Laying Up a Laminated Door Arch

An extended discussion with pictures goes into depth about various ways to make a laminated wood arch for a doorway. September 5, 2006

I'm picking up on a project from last fall that was put on hold. It is an archway entryway that has a 36" arch. I've decided on alder, and have resawn and planed the laminations - 10 (with a few spares) at 1/8" thick for a total thickness of 1.25".

Two questions:
1) I've always believed that a hard glue such as plastic resin/resorcinol would be the best, but I'm having a very hard time finding it locally, and I wanted to do the glue up tomorrow. Epoxy would be my next choice, but the finished arch will be run through the tablesaw, or planer, or both - depending on how evenly it comes out of the clamps. I'm quite concerned that the epoxy glue lines will tear up my planer blades. The blades are carbide, if that makes a difference. Is this a real concern? Or is there so little of the epoxy at that point that it is not a problem? I've also read a post or two that used yellow PVA, but since this arch is largely not fastened, I'm concerned about creep.

2) Based on advice from last fall, I will be doing the layout in a vacuum bag, bent around a form. The slats will go into the bag, and then the whole thing bent around the form. I saw a picture that someone posted showing only a very few clamps (3 in fact) that were used to pull the thing into shape, and then the vacuum bag does its thing. Do you pull the thing into shape before turning on the vacuum, or after? Or a little of both i.e. suck out some air, bend a little, suck out some more air, bend some more, etc. This is my first vacuum bag lamination, and I don't want to mess it up. By the way, each slat is 1/8" thick by 3.75" wide by 68" long.

Forum Responses
(Architectural Woodworking Forum)
From contributor A:
We use a white PVA exterior glue. I wouldn't do all of the laminations as lumber. We always use bending ply for the majority of the header and use only enough lumber to cover the reveal of the casing. A little spring back isn't a problem since a properly set jamb will be shimmed and nailed solidly. As long as the legs of the jamb are set plumb the arch will be correct. You shouldn't need any clamps to pull the header to the form. Sometimes we will wrap a piece of banding strap or even twine to pull it close and then let the vacuum do the rest. As far as glue affecting blades, I would join one edge on the jointer with the fence set to an area that is never used (all the way in or all the way out) and then saw to size on the table saw.

From contributor B:
We use plastic resin glue. We buy it in 500 lbs barrels, if you want to know how many arches we do. I would not use any bendy wood in an arch. You want the arch to be completely stable on its own. You need to fit the door arch to the jamb arch and if the jamb arch is flexible, youve got a problem. I know that you can shim it later but believe me it can be a real nightmare. We finish our doors in the shop and during the install it's a breeze if the arch is stiff. You can use the epoxy but you should use a filler to help stiffen the epoxy. Without filler you will get springback.

From contributor A:
I build tons of arches as well and never have any problems with the bending ply. The benefits of doing a 4 or 5 piece lamination far outweigh the benefits of the stiffness of a 10 piece lamination. Once you nail one side of casing on the jamb it is pretty rigid. 10 pieces of wood with glue on them sounds like a nightmare to me.

From contributor C:
For another point of view, we never use bending plywood for exterior applications. We use plys of solid wood and assemble them with West System Epoxy. We've never had a problem with sawing to size later and the epoxy will not fail.

From contributor D:
All of our laminations are made of solid wood - 1/8 inch strips to achieve whatever thickness is desired (jamb or arch). We use Titebond 2 blue top. We have made thousands and thousands and thousands of these bent laminations since 1990 with very few problems. We have used poplar, oak, cherry, mahogany, white oak hickory, maple, Brazilian cherry and a few other species - all glued up on custom bending tables. The one thing I read on here all of the time is spring back. We have more of a problem adjusting for arches drawing in across the ID, never springback.

From contributor E:
This is great - always lots of ways and opinions. We do ours with one layer of bending ply and 4 layers of 1/8" solid (three on the face and one on the rear for a nice looking product prior to installation). We use regular Titebond and rarely have a problem. I have one suggestion not yet brought up though. Do your vacuum pressing dry (no glue) a couple of times. There are many things all the folks here could tell you to look out for, but nothing beats a little hands-on experience to figure it out. Plus, after bending a couple of times during dry runs the strips will have a little bend to them already when you do the final glue up.

From the original questioner:
Thanks for all the responses. Sorry, I guess I wasn't clear enough. The attached image is what I'm aiming for. It is in the entry area of a condo, and is intended to provide some visual separation of the foyer from the rest of the condo (it opens into the dining area). Not the best of drawing efforts, but you should get the idea. The arch itself will be then attached to its vertical legs - probably a bridle joint. The glass panels are custom floated and then tempered.

As you can see, it really needs to be all plies of the actual wood. No bendy, unless I was to veneer the resultant edge, which I don't think is the way to go. Again, this is extremely sensitive to creep. The glass panels won't tolerate much compression. Because of the required focus on the glass panels, I need to minimize the amount of wood, thus just the three braces.

To contributor A: 6" jointer, there is no unused area. It was a nightmare indeed - my last bent lamination was 8 plys and 2" wide and not a full semi-circle. I did it all with clamps and was a bit of handful, but manageable. 10 plys at 4" wide and full semi-circle is a little more daunting.

To contributor B: Why thickener? I've used a fair bit of epoxy, both thickened and straight - and only thickened when I was concerned about gap filling. In this case, I'd be a little nervous about thickening the epoxy because of the potential increased thickness of the glue line. By the way, I'm not too worried about a little springback. I've allowed for it by overbending - my arch form is more than a semi-circle, more like 5/8 of a circle. So if there is some springback, I should be ok. If not, I've got room to trim back to where the ends of the arch will be tangential to the legs that will be attached. The reason I'm thinking of a hard glue is because of creep.

To contributor D(and all other PVA officinados): Do you not have problems with creep i.e. the arch relaxing over time? Or are your arches subsequently installed in an environment where they are supported (e.g. by framing) which prevents the creep? Also what about open time? I'm having some difficulty envisioning being able to get all these plys covered with adhesive, stuck in the bag, lined up as best as one can, then vacuum applied and bent around the form before the glue sets up. The long open time of plastic resin or epoxy looks very attractive from this perspective.

To contributor E: Excellent idea, and already in my plan. I usually do a dry run for any glue-up or joinery that I'm not familiar with. In fact, for this project I sliced up a 2 x 4 and went through the motions of aligning and clamping. I will probably glue it up using the vacuum bag to round out the dry run experience, then on to the actual plys.

Click here for full size image

From contributor F:
I read where you said that you need to use all solid wood plys because the entire surface of the laminations will be seen. You were correct in that some folks thought that most of the edge would be hidden by a casing molding. I want to say that I have had some failures when gluing solid wood plys to bendable plywood plys. In the instances where it happened, the overall dimension across the grain was about 8 inches. The thickness of the outer show wood ply was about 3/16". The failure was some delamination between the outer solid wood lamination and the plywood lamination.

As far as glue type, I think you can use anything you like that has an open time you are able to work within. I use regular old aliphatic resin (yellow woodworkers glue) with satisfactory results. No creeping, no excessive spring back.

As to what to do about cleaning off the excess glue after the laminating is finished, naturally you need to make the part oversized across the grain so it can be trimmed to net size after glue up. Make some pencil lines at a couple inches in from the edge of the outer lamination on the concave face. Apply some pieces of double sided tape to that area and then align a 1/8" thick by three inches wide scrap laminate with straight edges on that pencil line and stuck to the tape. Now you can run the glued up curved piece through the saw with the temporary straight edge against the fence and remove the glue while straightening one edge of the curved part. Now remove the temporary straight edge and run the glueless and straight edge of the part against the face while ripping the curved part parallel and removing the glue mess from the second edge.

If you have ripped the part 1/16" over net width you can use the planer to dress both edges by planing 1/32" off of each edge. The small amount of glue you plane will have little effect on the planer knives.

From contributor J:
I agree with both contributor D and contributor F. I don't understand why you need to use a vacuum bag, though. Form and clamps do the trick for us.

From contributor K:
The West system epoxy is easily absorbed by the wood, and if simply applied to the two faces of a joint and then clamped, often results in a starved joint. The normal application process is to apply straight (unthickened) epoxy to the surfaces to wet them out, and then mix a small amount of thickener to the rest of the batch and use that as the adhesive. It's not so thick as to leave a noticeable glue line, just enough to ensure that you get a good bond. It has a much longer open time than Titebond, which might help in your application.

From the original questioner:

To contributor F: Thanks - good tip about setting up a reference edge. That will come in handy.

To contributor J: Im not sure it's entirely necessary to vacuum bag. I've done it before with just clamps. But on occasion the glue line wasn't as tight as it could be in certain places. I believe that's a function not only of the clamps (assuming you have enough on), but the wood. Depending on the grain of the wood, it may not bend into a fair arc (you discover this real quick using battens to loft full size hull shapes). So there can be the odd pucker in the glue line(s). I believe the vacuum bag provides the ultimate in equal pressure over the entire surface, and thus is the best way to assure completely even and powerful clamping pressure. This project is as much a learning experience as anything, and I'm hoping to be able to incorporate the process into my tool bag of techniques.

To contributor K: Thanks for the explanation. In fact, that has always been the way I've used it, but as I said generally where I had some potential gap filling concerns. I figured (perhaps incorrectly) that the totally flat and smooth faces of these laminations wouldn't require any thickening. I'd guess I'd better re-think that and use some thickener.

From the original questioner:
I may have to go to plan B - all clamps. I had a go at it this afternoon, and not at all as slick as I had hoped. A major wrinkle (pun intended) was the bag wrinkling under the layup - badly enough to distort the arch. This is a new bag for me, got it for this project. I got a 30 mil vinyl - I figured what the heck, thicker is better. Obviously I didn't count on the wrinkling factor. I'm thinking thinner would be better for this purpose. This also helped answer one of my original questions - suck first or bend. I tried applying the vacuum first, and then bending, and got the worst of the bag wrinkling. I then released the vacuum, and straightened things out as best I could; then bent it, then applied the vacuum. Better, but still a concern maybe. I'll have another go at it tomorrow.

I also got some skewing, but that may be solvable. It's a pain trying to work through a 30 mil bag, where you can't see exactly what your doing and the work is being moved around by the seams and wrinkles in the bag. On a positive note, the plies were pulled absolutely tightly together - all along the entire arch - just perfect.

The attached picture was sent to me by a forum member last fall in response to my original posting on this project. What an excellent, clean looking setup. That's what I was aiming for.

Click here for full size image

From the original questioner:
And here's my effort -

Click here for full size image

From contributor D:
We do not have any problems with spring back or relaxing over time. As I said in my earlier post, we battle drawing up across the ID. And yes, a vacuum bag would be a nightmare for some of the casings we do. Our system works kind of like a poor mans pro-tec bender, if you have seen one of those - a big table that brackets are set up on. We use 6 tables and some are as long as 14 feet. We sometimes glue as many as 30 blanks a day.

Today we started on the following.
Eighteen 11/16 x 5-3/4 cherry extension jambs
Three 3/4 x 7-1/4 poplar jambs
Two 11/16 x 5-3/4 poplar extension jambs
One 3/4 x 1-3/4 extension jamb
Two 1-1/16 x 3-1/2 pop casings
One 1-1/16 x 5-1/4 mahogany casing

These will easily be finished by Friday, along with a lot of regular bandsawn casings. It takes a lot of C-clamps. A 36'' ID alder jamb can go from order to being wrapped in plastic wrap for shipping in as little as 3 hours.

From contributor E:
To contributor D: If I understand correctly sometimes you get "spring in" versus "spring out" on your jambs. This has happened to us occasionally on jambs and we anticipate it if were gluing up a laminated casing. I don't have any proof as to what causes it, but my suspicion is the moisture in the water soluble glues like regular TB and TB2. That moisture will cause the wood to swell, even along the length, to a small degree. It is that lengthwise swelling that I figure causes the spring in. The more layers involved, the more moisture involved, and subsequently, the more spring in.

From contributor G:

I didn't see where it was mentioned but if you use plastic resin glue you will get little or no spring back.

From contributor D:
To contributor E: I was thinking the same thing but I also cannot prove the cause. At least it is consistent and is easily allowed for.

From the original questioner:
Well, I had to go to plan B - modified. The vacuum bag just wasn't working for me. I think a combination of too thick bag and too tight a radius and too narrow. It might be doable, but after a lot of fussing around and plies squirming around in the bag, I gave up on the idea.

So I went with clamps, using a 3.5" wide piece of 1/8" thick flat bar as a caul. This worked beautifully. The steel strap evened out the clamping pressure very well. I used epoxy, so had the working time. The final word won't be spoken until I pop the clamps and see how it looks, but I have to go out of town for several days and it will have to wait until I return. But as I said, just looking at it the form it looks very good. I posted the above to help anyone who hasn't done this yet, and also acknowledge the help of all of you who posted suggestions.

From the original questioner:
Out of the clamps and looking as good as I would have hoped for. I'm glad I went with the steel strap. Contributor Fs tip for getting the first edge ripped true worked great.

From contributor B:
I would like to add a few photos from today. As you see, we just lay out the curve on a piece of plywood and attach the form to the line of the curve. We have been using the same blocks for a long time and they are cheap to build from scraps. Glue up (plastic resin glue) the veneers and stack um into one pile. We wax the plywood and run a little plastic wrap around the form as release agent. Start at the top of the arch and work left and right of center. This takes maybe 10 minutes. Take a brush and clean up any squeeze out and it's done. This was completed this morning from 9 to 12:30 - that includes setting up the form, resawing and planing the mahogany and gluing up the strips.

Click here for full size image

From contributor B:
Apply glue -

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From contributor B:
Wax and plastic -

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From contributor B:
Cleaned up and ready for lunch -

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From contributor H:
A bent laminate recurve bow was a junior high shop class project when I was teaching. A lot of the issues are the same. We used a form that was the shape needed on one side and then one that was a bit over sized on the other. We put the glued-up laminates between the forms along with a fire-hose that was sealed at both ends (one end had an air valve threaded into it like one would find on a pneumatic tire). Once all was put together the hose was pumped up until we saw a good ooze of glue come out from the edges. This was very effective and very quick to clamp up and as I recall, very little in the way of spring-back.

I need to build a bent wood table skirt and plan to use this method with a few modifications. I plan to use a cut form on the inside with a metal band to even out arch. I then plan to use a metal band on the outside of the arc with a hose between the outside laminate and the outside metal band. I will use a few clamps to hold things in the approximate position while I inflate the hose.

For the recurve bow we would put masking tape on the outside face of the last laminate and layout the cut profile while the piece was flat. I think this would work for laying out a straight edge as well. For a narrow project, say up to 3 inches, I think a single 2-1/2" hose would work just fine. For those that are doing a wider project I could see using more than one hose and working from the middle to the outside in applying pressure to cause the glue to work out to the edge and reduce the possibility of voids in the laminate. I think that this method might be a bit faster in clamp time because the air compressor does the work for you and can deliver a large amount of air quickly. It also provides an even amount of pressure throughout the arch.

To contributor B: Do you get any movement on the ends of your form pieces? From what I see, I would think that you would get some bulging out a bit at the outer (unsupported edge of the form) from the spring-back of the wood. I am also surprised that there is no talk of steaming the wood and bending it as a whole. What are your experiences with steaming and would it have an application here?

From contributor I:
To contributor H: I've not done bent lamination before, and that sounds like an interesting way to add a new skill. I'll have to look for a plan. Not being an archer, I have no idea of the proper dimensions or materials.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor M:
We make a form similar to contributor B. Instead of wrapping the form with plastic wrap we stack the glued laminations with the plastic wrap. This is cleaner and the wrap applies some even pressure everywhere on the glue-up. It does take longer to dry however. We adjust the thickness of the laminations according to the severity of the bend rather than being wedded to the 1/8" size.