I am in the process of attaching a curved back rest to the hind legs of a few chairs. The joint would most likely qualify as a half lap joint. Due to the curve of the back rest, I had to hand cut the curve on the back two legs. This joint has been difficult. It looks good when together, but I know that the faces don’t mate perfectly inside.
My question is as folows: will yellow glue work in this situation? I assume that it is not the best option, so what is? There will be two screws holding the back to each leg (4 total.) The long grain mating surfaces are approximately 1 1/2″ x 3 1/2″. There are no huge gaps on the inside. The largest deviation is probably 1/32″ with the average being much smaller than that.
Replies
Epoxy is the best choice for joints that have been prepared in a less than ideal fashion. Add some silica powderer to the adhesive, and it is capable of filling fairly large gaps.
Good Luck,
Todd
Matt,
Given your criteria, I see no problem in using yellow glue and screws.
-Paul
Since there is little stress on this joint, simply use the screws. If the future shows this to be insufficient, then add glue.
Matt,
A 32d is a lot of gap for yellow glue to fill, with any strength. And if I understand your description, a back rest will have a good deal of stress on it, leveraging off the leg joints. Chairs take a lot of strain, bearing live weight. I agree with Antzy that epoxy will be your best bet. Wood dust is also a good thickener for the glue, so it won't just run out of the gap before it hardens. Mix to the consistancy of peanut butter, spread on the joint, and assemble.
Good luck,
Ray Pine
Ray is right, epoxy is the only choice with a prayer of working. Frankly I don't think the screws add much, except to create stresses and thw wood shrinks and swells with the seasons.
I am a bit curious at how this construction came about. Traditionally chair legs are cut from one piece, aligned so that the critical junction where the seat attaches to the back legs has the grain running perfectly in line with the leg.
In addition, joinery on chairs and other curved parts is best accomplished before the curves are carved or sawn, or at least while there are good, square surfaces for reference. Makes getting good fitting joints a lot easier.
A chair back can get a lot of bending stress if someone leans back while sitting. Screws mean holes in the wood, holes mean stress concentrations, and stress concentrations result in cracks.
Use a decent grade epoxy well mixed and thickened with sanding dust from the wood which you are using. The dried glue will be slightly darker than the surrounding wood after finishing with a clear finish.
Matt,
Epoxy, no screws. Aside from appearance, they'll weaken the back leg.
Cheers.
eddie
Thanks for the replies. It sounds like the general consensus is Epoxy. Unfortunately, I had committed to the screws with holes. Maybe I will just plug the empty holes.
If I understand what you are doing correctly.....I think you are attaching a back (maybe 6 or 8 inches wide by 15 inches long?) to the top ends of chair legs that have already been fastened at the seat. If so, I have used screws on this type of chair back for at least four different sets of chairs and have never had a problem. I think the use of epoxy here is nuts.
The dimensions are different, but that's exactly what I'm trying to do. You probably explained it better than I did. Did you only use screws? The two legs will support any weight from leaning back. Due to the half lap, the chair back's weight is supported by the legs too. The only time this joint should be stressed is when the chair is picked up to be moved.
I believe that the screws alone will be just fine. If I were doing this, that's what I'd do. Although without seeing the piece or even a photo, I'm doing some guessing on your behalf. I doubt it is necessary, but to make a better seat for the half-lap you could do a fill with thickened epoxy. Wrap the leg with thin plastic like Saran food wrap. Put the thickend epoxy into the chair back lap the put the two together.... no screw yet just lightly clamp or even use masking tape to bind it together. When the epoxy cures you will have a precise joint. As I said in my early post, the use of screws here will not harm anything, you can always glue the joint later. I really doubt the chair back will crack due to stress from the screws.
Thanks. For my situation, your last point is probably the best (since I already have the holes drilled). I will start with screws and make any necessary changes in the future.
Matt, I use a lot of epoxy, and that was my first thought, but there is another glue that bridges good, is easy to use, and is tough when it is sets, and has enough elasticity to not break from the opposing grain directions. Liquid nails is a good adhesive that is used a lot in construction, but overlooked a lot in the shop.
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