Apr 06 2008
Precision refers to the amount of dimensional accuracy or incremental refinement used when something is made, and can be attributed to the quality of the layout, workmanship, or machine set up.
Accuracy refers to the confirmation of dimensional tolerances.
Dimensional tolerances differ with the various types of projects a woodworker will commonly undertake. The set up of shop machines and precision hand tools often requires the precision of accuracy to be at the thousandth of an inch level, however most woodworking projects require accuracy at a level which is commonly referred to by fractions, and is often referred to in the 1/32nd (.031) to 1/64th (.016) range.
The quality in our craftsmanship is inherent in our understanding of these constructs, and our personal stake in setting for ourselves, a level of tolerances. These tolerances are the differences between woodworking, and fine woodworking.
It may not seem relevant, but here is an analogy for higher accuracy. A surveyor will set up an optical instrument, and first, sight their back sight. What they are establishing is a couple of different things, but what is important for us to know for this discussion is that the further away the back sight is from the instrument, the higher the precision of accuracy will be when the surveyor makes other measurements that are shorter than the distance between the instrument and the back sight. The practice is sometimes referred to as going long, and is meant to create higher precision.
One of the common things I have heard over the years, is that in woodwork, a high degree of accuracy is not needed, and then there is the ever ubiquitous, “wood moves anyway”. The understanding being overlooked here is that a lot of assumed accuracy is inherent in the process, because it has been manufactured into the tools we buy, as well as a lot of the lumber we purchase, and we take for granted that it is already “there”. Even wood movement is understood and can be compensated for with relatively high accuracy. None of these assumptions fully get us off the hook.
Consider the ruler. Sure, the ruler has the increments we need, the 1/32nd, and the 1/64th… But we rely on the very same precision accuracy at the fractional level to be consistent to the thousandth of an inch, to assure each of those graduations are where they’re supposed to be. Someone in some lab and factory put all that accuracy into our tools. If we want our precision to maintain 1/64th accuracy, it has to be consistently maintained to 1/64th, plus or minus .001-.002, otherwise the eye will be drawn to errors. After the tool has done its part, the rest is up to us.
Unfortunately, not all levels of woodworking accuracy can be assumed. There are some levels that each of us working the tradecrafts are personally responsible for, and things go better when we are mindful of them.
Take for instance, straightness. The layout of lines is many things, but few things in the layout of lines are required to have the precision of accuracy we have come to expect from straightness. From precision straightness, we can evolve precision flatness, and also use precision straightness as a construct of precision squareness. Parallelism is yet another important derivative of straightness. How straight, straight is, is a pretty important matter. It is always best to start from the best we can do, as it will surely be degraded from there.
Think about the tooling we use to create straightness and flatness. It is inherent in the tooling and machinery. It had to get there somehow. We have to accept that the industrial designers, engineers and machinists did their part, and many woodworkers rely on the good graces of a millwright they never met for a lot of built in accuracy, but there is another part, which they left to us.
One of the more important tools a woodworker can own is a good straightedge. You can have them short or long and there are a number of makers offering them, but if you choose only one, a two-foot straight edge offers a lot of well-rounded utility to the Woodworker. Once you have one, what I want to encourage is; the use of it. Sure they are high accuracy, but it isn’t just for hanging on a peg and looking at.
Straightedges in the woodworking shop have a lot of application. They are available in both steel and aluminum, however they all have more utility if they are made from flat bar stock. Steel straightedges are generally made from stress relieved, 01 steel and are hardened. They are precision milled straight and parallel, and often offer accuracy generally to .001 over the length of the tool. The manufacturer will state the accuracy of their tool, sometimes offering a letter of certification as well. They lay flat on their backs for scribing or drawing lines, and stand on their edges for the comparison of surfaces. They are available with or without beveled edges, and with or without graduations for measurement, but these upgrades are not a necessary requirement, and usually add cost. If you can only afford one, it is better to leave measuring to steel rulers and tape measures.
I find the non-beveled, non-graduated types are less expensive and if it is less specialized, then it usually will offer more utility. Another rational is, that the less it costs the more likely a craftsperson will own it, and if you don’t have one, you can’t put it to good use. There is a lot of good use to be had. Longer than the average ruler and better quality ones are thick enough most generally to stand on edge.
For layout work, the straight edge is a heavy, wide tool, which stays where you put it and has a tall side, which is great for the marking of your work. It is very comfortable for use with any pencil, and it really shines while a marking knife is registered against it. It is an excellent way to connect all the straight lines after you have laid them out. It is also a very nice extension for use with the squares you have and will extend the reach of shorter tools when more reach is needed.
To the hand tool user, the straightedge brings a lot of utility. It can be used to verify the soles of hand planes. After you see where the work needs done, you can then lap the soles to correct the issues and verify as you go. Feel free to verify the flatness of your honing equipment. Flattening the workbench with the use of feeler gauges, a straightedge, and marking is a great use of the tool, because the high spots can be found and removed. The flatness of the workbench is a frame of reference for all future work that comes off it.
Is your board, especially when prepped by hand ready to accept the joinery profiles you intend to put in them? The flatness and trueness of boards is crucial for the fit and finish of dovetails. The plowing of slots and grooves such as dados and sliding dovetails, as well as the treatment provided by hollows and rounds are always made to look a lot better on boards that have been properly evaluated as ready by a straightedge. Handwork is a challenging process; why not evaluate the needed quality before moving to the next part of the process? Besides, the evaluation of a freshly jointed board edge, is just a quick quality assurance check, and a savior before you find an error in mid glue up.
The straightedge is also useful when evaluating the cup, twist, and wind in boards as well as evaluating the flatness of panel surfaces. A pair can even be used as winding sticks. Another good use is for establishing the straightness of the chute edge and fences on a shooting board as well as the overall flatness of its surfaces. While you are at it, evaluate your other shop built jigs from time to time as well.
For machine setups, routine adjustments and maintenance, the straightedge is a great tool. It is invaluable for evaluating the surfaces of the jointer beds for parallel and coplanarity as well as the proper calibration of its vernier settings.
The table saw can be evaluated for table flatness, which is not uncommonly found to be less than perfect yet in some cases correctable. There is also the adjustment of side tables, out feed tables and the trueness of miter slots. It is also valuable to know what the relative flatness and straightness the fence faces have. If there are anomalies, you can then compensate or adjust for them.
Miter saws can use the straightedge for evaluating the trueness of the fence, and are also aided by the straightedge when side wings, when used, are leveled with the main surface of the saw.
A straightedge can also be used for the routine set up of roller stands when used as an in feed or out feed support on any shop machine.
The router table is a high precision shop machine which is commonly shop made. There are many uses for the straightedge with this tool. Evaluation of the tabletop is a constant need with some designs due to the weight the tabletop supports. Many designs are under built and table sag is an error inducing issue. The plates often used to fit the router to the table can be ill fitting in their mortise, and require fine adjustments be made, in order to be brought flush with the table surface.
The router table fence is often in need of straightedge evaluation as well. It needs to be flat and straight, if split, it also has a need for coplanarity. It also must be evaluated to determine if it has any tendency for deflection. The router fence is also a candidate for using a straightedge along with 1-2-3 blocks, gauge blocks and feeler gauges for the settings of the fence and router bit height. With these tools in use, on a well-made table, one can expect fully repeatable accuracy from a router table to be in the .001 range.
The evaluation of any wood, which has been prepared for milling, is important as well. Any cup, twist, warp or wind is something that will throw off the fit and finish of the simplest joinery, and even make edge treatments like bevels, round-overs and more sophisticated profiles look awful. Further, these evaluations can make a lot of difference as to how safe a milling process may be. Knowing ahead of time saves a lot of needless frustration. There are few tools available to the woodworker which can assure things go right, and evaluate why things go wrong, with more power than a straightedge.
If I thought about it, there is probably much more which could be said about such a simple tool, but this is a reasonable well-rounded look at it. It may seem to be a cost prohibitive tool to some, but after thinking outside the box with me awhile, you see it has so much application, and with its evaluatory prowess, how much money could it save you in error free or error caught woodworking, even over the short run? In my shop, it has more than earned its keep and continues to, as I find that wood is costly, even more so than tools. In fact, around my shop, the straight edge offers more value than many other needed tools, and if you can get your mind around that, one will serve you just as well. It can touch so many aspects of your woodworking, that is, if you give it a chance!
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