Q. We have some cabinet doors that were flat but now, after installation, have warped a slight bit, enough to make them a reject in the customer’s eyes, and rightly so. Two questions: What caused this and what can we do about it to straighten the doors?
A: Moisture. Wood only changes size or shape in use because of amoisture change. (Of course, we could have a large outside force that wouldbend the wood too, but that is not a reasonable or likely event for cabinetdoors.) So, we know that there was a moisture change between the time youmanufactured the doors until they were put into use and the warp was noted. Now,the moisture in wood changes ONLY because the moisture content (MC) of the woodis not equal to (or in equilibrium with) the moisture in the air. Here is abrief summary of the relationship between MC of wood and the RH (relativehumidity) of the air. Note that temperature is not a factor…it is only humidity. For convenience, we sometimesdefine the air’s moisture level as EMC, equilibrium moisture contentof wood. If the air is at 7 percent EMC, the wood will be trying to achieve 7percent MC.
0 percent 0 percent
In the wintertime, the interiorconditions in most homes and offices are around 6 percent EMC. In thesummertime, 9 percent EMC. Further, the outside conditions are 12 percent EMC,summer and winter, in much of North America.
Warp. So, with a MC change comes a sizechange or change in shape. Usually, we can tolerate a change of + or - 1percent MC without any problems. It is with larger changes that we begin to seeproblems. Further, an abrupt or rapid change in conditions (going from yourhumid shop to a dry customer’s office for example in justa few days) will cause more problems that a slow change taking many months(such as the change from summer to winter). Hence, it is critical to have thewood, at the time of manufacturing, at a moisture level that the product willsee when it is installed in the home or office. In most cases, this means youneed to be certain that your wood supply is kiln dried to 6.5 to 7.2 percentMC, and that your storage area for wood and components is kept at about 35percent RH (6.8 percent EMC).
The assumption often is that therails or stiles are causing the warp. However, it can be the inserted panel. Infact, the inserted panel can be cut out of the door and then you will see if itwas the panel or the frame that caused the warp. Of course, this is adestructive test, with no chance for repair.
So, why does wood warp? There aremany reasons, but from a practical point of view, almost all warp resultsbecause
1) The grain of the wood within apiece is not parallel to the sides; that is, there is slope of grain. Thisslope can be due to the grain angle in the tree or due to the techniques usedwhen sawing the log into lumber. If the MC changes in a piece of wood withslope of grain, then the piece will warp, especially twisting type of warp. Whenperfectly straight grain is required for expensive items, such as a pool cue,the wood is first split (which will follow the grain) and then sawn parallel tothe pith. This is obviously too expensive for most lumber, so we need to workon minimizing the MC changes instead of grain selection.
2) The EMC on one side isdifferent than on the other side, creating a MC gradient and therefore ashrinkage gradient even when the grain is straight. Note that this differencecan be due to two different finishes on the two faces—one finish lets moisture through quickly while theother retards vapor flow fairly well. (Note that no finish seals the wood evenclose to perfect, so in reality if the outside RH changes, the wood MC willchange slowly (which is better) or quickly, depending on the finishproperties.)
3) A less common reason forcupping type of warp (warp across the width of a panel) is because for everypiece of wood, the side or face closest to the bark will shrink more than theother side or face. This difference between faces is greater as the piece iscut from a region in the log close to the center of the log. As the lowergrades of hardwoods, No.2 and No.3 Common, typically come from this region,this would be the wood most likely to cup, especially wide pieces. For thisreason, wide panels are glued up using narrower pieces, rather than two orthree wide pieces. This is also partly why MDF offers a more stable corematerial than solid lumber. (Note: The idea of reversing the grain of theadjacent staves in a glued-up panel has little or no effect on cupping.)
4) An inserted panel can bemis-manufactured so that it has unbalanced construction; that is, the face andthe back will not swell or shrink the same when the MC changes. If the insertedpanel has a veneer surface on the face, the back also needs a similar behavingpiece of veneer to keep things balanced. Also, the veneer itself can have grainangle issues with enough movement when the moisture changes to warp the core.
KEY: All of these reasons requirea moisture change of some sort. If there is no MC change, there will be nowarp.
Twomore ideas (correct MC is the first idea) for reducing the likelihood of warp.
5)When making the door initially, we can install stiffeners (usually metal) inthe rails and stiles that will better help the wood resist warping. This isespecially effective on large doors.
6)Use an MDF core with a veneer surface laminate rather than solid wood core withveneer or a solid wood core that is also exposed.
Straightening. I have listed here several ideason how to straighten a door, but you will likely agree that some are not toopractical all the time. Some basic concepts are: once warped, even when the MCreturns back to the original value, the warp does not disappear- - that is,some warp is semi-permanent. Plus, evenif the door is flattened, a subsequent moisture change can cause the door towarp again. As a result, most cures will not work too well. Spend time andeffort avoiding the problem in the first place rather than repairing the warponce it occurs. Also, the effort involved in straightening can be veryexpensive…it might be cheaper to make or order a new door.
Method#1. Rout a groove (perhaps ½ inch x ½inch x nearly fulllength) in the back of the stile. Then make a curved piece that will fit intothe groove that has about twice the curvature of the door, but in the reversedirection. Clamp this piece into the the groove and see if it straightens thedoor and perhaps even reverse the warp slightly. If it puts in too much reversewarp, then shorten the piece. But, if this piece does work, then glue it intothe slot and after the glue dries, remove the clamps.
Method#2. Rewet the wood partially and then bend it flat and continue bending it abit more. Then dry it in this reversed warp condition. When the bending clampsare removed, the wood will spring back a little bit…hopefully it willspring back enough so that the door is flat. This is somewhat “hit and miss.”
Method#3. Cut a bunch of small grooves crosswise (90 degrees) to the warp in the railor stile. These would be perhaps 3/8 inch deep- -experiment to find out how deep and how many. Thesegrooves will weaken the rail or stile so it might come back to flatautomatically, but if the warp is caused by the inserted pane, the warp will beworse. If not automatically flat, the door, with the weakened rail or stile,can be easily bent into a flat position. Then a brace can be fastened to holdthe door flat (see #6) or epoxy glue can be put into the grooves while the dooris flat, restoring the strength and keeping the door flat.
Method#4. There are commercially available jigs that can be put on a door, runningfrom one corner to the diagonally opposite corner. This is similar to thediagonal piece of wire that is used to straighten a swinging gate in a fence. Itworks, but might not look very cute.
Method#5. We can wet the door and then bend it flat plus a little more. It isdifficult to know how much to bend the door beyond flat so that when thepressure is removed, the spring-back will result in a perfectly flat door. Repeating:This is not a permanent fix, as a change in humidity can cause the warp toreturn.
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