Feb 05 2015

The Woodworking Accuracy Conundrum Part 1

Published by at 5:00 am under Design,Layout Strategy,Metrology,Skill Development,Wood

There is a lot of information about wood movement on the information highway, and some of it has painted itself in a corner. That’s ok, We can help sort this out.

More often than I wish were true, I find myself reading generalizations about wood as a building material and woodworking methods that I wish were better understood, because they create incorrect impressions and misunderstandings. Some we may have seen are: “Wood moves, so being very accurate with it isn’t all that important”, “Wood has seasonal movement you know, so there is not too much concern for high accuracy”. “Just get it close” and “Wood moves, so you can’t really rely on it as an accurate or precision material”. Sound Familiar? It’s unfortunately not the best of advice.

There are many other statements often made along these lines, but they don’t even tell half the story. In fact, statements like these have influenced many who read them that wood is a terribly inaccurate material and is difficult to use for making fine things. Some go on to reiterate what they have heard: “Well you know, wood moves and so there is that accuracy issue”, or “Wood isn’t really a precision material”, without really understanding that this issue is really not nearly as big as they have been lead to believe. See, it looks a lot like what was said in the preceding paragraph, only paraphrased, and not at all any better understood. Understanding this is one of the most important things in woodworking. Sure, knowing how to use tools is great, but what good is that if we don’t understand the material the tools are used for?

It is best not to pay too much attention to what people say off the cuff about wood movement, but forgive them, for they often know not what they say. It is best to study this subject for yourself and know for sure because it will empower you to create a lot of cool pieces in your woodworking future. Here is a new mantra: “We can understand Wood Movement and work wood with precision and accuracy.” It’s worth keeping in mind, because it is a lot closer to what really is possible and capable with wood. You’ll become a far better woodworker when you really understand wood.

Wood has a higher calling. Don’t believe? ok, but first consider how an infill plane maker fits some wildly dazzling pieces of exotic wood for infills into a steel plane body with 0.001-2 inch clearances. Then consider the friction fits of mortice and tenon or dovetail joinery. Finally consider why a picture frame’s miter joints still look great when the frame is a century old. How does all this precision occur in the face of such movement? It occurs beyond the realm of statements that profess wood’s inadequacies as a precision material. There is a lot of very old furniture in the world, and the fitment in it’s making is precision. It worked then and it works now.

There is nothing cavalier about learning more about such things, because the woodworkers who have mastered an understanding of wood movement have gone on to make amazing things from wood with masterful precision. What’s most important is that anyone can go on to make with wood through knowledge and practice. We can all make great things!

There is actually a lot about wood movement to understand, both generally and with specific species. I think we owe it to ourselves to personally endeavor to understand it better than we already may for ourselves, because it is a gateway form of knowledge to understand the materials we make with, and knowing it as well as we can will make us better makers. I think we can do better, so let’s try.

For well more than a century, scientists have researched most wood species on the earth and have determined quite specifically woods actually do move. We can know this as specifically as they do because they have published nearly everything they know, and provide free access to the information. It’s helpful to research this for yourself. We have several books covering this in the Woodworks Library in the Understanding Wood Section.

Here are a few nutshell points of interest.

Seasonal wood movement is basically a hygroscopic rhythm. While the hydroscopic aspect is universal to all woods generally speaking, each species has it’s own observed particulars. The rhythm timing of this is specific to every geographic locality. The wood breathes in moisture during periods of higher “moisture vapor in air” humidity, and then dries to a lower moisture content during periods of lower humidity. This rhythm when taken over the course of time affects the moisture content of woods, and this moisture content is a measurable quantity at any given time.

There are three axises of movement in wood. What this movement is and how it affects boards sawn for lumber depend on the sizes of the boards and how the boards were sawn from the log.

First, wood movement is usually described in the directions it affects the dimensional size of wood, for use as lumber. For the sake of this understanding, the log is a log at this point and has not been cut into boards yet.

Tangential movement describes the movement in a direction that is tangent to, or with the growth ring as it circles the log . Radial movement describes the movement that is radial, or across the growth ring in terms of the width of the ring itself within the log. Longitudinal movement describes the movement that affects the overall length.

Generally speaking, The percentages or ratios are taken from any wood of a given volume of length, width and thickness. It can be a log, but it can also be a sawn board of any given size. In boards however, grain orientation can become specifically sawn to enhance different qualities, such as economy, appearance, or to influence the accentuation or minimization of a particular kind of movement.

The descending order of maximum movement influence is; Tangential, most commonly found at 6-10% on average from the tangential proportion, Radial, with about half the movement of Tangential at 3-5% on average, and Longitudinal being the least at 0.1-0.2% of the total length. This relationship is commonly referred to as the T/R Ratio, (Tangential/Radial) and the Ratio for most wood species, is most commonly considered to be 2:1.

All this remains true no matter how you cut any log, however, as I mentioned, how you cut the log creates boards which are influenced specifically by the major movement influence in that board based on it’s major grain direction from how it was sawn from the log.

Some other notable points about wood movement are: Usually much of the movement considered in these percentages is shrinkage that occurs just once from the initial drying, kiln or air. Once wood is cut into boards and kiln or air dried, it is commonly stored covered yet exposed to outdoor relative humidity and not yet put into service. (e.g storage at the lumber yard) The EMC (Equilibrium Moisture Content) can vary as to location on a seasonal basis. 12-13 percent EMC is a common average over much of the US but there are both wet and dry extremes.

Consider this “outdoor wood” to be what wood is when you start your project with before you bring it in the shop.

Stay tuned. This is part one, but there will be more in part two…

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