Choice of bent lamination technique?

[DavidF]

Hi everyone, long time no post

I am starting a project that involves curved tapered legs on a small side table. They are approx 16" high by 12" wide and curved in a 31" radius. Tapered from 3/4 at the bottom to 1 1/2" at the top. I have two choices: the first is the traditional way, by making tapered laminations from solid stock and then laying them up on a form in a vacuum bag. The other, is as shown in FWW 231 Feb 2013 where the guy uses solid stock kerfed, for the former, outer veneers from the show stock and then edge banding on the sides (forgetting the steam bent bit!) My worry with the latter is spring back. With this method you have essentially 3 plys - the remaining material after kerfing the core then a sawn veneer on either side. I could end up with more than 1/4" spring back. where as with tapered laminations I could reduce the spring to less than 1/32 by using 9 plys. Perhaps the core is "floppy" enough to not spring back but still be held by the face veneers. The down side of the tapered laminations is that the thickness varies quite a lot over a short distance so even starting at 2mm at one end they need to be 4mm at the top which is still thick for a bending form (sorry about converting to metric!).... Any thoughts or experience welcome.

Thanks. David

 

[Henry W]

David: I can't answer your question, as I have no experience with either technique. It is however good to you on the NCWW board again.

Henry

 

[ScottM]

David welcome back. Glad to hear from you. Are you still on the far side of the pond?

 

[Mike Davis]

Hi David,

I have done a bit of lamination just not tapered. My thickest parts were 5mm and the radius was about 30cm, so I think you will be OK with 4mm and 31 inch radius. The tapering of a 12 inch wide piece from 4mm to 2mm does concern me though. If you are comfortable with that part then I say go ahead.

Would love to see pics when done.

 

[zapdafish]

I think you are going to have springback whichever method you used. I think the key is to keep them under the same pressure and temperature, (maybe a few other variables) for the same amount of time so the springback is as uniform as possible with each glue up. Another option is to do a wide lamination and then cut it into 4 legs so they hopefully all have the same curve. Also because of the taper, I might consider making the laminates extra long so that I could drive a screw into one of the ends to keep creep to a minimum.

For my only lamination experience I used 2 mdf forms and clamped them together with the laminates in between. I made 4 items and it worked out pretty well but I didn't need to be so careful of my tolerances as I think you will need to.

 

[Michael Dresdner]

There is a third option. You can make the legs from solid pieces of Cold Bend wood, bending them either before or after cutting. Cold Bend wood will bend in any direction without heat or moisture added. Once left to fully dry, it will stay in the bent shape, and unlike hot bent or pressed lamination, has little or no springback. If you are not familiar with it, you can read about it here: http://www.puretimber.com/bending/

 

[cyclopentadiene]

In preparing the rockers for a Maloof rocker, I have used 1/8" thick strips 4" wide in a form using lots of clamps and no steam. The laminations are glued together using plastic resin glue. The radius is slightly larger than you indicate but these have literally no springback when using walnut or maple. I have not used any other woods.

 

[Jeff]

I am starting a project that involves curved tapered legs on a small side table. They are approx 16" high by 12" wide and curved in a 31" radius. Tapered from 3/4 at the bottom to 1 1/2" at the top.

David,

I'm being dense with the dimensions and don't understand them. So the table is 16" h x 12" w. Correct? Where do these curved tapers come into play at 3/4" on the bottom and 1.5" at the top? What is your 31" radius?

A drawing or picture would help to define your plan because I'm just not understanding it.

 

[Sealeveler]

I used a 2 piece mold for my Father's cane (pic in avatar).Only used cold water to soak the walnut and sassafras then clamped in mold,allowed to dry in mold for a couple days then removed,glued,plastic between cane handle and mold and reclamped for couple days.After 7 years or so there has been zero spring back,it still fits the jig perfect.
Tony

 

[DavidF]

David welcome back. Glad to hear from you. Are you still on the far side of the pond?

Hi Scott, yes still in Blighty! Since coming back 3 years ago life has been a maelstrom of international travel leaving very little time for anything else. Although the new shop has been up and running for just over a year, I have had no time to do anything but turning. This table project is the first furniture that I have taken on. Amazing how the skills need a slight re-kindle after such a long time!!!

 

[DavidF]

Here you go! A Solidworks rendering of the design. You can see the taper on the "legs/sides" I am tempted by the kerfed core method as it lends itself to this design, but not something I have tried before. I may get hold of a piece of Poplar or similar this weekend and have a play.

 

[DavidF]

I think you are going to have springback whichever method you used. I think the key is to keep them under the same pressure and temperature, (maybe a few other variables) for the same amount of time so the springback is as uniform as possible with each glue up. Another option is to do a wide lamination and then cut it into 4 legs so they hopefully all have the same curve. Also because of the taper, I might consider making the laminates extra long so that I could drive a screw into one of the ends to keep creep to a minimum.

For my only lamination experience I used 2 mdf forms and clamped them together with the laminates in between. I made 4 items and it worked out pretty well but I didn't need to be so careful of my tolerances as I think you will need to.

I have done quite a bit of straight laminating and never had a particular problem with spring back. In this case I would use 9 laminations starting at 2mm at the thin end and 4mm at the other. Using the normal springback calculation of Y = X/n^2 I would get roughly 2mm of spring which I can deal with. But making all the laminations then making a jig to run them through the sander is a bit of a workup!!

 

[DavidF]

There is a third option. You can make the legs from solid pieces of Cold Bend wood, bending them either before or after cutting. Cold Bend wood will bend in any direction without heat or moisture added. Once left to fully dry, it will stay in the bent shape, and unlike hot bent or pressed lamination, has little or no springback. If you are not familiar with it, you can read about it here: http://www.puretimber.com/bending/

Just had a quick look here - fascinating stuff! I don't think it will help me in this project but looks as though one could have fun with it...

 

[DavidF]

I think I am going to try the kerfed solid former method and see how it works out, I'll let you know....

Cheers,

David

 

[Jeff]

I think I am going to try the kerfed solid former method and see how it works out, I'll let you know....

Cheers,

David

No experience with laminating here, but you're project is interesting in concept. I read the article that you cited. Wow! It's not for the faint of heart but doable. He doesn't explain in detail how to make the bending form with the 31 degree radius.

I used Sketch Up to construct the model from your 16" x 12" dimensions with 1.5" thick untapered and kerfed poplar core. That was an interesting exercise by itself.

 

[DavidF]

Yes it is an interesting project. I have designed the whole thing in Solidworks and designed the bending forms from there. I have made the side/leg and base forms already. Not really too many problems with those. In fact I made the form top by kerfing some 1/2" mdf and it worked just fine. I will get some poplar at the weekend and see what happens. Worst that can happen is that I'll get some spring back. I intend to use a 3mm thick sawn veneer on each face and pop in to the vac bag as a sandwich, then lip with solid. An alternative was to veneer initially with commercial veneer, then lip the edges, before putting a face veneer on, but with spring back it would not have fitted the form second time round, so that sort of fixed the method. I will then use through tenon to fit the top to the sides. The base is fixed with sliding dovetails - How hard can it be

 

[Dave Richards]

Try to cut the kerfs so they'll just close up when the core is bent. If you can figure out the length of the curves inside and out, this will be easier. It would be a simple matter in SketchUp to figure those lengths out; I expect you'll be able to do it in SW as well. The difference divided by the saw kerf width will tell you how many kerfs you need.

 

[DavidF]

Try to cut the kerfs so they'll just close up when the core is bent. If you can figure out the length of the curves inside and out, this will be easier. It would be a simple matter in SketchUp to figure those lengths out; I expect you'll be able to do it in SW as well. The difference divided by the saw kerf width will tell you how many kerfs you need.

Hi Dave, The curve is so shallow that even one kerf wouldn't close up!! I have found that a kerf pitch of 20mm seems good. Flexible enough without being floppy.

 

[Jeff]

Dado blade for kerf close up without losing structural integrity?