Q: What is the best grain orientation for bending wood? What is the best MC?

 

A: When looking at a piece of wood that is to be bent, there will be four sides. Two of the sides will be flatsawn and two of the sides will be radial or quartersawn. A flatsawn face will have the rings parallel or at 0 degrees to the face. A quartersawn face will have the annual rings at 90 degrees to the face.

If you bend a piece so that the flat sawn faces are the inside and outside radii, then it is likely that the grain will separate.

High end pressure can be used to reduce this tendency. If you bend so that the flatsawn faces are the sides and the quartersawn faces are the outside and inside radii, then grain separation is not likely. (In other words, the best choice is to have the rings running from inside to outside and not edge to edge.)

The best MC

For severe bends, the best MC is 22 percent to 28 percent MC. The wood should not be dried and then brought back to this moisture level. Rather, the wood should be dried down to this level and then bent before additional drying occurs.

For gentle bends, sometimes called pan bends, 15 percent to 18 percent MC is great. Drier wood is too strong and will start to get brittle.

 

Q: We have some hard maple turnings that come out of the lathe straight, but in a few days (or maybe longer) some of them warp enough that they're no longer acceptable. We're thinking of including some of those moisture packets when we ship them. What comments or advice can you provide?

 

A: I can tell you that warping of a manufactured product, as you describe, is caused by moisture content change. Nothing else, including heat or cold, or drying stresses called casehardening, causes wood to warp over time. So, you're apparently turning the items at a moisture content that is not in equilibrium with the air. The pieces are then drying out a bit in the dry air and such drying results in shrinkage and warp. (Note: The turnings could be gaining moisture because they're drier than the air, but this would be rare. Almost always, the turnings are wetter than the air.)

To correct this warping problem, you need to tighten up your purchasing specification to include that no pieces of lumber are above 7.5 percent average MC and that includes also that the shell and core moisture are within 1 percent MC. (Oftentimes, the core is wetter than the shell, so when you turn the wood, you expose the wetter core, which is what shrinks in the dry air.) I'm basing my suggestion on the fact that the air in manufacturing and shipment is around 40 percent RH. If it's 35 percent RH, then you need to target 7.0 percent MC. If 30 percent RH, then 6.0 percent MC. Your specification should include how you're going to measure MC (moisture meter brand and model). Obviously, once you have this specification, you'll indeed measure the MC of every piece of lumber, if you're serious about eliminating post-manufacturing warp. Your specification should also include what you're going to do with any wet pieces you discover.

Getting the right MC

Can you put the turnings in a plastic bag that's closed soon after they are manufactured? If so, there'll be no moisture entering or leaving the bag, so the MC of the sticks will remain at a constant MC, overall, forever. No moisture change means no size change or warp. Note, however, if one turning in the bag is wetter than the others, that turning will lose moisture to the drier pieces and may warp, while the others will gain a slight amount of moisture. It is critical. Therefore, make sure that the incoming MC of your lumber is indeed uniform. (The drying process to accomplish this uniformity is called equalization. Even though the drying process may take an extra day, increasing the cost slightly, I guarantee that your rejects due to warp will be greatly reduced.) You don't need a moisture packet or desiccant. In fact, the desiccant may dry the turnings further and cause warp.

One other comment: The warp you're experiencing results because of a moisture change and because the grain of the turnings isn't perfectly parallel to the sides. In other words, your turnings have too much slope of grain (SOG). One critical step in getting zero SOG is for the sawing operation to saw parallel to the bark of the tree rather than parallel to the center axis of the log. Also, they must use straight logs when sawing lumber for you. Next time you have some warped turnings, try and split them lengthwise. The split will follow the grain. If there is zero SOG, then you should be able to split the turning from end to end; if there is SOG, the split will travel to the edge of the turning in a few inches. Because some moisture change is possible in wood due to seasonal variations in humidity (the finishes we use are not perfect moisture barriers), if we have zero SOG, it will be possible for small MC changes to occur without objectionable warp developing once the turning is in use.

 

Q: You've mentioned that air-dried lumber will only go to 12 percent in most places in Canada and the United States. I've had pieces of 12/4 and 16/4 dry to 7 percent at the core after two years. I air dry lumber stickered, indoors. This way I don't have to use a kiln.

 

A: I think that we have a definition problem. Air drying is the process of drying lumber outside using the natural air temperature, humidity and air flow. Sometimes we'll shed air dry using a roof to keep the sun and rain off the lumber, but there are no walls in the shed so the outside air can blow through. Because the average outside conditions are 65 percent RH in most of North America, which is equivalent to 12 percent MC in wood, you can't dry under 12 percent MC in air drying. In fact, achieving 12 percent MC will take quite a long time, especially for thick stock.

You mention that you're drying indoors; I wouldn't call this air drying. You must be averaging 30 percent RH or drier in order to achieve 7 percent MC in the thick lumber within two years.

Drying lumber indoors in this manner (or even putting a few pieces in a hot attic to speed up the process) will achieve low MCs indeed. The advantage of using a commercial kiln is that the process will be faster and under close control. Further, temperatures above 130 F in the kiln will sterilize the lumber. For softwood lumber, temperatures in the kiln above 160 F will set the resin so it doesn't ooze out later in the finished product.

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