Boring a straight hole with a brace and bit is a task that takes a little bit of technique and a lot of practice. There are a few ways that I do it without any kind of guide or aid, and each of them has their place. But all of them are reliant upon having a practiced ability to see plumb and square. This ability comes with time and practice. The more you use your brace and bit in different situations, and at different angles, the better you will get with it.
However, there are a couple of aids you can use that will help you along as you practice. You can use a try square or two stood up on the surface when you’re boring vertically. Sighting along the squares will help to guide you nice and plumb. If you don’t have two try squares, you can do the same thing with a couple of blocks of wood that have a square corner.
Another trick that I quite like is to bore horizontally instead of vertically. By clamping the piece in the bench vise, you can approach the work from a more comfortable position rather than having to get right over top of it. Working this way, you can site along the bit to make sure you are boring square to the surface. This is easier than you think as you can naturally see “square” fairly well. If you’re unsure of yourself, it’s easy to clamp a square alongside the bit as a guide.
To help you with keeping the bit level, slip a ring over the shaft of the bit. Here I’m using my wedding ring, but any ring will work. It’s actually better to use something with some weight to it as a heavier ring will wobble less, but a finger ring works in a pinch. The goal is to bore the hole while keeping the bit approximately centered on the straight area of the shaft.
Put the ring on the bit and start boring your hole while watching the ring. If the ring begins to move away from you as you are boring, it means that you’re holding the brace end of the bit too high. Drop the brace lower to level the bit, re-position the ring at the center of the shaft and, try again.
If the ring begins to move toward you as you are boring, it means that you’re holding the brace end of the bit too low. Raise the brace higher to level the bit, re-position the ring at the center of the shaft and, try again. Eventually you will find the sweet spot where the ring will just kind of dance back and forth around the center of the shaft and you’ll know that you’re holding the bit level.
So if you’re having trouble boring straight with your brace and bit, try boring horizontally using the ring trick. It may seem like a parlor trick, but it really works well when you have to be very precise with your boring.
To see the ring trick in action, you can check out a video I did about 8 or 9 years ago when I built my English workbench on my YouTube channel here https://youtu.be/pyfmITHGKpI.