Making a form for an arch

lanwater
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I need to make an arch with a cross section of 3″×3″ for an outdoor table legs using bent lamination
I plan to experiment with 3/16 to 1/4 laminates.

The arch will have about 26" radius since the table will be about 30" high.

for 1/4 " thick laminates I would need 12 of them. Each laminate would be about 90" long.
That’s pi*radius for half a circle: 3.14*26 = 82.
Since I am going to have a stair effect on the ends during lamination I will add another 8" for the ends waste.

All these number are rough number but pretty close to final.

I need to know the best way to make the form so as to minimize the wood needed for the form itself and yet making sure it does not collapse during clamping.
I thought about using plywood or MDF, cut the arches on the bandsaw, stack them about 3.5" high and glue them. That is a lot of waste and probably heavy.

The other way would be out of 2×6 lumber like below. Staking them until I get the right height(3"+)
I need ideas/suggestion please.

Abbas, Castro Valley, CA

Use the same segmented approach with 6" wide strips of mdf. Staggering the joints will add to the strength and stability.

For bent laminations you really do want heavy!

Artisan Woodworks of Texas- www.awwtx.com

I would glue two pieces of 3/4" plywood together bigger that the finished arch. Then draw the inside of the arch. After that glue and screw blocks of wood to the line very 12" say. Then bend your strips around these blocks and clamp.

Tor and Odin are the greatest of gods.

You will develop substantial force with the laminations. I would bite the bullet and make a solid half circle (a little more than half actually) of two layers of plywood with spacer blocks to give you a little over three inch thickness, then cut several holes as required to allow access for clamps. The pieced up mold may work but if it doesn’t, you will find out too late. With a solid mold. You know it won’t move.

The early bird gets the worm but its the second mouse that gets the cheese.

Thanks for the input.

I was afraid I would have to go the Plywood route.

how wide (deep) the spacer between the ply would have to be? Can I get by with 4" deep with glue and screws?

Also, in theory 1/4" laminate should not be a problem for that radius. However, from your experience, will that be a problem when I am pulling 12 of those with the clamps?

Can /should I bend the first few, let the glue set then add some more?
Can I get buy with a ratcheting belt just to pull the laminate and sacrifice it by adding clamps while it’s in place?

Too many questions but I feel your experience is much more valuable than the books here.

Abbas, Castro Valley, CA

I’d go a few at a time both because the force will be more manageable and because a lot of layers with the equivalent of ball bearings in between will make you think of taking up herding cats as an easier alternative.
As for thickness, I would make the mold about 3 1/2" and cut my pieces just less than that. Then I would screw a few sticks on the sides of the mold to control the side slip. You should end up with a little trim room for the inevitable misalignment you will get and still have your 3".

About using a strap….. I think the reel would get too full with the amount of distance you’d have to pull. I haven’t done that but ratchet straps really only work well if you can snug them up empty and only tighten the last bit.
Is that at all clear?

Stack three pieces with glue and put a tack through one end to keep them from sliding lengthwise, then clamp that end and work toward the other.

The early bird gets the worm but its the second mouse that gets the cheese.

Thanks Paul, all clear.

What about the spacer between the 2 ply? it looks like it would have to be 2" thick. I can manage that. How deep would it have to be?
I realize that the laminates will be supported by the 2 ply as well but I am concern that it could give in under clumping pressure and that will translate to the laminates.

Abbas, Castro Valley, CA

Two inches is what you would want, yes. There won’t be a lot of strain on it if you clamp on the plywood itself. A “C” clamp on each side and a stick across the lamination is the way to go. There will be a tendency for the pieces to cup on you so the more clamps the better. This may be a place for some all thread, some wingnuts, and some creative thinking.

The early bird gets the worm but its the second mouse that gets the cheese.

Got it!

Thanks!

Abbas, Castro Valley, CA

I’m using the attached form for my rocker laminations (1 of the rocker arches is shown). To form the arch form, you can use a circle-cutting jig for either the bandsaw (my preference) or via a router.
MJCD

To ensure the arch is properly formed, I have a ‘backer’ laminated arch which sandwiches the 1/4" strips between the form and the pre-formed laminated arch.

I use C clamps and large Bessey clamps to draw-in the lams for gluing.