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Question about furniture back attachment

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Forum topic by TuffO posted 02-07-2015 10:40 PM 725 views 0 times favorited 11 replies Add to Favorites Watch
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TuffO

15 posts in 671 days


02-07-2015 10:40 PM

Hello Im working on a design of a work desk and I am wondering what suggestions you guys have for attaching the back panel to the piece. It could potentially be viewed from the back so it has to look as good as the rest of the project. All of the joints in the piece will use traditional joinery so I don’t want to just finish nail the back on. I was thinking about dadoing the back panels in but I don’t know if I really want to. Maybe do a raised panel design if I dado them in? Is there another way that looks really presentable?


11 replies so far

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firefighterontheside

13448 posts in 1317 days


#1 posted 02-08-2015 12:12 AM

Do you want to be able to disassemble? If not then either some biscuits and glue or something like this. I glued one panel in and the other has some screws from the back side so I can disassemble if I want. You could glue both in and either clamp til glue sets or use some pin nails on the inside to pin the panels into the dadoes.

-- Bill M. "People change, walnut doesn't" by Gene.

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TuffO

15 posts in 671 days


#2 posted 02-08-2015 12:58 AM

Ok Im seeing that pretty much the only way is to do a dadoed in panel. What additional “wiggle” room is needed for the expansion of the panel since it will be made out of hardwood? They would be no more than 18” wide.

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firefighterontheside

13448 posts in 1317 days


#3 posted 02-08-2015 01:39 AM

Do you mean how tight should the panel be in the dado or should you make it oversized to have room for expansion? It should be sized such that the panel fairly easily slides in.

-- Bill M. "People change, walnut doesn't" by Gene.

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TuffO

15 posts in 671 days


#4 posted 02-08-2015 03:44 AM

Yes how oversized the dadoes should be to account humidity expansion (I know its highly subjective to its environment). Is there a general “rule of thumb” for this or will making it easy to slide in take care of it?

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firefighterontheside

13448 posts in 1317 days


#5 posted 02-08-2015 04:20 AM

It just needs to fit in with some glue if you want it permanent. You don’t have to worry about wood movement, because you are putting hardwood into hardwood and they will move the same.

-- Bill M. "People change, walnut doesn't" by Gene.

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LeTurbo

217 posts in 1046 days


#6 posted 02-08-2015 05:49 AM

I can’t quite agree with firefighterontheside’s last comment – if you’re using hardwood, there has to be room for expansion where grain meets perpendicularly. The grain on the back would probably be vertical so needs to be able to slide top-and-bottom in its dado, with expansion space to the left and right. I don’t know that there is a rule of thumb; it’s usually guesswork depending on the weather when you’re building, the type of wood, and the size of the panel. Make the expansion dado a bit deeper than you think you need. The panel should only be glued, top and bottom, right in the middle.

Another suggestion would perhaps be this: give the back a kind of face frame, then come in from the drawer side or underneath and attach your panel with four screws. The holes in the panel need to allow for the expansion and contraction, so you can either route small slots, or simply wiggle your drill from side to side so the hole is slightly elongated. The closer the holes are to the centre, the less the elongation needs to be.

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TuffO

15 posts in 671 days


#7 posted 02-08-2015 06:10 AM

Oh interesting I had not thought of doing frame like that and assembling in that way. I will have to play around with my design to see if that is possible. As for the raised panels I was planning on making them out of walnut, and I’m in an Arid Grasslands environment currently but possibly moving to a humid forest environment. Im sure I’m safe to assume that I will need to account for a decent amount of humidity shift.

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firefighterontheside

13448 posts in 1317 days


#8 posted 02-08-2015 01:51 PM

I was assuming both pieces would be vertical, a vertical piece with a dado and a vertical piece like a stile of a panel. We would need to know what your final design is to say what to do for sure.

-- Bill M. "People change, walnut doesn't" by Gene.

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bobro

308 posts in 771 days


#9 posted 02-09-2015 09:51 PM

The usual rule of thumb is 1/4” per 12” wide for flatsawn, half that for quartersawn. That’s as far as I’ve ever heard and in keeping with observation. BUT that obviously assumes finished wood with the end grain covered: an open piece of unfinished 12” wide white oak can move almost a half inch over the seasons, even without extreme environmental changes. And it adds up when boards are glued together, basically identical to a single wide board. And don’t forget tree roots cracking concrete: hydraulic power is immense.

In the case of finshed walnut with the end grain covered, the rule of thumb should be just about right on, but since you’re moving to a different climate, in my opinion you should play it safe and give the 18” panel a 1/4 a side to move and don’t forget to have the dado deep enough to account for the inevtiable shrinkage 100 years later. If anyone think this sounds extreme, then they haven’t seen enough antiques or furniture moved from SE Asia to the US Southwest.

-- Lao Ma: You are so full of anger and hatred. Xena: Everybody's gotta be full of something.

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TuffO

15 posts in 671 days


#10 posted 02-10-2015 03:04 AM

Ok thank you, that is what I was thinking. I will make sure that which ever way I do it to make sure I cover the end grain. I will also check to see if I can get quartersawn from my local sawyer. Any other words of advice or lessons learned?

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bobro

308 posts in 771 days


#11 posted 02-10-2015 06:43 AM

Sure- the dimensions you need to compensate for wood movement are not as dramatic as the numbers might suggest. When you say a 12” board moves a quarter of an inch, unless you’re using wood at its driest in the driest part of the year, it’s not like it’s going to go from 12” to 12 1/4”, or air dried wood in the muggiest part of the year doing the reverse. In practice the wood is moving about a medium point of moisture content, which in temperate zones is right about where your kiln-dried wood should be at.

In other words you’re compensating for a board varying between 11 + 7/8s and 12 + 1/8 maximum. That means in typical circumstances, when you put the panel in there’s just a 1/16” space on either side of the board in the dado. That’s why firefighterontheside’s panels don’t explode: with a loose fit, he’s probably got 1/8” of actual total movement allowed, and indoors in his climate might never even have a problem.

In your case, compensating for a full 1/2” altogether on your panel works out to 1/8” space on either side of the panel, and as you see that’s no big deal.

With an unheated shop and air-dried wood, I get to see first hand some big changes when the stuff goes indoors, and have learned to really appreciate how much wood can move, and old-school “over-engineering”. Mostly people can use a moderate rule of thumb and be fine.

-- Lao Ma: You are so full of anger and hatred. Xena: Everybody's gotta be full of something.

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