Making bent lamination form.

[Bri in Mtl]

Most of my bent laminations have been done by simply having lots of clamps to a single form.
I'd like to make a form that has 2 halves(male/female?)
The bent laminations required are 3/8" thick, 2" wide, 3 ft long and are S-shaped. (Think back splats for a chair.)
I've already glued up some plywood for the rough form 2-1/2" thick. Can I simply make a S shaped cut in the ply form to split it and that's it?
OR do I somehow have to remove 3/8" in the cutting of the form so that when I clamp the 2 halves together the pressure will be even.
If so how to I remove the 3/8"? What type of router bit do I need? I can't figure out the process.
TIA

[John Bartley]

My initial thought would be that you should cut the thickness of the intended piece out of the form. Visualise the bent lamination replacing the part that you cut out of the plywood. That should provide just the right amount of curve and space betweeen both halves of the form.

cheers

[Rob Jackson]

Although I have yet to do any bending, I have thought on how to create a form such as what you describe. I have come up with something that may work and could easily be adapted to remove a specific amount of material such as 3/8"

First I would cut the final shape into 1/4 ply or MDF to use as a templete. I would then use the template and a guide bushing in my router with a 3/8" spiral bit and route the shape down the center of a piece of ply gradually increasing depth until you cut right through. Repeat the process on as many individual pieces of ply until you have enough to create the thickness of form that you need. Glue them together and if you take your time, you should have the form without any sanding needed.

I'm not sure if i have explained that correctly but hopefully you can understand what I mean.

Might work, might not

[billyt]

I have done bent laminations in white oak for an Arts and Crafts style rocking chair that worked out very well. I made the form out of laminated MDF so that it was a wee bit wider than required. I found the MDF worked better since it can be smoothed down nicely either with a router and template or just sanding. It is important to make the matching side smaller to accomodate the thickness of the lamination otherwise you don't get complete and even pressure. Leave the side of the form opposite the curve flat for good clamp placement..very important. But don't make it to narrow or you loose strength. Line the form with wide cellotape or waxed paper before starting to glue. and get all the calmps lined up and opened to the correct size first..its amazing how fast the glue will start to kick. Start clamping in the centre and go to the ends equally, i.e., one left then one right. If you can get it clamped along the edge of a bench it helps to keep it straight. Be prepared for some sideways slippage as well.

Good luck...Bill

[Chris Fournier]

You must allow for the thickness of the lamination. If you have a router table and a bearing kit, you can simply make one side of the form and change bearings to create the 3/8" gap on the other side of the form. You could free had the two sides on a bandsaw if you had a template of the splat made up...

[Edward G. Burlington]

Bryan: I do a fair bit of bent laminations as I find it more flexible and easier to do than steam bending. I find that usually you need to build a male and female form if you want to have good looking results. It is also important to make your lamination strips from one piece of wood and keep the order the same as it came off the flitch. That way the final piece looks like one piece of wood and the seams are almost invisible.

Make the form out of MDF and Baltic birch (or some kind of ply). I don’t believe MDF alone can stand up to the pressure over time but with some ply built into the mix it makes it much stronger and it will last much longer.

In addition to removing material equal to the thickness of the lamination you should also cut the female into segments. For and S curve pick either piece. The segmentation allows for two things. First a small variation in thickness of the lamination is not as important and second you can start clamping wherever it is easy to start. A solid piece on both sides is not always easy to get started and sometime a lot of strain is introduced if you don’t clamp evenly throughout the process.

Creating a form with a lip underneath also helps to keep the strips registered vertically.

Finally when I make a form like this I find it is relatively easy to make on the band saw. I first make up the sandwich a little thicker than the lamination width. I cut to shape one side first leaving enough room to make the clamp holes. Put some shims under the first piece, and then move the cut off away from the first piece a little less than the thickness of the lamination. I then run a home made washer ( a round disk with a hole for a pencil led that makes a mark equal to the thickness of the lamination) along the original cut so a mark is made on the cutoff at the correct spacing. Then I cut to the mark on the band saw. A little sanding is all that is needed to smooth out the surface. After putting packing tape on the face of the original piece I fasten it down to the lip (a flat piece of plywood larger than the original piece) so enough sticks out to support the segmented part. I then plot out the clamp holes and cut the female into segments to match the clamping holes. The segments should be equally spaced on each side of the clamps.

The question of what glue to use is a difficult one to answer. I like to use a glue that will cure very hard so there will be no creep. I originally used yellow glue but some experts say that is too soft and will creep over time. So far my older stuff hasn’t suffered but it is only 3 or 4 years since I did them. Currently I use a urea-formaldehyde resin (P-PPR) veneer glue </SPAN>that I purchased from Joe Woodworker. It has its issues (toxic to some degree) but is not going to creep. See link below:

http://www.joewoodworker.com/catalog/product_info.php?cPath=40&products_id=186

I don’t like epoxy because it does not like to be squeezed too much and this is hard to manage if you want real tight joints. Your current requirements should allow the use of yellow glue since the slats are trapped in the chair frame and should not have any serious straightening stress on them. However If you do your chair legs in bent laminations I would be sure to get a special glue for that purpose.

The picture below should answer any questions hopefully but ask anything you like.
MusicStandLegForm02.jpg
E.G.

[J.P. Rap]

I was thinking along the same line as Rob but I can see how that could be difficult without having to clean up the face a bit. Not impossible but difficult.
Since you already have your form blank glued up, it's a bit late for that but you could start over if you wanted.
While reading the answers, I came up with an idea.
This will require making two templates. First cut and refine a template outlining one side of the form. Use that templat and a 3/8" router bit to cut a second template giving you the two sides needed (male and female) much the same as Rob described.
Now you can use the pair of templates to refine your form.
If you have a spindle sander, set it up like a router with a bushing. How you do that is up to you but it shouldn't be too difficult. Lay out the shape of the form (3/8" wide) and cut it down the center on the bandsaw. Fasten the template halves to the form just back from the edge and sand it to the final shape with the spindle sander and guide bushing. It won't matter if the template isn't positioned perfectly because you will always end up with the same profile.
You could do this to make several identical forms so that you can bend more than one splat and leave them in the forms.

This is just an idea I just came up with so it may or may not work. It won't work if you don't have a spindle sander.
Has anyone tried this?
Is there a downside that Im not thinking of?



Another thought. Any industrial forms I have seen have alignment pins. Make your form longer than you need it and install dowels as alignment pins in the ends. That should help a lot at glue up time because it gives you one less thing to worry about sliding around.

HTH