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Split Tube Design Question

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#1 John Miele

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Posted 22 October 2021 - 02:32 PM

Hi,

 

I am building a 10" f6 dob and plan to split the solid tube for transportability. I have narrowed it down to two very different schemes for how to reassemble the separate halves. I would like some feedback on which method looks best from a rigidity point pf view or if anyone has used a similar (or better!) method with success.

 

The tube is very strong and rigid. The walls are approximately 0.32" thick. It was bought from Yazoo Mills.

 

1) The first method will be to cut a 6" long section of leftover tube and use it to create an internal sleeve that will overlap each section by 3 inches. This will tightly guide the upper section down and react the lateral loads. Then I will use four equally spaced adjustable latches to pull the sections together and react the tension loads from the overturning moment of the upper tube half. I calculated the highest tension load on a clamp will be about 21 lbs.

 

Pros: Should be very rigid because the two halves are being pulled together and no loose hardware. 

 

Cons: Might be tricky to get the sleeve to fit just right. If the sleeve swells over time, it might become very hard to assemble and disassemble.

 

2) The second method is to cut 4 pieces of red oak 9" long x 1" wide x 0.5" thick and bolt these "pseudo struts" to the lower half of the tube with 4 inches sticking above the joint. The upper half will slide down over these four pieces. Each piece will have a brass insert to accept a 1/4-20 knob that screws through the upper tube. 

 

Pros: The knobs can be tightened very hard so there will be a lot of clamping and friction force generated to help stabilize the joint. 

 

Cons: The knobs will be loose hardware to keep track of. If the holes in the upper tube become worn or elongated, that may allow some motion. Each strut will react lateral loads and tension loads...could they deflect a little?

 

Please excuse my terrible drafting skills...this figure is just to convey the ideas...

 

Thanks!

 

cs...John

Attached Files


Edited by John Miele, 22 October 2021 - 02:54 PM.


#2 MitchAlsup

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Posted 22 October 2021 - 02:57 PM

A lot of it is going to depend on where the altitude bearings will want to be.

 

If they are rather close to the primary end, 3) you could make a real truss for the middle instead of struts. This would make the upper and lower sections more compact and take less space in transport and storage.

 

If the balance point is towards the middle, then you have to pick up the gravitational loads from the center. And make both ends of the assembly stiff.

 

If your intent is to make the telescope into a 2 piece tube (1) is probably best.



#3 John Miele

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Posted 22 October 2021 - 03:38 PM

Right now it is the two piece concept. The bearing cradle position will be very close to what is shown in this sketch...the split will be 4 inches in front of the cradle.

 

   

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#4 Oberon

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Posted 22 October 2021 - 04:33 PM

I would avoid anything that smacks of a sliding snug fit. It will never go together exactly the same, it will give you collimation woes.

Suggest instead looking at a 3 ball and radial groove kinematic mount, and held together with your over-center latches.

 

Sorta like here https://practicalpre...well-criterion/

 

…only you want those three ball and groove joints fitted to rings and equally spaced (as shown) snugly around your tube (not shown).

 

And if you use ball heads, knobs or acorn nuts as a way of mounting the ball to a screw then your joints can be adjustable. This means your construction tolerances can be loose and you can adjust the screws to get the tube exactly straight, never to be adjusted again.


Edited by Oberon, 22 October 2021 - 04:39 PM.

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#5 bigbangbaby

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Posted 22 October 2021 - 05:42 PM

Look at the Discovery split-tube Dob pin joint. It's simple and it works. I'm using it for my 12.5 in. f/5 Newtonian build.


Edited by bigbangbaby, 22 October 2021 - 05:42 PM.

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#6 John Miele

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Posted 22 October 2021 - 05:45 PM

Look at the Discovery split-tube Dob pin joint. It's simple and it works. I'm using it for my 12.5 in. f/5 Newtonian build.

I saw that but I don't think I can fabricate aluminum bars and drill deep accurate holes for the alignment pins. If I could buy some already made I would! Did you make your own bars?



#7 bigbangbaby

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Posted 22 October 2021 - 07:51 PM

I haven't yet, but that's the plan. Find someone with a mill to bore and cross drill the blocks.



#8 Oberon

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Posted 23 October 2021 - 12:17 AM

I saw that but I don't think I can fabricate aluminum bars and drill deep accurate holes for the alignment pins. If I could buy some already made I would! Did you make your own bars?

Thats why I suggested the kinematic mount. Its simpler to make, and fundamentally superior.


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#9 dave brock

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Posted 23 October 2021 - 02:49 AM

I've made a number of split tube scopes up to 16". I use aluminium strip at the join plus suitcase

latches. Locks together solidly and very repeatable. The early ones used aluminium on the

inside as well as the outside but then decided the inside one isn't needed.

 

new pics 81.jpg

 

16inch at stardate sm.JPG

 

latch 018 sm.JPG

 

latch 005 sm.JPG

 

latch 001 sm.JPG


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#10 bigbangbaby

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Posted 23 October 2021 - 05:50 AM

Thats why I suggested the kinematic mount. Its simpler to make, and fundamentally superior.

What you're suggesting will work too. The Discovery setup is easy to make and use, which is probably why they did it that way. I had a 12.5 f/5 from them. The scope had other things I didn’t care for but the split joint worked perfectly.


Edited by bigbangbaby, 23 October 2021 - 07:06 AM.


#11 hamishbarker

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Posted 23 October 2021 - 06:46 AM

view towards secondary with tube folded
Album: 10" f5 folding dobsonian
10 images
0 comments

 
I know you already have a tube, but for ideas, the link above is my folding 10" f5 dob. It's a bit heavy but otherwise excellent, keeps collimation, zero vibration, works with my existing EQ platform, is its own "storage case" when folded, and now has an extra finder baseplate inside the tube to store the finder when stowed (folded).
 
I didn't quite locate trunnions correctly so may have to remove them and make a rocker a little taller, but as far as the folding concept, it's exactly what I wanted (except a bit heavy.)

 

Cheers,

Hamish


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#12 John Miele

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Posted 23 October 2021 - 08:23 AM

I would avoid anything that smacks of a sliding snug fit. It will never go together exactly the same, it will give you collimation woes.

Suggest instead looking at a 3 ball and radial groove kinematic mount, and held together with your over-center latches.

 

Sorta like here https://practicalpre...well-criterion/

 

…only you want those three ball and groove joints fitted to rings and equally spaced (as shown) snugly around your tube (not shown).

 

And if you use ball heads, knobs or acorn nuts as a way of mounting the ball to a screw then your joints can be adjustable. This means your construction tolerances can be loose and you can adjust the screws to get the tube exactly straight, never to be adjusted again.

Hi Oberon,

 

At first glance this looked like it would be difficult to make as well. But as I studied it more, it does appear doable. I'm getting a lot of good ideas  here and I appreciate them all. If I went with this concept. I could route out the two rings. I can order threaded phenolic balls from Mcmaster Carr, and I could use short lengths of channel to serve as the "V-groove". I would like to pull the rings together through the ball joint. Do you think I could extend the threaded hole all the way through the ball and use it to not only position the halves but to pull it up tight with a threaded knob?

 

cs...John



#13 John Miele

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Posted 23 October 2021 - 08:28 AM

I've made a number of split tube scopes up to 16". I use aluminium strip at the join plus suitcase

latches. Locks together solidly and very repeatable. The early ones used aluminium on the

inside as well as the outside but then decided the inside one isn't needed.

 

attachicon.gifnew pics 81.jpg

 

attachicon.gif16inch at stardate sm.JPG

 

attachicon.giflatch 018 sm.JPG

 

attachicon.giflatch 005 sm.JPG

 

attachicon.giflatch 001 sm.JPG

Hi Dave,

 

Thank you for thes images. Is your tube a paper based tube or some type of fiberglass? Does this joint hold collimation well as you push the scope around?



#14 John Miele

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Posted 23 October 2021 - 08:30 AM

 

 
 
I know you already have a tube, but for ideas, the link above is my folding 10" f5 dob. It's a bit heavy but otherwise excellent, keeps collimation, zero vibration, works with my existing EQ platform, is its own "storage case" when folded, and now has an extra finder baseplate inside the tube to store the finder when stowed (folded).
 
I didn't quite locate trunnions correctly so may have to remove them and make a rocker a little taller, but as far as the folding concept, it's exactly what I wanted (except a bit heavy.)

 

Cheers,

Hamish

 

Hi Hamish,

 

I don't think I can switch gears to go this route, but I have to say what an innovative concept you came up with! Truly thinking outside the box...lol!

 

Thanks for sharing the idea!

 

cs...John



#15 DAVIDG

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Posted 23 October 2021 - 09:46 AM

 Here is how I made my split tube for my 8" f/7. It is 18 years old and I never had a problem joining the tubes.

 I made  a coupling by taking a piece of the tube that is about 20" long and splitting  it down the size to remove a strip of material so it fits inside the ID of the main tube.  The coupling is glued into the bottom section. In the slit I glued a thin piece of wood that has a slight taper to it . So the coupling is slightly smaller in ID at the top then the  bottom  but were the two tube selection meet  when seated it is the same size of the ID of the main tube. The slight taper makes easily to joint the top and bottom section of the tube and once joined there is no play. The result is that the upper section can be rotated to put the eyepiece in any position and the scope stay collimated. 

  Here are some pictures showing the coupling with the wedge to make it taper slightly.

 

                  - Dave 

split tube 1.jpg

split tube 2.jpg

 

8 f7 5 30 2020.jpg


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#16 dave brock

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Posted 23 October 2021 - 01:58 PM

Hi Dave,

Thank you for thes images. Is your tube a paper based tube or some type of fiberglass? Does this joint hold collimation well as you push the scope around?


The tubes have all been Formatube (Sonotube in the USA) so yes, paper based.
I soak the edges to be joined (in fact all ends are done) with West System epoxy glue to give strength and rigidity.
The joint definitely holds collimation while observing and most times little or no collimation is needed after joining. The suitcase latches do a good job of positioning the tubes back in the exact same position.

#17 Oberon

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Posted 23 October 2021 - 04:35 PM

Hi Oberon,

 

At first glance this looked like it would be difficult to make as well. But as I studied it more, it does appear doable. I'm getting a lot of good ideas  here and I appreciate them all. If I went with this concept. I could route out the two rings. I can order threaded phenolic balls from Mcmaster Carr, and I could use short lengths of channel to serve as the "V-groove". I would like to pull the rings together through the ball joint. Do you think I could extend the threaded hole all the way through the ball and use it to not only position the halves but to pull it up tight with a threaded knob?

 

cs...John

Yes



#18 PETER DREW

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Posted 23 October 2021 - 05:13 PM

I used method 1 for joining two tubes for a 8.5" F12.5 refractor and method 2 for a 17" Newtonian.  I did this, not to separate them but to provide a permanent long enough tube.  Both systems worked well with little difference in the amount of work. .  



#19 bigbangbaby

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Posted 24 October 2021 - 07:46 AM

OP, going with your idea 2; to fix the tube wear issue, install blocks on the outside of the mating tube. Make some steel tube inserts for the threaded knobs to pass through. Slightly counterbore the holes and place a steel washer there. The washer should bear on the spacer. Do the same washer install on outward facing surfaces of the internal supports. This way when you crank down on the knobs you’re not crushing the wood but are metal to metal. 


Edited by bigbangbaby, 24 October 2021 - 07:47 AM.


#20 John Miele

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Posted 01 November 2021 - 07:24 PM

Well...what was once a 66" long single tube has been split in two! No going back now...lol!

 

Mulling over all options, I decided to try Oberon's suggestion of the 3 piece kinematic design. I think all these various options can work but I believe the kinematic design is buildable and has the potential to be very rigid and offer a repeatable connection that will not wear out over time.

 

So in my typical overkill fashion, I laminated up two 3/4" plywood pieces to make 1.5 inch thick rings. I ordered some 1" diameter phenolic balls with 1/4-20 tapped inserts and a piece of aluminum channel. I counterbored 0.5 inch deep holes for the balls into one ring and cut 0.5 inch deep slots in the mating ring for the channel. I snuck up on the ID of each ring with the router and they fit snug as can be over the tube. This joint is going to be very stiff. The next step is to get the rings on, align the tubes and fasten the rings in position. I left the bolts holes in the channel a little oversized so the channels can "float" into the proper final position based on the tube halves being lined up. With the tube halves temporarily clamped up, I'll tighten the channel bolts to lock them in place. 

 

This design has the benefit of also being easily removed and reworked into a index pin with clamps type of joint if it does not work as expected.

 

cs...John

 

 

Attached Thumbnails

  • IMG_20211024_104033633.jpg
  • IMG_20211101_155549707.jpg
  • IMG_20211101_155604834.jpg

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#21 John Miele

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Posted 01 November 2021 - 07:24 PM

Mating ring...

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#22 hamishbarker

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Posted 02 November 2021 - 01:19 AM

looking good. will you glue the ply rings onto the sonotube or just clamp them, or screw from the inside out into them? 



#23 bigbangbaby

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Posted 02 November 2021 - 07:37 AM

It should positionally repeat as a kinematic joint is designed to do. My only concern is phenolic isn’t that hard and will be bearing on a small area, so high stresses when you clamp the joint together. You may see the phenolic deform where it touches the aluminum channel. I look forward to hearing how it works. Nice job on the wood pieces.



#24 John Miele

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Posted 02 November 2021 - 07:39 AM

I will screw the rings to the tube by running screws from the inside outward as you mentioned. No glue at first. That way I can remove the rings and make adjustments if needed. Once I confirm the joint is acting exactly as I want then I'll probably use glue and screws to make it permanent. I drew index lines on the tube halves so I'll slide the rings over the ends, line up each tube half, make sure I have a ring clamp location at the TDC position, let the channel pieces slightly move themselves into the exact position if needed (I lines up everything pretty closely so that should be a very small adjustment) and then run in a few screws. At least that is the plan...lol!



#25 John Miele

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Posted 02 November 2021 - 08:02 AM

That's a valid concern. I'm using adjustable SS latches with a threaded rotating hook piece that can be used to very the clamping force as you want. Which in this case will not be a lot. I'll first adjust the clamps to draw the joint up snug-tight. I can then rotate each latch hook in half turn increments which pull the latch hook in by 0.025 inch per half turn. This should let me creep up on a good pulling force in a controlled manner that will not permanently deform anything. I redid my latch force calculations and the highest tension force on a latch will only be around 21 lbs. So as long as I clamp up around 25 or 30 lbs of force, I'll get no gapping and I don't think that low of a load will permanently deform anything. But we will see, and luckily, I just checked at Mcmasterr Carr and I can switch to a steel ball with the exact same dimensions if needed.

 

John

 

edit: thinking on how to check for deformation, I'll clamp the joint as I intend to and then disassemble and examine the contact area on the balls. If I can see or feel any indentations then I'll switch them straightaway to steel.


Edited by John Miele, 02 November 2021 - 08:05 AM.

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