how to construct an arc from joined straight segments

[Jodi]

I am building a headboard that requires an arched border along the top edge. The border is 3 1/2 inches tall (the side that faces outward) and 1 1/2 inches thick (top edge). So it's not the type of arc that can be bent.

My plan was to piece 5 wide straight segments together with angled joints then cut the arc from this. The segments would be 22" long for the center and 2 14" segments on each side. I started worrying, however about the integrity of the joints. First, they would be end grain to end grain - I could do a lap joint or a dado to help with that (I don't think biscuits would be enough). Second, I was worried about the wood movement. As the segments swelled, on the convex side of the arc the joints would grow apart, and on the concave side they would push toward each other. I thought this might make the joint break.

Does anyone have experience with something like this? Is there any other way to do it? Should I have smaller segments between joints? Is there enough movement in 3 1/2 inches to cause any problems?

This is my first commissioned piece for someone else, so I really want to get it right.

Thanks for any advice you can give.

Jodi

[Wess23]

I would stack them long grain to long grain. If your gonna join end grain to end grain,a finger joint would be the best way to make the joint. However if the piece is to be finished natural or stained, it's very hard to make the joints dissapear.

[BUB 209]

You could do the end grain if you used a spline joint. You can make a deep spline cut using the table saw (and a lot of caution) I wouldn't worry too much about the joint shrinking apart if the piece is going to be kept indoors. Clamping might take a little engineering if you use aliphatic resin glue (titebond) For joints where only minimal clamping can be used, I use a heavy-bodied, 2 part epoxy from Clearcote Corp. They're in Florida, the number is 727-898-8611.

[Preston Andreas]

You can make it from 3/4" x 3 1/2" pieces, gluing them together side by side and alternating the joints. Or, you can make a form and glue up enough strips, say 1/4" x 1 1/2" to make the 3 1/2" high arch. If you do the side by side, make sure the joints that show are evenly spaced.

[Traves W. Coppock]

On Sat, 09 Aug 2003 23:38:11 GMT, "Preston Andreas" Crawled out of the shop and said. . .:

snip

gotta agree with Preston on this one, lamination bending would be my choice here. one caveat tho, use a poly urethane glue. it will hold better, and produces much less "springback" of the material when released from the clamping form. and, as an added bonus, it gives you much more open time when stacking the laminations in the form. use one clamp for every 5" of length, and do not, i repeat DO NOT apply too much clamping pressure. poly urethane glue expands alot! DAMHIKT

Traves

[Jodi]

If I did a stacked laminate, how much spring back should I expect? The width of the arc end to end is 75" and it has about a 6" difference between top of the arc and the bottom. Is it enough to need to build the form with more of an arc than I need in the end?

Also, I know the process that you go through to make a form and laminate the strips, but I've never done it. I can't picture what the final front face would look like when just finished naturally. I'd imagine that you could tell that it's a bunch of laminated strips from the stuttering of the grain pattern. Is that how it looks or does it look pretty much like a single naturally curved piece of wood? In other words, would the manner of construction be less obvious than the straight segment method?

And finally, I don't have enough clamps to do the laminate unless the quick clamps have enough pressure to work. Since you say to not use too much pressure I'm assuming that they'd be OK. Do you think?

Thanks,

Jodi

[Lew Hodgett]

I jumped into this late so may have missed something; however. if you are considering laminating, go to the library and get Fred Bingham's book, "Practical Yacht Joinery".

It is an impossibility to build a decent wooden boat without laminating some beams.

Bingham goes into considerable detail including sketches, describing the process.

HTH

[Traves W. Coppock]

On 12 Aug 2003 19:52:57 -0700, [e-mail] (Jodi) Crawled out of the shop and said. . .:

snip

however, with the poly glue, you can eliminate alot of the springback. make the arc about 10% more aggressive than the final form desired. M&T joinery will "lock" the final arc so it wont move.

cut the laminates from a single piece of stock. use the bandsaw to cut them, and you wont notice much of the seams at all. here are a few tips for doing just that.

1: after you have all the strips cut, align the grain so it matches the best, and mark a triangle across the face of the seams. this helps you to align them without much guesswork when glueing up the piece. 2: and this one is REAL important. as you cut the strips, mark each face with a number or letter. first cut, mark a "1" on the faces you just cut. this helps keep them in order if you mix them up.

quick clamps should have plenty of force to keep the form tight enough. remember, you have 2(two) opposing pieces of bending jig that helps spread the clamping force over the entire glueup. use plenty of them. if you don't have enough of them, BUY them BEFORE you start your glueup. DAMHIKT :(( bar clamps are cheap enough so that no one should be without enough of those. i own several of the "el-cheepo" brand sold at menards or HD

never a problem, that's why we do what we do! :)

Traves

[Larry Jaques]

On Wed, 13 Aug 2003 03:42:21 GMT, "Lew Hodgett" pixelated:

How do you feel about "Yacht Construction" by K. H. C. Jurd? It's in the local library while Bingham's isn't.

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[Lew Hodgett]

I'm not familiar with it but you can buy Bingham's book for about $20 on Amazon.

IMHO, it's a great reference for someone interested in general woodworking, not just boat building.

HTH