I’ve become convinced that if we are going to successfully revive pre-industrial tools and techniques, we are going to have to understand and come to grips with pre-industrial tolerances. We need to know how square our “square” ought to be and exactly how smooth is “smooth.”
When a machinist’s square is placed on the surfaces of period furniture, one is hard pressed to find anything we moderns would call “flat.” Even discounting instances of warpage from the ravages of time, the surfaces of rails, the flats of tapered legs, drawer faces, and even tabletops display a refreshingly human workmanship. It no longer surprises me to find that the undulations on a “flat” drawer face allow even my thickest feeler gauge (0.025") to freely slide underneath a straightedge. In practice, the wider the board is, the more acceptable extremities of variation will be. An edge joint requires exacting standards, but the taper of a table leg only needs to look straight.
And that is, by the way, the only legitimate and sensible (in the most literal meaning of the word) measure of any surface that has no more than aesthetic concerns: does it look flat, smooth, or lovely? If it does, then it is. So, let the exercises in this chapter be an opportunity to train your eye and trust your fingertips to tell you when enough is enough. Leave the Starrett in the tool chest and learn to embrace sensible tolerances.
-Joshua A. Klein, excerpt from his upcoming book Worked: A Bench Guide to Hand-Tool Efficiency open for pre-orders soon.