Although I was sure that I’d read somewhere that there was a way of using a mirror to help judge when a drill bit was truly vertically, I struggled to find an account of how it was actually done. Eventually, after a lot of googling, I came across this letter and illustration published in Popular Mechanics nearly 80 years ago.
To find out if it worked, I bought a cheap handbag mirror.
First I cut off the hinge and trimmed back the plastic mount along one edge.
Placed next to the drill bit, the mirror showed when the drill was vertical…
… and when it wasn’t.
It’s a simple idea but I was impressed by how well it worked. A problem though, is that the mirror only tells you whether the drill is vertical in one axis. You have to move the mirror around the drill to check whether it’s vertical in the other axis and while you’re doing this, it’s easy to lose the vertical on the first axis.
One solution might be to have an L-shaped mirror or, perhaps better still, a mirror with a hole in its centre. Then, all you’d have to do to check that the drill was truly vertical in both axes would be to move your head.
So I ground a small hole in the centre of the other mirror and tried it out.
This is the view when the drill is vertical.
And when it’s miles off.
Of course, you don’t need a mirror to see when the drill is as far out as that. The benefit is that it makes it easy to spot small deviations from vertical.
Does it work in practice? As a test, I drilled ten 2mm diameter holes at 10mm intervals along a line in a piece of MDF and stuck cocktail sticks into them.
Not perfect – but not bad either. Certainly better than I was able to do in a repeat of the experiment when I used a small try square instead of the mirror as a guide, as you can see below.
Obviously, the best way to drill a truly vertical hole is to use a drill press. But there are occasions when this is impossible because the work piece is too large or too awkwardly shaped. It’s then that this trick with mirrors might come in handy.
Finely Strung 