Brendan’s video demonstrating his “chairmaker’s sighting square” was a bit of a joke. But the square itself is something we use all the time in the shop when drilling tricky angles.
You don’t have to build one of these squares – an aluminum framing square works just as well – but the wooden ones are nice. Here’s how the square works.
First the driller places the bit on the crest rail and lines up the drill with the hole locations on the crest and the seat (totally by eye). The driller can easily see if he (or she) is tilted too far left or right. But he can’t see if he’s tilted too far forward or back. This is where the sighting square comes in handy.
Place the sighting square on the crest with one of its legs in line with the drill and the bit (shown above). The “boring buddy” then sits at the other end of the square – basically, 90° to the axis of drilling.
Then the boring buddy holds up the sighting square, lining up the point of the drill bit with the hole location in the seat and one leg of the square. She can easily see if the driller needs to lean forward or backward to achieve the correct angle.
Works every time (as long as the boring buddy isn’t blind).
— Christopher Schwarz