Help and advice on warping large panels


deav

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I've been making large art pieces for a local retailers.  These are upwards of 6'x6' and comprised of glued up 4/4 walnut, and routed with a CNC.

 

Boards are joined and glued horizontally and each board is between 6"-11".  Joints are biscuited, glued and Kregg screwed in the back.  End grain is alternated between boards.  Front back and sides are sealed with teak oil, and I'm fixing 4 pieces of C Channel (1 1/2"x3/4"x1/8") steel to the back of each piece, perpendicular to boards.

 

Originally they were hung on the wall with z clips (french cleats) on the top edge.  This resulted in the bottom of the piece coming away from the wall, as the entire piece began to curl.

Recently, I've started including z clips for the bottom as well, to prevent it from coming off the wall at the bottom.  Now the pieces are cracking across the middle.  Since the piece is pinned on the top and bottom, it appears it still wants to move and the only place it can is out from the middle.  And not a crack at a joint, big stress cracks right across the center of a board.

 

How can I glue up a large walnut panel and not have it flex?  Is it possible?  What am I doing wrong?

 

 

 

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I understand what you are saying, but the steel was a recent attempt to remedy the issue.  Before I was using walnut bracing on the backs and the warping was occuring.  That's what led me to the steel approach.  The walnut bracing was just moving with the panel.

 

Is it possible to glue up a large panel like this and not have warp?  Or do you need bracing of some kind?

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2) Front back and sides are sealed with teak oil, and I'm fixing 4 pieces of C Channel (1 1/2"x3/4"x1/8") steel to the back of each piece, perpendicular to boards.

Yes you need two pieces of C channel, one at the top and one at the bottom.  You need to elongate the holes in the C Channel so that the boards can still move a bit across the grain.  The C Channel will help keep it flat, but the wood still needs to move across the grain or it will crack.

 

I'm using a 3/8" drilled hole in the steel with a #10 screw (.19" diameter).  So the hole is about twice as large as the screw.  Maybe not enough room?

 

 

 

It's kiln dried walnut.  How do I acclimate it before gluing it?  When these pieces leave my shop they are straight and flat.  When they get on the wall they start to move.  It's a good point about the wall not allowing for air flow.  The wood is set off the wall 3/4" by the steel.  So there is air flow, just not as much as the front.

Also, these pieces are in coffee shops, so I don't think all the steam of brewing helps the situation much.

 

 

 

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One thing that can help is to apply a film finish to seal the wood. This should help with expansion and contraction and stabilize the piece. Think of it as putting a wet sponge in a zip lock bag. 

 

Does the piece get direct sun light? Is it near a heat source?

 

-Ace-

 

Also your obviously carving the wood with a design removing stock. Wood is thicker and thinner so you may be releasing some stress that is built up in the wood causing it to move with moisture at different rates than the thicker wood.  

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A 6x6 is a big panel. You need to acclimate the boards to the coffee shop environment. I assume these are in heated/air conditioned buildings is your shop the same? 

Personally I would not glue the boards together for a wall hanging of that size. I would look for a method of hanging so they can float with a tongue and groove not glued together and make all the boards narrower none over 5"

You will always have issues being against the wall with a solid glued up panel no different than leaving it laying on the floor.

 

 

 

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I really appreciate all the insight.  This is driving me crazy and I have a lot of these hanging in stores.

 

 

Believe it or not that panel could expand and contract by 2 inches or across its width, depending on local climate and humidity. Here in chicago indoor relative humidity will range from below 20% to above 50%. I'd make each slot at least 2" wide.

 

 

Most of these are in Chicago and DC, which are places with large swings in humidity, like you said.

 

The boards are oriented horizontally on the wall, left to right.  I have the steel perpendicular to the boards, up/down.  I don't know how to allow for that much flex with a 5/8" long screw holding the steel to the wood.  It wants to push it's middle out if the top and bottom are attached with z clips.

 

Here is a photo of the stress crack

unnamed.jpg

 

They're not just routed, but filled with resin as well.

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That is part of your problem your binding the panels with resin. Wrong wood for the job. Use veneered mdf and you wont have that problem.

 

Cant use veneered ply or mdf for this process.  Requires heavy sanding to remove excess resin, which will remove any veneer.

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You have vertical steel supports on the back....attach at the top as before....on the bottom, cut a long (like 3") slot and then position your screw in the middle of the slot...there will plenty of room for it to expand or contract as the screw moves up and down within the slot.

 

|    steel     |

|     ____     |

|    /    \    |

|    |slot|    |

|    |____|    |

|   /      \   |

|  | screw  |  |

|   \______/   |

|    |    |    |

|    |slot|    |

|    \____/    |

|              |

|              |

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Take a pic of the front and back. Let's see the whole piece in all its glory. Specifically how it's mounted.  

 

-Ace-

 

Here is the back with C channel,  before fastening

 

Full_Size_Render_1.jpg

 

 

And the backside of a different piece with t bar instead of c channel.  You can see the top hanging clips.  This arrangement mades the bottom of the piece pull away from the wall, due to the top being fixed.

 

IMG_2513.jpg

 

 

This one show the backside with top and bottom clips.  This makes the piece push out from the center and crack.

 

Full_Size_Render_2.jpg

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The back tells the story. Everyone has been telling you to elongate the holes that are away from center. This requires turning the holes in your C channel into slots. You have to do the same thing in the wood cleats that run across the grain (first pic.) This may not cure all checking as the other factors come into play that have been mentioned. But, this cracked in two because it was bound by screws and not allowed to move the way the grain must with seasonal temperature and humidity swings. The figure about two inches of movement means you need slots at least one inch long at the top and bottom.

Quick and dirty...look at how the peg holes in his tenons are tight in the middle but slotted to either side of center. He is chiseling a drilled hole into a slot in this super short clip.

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You have vertical steel supports on the back....attach at the top as before....on the bottom, cut a long (like 3") slot and then position your screw in the middle of the slot...there will plenty of room for it to expand or contract as the screw moves up and down within the slot.

 

|    steel     |

|     ____     |

|    /    \    |

|    |slot|    |

|    |____|    |

|   /      \   |

|  | screw  |  |

|   \______/   |

|    |    |    |

|    |slot|    |

|    \____/    |

|              |

|              |

 

I understand the concept.  Thank you for the detail.  I'm just not sure it will help when the piece wants to come several inches off the wall, either in the center if I secure the top and bottom to the wall, or at the bottom if I only secure the top.  It wants to move a lot!

 

I'm wondering if I z clip or french cleat the middle as well as the top and bottom to the wall if that will help.  Then the forces will move to the space between the middle and top and middle and bottom. But these spaces have less mass to move (only about 3 feet)

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Again, thank you all for the thoughts and advice. I'm taking it all in and I think you've all made great suggestions.

Most of what I understand is these things are going to move, and it's a matter of allowing them to move and not crack.

I have another, more poinent question:

How can I make them so they can't move?

I don't think my client will want a situation where they may bow 2" off the wall, in the middle. If I stay with solid stock, can I keep them completely stable?

You've seen what they're capable of, and the forces at play. Would a perpendicular tenion board on the perpendicular borders stop movement?

What about back bracing the entire piece with ply?

What about increasing steel two or three fold and not allowing any play between materials? Essentially giving it no room to move by reinforcing with a powerful amount of steel.

Essentially, if wood movement is not an option, what are my options, or is that a question that missed the point of the nature of wood?

Someone suggested using only engineered stock and masking. I want to table that at the moment because I've tried it before and ive found it to be harder than expected.

I've thought about making my panels with walnut backer board, or thin walnut about 3/8", and gluing that to ply. But perhaps even that will pull.

Thank you all for your patience. I'm trying to figure this out. This is a big client for me and a big problem I'm facing.

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I think Carus gave you the best tip...breadboards.  If I were tasked with this project, I would make the whole panel the way you would make a big, thin table top...with breadboard ends.  That way the panel can move freely but it will be restricted from bowing.  I would mount the whole thing with hardware through the breadboards, maybe three fasteners on each side, being careful that the fasteners don't impede movement of the panel.  And I would use spacers to keep it maybe a half inch or so off the wall so that air could circulate evenly on both sides.

 

If I had any sketchup skills I'd illustrate...but I don't.  Hopefully my words make sense on their own.

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The advice above is all good. You MUST allow the panel to expand and contract across the width of the grain. Trying to prevent that causes it to bow out from the wall and crack.

Let me offer one more option for future pieces. Assuming face grain patterns are not a required part of the piece, try making your own 'engineered' material. Take those wide boards, and rip them into pieces about 30-50% wider than they are thick. Then rotate them onto their edges, and glue the faces together, like a giant edge-grain cutting board. This effectively turns the wood from 'flat sawn' to 'quarter sawn', orienting the grain so as to vastly reduce the movement across the panel.

And it isn't MDF.

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Why are you corresponding movement with bowing? With proper braces or breadboards or whatever, you should be able to hold it perfectly flat...but it will change dimensions. It may be 72" tall today and 73.5" in a few months and 71" a few months after that...but that whole time it would stay flat...movement doesn't mean it has to bow.

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I really appreciate all the insight.  This is driving me crazy and I have a lot of these hanging in stores.

 

 

 

 

Most of these are in Chicago and DC, which are places with large swings in humidity, like you said.

 

The boards are oriented horizontally on the wall, left to right.  I have the steel perpendicular to the boards, up/down.  I don't know how to allow for that much flex with a 5/8" long screw holding the steel to the wood.  It wants to push it's middle out if the top and bottom are attached with z clips.

 

Here is a photo of the stress crack

unnamed.jpg

 

They're not just routed, but filled with resin as well.

Am I the only one that got bikini clad babes when I clicked to enlarge his pics. Made me forget about his problem completely.

Sorry OP

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Why are you corresponding movement with bowing? With proper braces or breadboards or whatever, you should be able to hold it perfectly flat...but it will change dimensions. It may be 72" tall today and 73.5" in a few months and 71" a few months after that...but that whole time it would stay flat...movement doesn't mean it has to bow.

 

Technically it's cupping, not bowing.  Cupping is caused by uneven movement of opposing faces due to uneven airflow or if it's a piece of wood cut near pith...bowing is more often a result of tension in wood.  If one side of the wood is against the wall and gets no air, but the other side is fully exposed to air, the exposed side will shrink and the wall side will not...cupping.  Same reason we never lay panels flat on our benches overnight but sticker & stack them instead.  Generally, bowing is curvature along the grain, cupping is curvature across it.  Twist is a combination of both.

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