By Robert W. Lang
One of the most important skills in woodworking is rarely discussed or considered as a thing that needs to be learned or practiced. The basic skills of measuring and its close cousin, layout, are essential to produce quality work. As a bonus, mastery of these basics reduces frustration during the building process.
But things aren’t always what they appear to be – measuring is a risky business. To be successful, you need to know what can be trusted and what is likely to lead you astray. Any measurement is only an approximation; no matter how precise you think you are, someone can come along with a better device and a finer unit.
Measuring in fine increments has diminishing returns when it comes to making things, and there are swampy areas on the road to precision that can make fitting one part to another more difficult. The goal in woodworking is to have pieces that fit each other and look nice when finished. That goal may or may not require hitting every desired dimension exactly.
The difference between a tenon that fits nicely and one that rattles around in the mortise is quite small, whether you refer to that difference by a decimal point followed by a few zeros, or as a “smidgen.” Instead of declaring one method or another as absolutely right, let’s look at where and how things can go horribly wrong – and how to keep on the right track.
Don’t Agree to Disagree
An easy way to get in trouble is to assume that a piece of wood is square, straight or the correct dimension. It doesn’t matter if it’s a piece of 1x purchased S4S, or a piece that you milled from rough lumber. You need to prove things, but you also need to prove that the tools you use are accurate.
The common suggestion is to use one measuring device throughout a project – but if one device was suitable for all measurements, there would be no need for small precision rules, calipers or folding rules. Different tasks demand different tools. The better approach is to check all of your measuring tools for consistency and get rid of the ones that disagree (see picture below). If you measure twice with the same tool and method, the odds are you’ll repeat a mistake instead of catching it.
Proving a square is simple; all you need is a straightedge and a sharp pencil. Draw a line against your square, then flip the square over and draw another line. If the lines coincide or are parallel, all is right with the world. If there is a gap at either end of the two lines, the distance between the lines is twice the actual error.
From the April 2013 issue #203
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