One warning we’re constantly making in our shop is: “Watch out for that accumulated error!” It is so often heard that people have started to roll their eyes. But, as accuracy is one of the cornerstones of the woodworking process, it bears repeating. The principle of accumulated error is well known to anyone in manufacturing, but many woodworkers aren’t as familiar with it as they should be. It boils down to this: an “acceptable error” made many times in the same piece will become an “unacceptable error.” Meaning, while it may be an acceptable error for a 2-inch board to have a mere 1/32 of an inch of variance in its overall width, if that same variance is repeated on each of 16 boards that combine to make up a 32 inch surface, the resulting accumulated error will be a full 1/2 inch. This accumulated error is a problem that ranges in scope from “not too bad” (say, of an isolated table base or top) up to a “disaster” in the case of a piece of tightly fitting joinery, or in a set of angles. Anyone who’s tried to make a picture frame for the first time or anything with a set of angles, knows that if the first corner cut is one degree off it looks okay. The second corner cut being one degree off may sort of look alright. However, once you put that third corner error together with the others you already know you’re in trouble and then when the fourth corner error is placed with the others you see that your are now off by a full 4 degrees.
That said, woodworkers are helped by the fact that as a medium wood can be reasonably forgiving. As long as all cuts are made “square” a variance of 1/16 of an inch will largely go unnoticed in most projects. Fortunately for us, barn wood is even a little more forgiving that younger cleaner woods. The highly featured surface of aged wood hides many imperfections but while that helps us it certainly doesn’t make our task easy. We still have to deal with the basic challenge of getting square cuts from vintage lumber whose hand-hewn history generally means that the boards contain no truly square angles, in any dimension. That’s the challenge and the fun … as long as we watch out for accumulated error.
And watch we do. Aside from following the old woodworkers adage: “measure twice, cut once,” the best way to correct an accumulated error problem (and others) is to avoid it to begin with. The more precise the calibration of our equipment is to begin with, the less likely it will be to have any problem. You can find out more about calibration technique by reading woodworking literature or watching internet videos of some pretty ingenious guys calibrating their tablesaws, crosscut sleds, etc. to have them be accurate to within a thousandth of an inch.
Keep your eye on the Rustic Grain blog, where soon we will post a quick clip from our shop dealing with this issue and showing off one of our new toys. Until then, be on the lookout in your own project work for that stealthy headache known as accumulated error. Attention to detail is the solution.