Panel Flatness Question

[JohnM Martin]

I recently glued up a farm table top from 2"x6" Reclaimed Pine. Things went pretty well up to the point of taking the top out of the clamps. During the gluing, the decently flat top became cupped significantly. I alternated the end grain and used cauls along with alternating pipe clamps. So the question I have is - what can cause this cupping. Can it be from too much clamp pressure? How much clamp pressure should I go for? This is my first table top glue up and the clamps were as tight as i could get them.

[Mel Fulks]

Fence might be out of square. Mark one end of all boards next time and joint one face toward fence , adjoining board face out, then face in.....Cancels the small error.

[Lee Schierer]

Cupping can be caused by several factors when making a glued up panel. Too much clamping pressure is possible, but other causes are more likely.

The first step is to plan your glue up. Dry fit your pieces to insure that the grain orientation is what you want and the the edges meet all along the length with no pressure applied. Any visible gaps can potentially create problems down the road even though the clamps my be strong enough to pull the gaps closed. Check your edges to insure they are perpendicular to the face. If there is a slight angle, this will cause your panel to curl when the clamps are removed.

As you place your clamps and start applying clamping pressure, you should do so incrementally. Clamps should be applied to both sides of the panel. Tighten the first clamp just enough to draw the wood together. Do the same with each clamp until all have some pressure applied. Then you can incrementally increase the pressure to all clamps. You need to apply enough pressure so you get the individual boards drawn together and you get glue squeeze out all along the length of each joint. This should be small beads of glue, top and bottom on your panel, not big sagging runs of glue. Clamps should be tight, but not crushing the wood.

Pipe clamps, I use them exclusively, tend to bow as pressure is applied which can lead to problems, that is why you should always place them alternately top and bottom of your panel. Check your panel for flatness with a straight edge as you clamp it up. If it is flat with clamp pressure it should remain flat.

So why does a panel that was clamped up flat cup when the clamps are removed. The most likely cause is moisture changes in the wood. If you removed more material from on face of the wood than the other and the wood was not acclimated to your shop you have potentially created stress in the wood which, may cause it to cup. When working with reclaimed wood it is a good idea to check the moisture levels as you work with it. This is particularly true with thicker pieces.

Once you remove the clamps, do not lay the panel flat on any surface or lean it against a wall over an extended period of time. Allow air to get evenly to both sides.

One more thing... Always make your glue up slightly longer and wider than required for the final dimension. This gives you the ability to cut your panel apart if things go wrong and start over. If things go right it gives you a little bit to trim off so you get perfect final dimensions and clean cuts.

I hope this helps.

[JohnM Martin]

Cupping can be caused by several factors when making a glued up panel. Too much clamping pressure is possible, but other causes are more likely.

The first step is to plan your glue up. Dry fit your pieces to insure that the grain orientation is what you want and the the edges meet all along the length with no pressure applied. Any visible gaps can potentially create problems down the road even though the clamps my be strong enough to pull the gaps closed. Check your edges to insure they are perpendicular to the face. If there is a slight angle, this will cause your panel to curl when the clamps are removed.

As you place your clamps and start applying clamping pressure, you should do so incrementally. Clamps should be applied to both sides of the panel. Tighten the first clamp just enough to draw the wood together. Do the same with each clamp until all have some pressure applied. Then you can incrementally increase the pressure to all clamps. You need to apply enough pressure so you get the individual boards drawn together and you get glue squeeze out all along the length of each joint. This should be small beads of glue, top and bottom on your panel, not big sagging runs of glue. Clamps should be tight, but not crushing the wood.

Pipe clamps, I use them exclusively, tend to bow as pressure is applied which can lead to problems, that is why you should always place them alternately top and bottom of your panel. Check your panel for flatness with a straight edge as you clamp it up. If it is flat with clamp pressure it should remain flat.

So why does a panel that was clamped up flat cup when the clamps are removed. The most likely cause is moisture changes in the wood. If you removed more material from on face of the wood than the other and the wood was not acclimated to your shop you have potentially created stress in the wood which, may cause it to cup. When working with reclaimed wood it is a good idea to check the moisture levels as you work with it. This is particularly true with thicker pieces.

Once you remove the clamps, do not lay the panel flat on any surface or lean it against a wall over an extended period of time. Allow air to get evenly to both sides.

One more thing... Always make your glue up slightly longer and wider than required for the final dimension. This gives you the ability to cut your panel apart if things go wrong and start over. If things go right it gives you a little bit to trim off so you get perfect final dimensions and clean cuts.

I hope this helps.

Thanks, Lee, for that good overview. I pretty much followed your summary, except that I think i over tightened and didn't incrementally tighten. The cupping on this top occurred while the boards were in the clamps... not after they were removed. I clamped it up in the evening and it was flat. When I came back the next morning, there was a bow in the top... a hill along the middle of the top and a valley along the middle of the bottom. It runs consistent throughout the length of the panel.

As a side question... are hardwoods less susceptible to this than softer woods?

[Mel Fulks]

Yes, that is a good overview. But never trust the accuracy of jointer fences when making panels. Cancel the error.

[JohnM Martin]

Yes, that is a good overview. But never trust the accuracy of jointer fences when making panels. Cancel the error.

For edge jointing, I actually used a tracksaw and a #7 jointer plane along with a combination square to joint the edges.

[Mel Fulks]

Then I would cancel the error of the track saw and try to avoid using the plane. YES a track saw has error.

[Martin Wasner]

Crushing wood is the only downside to over tightening clamps. I'd bet money it's cumulative error on the edges not being square. It doesn't take much.

It could also be a temporary thing, with one side having dried out more than that other. Let it rest a few days and it might bounce back.

[Martin Wasner]

Yes, that is a good overview. But never trust the accuracy of jointer fences when making panels. Cancel the error.

The handful of bigger table tops we've built, we use the jointer to get them straight, then run them through the shaper to dress the edges. Pretty low stress operation that way.

[Lee Schierer]

As a side question... are hardwoods less susceptible to this than softer woods?

I've had hardwood panels do the same thing, but soft wood is likely to crush easier. If the panel doesn't flatten on its own in a day or two, wet the concave side with a wet sponge and let it dry so that air gets equally as you can to both sides. It may flatten out if the problem was caused by slight crushing or moisture differential in your wood pieces.

Here's what happens if the edges of your boards are out of square by 0.5 degrees, which would be pretty hard to see with a combination square. If they were out of square by 0.25 degrees, the cup would be ~1/8".
cup.JPG

[JohnM Martin]

I've had hardwood panels do the same thing, but soft wood is likely to crush easier. If the panel doesn't flatten on its own in a day or two, wet the concave side with a wet sponge and let it dry so that air gets equally as you can to both sides. It may flatten out if the problem was caused by slight crushing or moisture differential in your wood pieces.

Here's what happens if the edges of your boards are out of square by 0.5 degrees, which would be pretty hard to see with a combination square. If they were out of square by 0.25 degrees, the cup would be ~1/8".
cup.JPG

Thats what mine looks like… with the “hill” on the top and the “valley” on the underside. Does this mean I didn’t get the edges jointed sufficiently square? Am i correct in interpreting your previous comment that this can happen if there are slight gaps in the seams that the clamps hold together? To be honest, I think there were some very slight gaps in some of the seams.

[Art Mann]

I have been gluing up panels since 1977 and I have never bothered to "cancel" the error by alternating the boards. I have had flatness problems a few times but not due to lack of squared edges. It is certainly possible to buckle a glue-up by over tightening the clamps. If there is any risk to bowing due to the geometry of the assembly, I use cauls. I am living proof that it is possible to set up a table saw and/or a jointer to cut edges that are square reliably.

[Mel Fulks]

Sure it's possible and there are times when a jointer must make a perfectly square cut. I've used some great old jointers and never seen one that holds a perfectly square setting for long. Most want to settle into a stable but slightly off position.
I use sprung joints for all panel glue ups ,so I'm not bothered by making adjustments that will improve quality.

[Tom Bender]

Lee hints at the correction, if it doesn't flatten in a few days, rip it into 3 pieces, flip the middle one and reglue. This should leave you with only a little flattening work to do.

I would add that perfection is not normally needed in a tabletop.

[Prashun Patel]

Can you post a picture? While proper jointing and clamping and grain direction are critical to getting flat panels, sometimes mildly out of flat panels are not noticeable to the naked eye.

You may be able to correct enough with a jointer plane if you are comfortable with hand planes.