Measure twice; cut once. Good advice, but it leaves out an important part: Do the math.
Math was always a good subject for me in school, and I typically got A’s in it. Sudoku bores me, because it’s just simple math. And for some reason I’ve always been able to count a bunch of stuff just by glancing at it. For example, with a single quick look at a small bowl of apples I can, assuming they’re all visible, tell you how many there are without actually counting them. Nice party trick.
But here’s the thing: I’m lazy and often don’t pay attention to numbers I come up with. I also tend to pop a number into my head, whether the number has any connection to reality or not, and just go with it. Not a problem if the number is correct, but if not…
This can be disastrous in the shop. Let’s say I want to add 5" and 7". Easy-peasy with my brain turned on: the answer is 12". But if I’m not paying attention I can far too easily add 5" and 7" and come up with 2". You see, floating around somewhere in what passes for my brain is the fact that 5+2=7, and I can mathematically transpose the equation and grasp 2" as the number I’m looking for. Kind of like a mathematical dyslexia. But the issue is that I don’t question it. I’m satisfied with 2" and go with it. Everything that follows based on that incorrect math ends up wrong, of course.
Sure, that’s a drastic example above – what sane person can mistake 12” for 2”? (Please, don’t answer this.) But the fact is that I do this often enough when I’m not paying attention that I try not to do math in my head.
Seriously. Whether it’s balancing the checkbook or laying out cut lines and dimensions in the woodshop, I use a calculator whenever possible. Other times while working, I’ll write the equation out on the back of a piece of scrap to visualize it. Then I’ll do the math again, just in case I wasn’t paying attention the first time.
Fortunately, since this is a personal shortcoming I know about, I almost always take steps to avoid it happening and things go pretty smoothly in the shop. Needless to say, however, those steps mean nothing if I’ve only measured once.