Lid cupping

[David E. Hutchins]

Hello everyone, so I'm pretty new to woodworking, less than a year and really like to make dovetail boxes. My shop is not heated and I live in the north east. The supplier I buy hardwood from keeps it in an unheated building also. After I put the finish on I have to bring the project into my house to dry and almost without fail, the next day the cover will be cupped. Sometimes pretty severely.
I've tried single piece lids and laminated ones and seem to have the same problem either way. I'm wondering if this will always happen because of the temperature and humidity change or if there is something I can do to stop it? Any help would be much appreciated. Thank you

[Warren Mickley]

Do you put finish on the inside?

[David E. Hutchins]

Yes, I put finish on the inside and the bottom of the box

[Stew Denton]

David, as you probably know, as wood dries out, it shrinks. However it does not shrink uniformly. It shrinks lengthwise only a little. Parallel to the growth rings it shrinks about twice as much as perpendicular to the growth rings.

What this means is that if the wood is worked wet, and brought inside where it will dry out, that it will cup if flat sawn. It will also split at the ends, lengthwise. If it dries out quickly because the ends will dry out before the center, the ends will shrink first, and then because something has to give, the ends will split.

The solution to the splitting is to force it to dry out through the sides of the lumber instead of the ends, and it is the natural inclination of the lumber to dry out through the ends. To force it to dry out through the sides, seal the end grain with something like Anchorseal or you can use canning or bees wax if the lumber is not going to get too warm. I have used a 50/50 mix of bees wax and canning wax until recently when I bought some Anchorseal 2.

There are limited fixes to cupping. One is to only use dry lumber. Thus it will be much more stable that way and most of the cupping due to drying out will have already occured.

The rule of thumb for air drying is that you have to let it dry 1 year for each inch of thickness. (If you read the "sawmills and Kiln Drying" board on this site you will find a lot on air drying lumber I am sure, as that is an issue always faced if you saw up tree sections into lumber, because freshly cut trees will be completely saturated with moisture.)

The battle is that even if you use dry lumber to build a project, if the humidity is high, the dried lumber will pick up moisture from the air and swell a little, and if the humidity is very low, the lumber will dry out some and shrink. Thus, as such the lumber will "move," and either shrink or swell, and it may also cup some as the humidity goes up and down with the changes of the seasons. Woodworkers must plan for this shrinkage and swelling, and there are various solutions to the problem that have been developed over the ages.

A second method is to use narrower strips of lumber glued up to the desired width. What is done is that the growth rings are alternated one rings pointing down, the next pointing up, next down, etc. That way the cuping of one strip sort of cancels out the opposite cupping of the next.

Another method is to use quarter sawn lumber, which will shrink as it dries out, but it won't cup like flat sawn lumber will.

A third method, often used by turners, is to treat the lumber with propylene glycol which will displace the moisture and then not dry out, thus stabilizing the lumber. There are major disadvantages with this approach also.

No matter what approach you take, you should only use dry lumber for furniture, cabinets, etc., as this will minimize the cupping. Some guys air dry their own lumber, others use thoroughly kiln dried lumber. No matter what kind of dry lumber you use, you should let it acclimate to your climate by letting it set in your location for a couple of weeks before using it.

If you use wet lumber it is going to move, cup, etc. Period.

Stew

[Jim Ritter]

Use only quarter sawn wood, It rarely cups. Flat sawn is almost guaranteed to cup due to different conditions inside the box versus outside.
Jim

[Warren Mickley]

There are a few causes of cupping.

One is uneven moisture on the two sides of the board. If you wet one side and put the other in the sun, it will become concave on the sunny dry side. Putting a finish on tends to retard the movement of moisture, so you may think that you want finish on both sides, so they shrink evenly in dry climate or expand evenly in a humid one. However the inside of a box or case is already slower to react to a new environment then the outside. So for traditional work we usually do not finish the inside and this helps even things out.

Another cause is the tendency for a plain sawn board to cup on the bark side as it loses moisture and to cup on the pith side as it gains moisture in the summer. In this case there is some warping even if both sides of the board have equal moisture. If you make your top so that the rings on the end curve upwards toward the edges, it will tend to become convex in a dryer environment.

There are also a number of ways we make a top that restrict its movement. For example we sometimes fix battens near the edges to keep it straight or restrict it in a frame. And others.

[Steve Altman]

It might be helpful if you could put a photo up showing the box(es). There's different ways of making a box...

[Phil Mueller]

You may want to try to take in an oversized piece of wood that you will make the lid out of and bring it into the house for a couple of weeks. If it cups, you can then plane it flat then cut it/plane it to size, then finish.

[Andrew Hughes]

I’ve found flat sawn is worst for staying flat quarter sawn is better but still will cup. Unless it’s really thick and not too wide. But that usually doesn’t look good. The best lids for boxes have some kind of frame and panel.
Theres is one secret trick I used to do with a slab style lid. Shape the wood so it’s thicker in the middle and tapers out toward the edges. It’s a very much like a pillow shape.
This can easily done with a handplane.
Good Luck

[David E. Hutchins]

Thank you all for the tips and information. I guess I assumed that since I was buying kiln dried lumber that it would be dry enough but apparently that's not always the case. I will try a couple options I read here and see what works. 15844400924691239419787.jpg
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[Derek Cohen]

For small pieces, I wonder how drying in a microwave would work? Anyone here?

This type of box lid is quite resistive to warping as it is supported all around ...






Regards from Perth

Derek

[Warren Mickley]

A lot of woodworkers don't understand moisture. They buy kiln dried lumber so that if there is a problem they can say "It's not my fault; it was kiln dried."

The truth is that wood that has been dried will swell with higher humidity and shrink with low humidity in cycles for years. In your case the lumber was stored and worked in an environment of moderate humidity so it was likely slightly higher in moisture content than when freshly dried. When you brought it into an environment with extremely low humidity, it lost moisture and wanted to shrink. The bottom of the lid, not being fully exposed to this dry air, did not shrink as much as the top, which made for warp. If you had taken your box to an area of high humidity, it could easily have warped the opposite way.

It could be that just turning the lid upside down and exposing the other side to the dry environment will mitigate the problem. It could also be that given time, the bottom of the lid will catch up to the top and even out. As I mentioned earlier, leaving the interior of a box unfinished will generally help the interior (or the bottom of the lid) stay even with the outside. If you take the lid outside and leave it bottom side up in the sunlight, it will move in a favorable way, but you should keep close watch.

[Robert Engel]

Probably related to unequal drying inside of lid vs. outside exposed to more air.

Before final milling try acclimating your project parts inside the house for a week or so. Also between phases of assembly/finishing.

[Scott Winners]

Hey David, you have several good pointers here. One I didn't see mentioned was your EMC, or equilibrium moisture content. You might try your county extension agent to see what it is for your area, your wood dealer might know also. When wood comes out of the kiln the wood moisture content is a number, probably about 6 to 8% moisture content. When it sits on a rack at your dealers unheated warehouse the MC (moisture content) of that board is going to move from whatever it was coming out of the kiln towards whatever is the outdoor ambient EMC for the town where your wood dealer is. Then when you bring the same board into your home, it is going to move towards the EMC value for your home's conditioned interior.

EMC looks at both ambient temperature and relative humidity to calculate what the eventual equilibrium moisture content will be if the board stays on the same place long enough. If you find a two inch thick board that was kiln dried to say 6% MC, but it has been on a rack at your dealers unconditioned warehouse for four years where the EMC is 15%, you can pretty much figure the board is at 15% MC.

I am not discounting the above advice about leaving the insides of your boxes unfinished, or using breadboards or laminating thinner strips together to make your tops, or using a treatment on the end grain of unfinished boards. All good strategies. My local dealer has a storage room that is pretty much unconditioned space as well.

When I have a project in mind for inside the house I tend to rough cut my stock, treat the end grain, and then sticker stack on the floor of my office so the cat can scratch it up and I have multiple opportunities to trip on it while I am letting the rough cut stock equalize. Clearly I am not a pro. If I was going to quit my job and try to make a buck woodworking I would be inclined to have a conditioned wood storage space so my rough stock would be as close as possible to the EMC of my average customers homes while it was waiting to be processed.

You might read up on and make a few of the "six board chest" there's tons and tons of writeups on those and endless discussion of how to put them together to allow for seasonal wood movement.

[Stew Denton]

David,

Excellent advise from Scott on the moisture situation of the wood. One point however, if you are using pine, fir, etc., instead of hardwood, the kiln dried lumber for that type of wood is normally 16% to 18%. This for ordinary dimension lumber. It is not dried as well as they dry the hardwoods.

However, that may not be the case with something like lumber that is for finish work or cabinets, etc., say something like clear white pine. CWP may be dried to more like hardwood standards, I don't know on that material.

Regards,

Ste