Thursday, February 26, 2009

Wood Moves

Today I want to start on the topic of accuracy in work, a facet of woodworking that is of particular interest to me. Whenever a person undertakes to cut pieces of wood so as to form joints, and then fit them together, the accuracy of the fit is a major concern. It is a critical concern actually, because the closeness of the fit determines in large measure the structural integrity of the joint. Joints that fail in loading are dangerous in buildings for obvious reasons.

Any joint, like a splice or scarf will be weaker than an uninterrupted section of timber, so the joined timber as a whole is only as strong as its weakest point - namely the joint. A fit of a mortise and tenon that has too much internal slop will allow for the pieces to move when loaded and strength will be greatly diminished as a result. If the fit is too tight, then the risk is that the receiving piece will split when the tenon is forced in, rendering the joint greatly weakened. This is less a concern in working with softwoods, as the wood inherently can be compressed more - nevertheless, I would say that a lot of timber frame joinery, as practiced, relies unduly on the power of multi-ton come-alongs and massive sledges to bring the joinery together, which is often a bit too tight to begin with.

In working with hardwoods, there is far less tolerance for error in the fit, and from working hardwoods in the process of furniture-making, I have learned much about achieving good fits, and these lessons have transferred well to timber work I think. In denser hardwoods, the difference between a joint that fits just right, and if glued has just the right amount of space for the glue, is on the order of 0.005". Thus, if you want a perfect fit, you need to be able to cut the joints with something close to that level of accuracy, more times than not.

When I returned from Japan to Canada in the summer of 1999, I managed to gain employment a day or two after touching down with a company on Vancouver Island called Daizen Log Tech. It was run by a Japanese owner and shared lot space with B. Allan Mackie's famous Log Building School. Several employees were Japanese, and the bulk of the production was log home shells made for export to the Japanese market. They also had a 'timber frame' division (I use that term loosely), and that is where I, er, slotted in.

I was told early on by my supervisor that their tolerance of fit for a mortise and tenon was +/- 0.25", which I thought was a lot. Their rationale for such slop in fit was that the parts were easier to trial assemble and take apart in the yard (which was true to an extent), and that after a one-month sea journey in a container, they didn't want their Japanese crew wrestling over joints that had swelled and become too tight to fit. I knew this was plain wrong, but I was the new guy so I said nothing. I was told that under no circumstances did they want me fussing over the inside of joints in places no one would ever see.

I was also told by a veteran worker there, after I asked him why there wasn't a jointer ( a woodworking machine for creating flat surfaces) in the timber shop, that the owner had injured his fingers using it, and that besides, "what did you need a jointer for? The tablesaw cuts a straight line". My eyes widened, and I knew this to be completely fallacious. While the saw can and does cut a straight line, the behavior of wood, its movement, after rip cutting is another matter. If the surface of the piece of wood if twisted or bowed, etc, then the process of cutting 90˚ (or whatever the angle) in relation to that at best will produce an uneven result, and at worst is dangerous to the machine operator. However, he was the veteran and I said nothing. I've since learned that this is a relatively common perception. Most shops do not have jointers.

A little while later, when I was discussing with another worker my aim to get a good fit with joinery, the fellow turned to me, and with a definite tinge of scorn in his voice said, "what are you worrying about that for - are you crazy? Wood moves!" I stared, and gulped.

Daizen was quite a scene. They had a crew of 20-odd people in the yard, along half a dozen in a window and door plant. At any given moment there were typically 3 log shells in production, along with one 'timber frame', and the cacophony of 10 chainsaws was omnipresent throughout the working day. New employees were expected to have their own tools, including chainsaw, and starting pay was a less-than generous $8.00/hour. I lasted 6 months, and moved on to working for a real timber frame company on another part of the island. Eventually Daizen closed down, laid off all its workers, and the owner moved, acquired a Hundegger automated timber cutting machine (thus solving the 'employee problem') and now, last I heard, produces timber components with that set up.

Anyhow, Daizen was an interesting place to start out my woodworking career, as I got to see that 'professionals' are often not all that knowledgeable or professional in their work, regardless of how long they had been doing it. I was surprised to find that people with 20 years experience, though accomplished in their work, often had a narrow grasp of wood and woodwork outside of the specifics of their jobs. I didn't realize then that for most woodworkers, the job ended at 5:00, and they didn't give it further thought until the following morning, thus they didn't learn much beyond the day-to-day concerns of their job-specific tasks.

Since the Daizen days, I have worked in a variety of shops and situations, along with having a long period of self-employment. My thoughts and desires to achieve fine tolerances for the fit of joints has often been greeted, I have found, with either dismissal, disbelief, or scorn by other woodworkers. I find these attitudes curious.

Another aspect of tight-fitting joinery is simply the appearance of tight-fitting joinery. When any joint is drawn up tight, most of the internal mechanism is concealed from view, and thus what is available to the eye are the intersections on the surface. One way to get tighter, gap-free intersections is the process of undercutting, as seen in the picture to the left. This picture I grabbed off a web site page explaining 'how to undercut', as a path to joinery success, however I would say the picture is better described as how NOT to undercut. To my eyes, the undercutting has been taken too far. If the tenoned vertical piece in the assembly were loaded downwards, then instead of having good bearing surface at the shoulder, it instead bears along a narrow edge at each side, and the tenon would probably bottom out to boot. This makes for a weaker joint, and one vulnerable to splitting if loaded. If the tenoned piece were loaded side to side, the knife-edge at the outside of the shoulder would dig into the surface (end grain being much harder then edge or face grain) and crush the fibers down, leaving a gap later and a loose fit. Additionally, if the lower receiving piece shrinks appreciably, the undercut will become quickly apparent, and in the effort to hide a gap by mean of undercutting, the result will be to show an even larger gap. If the receiving piece swelled, the grain on its face gets crushed again, and when it shrinks back, there will be a gap and thus a loose fit.

I'm impressed when I come across crisp-fitting, 'light-tight' joinery, however if I later learn that the joint was heavily undercut to achieve that, my esteem disappears. The mechanical connections achieved by joinery are only as good as the integrity of the fit. Placing too high an emphasis on the external visual aspect at the cost of a quality fit internally is a poor decision in my view. Taken too far, undercutting a joint results in little more than a candy shell of a connection, and this defeats the purpose of the joinery in the first place.

Undercutting in fact, is often a 'blunt instrument approach', a means of quickly resolving issues in a joint fit that is not closing cleanly. While working faster is, economically-speaking, of interest, this shortcut does nothing to improve the craftsman's ability to actually achieve a crisp fit, thus undercutting can become a bit of a crutch. More often than not, it seems to me, undercutting produces inferior joinery.

Undercutting can play a role in circumstances where relieving a surface slightly will help in dealing with seasonal wood movement or shrinkage between pieces in a joint. An example of this is the practice of gluing boards edge-to-edge, where a slight amount of material is planed out of the middle of the contact zone along the edges, thus the boards abut at their ends but have a slight opening between them at the middle. This is termed 'springing the joint' and it allows the construction to suffer moisture loss at the end grain zones of the board (where moisture is most rapidly lost or gained) without the joint splitting open.

I have studied joinery from both Western (English, German, French and Hungarian) and Asian (Japanese and Chinese) perspectives, and in general, the western joinery methods are relatively undeveloped in comparison to the joinery elaboration seen in Japan. Not in every case, but generally speaking. English scarf joints, for example, reached what might be argued as a higher pinnacle of development, obviously due to a chronic lack of long straight timber in that country from the 1400's onward. Necessity is the mother of invention, as they say.

A really excellent resource for the study of English joinery methods and developments are the works of the deceased Cecil A. Hewett, especially in his works "English Cathedral and Monastic Carpentry" and "English Historic Carpentry", which is linked at the right of this page in the book list.

Hewett undertook the study of old buildings in the Essex region (primarily) of England, and by showing the development patterns of joinery was often able to come to more refined understanding of the historic process of building. The interesting thing he notes, to my view, is that for any given type of joint, the development process is very similar: early crude forms are gradually improved upon, time and consideration, sometimes simple insights of brilliance, proving the superior forms, until after often hundreds of years the zenith in development of the joint form is reached. Often, it is the case that the 'best' joint from a structural perspective is not such a good joint from the economic perspective, as complex joints take more time to cut. Thus, after a zenith of development is reached, there typically follows a drop off where inferior imitations are made, either by subsequent carpenters who did not fully understand the brilliance of the penultimate form, or who were, surprise surprise, trying to cut corners with a quickly-cut imitation. Thus, from perfection follows degrade, technically-speaking.

The point of interest here is that a carpenter of today, ignorant of such matters, may well see an old timber building and seek to emulate the joinery and structural systems, when in fact he is copying something that was poor in the first place. An example of this are pockets for joists cut into the Summerbeam simply by chopping out a series of rectangular pockets along the arris of the summer beam to house the floor joist. The pockets cut into the upper surface of the timber and thus weaken it unduly. The carpenter who studied the mechanical concerns and history of development in forms of joinery for floor joist attachment to carry beam would learn in time that the surfaces to leave alone are the ones loaded in compression or tension (usu. top or bottom surface in a beam), and mortises are best placed in the neutral axis of the receiving timber. Thus, it can be seen that the tusk tenon, illustrated on the left, is the superior form of joint for this application, in most cases.

Another example of poor joinery in timber work are single housed dovetails. While these facilitate easy assembly in that the joists (or other similar part) can be simply dropped down into the receiving pockets on the beams, this joint, especially when cut out of green timber, has really poor performance characteristics. When loaded in tension (which is what the dovetail is supposed to resist), the long grain side of the dovetail is opposed by end grain in the dovetail mortise, and thus when the surfaces meet the side grain of the male dovetail is readily compressed. Thus, the joint can withdraw from its housing quite easily. Factor shrinkage into that when using green material, and what results is a near-useless connection, albeit one that is easy to assemble.

There's a book out there, the '"Craft" of Modular Post and Beam', which shows a housed dovetail proudly on the cover photo, as an example of 'craftsmanship' or something 'cool' I imagine. The joint is 'reinforced' with a pair of dowels driven down from the top, as if the designer of the joint intuited that it wouldn't actually resist withdrawal very well, so he put a couple of dowels in there to 'beef it up'. The dowels render the logic of the dovetail, weak to begin with, largely moot, and in fact under tension I doubt the dowels would add significantly to the load resistance of the joint anyway. Love of form has trumped functionality - sadly this is true in many places. The housed dovetail illustrated on the cover of that book is only a hint of the gems of information you will discover inside, if you know what I mean.

As I mentioned in an earlier thread, I avoid single dovetail construction in timber work, and try to limit them in furniture as well, particularly in places where loading on the joint might be higher - like a table or chair leg for instance. The dovetail performs best when used in multiples, like in carcase construction (see "Steps Along the Way" on this blog), or when used with a long siding abutment surface, illustrated to the left, such as the Chinese use for battens to reinforce the underside of a table top, or the battens medieval English carpenters used to reinforce planked doors (and so forth).

Anyhow, those are just some preliminary considerations about joinery - there's much that can be said on the topic. Let's not forget though that an accurately cut joint, if a poor design, results in poor construction.

In the next post I will look more at the topic of accuracy, and how I strive to achieve that in my joinery work.

6 comments:

  1. With stock that has been jointed true and appropriately seasoned for local climatic changes does not "springing the joint" in edge to edge work simply become erring on the side of caution?

    While this technique may leave you with the lesser of two evils does it really give the best joint?

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  2. Koot,

    I think the key thing is that "appropriately seasoned for local climatic changes" is not a particularly accurate term or way of thinking about the issue. If you live in a climate with a relatively steady amount of relative humidity year round, then the issue of moisture loss/gain is somewhat moot after the wood has been acclimatized to a given m.c., and one could be successful using a piece of wood that had dried to some given point.

    If you live in a climate with large swings in seasonal humidity levels, then the comment 'appropriately seasoned for local climatic changes' is somewhat meaningless. Wood is going to swell at the humid time of year and shrink at the dry time of year. After the initial drop of moisture content from saturation point at the time the tree is cut, there is a large change in wood volume, and when the wood has been later 'dried', the range of movement afterwards, plus/minus, is not going to be as great, but it still moves. The cells in the timber that held the moisture are damaged by the moisture loss and thus can never take up quite the volume of moisture they did while the tree was standing.

    So, at the humid time of year, which is summer in New England, the wood will be swelled up a bit. It doesn't matter if this is year 1 of the 'seasonal acclimatization' or season 10. Thus, the boards you joined on edge will loose moisture in the winter, most heavily from their end grain. If you use the term 'seasonally acclimatized' which season are you referring to?

    I think hollowing the edge is a good precaution against the season movement issue, and in this climatic zone in which I live a wise thing to do. What could be wrong with "erring on the side of caution"? That said, I probably wouldn't bother doing the springing on short pieces, say under 18" long.

    In fact, since one can have no idea what happens to a piece of furniture after it leaves the shop, it is wise, in my view, to design around the worst-case scenario. If you build your piece in an expectation of an ambient humidity such that a wood with 12% m.c. performs just fine, then that piece will suffer if the client takes it to their new home in Salt Lake City (as an extreme example).

    This has been borne out in observing what happens to old pieces of high quality furniture, which many would say used, in the 'good old days', wood that was 'seasoned properly' - put them in a modern home, heated all winter, and you will see these pieces crack and shrink and twist. This is a major issue in the conservation of such pieces. Another example would be a lot of the high end hardwood furniture I have seen out of Hong Kong - in Vancouver there is a store specializing in that stuff, and after 2 seasons in the store, there are cracks and gaps in the pieces, and I'm sure this wouldn't be the case in Hong Kong, where the humidity is more constant.

    Since wood moves, designing around that fact is a critical aspect of the process of creating, and in the production of pieces that are meant to last for years. I don't trust the glue in fact, so I don't even do edge-edge glue ups any more, I always use a tongue and groove AND glue, and hollow the joint very slightly. Overkill to some, but to me it means a piece with better integrity over the long term.

    ~Chris

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  3. You know after I posted I thought to myself, "that was a pretty off-handed generalization", and it was also completely based on my own local experiences, so I guess very subjective. I do agree with your comments about wood movement and its seasonal swelling and shrinking after drying.

    As a bit of background to where my thoughts come from I did most of my furniture making on Prince Edward Island which meant bone dry houses in winter (shrinkage) and lots of dampness in the summer (swelling). When I first started in the trade most of my wood (kiln dried) was trucked in from Ontario, and was often as low as 7% moisture content, maybe good for the desert but not the Maritimes so after a few hard lessons I learned to leave it stacked for a season or two to allow it to pick up a bit of moisture. This is what I meant by my off-hand "appropriatley seasoned for local climatic changes" quip. Over time I learned the 8 to 9% m.c. range was going to be the happy medium for where I lived if the furniture was for inside the average home. However I did send some major projects off to large cities in the "real world" and often wonder how they made out over the last 30 years. Kind of like Sheets mentioned earlier 'hope I raised my children right'.

    Over the years I started air drying my own rock maple, yellow birch and pine, the final drying stage being in an old school bus in the summer, and then giving it a spell of "sitting around" in the shop to pick up a bit of moisture, again hoping for that ever elusive happy medium. Really in some ways just a crap shoot all in all, but tempered by some hard experiences and careful observations.

    I now live in what is termed an Interior Rain Forest of British Columbia near the West Kootenay town of Nelson (a few miles up the West Arm from a local timberframe-gone-Hundegger outfit that you probably know Chris) and while the wood movement isn't quite as severe it is always an issue to plan for and build around. However, back to springing the boards, well I'm just not sure it's in my "grain".

    I'm really enjoying your blog Chris,(I discovered it through Tomohito Iiada's link) keep the fire burning!

    Marv

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  4. Hi Marv,

    nice to hear from you again. Sounds like you have had some diverse experiences with wood in a few different climates. You're now in a place with some nice wood - the mountain Doug Fir I much preferred to the coastal variety, and then there's a fair bit of Larch too. Nice spot to be in, especially Nelson!

    Speaking of difficult environment to design solid wood products for, a friend of mine who I worked with in California had just moved there from Salt Lake City. Apparently it is so dry in SLC that it is very difficult to get wood sufficiently dried in out of the kiln so that it will work well in service. Even wood taken down to 6% m.c. in the shop could develop shrinkage problems in service after a few years.

    The Northeast is a challenging environment because the humidity swings from near 0% in the winter to near 100% during parts of the summer. Besides dealing with that factor in the wood, there is dealing with that factor in the problems with tools. The heads of all my hammers are loose right now, for instance. I am thinking having a winter and summer set of tools might make sense here as many carpenters in Japan, an environment with severe swings as well,a do just that, even having winter and summer sharpening stones.

    Glad you're enjoying the blog, and I appreciate your comments.

    ~Chris

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  5. Got to watch the wooden handles on socket chisels in the summer, especially if your wearing sandals. ;^)

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  6. Now, I'm of a mixed mind when it comes to the scenario of the chisel dropping to the floor - do I sacrifice my foot/hand, or not? Anyone who has had to spend hours redoing the edge of a chisel after it found the concrete would understand the issue, I'm sure :^)

    ~Chris

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