Accuracy and Precision
Issue: Issue 2.20
Posted Date: 11/20/2001
Accuracy and Precision (WoodCentral)
The ability to accurately and precisely measure the wood your are cutting is aconstant challenge in woodworking. Wood moves, saws have runout, fences can be misaligned and countless other factors can conspire against your determination to cut the four legs of your latest project the same length. But what if the tools that you use to measure your lengths and widths aren't even right.
That was the crux of one discussion this week when a woodworker asked about the accuracy of using steel tape measures. Seems there are many schools of thought on the proper tools for measuring. One school swore by the steel tape measure as long as you measure from the 1" mark and always use the same tape. Otherwise the end tab, the thing you hook on the end of your board, tends to move and not give consistent results (depending on how loose it is).
Others swore by metal rules while still others were committed to the merits of 6' folding wooden rulers. One person said that metal rulers contracted and expanded too much based on temperature to give a good reading. Others countered that wooden rulers would expand and contract just as much during temperature changes and would also be susceptible to humidity changes. Still others said the joints on the folding rulers loosened over time, giving inaccurate results.
The debate was pretty well summed up by the person who cited this definition:
"Accuracy: The ability of a measurement to match the actual value of the quantity being measured. Precision: The ability of a measurement to be consistently reproduced."
The latter, argued many, is the real goal of woodworking. It's not as important that each leg of the table be 23" as it is that they all are the same length. This evolved into a short conversation about story sticks. Story sticks are measuring devices you build when you need to do a lot of the same cuts, e.g. the drawer fronts on the 10 jewelry boxes you are working on. You make one stick for those drawer fronts and use that to measure with. One woodworker wrote, "I'll add that it doesn't matter what you use. A garden hose will work as long as you use the same garden hose for transferring lengths. Story sticks are a great way to build stuff, and with story sticks you could avoid numbers altogether."
Another told of his machinist friend who has trouble with woodworking because he can't get it through his head that wood moves. He carefully measures the pieces, working with precise instruments and detailed drawings. But by the time he cuts the next piece, a few days later, the wood has moved and the pieces don't fit together well.
The lesson here seems to be: making consistently precise cuts is more important than making terribly accurate cuts. And if you're a stickler for details, wood can be an unforgiving mistress.