May 03 2008
Yup. The Challenges to Squareness.
You didn’t actually think this was going to be easy did you? Well, it isn’t always, but I think the endeavor of overcoming some of these challenges can make things better, especially if you like taking good to great. Knowing what some of the issues can be, and how to overcome them when and if they arise, can help our results better match our desires.
We learned in The Constructs of Squareness article that geometrically speaking, a right angle is 90 degrees, and if it isn’t 90, then it isn’t a right angle. Everything can be represented perfectly on paper, in CAD drawings and in theory, but in building, milling, and manufacturing there are a number of factors, which can affect the quality of accuracy. Some we have to accept, some we can learn to work with, and knowing the difference is how we approach closer to fine, if fine is the goal.
Things that affect the accuracy we use to build do vary. Goals, philosophy, materials and tooling all play a part.
Goals affecting accuracy are often production oriented, cost oriented, or what the intended use of a final product is. If the Goal is to build a doghouse, it needs done quickly, and the price of materials and labor needs kept low, then, the accuracy of squareness need only be relative. If the goal is to make a jewelry box, where scale is small and appearances will be highly scrutinized, then the accuracy of squareness becomes much more important, because the philosophy behind jewelry boxes is seeing how far craftsmanship can be taken. Close tolerance fit and finish is a very large part of how this type of work will be evaluated.
Philosophy does not always have to do with goals, but is often a party to goal-oriented work. All craftspeople over time develop an inner guide regarding the level of accuracy and craftsmanship that are acceptable for their work. Sometimes it is based on the kinds of work they most enjoy, the styles they work in, the level of patience and time they have to give towards their efforts, and if they are working to requirements which are or are not their own.
Materials are often a factor. Consider many different materials, and the methods that render them into a finished product. Casting, molding, rolling, extruding, machining cutting, grinding, all leave behind a surface quality which can affect accuracy, the very accuracy that may be needed to reach a goal. The layout work for a piece of rolled or ground steel may have a smooth surface and take place on a granite surface plate. This smoothness of the surface qualities are enhancements to accuracy.
Wood smoothness is variable, and dependent upon the state of milling it is in. Cutting marks on an 8/4 board from the hardwood dealer can easily be in the +/- .005 to .015 range, and some board surfaces can be found that are coarser than that. Saw tooth marks, planing snipe and other machining factors are the norm. It is up to us to render woods smoother with our own processes, and some woods are rendered smooth from machining processes easier than others.
Moisture content can also play a role. Beyond the limits, which the cellular structure of the wood itself inherently provides, the smoothness from our milling often determines how well we can do with the quality of accuracy we can render upon it. We can mark it for squareness anytime we like, but the quality of squareness we get, no matter how good the tool can be degraded or enhanced by the quality of the surface we are working with. This is why it is a good shop practice to sneak up on the final sizing you need as the board is milled to final dimensions, the process can become more accurate as you go.
Tooling can be a factor. Tooling is available in a number of levels of quality, and accuracy. The higher the quality, of course the higher the cost, and the level of accuracy is commonly better, yet it does not mean that lower quality tools can not be found to be, or made to be highly accurate. It should be evaluated case by case. There are budgets to consider but I’d like to advocate that when it comes to layout tools and approaches to Metrology, it never hurts to do the best you can, and buy the best tool you can afford.
Straightness is a factor, which I touched on, in an earlier article, called The Utility of the Straightedge. Too, squareness benefits from this same straightness, and angular precision is also brought into the mix. Consider a Starrett combination square. It is adjustable, blades are interchangeable on it, commonly to 24 inches, but 36 and 48-inch blades are obtainable. At 24-48 inches away from the squares head, one can begin to realize the value of having straight edges, and precision angular accuracy coming from the tool very easily. If a square with this capability were to contain error, imagine how amplified the error would become at three to four feet from the reference edge.
High accuracy in the tooling pays you. Even when wood surface quality is poor, the layout cannot be more accurate by any tool that is not accurate. The surface quality of the wood can be improved though accurate machine setups, various cutting, planing and machining methods to enhance layout accuracy, if the layout tool can “bring it” to begin with. This means finer accuracy from layout tooling is possible if layout is performed after the surface quality of wood is improved.
Certainly the doghouse we talked about earlier will not be rendered higher quality by using a Starrett precision square, but the jewelry box will suffer if the square used to lay it out was not accurate. If you choose to tool up well, then you are free to work at any level, choosing the level of accuracy you desire, and often even verify the quality of other tools you may own, so you can be aware of the quality of layout they offer, and you can account for, compensate, or restrict the tool for use where it is adequate for the work it is called upon to do.
Wood movement is often at issue, as a reason precision accuracy is not necessary, or desirable for woodworking. Most often, the argument stems from not knowing the ways which wood moves more specifically, therefore ruling out wood altogether as a material capable of high accuracy and precision. Yet those who endeavor to understand wood movement achieve very high quality, stable results from wood in as built conditions.
Here are a few notable thoughts regarding wood movement.
Select grain structure is important. If wood stability for a project is desirable, consider that quartersawn woods are more stable than plainsawn, because the board grain does not cross the pith;
Quartersawn wood orients the growth rings radially, that is, at 45 to 90 degrees to the wide surface. Wood movement is along the rings, and rings are kept short by half or greater in quartersawn boards than that of plainsawn, and the movement from moisture content expected from quartersawn is half that of plainsawn.
Plainsawn lumber deals with growth rings from zero to 45 degrees to the flat side of the board, otherwise called tangentially, it does cross the pith, and movement in these boards can be expected to be twice that of quartersawn.
Vertical, clear grain boards will be most predictable. Avoid boards with interlocked grains, variable grains, reaction woods, tension woods, and mineral deposits, as seasonal movement from moisture content in boards like these is not predictable even at equilibrium moisture. If the figure and beauty these woods can offer is desired, it is up to the woodworker to design with this in mind, and build into their project the compensations needed for these factors.
Once EMC, Equilibrium Moisture Content is achieved, and considering the common ranges of humidity for the area of the country, as well as where the wood will reside in regards to climate control or not, and whether the air handling has air conditioning which is capable of dehumidifying are other factors.
The good news is, wood movement in a climate-controlled area, such as indoors, often has very predictable movement in select, uniform grained boards. A great deal of research and observation has been conducted over the last century, dealing with the many species of woods that are used as building materials, and the data is freely available for use to the builder. Please see Wood As an Engineering Material, written by the US Department of Agriculture, Forest Products Laboratory as one of the foremost writings on this subject. There is a copy available in the Woodworks Library.
The details can be accurately worked out. Properly sawn, select lumber is also helpful when future predictability is desirable, and most boards will not shrink more than 0.2% longitudinally from green to kiln dry, so most any angle cut on the end of a board will remain accurate, as originally cut over the service life of the piece, meaning the accuracy of squareness, can be made highly precise, and can be counted on to remain that way.
Learn to familiarize yourself with the various appearances of grain in wood, and know where and when to put it to the best use. While they can be very, very beautiful, boards of any variety containing interlocked grains, variable grains, reaction woods, tension woods, and mineral deposits will not move consistently when the seasonal moisture in these boards swings. Cupping, twist, warp and wind are all plausible factors, which will affect dimensional accuracy in any direction and the best way to deal with this is to bring these boards to EMC and let them move all they want as they acclimatize.
Begin final milling difficult boards by starting a bit bigger than the intended final size. Work your way in, so as to relieve any of the stresses these boards may have, correcting as you go, so that when you have them at the final dimension, and in the realm of the target EMC, you have the best chance of knowing the future outcomes, and then design around the seasonal movement they will still require.
Once the boards have been milled to the flatness, squareness, and dimensions needed, the next step often includes joinery, and adhesives. Joinery inherently enjoys flatness, straightness and squareness as components of it’s fit and finish. The need for close tolerances is relatively high. Glues commonly call for joinery tolerances for squareness and parallelism of .002-.005 inch, for optimum adhesion, and clamping forces will not help you achieve better results from improper milling.
If the factors affecting wood movement and surface smoothness are observed, wood should be able to be worked and milled easily to accuracy approaching .001 inch, and certainly .001-.003 inch, depending on grain smoothness. As previously discussed, not every project will require this precision in every way, and it will be up to the builder to decide what works best for the project. However if the tooling you own cannot bring .001 inch accuracy, cutting tools cannot mill to accurate lines, and the bets for precision are off, whether the project would benefit from fine looking accuracy or not.
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