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Expansion and Contraction..Am I thinking correctly

7K views 63 replies 32 participants last post by  jerryminer 
#1 · (Edited by Moderator)
I am doing 3 board Walnut counter tops for my daughters kitchen.. ( interior decorator and her call) . I have to do a 90 corner, mitered. I want to glue the seam solid, I say it will be ok.. Your thoughts … She wants this seam tight and sealed. tops are 25 " wide ( 3 boards). I feel confident, but not 100% sure.
 
#6 · (Edited by Moderator)
As you know they will move across the grain how much probably will be determined by the width of the mitered
part of the countertop. it seems like I've seen you miter table tops with mitered edges 4 or 5" without concerns ? I guess another issue is if your using 1/4 sawn material or not ?
 
#7 ·
Just my thinking!
As they expand they will hit in the front so the pressure would want to open the miter in the back.
I don't know if they will expand enough to do that though. Controlled environment and acclimated wood, it may be OK.

Think to yourself, what would you tell someone if they were asking you?
Pretty much, that's probably the right answer.
Myself, if it were for me, I would probably do it.
For a client, might not want the liabilty to HAVE to re-do it, and then how do you re-do it any better?
 
#9 ·
jbay,

I totally agree, I have never done this, I do furniture, but to glue it seems logical, but then again,
Im going to domino it, glue it solid , and see what happens, if it has issues, Ill certainly hear about it and would make a new top..and so will you folks, one of those , live and learn things.
The wood is well acclimated and dry, I will finish it top/ bottom and seal it well. Time will tell,
 
#10 ·
If you putting you miter were two counters meet at a miter I would guess the will move in a similar manner but still would have concerns about it affecting the miter .would crystal allow a board at the miter were you could do a stop dovetail allowing both sides of the "L" to move at whatever each side wants to do?
 
#12 ·
yes, they're going to move since they are solid wood.. no doubt.. I think it all depends on the moisture content of the boards and if some are as dry as the others.. if it were me, I woukd miter them, route for a spline to go the width of the miter, but stopping anot inch or so from the edge.. and use those "T" bolts to draw the joint tight and use no glue.. the spline and T bolts should keep the seam tight and with no glue, allow for movement..
 
#14 ·
The critical thing here, IMHO is that with wood movement, the miter angle will change-so the attachment system needs to allow at least one leg to "swing" to accommodate.

Countertop bolts will keep the miter tight, but movement still needs to be accommodated.
 
#15 ·
. My thought Charles would be to use two 1/8 splines thru the mitres or 1 at 1/4" width using a slot cutter in a router. I would fit and glue the mitre at the shop if I could using a template from the wall to fit and also be sure to seal all ends, edges and bottom to cut down on taking on moisture.
 
#16 · (Edited by Moderator)
I've certainly never done this, but I think it may be a problem. As either piece expands along its considerable width the portion at the miter is bearing against the end grain of the other piece. I don't see how it can move freely without opening up the joint at the rear. On the other hand, if the pieces are contracting they would pull away from each other and just open up the entire joint unless the entire top is allowed to float freely.

Edit:
On second thought, I think that it might work as long as the entire top is allowed to move. We normally don't worry about expansion with the grain, but here the cross grain expansion of one piece will be transferred along the length of the other. Allowing the top to move lengthwise and not just across the grain might be critical.

I think I need to go build a mock up and dump some water on it.
 
#17 ·
Boards do not expand and contract equally in width and length (very little in length). Therefore, any angle other than a 90-degree crosscut will not stay the same angle, as the wood expands and contracts. If you cut a board at 45-degrees and it then expands, the angle will become more than 45-degress (move closer towards a 90-degree cut). If it shrinks, the angle will become shallower.

Assuming the back edge of the top is secured, this will force the countertop pieces to remain in a 90-degree angle, but as the wood expands, the joint will tend to open up on the outside corner of the joint. If the joint were not held down at the front, then the front could actually lift up and allow the corner to stay tight. I believe this is exactly the type of thing that happens on picture frames or other applications that don't force the wood to stay in the same plane.

A 25" wide miter is huge, I can't see how this could possibly work. It's not practical to let the whole thing float as the counter could then pull away from the wall or back splash. A 1% expansion would be 1/4" and the front would try to open up to almost 3/8". And I think any attempt to force the joint to stay, will put a lot of stress on the wood. Either causing it to distort, or more likely split. Somethings got to give.

I'd look at creating some type of joint that can move. Or go with a veneer over a stable substrate or at least one that moves the same in both directions.
 
#18 ·
What is the short length of the "L"? How thick is the back splash? Given zero constrictions the same wood will expand at the same rate through the miter. If you could secure it at the front edge of the cabinet and allow a maximum gap at the back to be covered by the back splash, there is no reason it shouldn't work. The difficult thing would then be effectively sealing between the top of the counter and the bottom of the back splash with a dynamic seal that wouldn't be torn apart (I.e. silicone caulk) throughout several expansion/contraction cycles.
 
#20 · (Edited by Moderator)
Thanks for the great explanation Clin.

Imagine cutting a square piece of stock across from corner to corner. That makes an equilateral triangle with two equal length sides and 45 degree angles in the corners. Now expand (or contract) along one of the sides and you no longer have an equilateral triangle and the 45's are gone. That is just what would happen in this case. The joint will fail.
 
#21 ·
Clearly explained in post #6 ;)

As far as clins explanation:
"If you cut a board at 45-degrees and it then expands, the angle will become more than 45-degress (move closer towards a 90-degree cut). If it shrinks, the angle will become shallower."

This is not true unless one side of the board is captured. Un-captured (theoretically) the board will expand equally and the angle will remain the same. (theoretically)
 
#22 ·
Well Charles if it were mine, this is what I would do:
I am more concerned about 3 boards in 25 inches cupping.
I would bisket or domino the miter to prevent the cupping.
I would also glue the joint.
Thirdly I would use "I bolt fasteners from the underside, like you would if it were a formed countertop"
Please keep us posted how this turns out…....
Best of luck !
 
#23 ·
Clearly explained in post #6 ;)

As far as clins explanation:
"If you cut a board at 45-degrees and it then expands, the angle will become more than 45-degress (move closer towards a 90-degree cut). If it shrinks, the angle will become shallower."

This is not true unless one side of the board is captured. Un-captured (theoretically) the board will expand equally and the angle will remain the same. (theoretically)

- jbay
Wood is not a homogenous material. Something like Corian would expand and contract equally in every dimension. Wood is going to move differently along X, Y, and Z axis. Thus the shape of a board with a miter will change over time.
 
#26 ·
Clearly explained in post #6 ;)

As far as clins explanation:
"If you cut a board at 45-degrees and it then expands, the angle will become more than 45-degress (move closer towards a 90-degree cut). If it shrinks, the angle will become shallower."

This is not true unless one side of the board is captured. Un-captured (theoretically) the board will expand equally and the angle will remain the same. (theoretically)

- jbay
As William said, this is only true of something that expands and contracts the same in both directions. Solid wood boards definitely do not expands and contract the same. Most boards move insignificant amounts in length and quite large amounts across the grain. Wood products like MDF or plywood move little, and also tend to move the same in all directions since there is no dominate grain direction.

You are correct that if the material moves in the same proportions in both directions, the angle will stay the same. But, solid wood boards do not move like that. I think it's common for miters to open up due to this, even in relatively small board like door trim.
 
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