Best way to make a wooden arch? - Woodworking Talk - Woodworkers Forum
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post #1 of 20 Old 03-10-2018, 08:25 PM Thread Starter
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Best way to make a wooden arch?

Hi,

I am building an observatory for a telescope on my own. I would like to build it entirely out of wood. I was trying to figure out what is the best way to make an arch from wood?

The arch is a full semicircle of radius 4 feet made of 4x4 inches thickness. What is the best way to do this? Can I do this with multiple layers of plywood or using small 4x4 segments glued together to create the arch?

Thanks for any pointers.

Vipin
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post #2 of 20 Old 03-10-2018, 08:52 PM
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With a 4' radius you won't be able to make the cut with a 4x4's. It would probably take something like a board 24" wide. You need to draw the radius out on a sheet of plywood to determine the size. You might be able to use 4x4 material but but make the parts in short lengths and dowel them together end to end. I would be more inclined to glue together wood in a rough shape of the arch and then cut it on a bandsaw.
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post #3 of 20 Old 03-10-2018, 09:40 PM
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Things like that go through my shop all the time (I'm not the one making them, I'm the guy annoying the guy making them). What he does is essentially what Steve drew except he does it with a router going around a fixed point. Either that or he'll glue up strips of wood together on one his radius jigs to get whatever he needs.

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post #4 of 20 Old 03-10-2018, 09:43 PM
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Do you mean an arch, or a dome?

If it's an arch, I'd laminate it from 6- 1x4's. You'd need 14 footers and I imagine they'd need to be fairly clear to bend to a 4' radius.
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post #5 of 20 Old 03-10-2018, 10:24 PM
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Now if you’re talking a dome, I can demonstrate how to make that out of a single piece of wood. Never tried making anything that big however.


In woodworking there is always more then one way to accomplish something.
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post #6 of 20 Old 03-11-2018, 12:42 AM Thread Starter
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Thanks for all the pointers. Ultimately I wish to make a dome.

If I were to glue them together would it hold its strength compared to a monolithic piece? Should I be concerned about glue coming off or something since the dome will be sitting outside in the elements 24 hrs?

Another question I should ask is: what kind of wood I should use for the project?

Essentially I am trying to build something as shown in this video:

https://youtu.be/uZK7aLrdsd8

Vipin
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post #7 of 20 Old 03-11-2018, 08:30 AM
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Quote:
Originally Posted by vipinpsharma View Post
Thanks for all the pointers. Ultimately I wish to make a dome.

If I were to glue them together would it hold its strength compared to a monolithic piece? Should I be concerned about glue coming off or something since the dome will be sitting outside in the elements 24 hrs?

Another question I should ask is: what kind of wood I should use for the project?

Essentially I am trying to build something as shown in this video:

https://youtu.be/uZK7aLrdsd8

Vipin
Gluing the wood together would be as strong as if you used a solid board but it would need some protection from the weather. It would have to be thoroughly primed and painted and maintained. For that application you would use a exterior glue such as Titebond III or Elmers Carpenters Glue Max. This type glue is not affected by water once dry. The glue works well but if the wood is allowed to get wet it swells up and then shrinks when it dries. This action pulls at the joints where over time it pulls the joints apart. The wood I would recommend is pressure treated pine. Now having said that there is a lot of problems working with treated wood if you just go to the store and start building. The wood has been put in a tank and saturated through and through with a chemical which is mostly water. If you use it fresh from the store it's like building with green lumber freshly cut from the tree. What needs to be done is stack the wood as though it was green with sticks between it and let it dry for more than a month before building.
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post #8 of 20 Old 03-11-2018, 10:30 AM
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Quote:
Originally Posted by vipinpsharma View Post
Hi,

I am building an observatory for a telescope on my own. I would like to build it entirely out of wood. I was trying to figure out what is the best way to make an arch from wood?

The arch is a full semicircle of radius 4 feet made of 4x4 inches thickness. What is the best way to do this? Can I do this with multiple layers of plywood or using small 4x4 segments glued together to create the arch?

Thanks for any pointers.

Vipin

Hello and Welcome,

I have had to design and build these from time to time, either to support masonry during the creation of a stone arch, or for the actual frame work of a structure...

I would, myself, suggest a "jointed wooden arch" of some type. These could be just timber frame style joints, or full on structurally glued laminated beams...The choices are pretty vast and many are superior to others structurally...

Could you pick some photos of what you have in mind or think you would like to build? The link above in read should give you some ideas perhaps?

j
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post #9 of 20 Old 03-11-2018, 10:38 AM
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Here perhaps are some additional concepts that would help...???...The foundational frame work is not as complicated as it may appear to you...There are some videos out there of these being assembled...





Links to google page covering more of this subject can be found...here. You may also want to look up Reciprocal Roofs...
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post #10 of 20 Old 03-11-2018, 11:02 AM
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Once complete, does the dome get covered with sheathing and roofing to keep the elements from the framing members?

Also, if the arcs are simply framing members, I don't see why they'd need to be 4" wide.
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post #11 of 20 Old 03-11-2018, 12:53 PM
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Ultimately, it makes a big difference!

Quote:
Originally Posted by vipinpsharma View Post
Thanks for all the pointers. Ultimately I wish to make a dome.

If I were to glue them together would it hold its strength compared to a monolithic piece? Should I be concerned about glue coming off or something since the dome will be sitting outside in the elements 24 hrs?

Another question I should ask is: what kind of wood I should use for the project?

Essentially I am trying to build something as shown in this video:

https://youtu.be/uZK7aLrdsd8

Vipin
The dome in the You Tube has 2 parallel arches for the telescope to see through. You may also want a sliding cover to protect the scope from the elements. This should all be designed as a system, not piece by piece, OR you be back tracking many design and construction decisions.

The dome can be made of "modular" elements coming together at the center, that way will all be uniform, smaller and therefore much easier to make. You might also consider heat bending PVC pipe over a semi-circular form. It is totally weatherproof and quite strong when bent in an arc. The sections can be smaller than if made from wood.
Just a thought .....

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)

Last edited by woodnthings; 03-11-2018 at 12:56 PM.
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post #12 of 20 Old 03-11-2018, 02:44 PM Thread Starter
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Quote:
Originally Posted by Steve Neul View Post
Gluing the wood together would be as strong as if you used a solid board but it would need some protection from the weather. It would have to be thoroughly primed and painted and maintained. For that application you would use a exterior glue such as Titebond III or Elmers Carpenters Glue Max. This type glue is not affected by water once dry. The glue works well but if the wood is allowed to get wet it swells up and then shrinks when it dries. This action pulls at the joints where over time it pulls the joints apart. The wood I would recommend is pressure treated pine. Now having said that there is a lot of problems working with treated wood if you just go to the store and start building. The wood has been put in a tank and saturated through and through with a chemical which is mostly water. If you use it fresh from the store it's like building with green lumber freshly cut from the tree. What needs to be done is stack the wood as though it was green with sticks between it and let it dry for more than a month before building.
I will keep a note of the recommended glue brands for exterior applications. Good suggestion on letting the new stock dry out before building as well. Thank you.
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post #13 of 20 Old 03-11-2018, 03:18 PM Thread Starter
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Originally Posted by Quickstep View Post
Once complete, does the dome get covered with sheathing and roofing to keep the elements from the framing members?
Yes, I was thinking of cover it with something which will help create a nice seal against rain/moisture but not the kind of roofing typically done for home (like insulation, roof tiles, etc]

Quote:
Originally Posted by Quickstep View Post
Also, if the arcs are simply framing members, I don't see why they'd need to be 4" wide.
I want it a little wider because I might need it some space to put a slot for the curved hatch to slide in and out.

Take a look at the attached drawing to give you some idea of what I am after.
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post #14 of 20 Old 03-11-2018, 03:21 PM Thread Starter
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Originally Posted by woodnthings View Post
The dome in the You Tube has 2 parallel arches for the telescope to see through. You may also want a sliding cover to protect the scope from the elements. This should all be designed as a system, not piece by piece, OR you be back tracking many design and construction decisions.

The dome can be made of "modular" elements coming together at the center, that way will all be uniform, smaller and therefore much easier to make. You might also consider heat bending PVC pipe over a semi-circular form. It is totally weatherproof and quite strong when bent in an arc. The sections can be smaller than if made from wood.
Just a thought .....
This is what I plan to build; I was thinking of starting out with the main center arcs (the two parallel semicircular shapes).
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post #15 of 20 Old 03-11-2018, 09:52 PM
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I didn't see anything in the UTube video that you could not build out of plywood. You can layout your arcs using pencil and string, cut them out using a saber saw, and glue-screw them together to make the necessary laminations.

I think making the ribs of plywood would allow you to make a light-weight but strong structure that you could cover with wood or even metal.
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post #16 of 20 Old 03-12-2018, 07:51 AM
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the pywood issue ......

Cutting large areas out of plywood is very wasteful, especially when the dimensions are 48", you only get a couple of pieces. It's better to start with narrow linear pieces and bend them over a large form or build up the arch from smaller segments, glued together.

This site may have posted previously, I donno?
http://www.thisiscarpentry.com/2012/...arches-part-1/

How to draw one from the same site:
http://www.thisiscarpentry.com/2008/...egmental-arch/
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The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
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post #17 of 20 Old 03-12-2018, 08:03 AM
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Plywood products not the solution here...

I didn't see Woodnthings' posting...will add this anyway...

Because of CNC machines and a myriad of DIYers jumping into the very weakly supported concept of "sustainable" and/or "green building" that plywood seems to offer. Many are valiantly trying to make just about everything from sheet good wood products because of its availability, ease of novice use and application, and false belief in its structural qualities. Most plywood typically does not have the structural capacity many believe it has. Few will meet standard architectural load parameters necessary for load baring structure unless attached to an an armature of framework of some kind...as a skin. Which is what plywood had always been intended to do. They work best in "skinning" a frame to form a structural diaphragm...Not create a structural frame work itself. Comparatively, plywood can be...made to work...but is not the most logical first choice for such application typically...

Last edited by 35015; 03-12-2018 at 08:53 AM.
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post #18 of 20 Old 03-12-2018, 08:43 AM
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There is an observatory on top of an old row home in Lancaster PA about two blocks north of the Courthouse. IIRC, It looks like a round cylinder topped with a metal dome from an old farm silo and part of the roof From the street level, it appears that two 1/4's of the dome slide in opposite directions like barn doors to open half of the observatory. With all the lights of the city, I can't imagine that much can be viewed from that particular observatory. .
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post #19 of 20 Old 03-12-2018, 03:02 PM
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The You Tube builder ....

Actually the dome builder uses some clever methods. This is a snapshot of his process. The arches look like they are strong and lightweight.

However, I don't understand the large holes in the perimeter ring, because they serve no purpose in the construction of the dome as far as I can tell. They would be more useful IF the arches were inserted into them as an anchor point. That would be a perfect method IF you were using PVC tubing and inserting it into the holes. This is a connector for the top of the dome from another You Tube video:
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The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)

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post #20 of 20 Old 03-14-2018, 06:32 AM
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This should all be designed as a system, not piece by piece, OR you be back tracking many design and construction decisions.
This is certainly true.
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