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Cupping?

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Forum topic by nschafer posted 08-01-2018 01:30 PM 1187 views 0 times favorited 19 replies Add to Favorites Watch
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nschafer

8 posts in 431 days


08-01-2018 01:30 PM

Topic tags/keywords: question joining

I am nearing the end of my first project using dowels and edge gluing. Before I put a finish on the table I wanted to address an issue with the joints. Two of the joints in areas are raised where the boards meet. Oddly enough only occasionally. It has been humid recently and I don’t know if that is causing it or not, but in the evenings you can feel the raised edges. It does not happen to all of the joints or along entire length of the joint. Under the hickory is a 3/4 inch sheet of plywood. I considered running screws along each board from the bottom to try and keep them from raising but didn’t know if that would fix the problem? In the pictures you can see the raised joint. Any ideas would be greatly appreciated!


19 replies so far

View Robert's profile

Robert

3436 posts in 1900 days


#1 posted 08-01-2018 01:46 PM

Couple things,

1) Keep stickers or slats underneath to allow air flow or there will be unequal humidity on the top and bottom.

Note: even if stored on slats, any given board can develop a cup simply because it is not acclimated.

2) When using dowels, there can be some swelling in the areas of the dowel. this is due to the dowel expanding from the moisture in the glue. Better to use biscuits or Dominoes. Keep the biscuit hole offset toward the bottom. With some experience, you will find it just as easy not to use any alignment.

3) When jointing edges for any type of panel glue up, you have to be sure of the angles. You can shoot for a perfect 90° or, a better method is to alternate faces against the fence on the jointer. Any slight error will be offset by the complimentary angle on the next board.

If you’re jointing by hand, fold the faces of two adjacent boards like a bookmatch, clamp and joint as one unit.

The solution in this case is to sand out the discrepancies.

Screwing down onto plywood is not going to solve the issue. Get some air circulating and leave it alone for at least a week. Not a bad idea to clamp it down to a flat bench top, etc.

-- Everything is a prototype thats why its one of a kind!!

View Bill_Steele's profile

Bill_Steele

524 posts in 2151 days


#2 posted 08-01-2018 01:56 PM

So the individual boards—which have been glued together along their length—are cupping, but not consistently. Are the boards attached in any way to the 3/4” plywood? It appears that you have breadboard ends—is that correct? If so, how are they attached?

I don’t think that screws would stop the cupping. I suppose if the cup was facing down (rather than up as it is now) then a screw that pulled the center of the board down might help even things out. I wonder if you were to take a hand plane and lightly plane on the top of the joint if that might knock the “peak” off and level things out a little better?

It’s a nice looking table top—sorry you’re having problems with it.

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Aj2

2321 posts in 2217 days


#3 posted 08-01-2018 02:10 PM

There shouldn’t be any plywood underneath glued or screwed .
There shouldn’t be any wood attached to the ends or the table that’s glued alongside length.
The boards can have dowels for alignment. But remember each dowel takes some long grain away and long grain is stronger than a dowel.
The boards jointed square to the face with gaps so small your hands will close them is idea.
I also like the look of your top. So let’s get it right.::)

-- Aj

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bondogaposis

5453 posts in 2771 days


#4 posted 08-01-2018 02:32 PM

What is the plywood underneath for? Solid wood should not be attached to plywood in any way. Why are we not seeing end grain in the picture? What did you do on the ends?

-- Bondo Gaposis

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Rich

4564 posts in 1009 days


#5 posted 08-01-2018 02:33 PM

I can’t say for sure since I don’t know enough about the environment or how the boards have been handled and treated, but there is a phenomenon called compression shrinkage that affects boards. It happens when the surface gets higher in moisture content (MC) than the core. The cells expand, but their movement is restricted which compresses them. Then when the MC returns to equilibrium, the surface shrinks, pulling it into a cup.

The solution is to force compression shrinkage to occur on the other side.

-- Knowledge is not skill. Knowledge plus ten thousand times is skill. -- Shinichi Suzuki

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nschafer

8 posts in 431 days


#6 posted 08-01-2018 05:58 PM

Thanks for all the feedback. I put the plywood underneath for some durability. It is a conference table and if people were leaning on the table I didn’t want to have any give. I put a “skirt” around the outside of the table to hide the plywood. I used dowels to keep that aligned and glued the skirt to the hardwood and plywood. I did screw the plywood to the hickory all the way around the table about 4 inches in. There was also one or two spots where the edges of the boards did not line up and the spacing was too far for glue to keep them together so I put a few screws in to keep the boards aligned. I had considered sanding the high point down but was afraid when I take the table into the air conditioning it would level out again and cause a dip at the seam.

Here is a picture before I put the skirts on.

View Woodknack's profile

Woodknack

12842 posts in 2799 days


#7 posted 08-01-2018 06:11 PM

Remove the plywood.

And I’m not sure if it’s a trick of the light but there appear to be gaps in your glueline (pics 1 & 2), there shouldn’t be any. I would consider ripping them apart, truing the edge again, and glue them back. If it’s a trick of the light or just my eyes, ignore that part.

-- Rick M, http://thewoodknack.blogspot.com/

View smitdog's profile

smitdog

433 posts in 2525 days


#8 posted 08-01-2018 06:24 PM

If the hickory is screwed to the plywood all the way around then that’s probably why you are getting the cupping. The hickory is trying to expand across the grain and it has nowhere to go because the plywood is trying to hold it still. If you are dead set on having the plywood underneath I only know a couple ways to handle it, maybe others can chime in with a better idea. #1 – you could enlarge the holes where the screws are through the plywood, then use a washer and just barely tighten the screw just so the washer touches the plywood. The idea behind this is when the hickory expands and contracts the screw with washer can slide back and forth in the oversized hole or slot in the plywood. #2 – screw the hickory on straight down the middle of the length of the table only so that the hickory can expand or contract outward from the center. Either way you need to cut the plywood back from the edge of the hickory a bit, maybe quarter or half an inch, otherwise when it contracts it will pull off the skirting which should be attached to the edge of the hickory only.

-- Jarrett - Mount Vernon, Ohio

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bondogaposis

5453 posts in 2771 days


#9 posted 08-01-2018 07:15 PM

“Well this is another fine mess you’ve gotten yourself into”, as Oliver used to say to Stan. Wood moves with changes in humidity, plywood is stable and does not move. That is the source of your problem. You can’t attach solid wood to plywood like that. The planks you are using are far to thin for the size of table you are making. I am afraid there is no fix for this but to use thicker boards for the table top. Dowels are also completely unnecessary, you are better off just edge gluing the top, any misalignment of the dowels can actually prevent closing gaps in the seams.

-- Bondo Gaposis

View OSU55's profile

OSU55

2359 posts in 2409 days


#10 posted 08-02-2018 03:07 PM

Get rid of the dowels and us cauls. How to make them. As others said lose the plywood. You can use vertically oriented members under the table, in addition to the apron, if the top flexes too much.

View Bill_Steele's profile

Bill_Steele

524 posts in 2151 days


#11 posted 08-02-2018 07:30 PM

I agree with the others—the plywood substrate could be problematic because of the different rates of expansion/contraction between it and the Hickory. As Jarrett (smitdog) stated—some sort of sliding screw slot might help you attach the plywood to the Hickory. There are router bits specifically designed to cut these slots.

I also think the skirt is likely causing problems. On the edge—the skirt is solidly glued to the plywood and when the Hickory tries to expand it is stopped by the skirt. The only place it has to expand is within the table—which results in the cup. If it ever contracts—it will likely split. The skirt glue-joint at the end-grain is also a problem. You need some sort of breadboard type of joinery here to allow the Hickory to expand. A reminder that wood expands across it’s grain—not so much along it’s length. If you try to constrain the wood from expanding it will distort (e.g. cup) or split. You’re not going to be able to stop the natural expansion and contraction with glue or fasteners—you have to design to allow for movement.

View nschafer's profile

nschafer

8 posts in 431 days


#12 posted 08-03-2018 05:23 PM

Thanks again for all the info. So as much as it breaks my heart it sounds as if I need to get rid of the skirt and use some sort of breadboard. I have never even heard that term but googled it and found several options. What would be the best and easiest for me to adapt to this table?

View Rich's profile

Rich

4564 posts in 1009 days


#13 posted 08-03-2018 05:59 PM


Thanks again for all the info. So as much as it breaks my heart it sounds as if I need to get rid of the skirt and use some sort of breadboard. I have never even heard that term but googled it and found several options. What would be the best and easiest for me to adapt to this table?

- nschafer

Breadboard ends will do little to strengthen the table top. You’ll get far better results with battens underneath. You can still do the breadboard end if you like that look, but it’s the battens that will keep the top flat.

-- Knowledge is not skill. Knowledge plus ten thousand times is skill. -- Shinichi Suzuki

View OSU55's profile

OSU55

2359 posts in 2409 days


#14 posted 08-04-2018 12:46 PM

BB ends are about style – hide end grain and the way they look – they are not needed or required. Agree that battens underneath are the way to reinforce the top. Allow for wood movement in perpendicular grain situations.

View Robert's profile

Robert

3436 posts in 1900 days


#15 posted 08-04-2018 06:47 PM

If you want it thicker, add another layer of hard wood to the bottom. As long as its the same wood you shouldn’t have an issue.


BB ends are about style – hide end grain and the way they look – they are not needed or required. Agree that battens underneath are the way to reinforce the top. Allow for wood movement in perpendicular grain situations.

- OSU55

I disagree with this somewhat. They definitely can be a structural part of a table or panel, IMO almost a necessity for certain applications. They are for me, anyway :-)

-- Everything is a prototype thats why its one of a kind!!

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