Bench top cupping

[Dan Karachio]

Hi All,

A while back I posted about my take on the $250 workbench. I used a 1 3/4" thick maple butcher block top with 4X4 Lowes fir legs with the 2 x 8 fir stretchers connected using threaded rod. It ain't pretty, but it is solid, heavy and stable. It has changed my life for the better and some day I will build something better.

I am 99% sure it was dead flat to start, but now I am seeing a sort of cupping along the entire top - the sides of the top are high, dipping in the middle, the entire length of the top. Maybe 1/32 to 1/16. I have a dehumidifier running 24/7 in the basement shop (set to maintain 55% humidity). I can deal with it, but can I ask a dumb question - why is this happening? Is it a support issue? Is it humidity? Settling? Anything I can do to arrest this? Things I could check? Honestly, I was thinking of flipping it and seeing if it cups back the other way.

The top was from a butcher block mfg here in PA. They sell cast offs (with scratches) at a great deal. My top is 6' by 2'. It is the type made from multiple pieces and strips. I imagine planing this would be fun, given the grain direction going in many directions. Then again, I have been itching to buy a LN or LV jointer! :-)

Thanks in advance.

[Stephen Musial]

Did you finish the top with anything? Is it open on the bottom and if so, did you finish the bottom as well?

If it was only finished on top, it could be absorbing moisture unevenly which could lead to cupping.

If it's easy to flip, I'd do that and see what happens. But then again, when I need something dead flat, I just use my tablesaw top so I may not be the best person to give advice.

[David Keller NC]

Dan - One thing to realize is that a laminated benchtop formed from 2" thick strips glued face-to-face isn't any more stable than a quarter-sawn board of the same width and species. What laminating does for you is allow you to build a quartersawn top out of strips of many smaller trees of a width that would be unobtainable in a solid board.

This, of course, assumes that the strips were carefully selected to have QS grain on thier edges when they were glued up. With factory-produced tops, that's often not the case - they're strips with random grain orientations, and the differential expansion/contraction can do weird things over time.

But, as Stephen mentions, you can substantially reduce the cupping/twisting/bowing tendency of a laminated benchtop by either applying no finish to either side, or a finish to both sides.

[Dan Karachio]

Thanks guys. This is a big "duh" moment for me. The slab came with a sort of glossy finish on one side. I have this on the up and though it is wearing away, it is nowhere near gone. Can I ask one more dumb question (or two) please.

If I sand this finish off today, is there any hope it will straighten out a bit on it's own? Anything I could do to get it back to close to flat? Sand it off then flip perhaps? I'm not looking for machine shop precision here.

The "duh" comes from recent experience. I oiled a counter I made of walnut boards on one side and it cupped within days. I oiled the other side and it straightened out. Why why why didn't I think of this with the butcher block? Duh!!! :-)

[Jim Koepke]

I am not sure about the straightening out on its own. It may depend on how the top is anchored to the base.

What appears to have taken place is the unfinished surface has sucked up some moisture and expanded. It may be possible to strip the topside and give it a little moisture to compensate until the block reaches equilibrium.

Pictures may be a help if you can post them.

jim

[David Keller NC]

"If I sand this finish off today, is there any hope it will straighten out a bit on it's own? Anything I could do to get it back to close to flat? Sand it off then flip perhaps? I'm not looking for machine shop precision here."

Maybe. If this has been going on for a while, the top may have taken a "set" where the wood fibers on the top have been compressed beyond their elasticity point, and they won't fully return to their original dimensions when the moisture in the top re-equilibrates. I'd take the finish off and wait a week or so. If the top returns to close to flat, you're golden. If it doesn't, you may need to flatten it with a plane.

[Dan Karachio]

Thanks all, I'll give it a shot.

[george wilson]

Strange as it might sound,we used a technique in the museum of wetting the CONVEX side of warped wide boards like table tops. The convex side gets more convex when it is wetted,but as it dries,it sucks down flatter than it was before. This even worked on 200 year old tables.

Sometimes,in severe cases,wet cloths would be piled up on the convex side to help the water sink in even deeper.

The most severe treatment I have seen is to clamp the table top down flat,with the convex side up,then wet it and soak it well for several days. Then,keep it clamped flat for weeks as the water completely dried out. This probably helped to squeeze the wood's cells flat across the grain as they were wetted,and to keep them that way while they dried out. This would cure some pretty severe cases of cupping.

I think as little cupping as you have could be cured by just wetting the convex side well,and letting it dry.

One question: is there NO finish on the underside? The butcher block tops I've seen had a coat of finish on the underside too,just not as much as on the top side. If so,you'll need to get rid of it,and simple sanding won't do unless you belt sand it,if you want water to penetrate.

[Dan Karachio]

Thanks George. If I pull it off, maybe I can get a job in a museum? :-) Seriously, that sounds like fun, but way past my level of detail or, well, smarts.

You are right. There is a lighter finish on the underside too. I just sanded both off. Not very Neanderthal, but my Festool sander did the trick on both sides. However, now I am starting to wonder, if both sides were finished, was the cupping more from how the bench is built, the materials in the bench or just plain bad luck? As for the bench design, the top is supported by 4 4x4's which are the legs. nothing too unique there. I was hoping the butcher block would be sturdy enough not to warp.

Again, this is just my get a bench to someday build a real nice bench, but I was enjoying that flat top and I am still learning about what IU want/need in a bench and how to use one. I can't say it enough - my bench, beater that it is, is the best most useful too I now have.

[george wilson]

Try wetting the bottom. If you actually got he finish off that soaked into the pores of the wood,it will work. Or,you could lag bolt 2 cross pieces of 4"X4" underneath the top. Add shims in the center to flatten the top. It isn't warped much,but you might want to arrest the warping while it's not too bad. The laminations might pop apart if you let it get worse,and try to bolt it down flat.

[David Keller NC]

"However, now I am starting to wonder, if both sides were finished, was the cupping more from how the bench is built, the materials in the bench or just plain bad luck? As for the bench design, the top is supported by 4 4x4's which are the legs. nothing too unique there. I was hoping the butcher block would be sturdy enough not to warp."

It's unlikely to be the way the bench is built, nor bad luck. What probably occurred is that the butcher block consists of fairly randomly oriented face/edge grain laminations, and it was built in a drier enviroment. As it's sat in your shop, it's absorbed water, and because it's not uniformly oriented as quartersawn, it's bowed a bit.

Think of it this way - you wouldn't be surprised if a 24" wide face grain board cupped 1/32nd of an inch, would you? Well, that's essentially what your (and mine, and everyone's that's made of real wood) bench is - a 24" wide face/edge grain board.

So - while what George suggests is a the standard way of dealing with a cupped or bowed board that has insufficient thickness to be squared by cutting tools (planer, wide belt sander or neander hand plane), I would suggest not wetting any of the bench - after all, you just spent several weeks allowing it to come to perfect equilibrium with the stable shop humidity.

Now is the time to flatten it - and the only way to really do this and keep your neander credentials is with a jointer plane, be it a #8, #7, or a wooden one. And if you don't already have one, start looking, because you will absolutely have to flatten your bench top occasionally - it's part of the game, and it's a lot cheaper and easier than buying a $3 k drum sander.

[george wilson]

Now there are 2 completely conflicting points of view,as is usual.

I'll give one last piece of advice: If you hand plane the top,you will have a whole bunch of trouble with some of the wooden strips needing to be planed in the opposite direction. What I do with curly or difficult to plane woods is to get the plane razor sharp.Set it to a fine cut,and plane directly ACROSS the grain. This will give better results than you might think if your plane is very sharp,and cuts are very light. Then,you can scrape the top with the grain,or finish up with a sander.

[Jim Koepke]

My knowledge of this is definitely no where near that of George or David.

My wondering is if it cupped due to one side taking on moisture and the other side not. Also, since that took place, the finish has been removed from the two sides. Now, wouldn't a wait see approach be a good way to go until more movement takes place? Then when that movement occurs, the angle of attack can be decided at that time. If equalization takes place and it goes back to flat, you will be happy you didn't try to flatten the top with a plane. If it wants to cup more, then you will want to get it to stability before you take a plane to it any way.

just some random thoughts as my eyelids get heavy.

jim

[george wilson]

Could be,Jim. I really think that he needs to lag bolt the top to some stout supports if he gets it flat.

[Dan Karachio]

Ah, what would SMC be without different opinions! :-) Alas, I wetted it last night and was about to go check it out. However, the good news is this. Birthday is coming up and there is nothing I want more than a jointer. I have a LN #4, a A Lee Valley LA Jack, so I am leaning toward a Lee Valley LA jointer. Given all this whacky grain, maybe a higher angle blade would be a smart extra purchase - for this and other things?

[Tom Adger]

Gravity works 24/7. If it has settled some in the middle, thats the way it is. If you think it has reached some sort of equilibrium, then flatten it. After all, you have 1 3/4" to work with. I consider myself a hybrid of a neander, and a user of power tools when they are appropriate, do a better job, and are faster. either use a flatten jig for a router(which you can make) or use a flattening plane. Whatever gives you the most pleasure and satisfaction. That's the whole point of the thing.

[Leo Graywacz]

Take a heater and place it under the table. Let it heat up and check it every couple of hours. Let it dry out enough to cup a little bit. After this happens remove the top and flip it over (turn off the heat) and coat it with a few coats of dewaxed shellac. After it dries put a coat of finish on it like polyurethane. After the table is sealed on both sides it should not warp again. However, wood does what it wants to and sometimes you can't stop it.

[David Keller NC]

Now there are 2 completely conflicting points of view,as is usual.

I'll give one last piece of advice: If you hand plane the top,you will have a whole bunch of trouble with some of the wooden strips needing to be planed in the opposite direction. What I do with curly or difficult to plane woods is to get the plane razor sharp.Set it to a fine cut,and plane directly ACROSS the grain. This will give better results than you might think if your plane is very sharp,and cuts are very light. Then,you can scrape the top with the grain,or finish up with a sander.

Actually, George - I'm in agreement in general on the method you describe - I use it very often on very wide, unstable boards like maple where flattening the board would reduce the thickness to 3/8". I wet the concave side, put a gentle heat on the convex side, and when I get the board close to flat, stack and sticker it on a flat surface, and heavily weight it. In about 2 weeks, the board's usually taken the "set", and will stay flat enough for me to plane it and get it to furniture-sized parts.

On a bench, though, I suspect that even if he wets the concave side, without any restraint the bench will just re-cup when it dries out again. And since the bench top's thick and the degree of out-of-flat is minor, significantly reducing the thickness by planing it flat isn't much of a concern.

And you're quite right about planing the bench at 90 degrees to the grain for the first couple of passes. The most critical flatness tolerance on a bench is across the bench - not so much along the length.

[David Keller NC]

Ah, what would SMC be without different opinions! :-) Alas, I wetted it last night and was about to go check it out. However, the good news is this. Birthday is coming up and there is nothing I want more than a jointer. I have a LN #4, a A Lee Valley LA Jack, so I am leaning toward a Lee Valley LA jointer. Given all this whacky grain, maybe a higher angle blade would be a smart extra purchase - for this and other things?

A jointer will wind up being one of the most-used planes in your shop if you're interested in using wide boards that would take a $10k machine to process. As to usefullness, there's no reason that a LA jointer would be any more/less useful than a frog-equipped plane with a 45 degree bed angle (you can get the same result out of a 45 degree plane by putting a back-bevel on the blade as you can with a bevel-up and a high planing angle). One thing that I'll caution on is that high planing angles take a good deal more "oomph" to push the plane down the board than a lower one, and on a huge plane like a jointer, the effect is even more pronounced when used on a flat surface (rather than an edge). The point here is that if you want a LA jointer with a high-bevel grind, buy a second blade that you'll keep at a standard angle, which you'll want on boards without a lot of wild grain.

[george wilson]

I plane straight across the grain to avoid tearing wood that cannot be planed in either direction. The many pieces of which his butcher block top is made will have different orientations,and one good tear out will mean a lot more planing still without success if he keeps tearing the wood.

Another solution would be to grind and hone a steep angle as a secondary bevel on his plane,so it scrapes as much as it cuts. The plane will be harder to push,but will not tear out the grain. This is how Leonard Lee recommends to sharpen a plane for difficult wood.

Perhaps after he gets the top flat by planing across the grain,he might try grinding a steep angle,and planing with the grain to smooth out the cross grain plane cuts.