbnails

  • CYLINDER.jpg

Edited by BwannaDon, 13 January 2018 - 08:38 AM.


#6 beggarly

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Posted 13 January 2018 - 08:50 AM

I can't imagine that the cylinder needs to be load bearing all around 360°.

 

Wouldn't something like https://makezine.com...nian-telescope/ be sufficient? (2. Build the secondary cage)



#7 dan_h

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Posted 13 January 2018 - 10:46 AM

How about a commercially available drum shell? they are made of wood, lightweight and strong. Lots of sizes available unfinished.

 

http://precisiondrum...ex.php/contact/

 

dan


Edited by dan_h, 13 January 2018 - 10:48 AM.

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#8 steveastrouk

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Posted 13 January 2018 - 11:06 AM

How about filling the space between two plywood cylinders with 2-part expandable urethane foam?  It's quite strong and stiff when cured.  Ply-over-foam or fiberglass-over-foam is very popular for airplanes (real and model) because of its excellent strength/weight ratio.

Or make your rings out of foam rigid boards ?



#9 howardcano

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Posted 13 January 2018 - 11:17 AM

How about a commercially available drum shell? they are made of wood, lightweight and strong. Lots of sizes available unfinished.

 

http://precisiondrum...ex.php/contact/

 

dan

These are often  available quite inexpensively on Craigslist, or from your local drum shop-- or if you live near me, right out of my basement!

 

I've used them on the mirror end to effectively lengthen a tube, to move the focal plane closer to the diagonal for an increased illumination circle.


Edited by howardcano, 13 January 2018 - 11:19 AM.


#10 MitchAlsup

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Posted 14 January 2018 - 02:07 PM

Plywood doesn't seem like a good option for the sheets, does it? If it's thin enough to bend, it's probably thin enough to crack when pushed or bumped into.

Drum hoop/shell epoxied to 2 rings:

 

cell-stuff61.JPG

 

The drum hoop is <about> 5-6 layers of ply and close to 1/4" (6mm) thick.

It happens to be maple if anyone cares, where as the rings are baltic birch.


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#11 FlorinAndrei

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Posted 14 January 2018 - 06:36 PM

Lots of food for thought in this thread, thanks everyone. I'm considering all the replied I've got. Need to think some more.

 

Why do you want thick rings? Especially when the cylinders is load bearing?

On one hand I want to keep things simple and move the project forward quickly. OTOH, I never seem to be able to leave well enough alone in the design phase.

 

What I have in mind is a truss dob with top and bottom boxes (tube assemblies, yadda-yadda) that look very similar; the top and the bottom are visually hard to distinguish. I know it's the bottom TA that carries the biggest load, so for the top TA I'll just use thinner materials or whatever.

 

The whole design has hexagonal symmetry, all the way down to things like the spider vanes (I'll use a 3-vane spider). 6 truss poles. The outside of the TAs is hexagonal, the inside is round.

 

I just don't want to use the classic square mirror box. Call it a whim if you like.

 

What size? 

12.5" f/4.7

 

That sounds difficult, time consuming, and complex in execution. You might look into strip boat building methods. They're built on a strong back armature, laid up in narrow strips with two opposite joints fitted closely but left unglued. Then the assembly it is split at the unglued joints to remove the armature and joined back together (with epoxy or wood glue), sanded to a smooth round profile, and laminated with fiberglass cloth inside and out. You end up with a very strong composite, but the core is wood, and.... there is a ton of guidance available online.

One thing I could never figure out with plywood frames laminated with fiber composites is how does it work in terms of differential thermal expansion / contraction? I looked up the coefficients for plywood and for carbon fiber, and they're pretty different.

 

Let's say I have two rings made of plywood. Then I make a cylinder from two sheets of carbon fiber with a core of cellular kevlar in between. I glue the plywood rings to each end of the carbon cylinder. Then one winter night I take the assembly from the warm temperature indoors to the frozen atmosphere outside. Where does all the thermal stress go?

 

 

Plywood doesn't seem like a good option for the sheets, does it? If it's thin enough to bend, it's probably thin enough to crack when pushed or bumped into.

Drum hoop/shell epoxied to 2 rings:

 

attachicon.gifcell-stuff61.JPG

 

The drum hoop is <about> 5-6 layers of ply and close to 1/4" (6mm) thick.

It happens to be maple if anyone cares, where as the rings are baltic birch.

 

Wait, really? Is the 5 ply material pliable enough to bend at that radius? Did you soften it with steam or something before bending it?



#12 Pinbout

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Posted 14 January 2018 - 06:52 PM

  Aitwood.com


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#13 Rusted

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Posted 15 January 2018 - 01:32 PM

Two 1.5mm plies of aircraft, birch multiply make one very stiff piece of 1/8" ply when glued together.

The two can easily be rolled into a 6" or smaller tube before being glued together.

Keep adding more plies depending on desired resistance to a herd of trampling elephants.


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#14 MitchAlsup

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Posted 15 January 2018 - 04:48 PM

 

Drum hoop/shell epoxied to 2 rings:

 

attachicon.gifcell-stuff61.JPG

 

The drum hoop is <about> 5-6 layers of ply and close to 1/4" (6mm) thick.

It happens to be maple if anyone cares, where as the rings are baltic birch.

 

Wait, really? Is the 5 ply material pliable enough to bend at that radius? Did you soften it with steam or something before bending it?

I bought the drum shell already as a round cylinder of plywood.

http://precisiondrum.../drum-shells-2/

 

All I did with it was cut to length, epoxy and tung oil.

I would not have the tools or strength to bend plywood like that.


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#15 Simon Alderman

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Posted 18 January 2018 - 09:06 AM

Two 1.5mm plies of aircraft, birch multiply make one very stiff piece of 1/8" ply when glued together.

The two can easily be rolled into a 6" or smaller tube before being glued together.

Keep adding more plies depending on desired resistance to a herd of trampling elephants.

Really? I’m not trying to be difficult but the 1.5mm is equivalent to 1/16” or .064” ply and the aircraft ply I’ve worked with would not roll into something that small (6”). Could you possibly mean the .4mm? That stuff is much more flexible and I could see making tubes with it in multiple layers. 

Again, not trying to troll, I’m just very curious! 



#16 Pinbout

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Posted 18 January 2018 - 09:14 AM

bendy board or wiggle wood all bend extremely easily...



#17 Fly2High

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Posted 18 January 2018 - 09:21 AM

my thinking is this.  my past interests include building R/C composite model aircraft.  We would cut Plazamate foam (usually 60psi) to use for cores for wings.  We would then laminate obeche, balsa or some form of composite (fiberglass, or carbon fiber) onto the cores.

 

My thinking is to make a stack of foam and cut two thin pieces of plywood into the shape you want  and place them on the outside of the foam as a template.

 

Then cover them with your desired sheeting.  a little steam or wetting the wood and then apply steam will make it bend without breaking, if memory serves.  Obeche can be purchased rather thin.  I think it can come as thin as 1/64".  I am certain wing guys have made to cover more complex surfaces than your cylinder.  I also have covered wings with 1/32" balsa.

 

A few well placed 'ribs' within the foam can tie the inner and outer surfaces prior to sheeting and make for a light yet rigid structure.

 

Oddly enough, I am thinking of building a telescope, but using carbon fiber cloth, in much this way.  If we can make R/C gliders fly using these techniques then I see no reason it would not work for a telescope.



#18 BwannaDon

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Posted 18 January 2018 - 11:21 AM

If you look at a raw thermal coefficients there may appear to be a great deal of difference. Steel is 7.2 (x10 to the minus 6) Plywood 3.4 (x10 to the minus 6). One might think then that this difference could become problematic but for steel, we are talking about .0000072" per inch per degree or... .00072" per inch with an 100 degree temperature increase / decrease. Plywood... .00036" per inch with 100 degree change. I really don't think it is a factor.
Wood is affected to a far greater degree by moisture content therefore must be sealed from the atmosphere to maintain relative dimensional stability but even still... in the scale you are working not so much of a concern. The recommended gap in siding is typically 1/8" in four feet.
PVC pipe (schedule 40 and 80) is available in sizes up to 14" inside diameter and by the foot. It also glues into standard fittings.

#19 BwannaDon

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Posted 18 January 2018 - 11:22 AM

https://flexpvc.com/...40PVCPipe.shtml

#20 Dale Eason

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Posted 18 January 2018 - 11:01 PM

 

 

Oddly enough, I am thinking of building a telescope, but using carbon fiber cloth, in much this way.  If we can make R/C gliders fly using these techniques then I see no reason it would not work for a telescope.

Having done all of that, build and fly R/C gliders from composites and build and use a 16 in ultralight dob telescope.  I know that the design constraints for wings and fuselage are much different than the weight carrying and stiffness requirements of a good telescope.  The two are very different in their needs.

 

I used a aluminum bicycle wheel rim for my upper tube ring that my 3 aluminum trusses, focuser, and finder attach to.  The mirror box is an aluminum rectangle because that matches with the more or less rectangular requirements of the base and altitude bearing attachment.

 

Dale



#21 dan_h

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Posted 19 January 2018 - 12:58 PM

<<   The whole design has hexagonal symmetry, all the way down to things like the spider vanes (I'll use a 3-vane spider). 6 truss poles. The outside of the TAs is hexagonal, the inside is round.

 

I just don't want to use the classic square mirror box. Call it a whim if you like.  >>

 

Re Hexagonal tubes.

 

I made one hexagonal refractor tube. You would think this would be nice. Two flat sides to mount rings and other hardware. Nice sloped sides top and bottom to mount finder, focuser etc. 

The reality was that it was the ugliest tube I ever saw. I never mounted the optics and immediately began work on an oxagonal tube with 8 sides. Much more eye appeal and just as easy to construct, work with, and mount. And I suspect the octagon flexes less than the hex.

 

You might like the idea of a hex tube, but once you have one, you may change your mind. There is a reason you don't see many of these around. 

 

dan 



#22 donlism

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Posted 20 January 2018 - 07:13 PM

Wood bends quite easily if you get it hot and have a pretty good idea what you're doing.  Look at the lower-bout cutout on an acoustic guitar, for example, and that's a solid piece about 3mm thick or so.

 

Albert Highe likes drum shells.  His suggestion is to use a thin, light tubular shell with stiffening rings on the ends.  The rings keep it round, and the thin tube holds the rest of the shape.  Apparently the whole thing ends up very strong, and relatively light.


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#23 MitchAlsup

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Posted 20 January 2018 - 07:17 PM

I like them, too, a circular hoop with stiffening rings top and bottom is essentially a round I-beam.

 

cell-stuff61.JPG


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#24 FlorinAndrei

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Posted 20 January 2018 - 08:35 PM

Oddly enough, I am thinking of building a telescope, but using carbon fiber cloth, in much this way.  If we can make R/C gliders fly using these techniques then I see no reason it would not work for a telescope.

That was exactly my initial plan. But I don't have as much experience with composites, so I did not think I could afford the inevitable delays that come with the learning process. Also, the mirror is full thickness, so it's probably not worth making a fancy carbon fiber scope for it.

 

It will have to wait for the next project.

 

You might like the idea of a hex tube, but once you have one, you may change your mind. There is a reason you don't see many of these around. 

I misspoke. The tubes per se are round. It's the rings on the ends of both tubes that are hexagonal.

 

If it turns out to be the ugly duckling of ATM, I'll just have to live with it for a while. smile.gif

 

Albert Highe likes drum shells.  His suggestion is to use a thin, light tubular shell with stiffening rings on the ends.  The rings keep it round, and the thin tube holds the rest of the shape.  Apparently the whole thing ends up very strong, and relatively light.

I think I'll end up borrowing quite a few points from Highe's book. The time is nigh and I can't experiment too much with unusual designs. I already got a lot of good ideas from everyone posting in this thread.



#25 Rusted

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Posted 21 January 2018 - 02:53 AM

 

Two 1.5mm plies of aircraft, birch multiply make one very stiff piece of 1/8" ply when glued together.

The two can easily be rolled into a 6" or smaller tube before being glued together.

Keep adding more plies depending on desired resistance to a herd of trampling elephants.

Really? I’m not trying to be difficult but the 1.5mm is equivalent to 1/16” or .064” ply and the aircraft ply I’ve worked with would not roll into something that small (6”). Could you possibly mean the .4mm? That stuff is much more flexible and I could see making tubes with it in multiple layers. 

Again, not trying to troll, I’m just very curious! 

 

I thought I hadn't a picture of my 6" home laminated marine ply tube for my 5" home made achromat but I have just found an old film print:

When I gave up on the project the tube lasted for years just lying on the ground year round.

The thin plywood flexes easily one way but not the other. Roll accordingly!  wink.gif

This was a failed attempt at an all-plywood, Dobson-style equatorial. Too much friction on the large bearings!

 

Telescope5in 6in plytube  equatorial rsz 500.jpg




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