Wood movement question of a seat

[AndrewPritchard]

I am building an entry seat for my apartment out of solid cherry. The basic design is attached. I have come to a difficult point however, because I've realised the seat (not shown) might end up as one solid panel, which will have some wood movement issues. As it expands, it runs the risk of blowing out the mortise and tenons at the front of the piece. How can I solve this issue?

I don't want to buy a sheet of cherry plywood - that'll double the cost of the project and frankly I don't have the money for that.

Do I make a frame and panel, which will allow the wood to move?

Do I put a mortise into the legs, to allow the wood to move into those mortises? If so, how deep should they be?

Would it better to remove the arms, and put a solid panel on, like a table top? I rather like the idea of the arms, as it makes it easier to stand up if you have something to push off against.

I'm open to ideas here, so what do you think I should do?

 

NB There are a bunch of features not on this design. I'm probably going to make the top rail curved as I think it'll look nicer. There will be a french cleat running across the middle and top rails at the back to add coat/bag hooks etc. The panels in the frames will have a large ogee cut around them to hold them in the frame. The arms will be shaped to make them rounded, but I don't know how to do that on Sketchup.

[Chestnut]

Attach it like a table top with zclips or figure 8s.

If the top is goign to be constrained between the 2 sides and not sitting on top of something you'l basically attach the sides like 2 breadboard ends. A center mortise that you glue with front and rear mortises that you will make longer and allow the tenon to float back and forth.

If you want the front to stay consistent then glue that tenon and float the middle and rear. I've done this quite a few times in various projects.

The shelf below the top on this guy is attached with a M&T at the front and 3 floating M&T in the middle and back.

*Edit you'l need to make sure there is clearance for the seat to extend out the back/front or both the front and back depending on how you attach it

Edited April 10, 2019 by Chestnut

[AndrewPritchard]

23 minutes ago, Chestnut said:

Attach it like a table top with zclips or figure 8s.

If the top is going to be constrained between the 2 sides and not sitting on top of something you'll basically attach the sides like 2 breadboard ends. A center mortise that you glue with front and rear mortises that you will make longer and allow the tenon to float back and forth.

If you want the front to stay consistent then glue that tenon and float the middle and rear. I've done this quite a few times in various projects.

The shelf below the top on this guy is attached with a M&T at the front and 3 floating M&T in the middle and back.

 

It will be constrained by the legs so a breadboard like solution would probably be best. The front edge would probably be best as a fixed edge, so I can guarantee it'll be comfortable to sit on. This would certainly solve the problem. I hadn't considered doing it like a breadboard. Never done a breadboard before, so this will make for some interest skill building.

Would a 1/4" thick tenon be strong enough to support the weight of a 250lb human? Should I make the tenon 1/2" thick to be sure? The plan was to make it from 3/4" stock. I could even drop the seat by 1/4" to make sure there's plenty of wood above and below the seat.

Would there be any benefit to making it one long tenon? What is the advantage to dividing it into 3 tenons?

(Nice piece BTW)

[Chestnut]

Going a long way in making the seat strong is to have some bracing under neath the back of the seat. Basically it's a rear apron that sits under the seat. The apron stops the sides from pulling away from the seat. If you can accomplish this 1/4" tenons are more than enough. You could up it to 1/2" if it makes you feel better but I'm not sure it's necessary.

You could do as many tenons as you like the 3 was an example because it splits the seat in to regions front middle and rear. you could do 2 tenons in front 2 in the middle 2 in the rear or any combination you'd like.

1 long tenon would be your strongest and best option. It's also going to require a stopped groove/dado on both the front and rear. Just make sure to not glue more than a couple inches near the front or you'll run into wood movement issues. This will also ensure taht the seat stays flat over time. The downside is it makes fitment critical and glue up a bit more tricky. If your seat panel isn't perfectly flat when you cut the tenon the fit could be problematic.

[AndrewPritchard]

You can see I've added some stretchers/aprons under the front and back of the seat to support those edges. I assume I don't glue the seat to the front one? Definitely not to the back one.

I could add a third in the middle, but I'm not sure I need that. The seat will be around 16" deep. I'm going to have find the flattest boards in my collection to limit fitting issues.

The side rails can have a continuous dado, and the stops front and back, can be the legs

[Chestnut]

If you can put a rail under the front and back and connect them on the 2 sides (basically an internal frame) nix the joinery and attach the seat to that frame with figure 8s or z clips. This is essentially how a dining chair with a drop in seat is made.

The benefit to the joinery on the seat is you don't have to do the cross brace in the front or the braces under neath along the sides. The brace in the rear doesn't support the seat as much as it stops the sides from spreading apart.

Hope this doesn't seem like we're goign in circles.

[AndrewPritchard]

1 hour ago, Chestnut said:

If you can put a rail under the front and back and connect them on the 2 sides (basically an internal frame) nix the joinery and attach the seat to that frame with figure 8s or z clips. This is essentially how a dining chair with a drop in seat is made.

The benefit to the joinery on the seat is you don't have to do the cross brace in the front or the braces under neath along the sides. The brace in the rear doesn't support the seat as much as it stops the sides from spreading apart.

Hope this doesn't seem like we're goign in circles.

I had initially mis-interpreted what you were saying about the figure 8s etc - but you have clarified it nicely. I still think a breadboard joint would be the way to go, mostly because it's a new technique for me

[Chet]

23 minutes ago, AndrewPritchard said:

mostly because it's a new technique for me

This is always a good reason for doing something.  

[Ronn W]

Looks like your framing (if mortise and tenon) will be ridgid enouhg to be stable.  Your sde panels (if glued in place) will and a lot fo rigidity and if you also put a panel in the back - you got no worries then the seat does not need to contribute to the stability.

 

[wtnhighlander]

This is an interesting problem, with good solutions already provided.

I believe this is the first "hall tree" style bench I have seen that DIDN'T incorporate the base section as a storage chest, with the seat as a hinged lid. 

[AndrewPritchard]

On 4/10/2019 at 9:56 PM, wtnhighlander said:

This is an interesting problem, with good solutions already provided.

I believe this is the first "hall tree" style bench I have seen that DIDN'T incorporate the base section as a storage chest, with the seat as a hinged lid. 

The area underneath is for storage, but here in Nova Scotia around half to a third of the year we have snow on the ground. The area under the seat will have a removable drip tray. The idea is that I can remove the tray without having to take all the boots and shoes out first. Just slide it out from underneath the bottom stretcher, clean and replace.