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Wood expansion question - AlanS - 11-09-2020
[attachment=30919]So, Building a small jewelry box 6"x3". Sides are curly maple, 1/4" thick. Top is Padauk..1/4" thick, set into a dado in the sides of the box. Bottom is veneered...no issue there. The top panel...I want to make a rabbet around the edges, so that the top will end up flush with side edges. I would love to have the panel fit tight to the sides, with no space around the rabbet. I fear that, given it's low humidity now, that in the next summer, the panel will expand, and without any wiggle room, it will explode. Should I really be concerned about wood movement in a panel that is 1/4" thick? Here is a quick pic...it's not glued, and you may be able to see the rabbet along the edge of the top...When glued tight, there would be no space around the rabbet. You would only view the 'raised' panel portion...
RE: Wood expansion question - AlanS - 11-09-2020
[attachment=30920]Just to clarify.... I don't want any spaces between the top and the sides. Am I asking for disaster?
RE: Wood expansion question - jteneyck - 11-09-2020
(11-09-2020, 02:07 PM)AlanS Wrote: Just to clarify.... I don't want any spaces between the top and the sides. Am I asking for disaster?
Yes. You need to allow for probably 1/16" all around, especially on the short ends as that's the direction the panel will expand/contract. You must live somewhere opposite of the weather where I live. In the Summer in the northeast the RH is high; in the Winter it's low. That's why people often say build tight in the Summer and loose in the Winter. In any case, you need to allow for the expected expansion.
John
RE: Wood expansion question - AlanS - 11-09-2020
How is it that some builders will have a tight panel...no space ? Or glue a solid panel on top of a box frame. There is no place for movement when you do that...glued with and cross grain. And, I have seen inlays that run cross grain. How do you accommodate for expansion with that process? Questions, questions, questions....
RE: Wood expansion question - Alan S - 11-09-2020
(11-09-2020, 05:51 PM)AlanS Wrote: How is it that some builders will have a tight panel...no space ? Or glue a solid panel on top of a box frame. There is no place for movement when you do that...glued with and cross grain. And, I have seen inlays that run cross grain. How do you accommodate for expansion with that process? Questions, questions, questions....
You can get away without leaving space along the grain of the panel, but not across the grain.
The reason you can sometimes get away with it is the difference between how much it might expand and contract if it were not constrained, and how well it's constrained. The width of the panel, type of wood, whether it's quartersawn or flatsawn, and humidity change all determine how much the wood wants to move.
The wood thickness matters to how hard it pushes. If it's thin, the force may not be enough to break a glue bond, so plywood exists. The smaller the glue surface, the less force it can resist. It's unlikely gluing a little bit of a panel edge will resist expansion and contraction if the panel wants to move a lot. Running your panel's grain lengthwise would give it more chance.
RE: Wood expansion question - oakey - 11-09-2020
use plywood
or veneer particle board or similar sub strait
or take your chance on building it tight
and having it open up at the corner miters when it expands
RE: Wood expansion question - ianab - 11-10-2020
Quote:The width of the panel, type of wood, whether it's quartersawn or flatsawn, and humidity change all determine how much the wood wants to move.
This ^
The individual numbers in that equation can change by 2 or 3X their value. So you can't make a hard and fast rule. So it's hard to say if 4" or 6" will be "too much".
If you want to be sure, veneer the fancy wood onto a ply backing which wont move enough to matter. Ply is basically a stack of thin veneers glued together. Each thin layer cant' exert enough force on the next long grain layer to cause movement, so the sheet stays pretty much stable. Once you go over ~1/8", it's not a veneer any more, and expansion forces start to matter.
RE: Wood expansion question - rwe2156 - 11-11-2020
I'm building a bunch of boxes for Christmas presents.
They will all be veneered for just the reason you're asking.
So, I would either buy or make veneer out of this, and you'll be ok.
RE: Wood expansion question - Don_M - 11-11-2020
(11-09-2020, 01:43 PM)AlanS Wrote: So, Building a small jewelry box 6"x3". Sides are curly maple, 1/4" thick. Top is Padauk..1/4" thick, set into a dado in the sides of the box. Bottom is veneered...no issue there. The top panel...I want to make a rabbet around the edges, so that the top will end up flush with side edges. I would love to have the panel fit tight to the sides, with no space around the rabbet. I fear that, given it's low humidity now, that in the next summer, the panel will expand, and without any wiggle room, it will explode. Should I really be concerned about wood movement in a panel that is 1/4" thick? Here is a quick pic...it's not glued, and you may be able to see the rabbet along the edge of the top...When glued tight, there would be no space around the rabbet. You would only view the 'raised' panel portion...
I ‘ve built about a dozen or so boxes and the top lid joint (for a panel construction) is usually as shown below. It doesn’t achieve your goal of a “flush” top – but eliminates the “gap”. About the only other technique I can think of is a plywood top panel with a veneer. That might be stable enough to “frame” the panel w/o any gaps.
[attachment=30956]