Arch Construction Method for Arbor

[Frank Snyder]

I'm making this arbor for a friend (their design) and I've been pondering over the various methods one can make an arch like this. My original thought was to use the bent lamination method, but I was unable to find clear Cedar with which to make it. My next thought was to use the more common method of cutting staves and piecing them together to make the arch. My concern here was the end grain connections separating, so I thought by adding half-lap joints to expose more side grain for glue, that might prevent this from happening. I'd use a finger joint bit, but that can only handle 1-1/2" thick, whereas each layer of this 2-ply is 2-3/4" thick. Any thoughts or suggestions are appreciated.

Also, can anyone recommend the best exterior glue for Cedar? (TB3?)

Arbor.jpgArch Construction.jpg


Frank :-)

[Jim Becker]

Typically, I've seen these done with mitered sections wide enough to support the width of the arch and joined together with loose tenon joinery. The more segments you do, the narrower the stock you can use. If you leave "wings" on the segments, you can make the assembly easier to clamp while joining it up and then use a circle cutter arrangement on a band saw or a trammel with a router to cut the finished arch...doing the outside (larger) arc first.

[Frank Snyder]

Hi Jim,

Thanks for the feedback. From what you've described, it sounds like sections already have the full thickness of the arch (5-1/2" in this case) and are not layered together in the example I provided. I like the "wing" idea for clamping.

Good idea for using a trammel. I'd need some substantial outrigger support for an arch this size (and concoct something to act as the center of the arch). I'm not sure how I would cut the inside radius.

Using a router, I'd need to find a really long straight bit. At least using 2 layers, it knocks a ply's thickness down to 2-3/4" which makes it a little easier to work with.

Thanks again for the ideas.

Frank

[Jim Becker]

Yes, full thickness, Frank.

You just move the pivot point to cut the inside arc after doing the outside one. For the band saw, you can use some inexpensive (if there is such a thing) sheet good stock and temporarily fasten the blank to it, making the workpiece plus the sheet goods the "trammel". For the router method, the workpiece stays stationary and you swing the router on a trammel. It is unlikely that you can cut all the way through such thick material (due to cutter length), so make your arcs with several passes to max depth, rough cut the remainder off at the band saw or with a jigsaw and a long blade and then pattern route the rest flush with the surfaces originally cut by the router.

[Greg Funk]

I think I would spend some more time trying to source clear cedar. That thick arch would be easier to laminate. We have lots of arched trim on our house and it was made with mitered segements of 1-1/2" boards glued together with biscuits which was very strong. The segments were trimmed with a bandsaw and finished with a belt sander. Too bad you didn't ask a few years ago I had 1000's of feet of cedar at least half was clear.

Greg

[Frank Snyder]

Jim - The plywood backer makes the most sense for cutting the radius with the bandsaw. I was envisioning using a simple 1x brace to act as the center for the trammel, but it would lose its connection to the arch once the bandsaw cut through it. The plywood backer should work better. I guess we just discovered a good use for that Chinese plywood they sell at the Borg ;-)

Greg - A bent lam was my first choice, and the biggest Cedar dealer in our area said they could normally get clear Cedar. However, their suppliers get all of their Cedar from up by you (Canada) and apparently there was a labor strike which caused a serious disruption in supply. So for right now at least, clear Cedar is currently unavailable for me. In fact, I was only able to get rough sawn Cedar 4x6 posts in 10 and 12 foot lengths which I need to mill S4S. It's a real workout running these beasts across my 8" jointer trying to flatten them out (today's activity).

I've never used biscuits for exterior applications. I'd worry about their long term strength being outdoors. I've got a Domino joiner which I could try using their exterior rated Dominos for this project. Hmmm...

Thanks anyway for the retrospective condolence. I would've loved an excuse to go mountain biking up in the B.C.

[Jean-Franco Tracogna]

Frank, here the method I used for a demi-lune table top. Joinery is with loose tenons. You can adjust the thickness gluing more pieces together or starting from a piece of wood having the right thickness. IMO it's less work to do this that way ?

[Curt Harms]

and did it much like Jim said-2 layers of 5/4 P.T. stock (as I recall) fastened with splines in the ends and staggered one on top of the other then cut out on a bandsaw using a circle cutting jig, I think.

HTH

Curt

[Frank Snyder]

Jean-Franco - Thanks for the Sketchup drawing. I need to use shorter sections due to the material's dimensions (not enough there to make a harder radius). It'll be more work, but I don't mind.

Curt - Norm's method is essentially the same as mine, except I added half-lap joints to add strength at the end grain connections. Internal tenons would also probably work just as well, if not better.

Do you think internal tenons would be a better connection than half-lap joints?

Thanks again,

Frank

[Jamie Buxton]

If you lay up the arch from short sections and do nothing more, you'll have lots of glue lines looking up at the rain. Even the best glue lines are somehow the place where water infiltration starts. I'd look for one more piece of thin wood which I could wrap around the curve, and use it to "veneer" the outside of the curve. If it is perhaps 3/16" thick, it will bend around the curve, and you don't have to get obsessive about minimizing the glue line between it and the laid-up pieces.

[Frank Snyder]

Jamie - Good idea. I just wish I could find some clear Cedar I could use to do something like that. I'm not sure something with knots would hold up during a bend.

Frank

[Frank Snyder]

Finished project at http://www.sawmillcreek.org/showthread.php?p=855838