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Internal Dovetails to make a long curve - Is this doomed to fail?

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Forum topic by HibbittyDibbitty posted 01-16-2014 09:30 PM 1181 views 1 time favorited 13 replies Add to Favorites Watch
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HibbittyDibbitty

8 posts in 1054 days


01-16-2014 09:30 PM

Topic tags/keywords: swing joinery pine joining question dovetails

Fairly novice woodworker here, and I have a question for you more seasoned folk regarding my current project.

I’m making a porch swing for my wife and decided to completely design it myself. I’m making the first one out of SYP just to test it out, but will be making another version out of some wood I had milled from a special-to-me oak that died on the property where I grew up. Even though the test version of the swing that I’m making now won’t have the sentimental value of the final version, I’d still like to either put it to use elsewhere in the yard, or give it to someone as a gift.

I’ve designed the armrests to be big, swooping curves, and am making them so that they assemble from three sections, with the grain running horizontally at the base and top, and vertically at the front to join the curve together. I’m hoping to use as little metal hardware as possible (hopefully none), so I’ve designed these pieces to fit together via large hidden dovetails. Here’s the idea in sketchup:

The top and base sections are each made from 2 pieces, so the dovetailed upright will be sandwiched between the outside/inside pieces. The entire armrest will be 3” wide. The finished product would look something like this:

You can also see that I have the same idea incorporated into the back of the armrest where it will attach to the rest of the swing.

Again, I’m pretty inexperienced, but I couldn’t find any good info online about making this type of curve, and my limited understanding of wood tells me that this should be fairly solid. But I figure some of you wise elders might be able to stop me from making a huge mistake. Thoughts?


13 replies so far

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Nicky

695 posts in 3554 days


#1 posted 01-16-2014 09:35 PM

Interesting design. Will these armrests carry the load of the seats (and its occupants?) Or are they decorative armrests?

-- Nicky

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HibbittyDibbitty

8 posts in 1054 days


#2 posted 01-16-2014 09:46 PM

They won’t be supporting the weight of the swing; I’m planning to attach rope/chains to the front seat rail at the bottom and to the seat back at the top. What I’m worried about is how much force they’ll encounter from the flex between the seat bottom and back. The interface between the seat bottom and back is not half-laps like I’ve seen on many swings, but rather the back rail of the seat bottom will be tenoned into a mortise at the bottom of the uprights on the seat back.

Like so:

So I’m anticipating a little bit of racking once weighted. Don’t know how much though. I’ve given some thought to attaching the rope/chain to the bottom of the seat back upright piece and having it go through an eye bolt or something at the top, so that I can disperse some of that force, but not sure yet.

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HibbittyDibbitty

8 posts in 1054 days


#3 posted 01-16-2014 09:57 PM

Now that I’m looking at this with fresh eyes, I think I could probably turn that bottom joint (where the seat back meets the bottom of the armrest) into either a half-lap or a bridle. I imagine that would go a long way towards preventing the racking. I just wonder if it would be enough.

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Kryptic

294 posts in 1122 days


#4 posted 01-16-2014 10:12 PM

aesthetically it wouldn’t matter, but structurally, I think its a good idea.

That said, I think its a lot of work, for a joint, that cannot be seen.

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HibbittyDibbitty

8 posts in 1054 days


#5 posted 01-16-2014 10:23 PM

Kryptic, the curved armrests are the aesthetic part that I’m interested in; the dovetails were what I was thinking would be the most structurally sound. I’m just wondering if I’m missing something more secure than these.

As far as simplicity is concerned, I don’t think these will be too difficult. I’m planning to use my bandsaw to cut out the outline, then it’s just a matter of using my router to remove half the thickness on the dovetails, as well as the “dovetail mortises” (is that a thing?). I had my router in mind when I was designing these, thus the contoured corners of the dovetails.

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Jorge G.

1537 posts in 1937 days


#6 posted 01-16-2014 10:27 PM

Why not do the arm rest as a bent lamination? You can make an U and then cut to shape with the band saw and some spokeshave work.

-- To surrender a dream leaves life as it is — and not as it could be.

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Kryptic

294 posts in 1122 days


#7 posted 01-16-2014 10:33 PM

This is my last post to try and help. I get bored easy.

It easy to over complicate a project. If you study chairs, swings, that type of project, you should have an understanding of vector mechanics. That said, all the forces that are exerted onto the arms, do not require a dovetail, with the exception of where arm rest meets back of seat, even then its debatable as we will both be long dead before the joint fails.

Good Luck,…. I like your design as it would offer a learning curve, that most wouldn’t attempt.

Best

View DKV's profile

DKV

3940 posts in 1966 days


#8 posted 01-16-2014 10:47 PM

I like Jorge’s bent lamination and then sculpt to your desired design/look. Your swing looks nice.

-- This is a Troll Free zone.

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HibbittyDibbitty

8 posts in 1054 days


#9 posted 01-16-2014 10:48 PM

Thanks for the input. I wouldn’t say that I have a thorough understanding of vector mechanics, but from your prediction of the learning curve on this project, I’d wager I’ll have more of an understanding when I get done. I had pretty high confidence in the simplicity of the build an hour ago; I hope I haven’t designed a check that my skill can’t cash…

View rum's profile

rum

148 posts in 2048 days


#10 posted 01-16-2014 11:16 PM

imho I would resize the dovetail ends smaller and align it so that you can do all of the cuts straight and square and then cut the curves around that. That will still be plenty strong and be MUCH easier to lay out.

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Kryptic

294 posts in 1122 days


#11 posted 01-17-2014 12:46 AM

with all the different climate zones I always think within the one I live in. That said, my climate is eternally brutal but for the blissful short days of summer : )

A porch swing, if hung by rope or chain, and the only thing the rope of the chain hold, is the seat, and all other appendages of, go from there up, which are the arm rests, and the weight of your arm bares down as your back leans to the stern, and she becomes a bit like a boat.

You would be better to take a quick perusal and study the way Sam Maloof made his rocking chairs. Combine that with a quick study of the “Muskoka Chair”, or “Anirondack” …… decide your fate and pick chains or rope for I cannot imagine a porch swing that isn’t suspended from the ceiling ? The chains attach to the seat, and the chains swing, the backrest and arms rests ….. both have to be comfortable and functional, with the structural integrity of a woman, a daughter, a son and his girlfriend, an old, very old grandmother with a great grandson at risk. Precious for sure, pure Kodak moments.

at the end of the day. its all about what you want but if u asked me if I wanted to invest in this design other then words of experience, I would have to say no lol. Other then an exercise in perseverance, I would carry on, for tradition bares little onto the outcome ? I would prefer to put a sliding, and locking dovetail in the odd right spot.

the design reminds me of the old cog wheels that were once powered by a river, and so on, and so forth. I kinda like it.

Best

View Loren's profile

Loren

8301 posts in 3110 days


#12 posted 01-17-2014 01:15 AM

I think this is an unnecessary exercise in joint making,
but if it intrigues you, try it out. You may be right
about the durability, but I would use regular tenons
myself just because I would be able to skip truing up
the insides of the arms where they are glued together
that way. Another drawback is you may find dry
assembly awkward.

What you’re proposing is like a variant of the blind
fox wedged tenon. If you’re looking for a tricky
joint to make, the blind fox wedge is one to try.

Another way to do it is a sliding dovetail you pop in from
the front. The joinery is visible this way, which
makes some designs more interesting.

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Kryptic

294 posts in 1122 days


#13 posted 01-17-2014 02:34 AM

a sliding dovetail is blind by nature

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