Can You Explain Why a Board Twists?

conversion-lines-on-log-fan-IMG_5076

Today I was working on the layout for “Honest Labour” and had to revisit the 1936 volume of The Woodworker magazines. I stumbled on this delightful and ingenious way to explain and demonstrate how wood twists as it dries. Read the original text below and check out the illustration.

— Christopher Schwarz 

Every woodworker knows that a certain shrinkage in wood is inevitable, and most know (to their cost) that a board will sometimes twist. Probably the majority connect the two phenomena, and say that a board twists because it shrinks. But this is only a half truth. It is true that the twisting would not take place if the wood did not shrink, but it is quite possible for a board to shrink without twisting. In fact, every well-seasoned board does so. Shrinkage has to be accepted as inevitable, and the fact that a board has remained flat goes to prove that the shrinkage can take place without twisting.

To revert to our subject, however, assuming that a board has twisted, that is become hollow, who can explain why this has taken place? An excellent practical demonstration of what happens is shown in the accompanying photographs. First a piece of paper about 6″ wide and 2′ long is folded up across its width in a series of folds, rather like a fan. The whole thing is then opened out at one side so that a circle is formed (like a double fan) as in Fig. 1, and the joining edges are glued together.

Across the face of this a series of lines is drawn with a brush and black ink. The lines at A are meant to represent the cuts that would be made in a log to produce plain (flatsawn) oak. That at B is a solid square of timber, whilst the C boards represent figured boards (quartersawn) cut radially from the centre.

Now shrinkage takes place around the annual rings, and it is obvious that if a log were never converted it would have to split, because the shrinkage would mean that the length of its circumference was becoming less. In the demonstration it is assumed that the splits have taken place at the two sides, and consequently two cuts are made at these two points. The spring of the folded paper will cause the whole to assume the shape shown in Fig. 2, and this is precisely the shape a split log would assume.

The originally straight lines of the conversion of the plain (flatsawn) boards A are now all curved, the square at B has shrunk badly at one side, whereas the figured (quartersawn) boards, C, remain straight. Thus we can see why plain oak is so much more liable to twist than figured oak, and why the boards always twist with their edges away from the heart. Thus in a twisted board it is always safe to say that the rounded side is the heart side. Furthermore, by an examination of the end grain is is always possible to say which is the heart side, and which way it is liable to twist if at all.

About Lost Art Press

Publisher of woodworking books and videos specializing in hand tool techniques.
This entry was posted in Honest Labour, Uncategorized. Bookmark the permalink.

21 Responses to Can You Explain Why a Board Twists?

  1. Helen Welch says:

    Explanation noted, and now I’m off to make my fans for the next class.

    Like

  2. Steve P says:

    Maybe i am misunderstanding something, or terminology, but doesn’t this explain why a board cups not twists? Because this is a thin slice, but if the paper were lets say 4 feet thick, and all the same diameter all the way down would it still “twist”? Or is the twist due to it cupping but the diameters being slightly varied across the length(or tapered as certain parts of a trunk are)?

    Liked by 1 person

    • Henrik says:

      In my understanding: yes, and yes. Other factors contribute to twist (as opposed to cupping); tension-wood on one side of the tree, a result of strong winds or growing on a slope, tends to give strong and often unpredictable twist, something that not always can be removed through planing. No idea why, but my experience is that such wood sometimes continues twisting, probably because planing then disturbs some form of equilibrium… Best destined for the burn pile.

      Like

    • Terminology changes, across time and cultures. When I read early woodworking texts, terms such as twist, warp, cast etc. have different meanings. Clearly, the author is talking about what we today would call “cupping.”

      Another example: plain vs. figured oak. I added “flatsawn” and “quartersawn” to the piece to reduce confusion. “Figured” oak means something different today than 100 years ago.

      Like

      • Steve P says:

        Yeah its kind of fascinating. I’m finally getting the hang of the differences used in wording between UK and US, such as rabbet and rebate, but throw in different time periods and you almost need a rosetta stone. Though this post here makes me realize I should probably pick up “With the Grain”, or “Cut and Dried” sooner rather than later. I remember watching one of your videos/DVDs on dimensioning stock by hand, where you explained the twist. The only bad part is I watched this AFTER I had made a hutch where one of the door frames had twist, even though I was sure I had “flattened” all of the boards(I only knew about cupping). Its a learning process, that’s for sure.

        Like

        • Count_Sudoku says:

          To translate into today’s speech:

          When the author says “figured boards” what he means is “literal boards”

          Like

  3. That really is ingenius.

    Like

  4. Pascal Teste says:

    I like it. Very smart way of demonstrating the phenomenon.

    Like

  5. I must say that seeing only the medullary rays on those paper “boards” is a bit confusing!

    The growth rings could’ve been drawn with the dark pen thinner than the board outlines for more clarity of the end result.

    Like

  6. Chris says:

    “Thus in a twisted board it is always safe to say that the rounded side is the heart side.”

    Not to be that guy, but….. If a board is allowed to fully acclimlate, then machined flat, then moved to a different climate, it can round EITHER towards or away from the heart side, depending on the humidity of the new environment relative to the old.

    Like

  7. Steve P says:

    I also realize now why “winding sticks” are called “winding sticks” and not “twist sticks”. They most likely called twist “wind”. But does that mean that a straight edge is a twist stick?

    Like

  8. Rob Porcaro says:

    The pleated paper is a nifty demonstration to understand why a flatsawn board cups toward the bark side as it dries. But it is definitely about cupping, not what we currently call “twist.”

    However, we can take this a step further to explain actual twist. Imagine a long series of the circular pleated paper discs arranged on the same axis going in and out of the screen. In other words, like a tree. Now rotate, let’s say clockwise, the disc below the top one. Rotate the third disc a bit more, and so forth down the “tree.”

    Now imagine a flatsawn board cut in an ordinary through-and-through fashion from our twisted tree. At the top, its right and left sides are symmetrical. But as we proceed further down the board, its left side is increasingly different from its right side. The left side is relatively more quartersawn compared to the right side, which is relatively more flatsawn.

    And so, we have a board that cups differently as we look down its length. Furthermore, the cupping is increasingly asymmetrical between the right and left sides of the board.

    That results in a twisted board. And, of course, we can now imagine all sorts of variations and inconsistencies that nature produces as a tree grows, along with the consequences for the wood that we are so blessed to use.

    But I don’t like twisted boards.

    Like

  9. Paul Bouchard says:

    In one of the The English Woodworker’s videos, he passes on a good rule of thumb: the growth rings will want to straighten as the board dries. I’ve found that very easy to visualize.

    Regarding wind – I’ve found that nearly every pine board I’ve worked on had the same direction of twist – the top right and bottom left corners sat proud…just wondering if I’m an outlier of if it’s a northern hemisphere phenomenon.

    Liked by 2 people

    • Wow. I’ll have to start watching for that.

      Liked by 1 person

    • David Germeroth says:

      Paul,
      In general, you’re correct. Most trees, if they twist, twist with the sun. Tendency to twist varies between species and within species. Some species, like the elms, like to mix it up. Twisting clockwise for a few years and then flipping to counter clockwise and vice versa. That is why elm is so tough to split. Twist in individual tees can also be caused by an unsymetrical crown, leaning and anything else that causes a consistent unbalanced load on the stem.

      Like

    • Steve P says:

      You know, its funny, i noticed something similar when planing a bunch of cherry. That it was always the same corners that were high spots. But I think it was opposite of what your pine does. But now i am curious. I’ll have to dig out some of the rough boards. You mention northern hemisphere, that would be wild if it was related to the Bernoulli effect. But nature never ceases to amaze me. I’ll report back if I find a pattern.

      Like

  10. Quercus Robur says:

    Sweet! The natural tendency of _any_ paper fan to collapse unto itself is something we all encountered as kids – which makes this demonstration immediately memorable.

    Like

  11. Larry Steinbach says:

    I have harvested a lot of city trees being removed and make lumber for my own projects. I have collected many large side branches of cherry and found that their lumber seldom behaves. Their load is totally on the top and I am guessing they are pre-sprung, so to say. I hate to waste beautiful wood and through experiment have lessened this effect by milling the logs so the boards are cut in the vertical plain as the limb grew. They distort but remain mostly flat with some bow.

    Liked by 2 people

  12. Chester Arnold says:

    This is also why when installing deck boards we always put them “crown down”, so that when they get wet the bow is up and the water runs off better.

    Like

Comments are closed.