There are two styles of drill bits used for timber framing, Jennings style auger bits and Forstner bits. A Jennings spiral bit has a central tip that looks like a screw, whereas a Forstner bit is guided by cutting teeth on the outside rim. I prefer the Jennings style bits because of the nature of my work. I need to drill out mortises before I pick up my chisel. I need to be able to drill 6″ into my beam without the hole clogging from debris. A Jennings style bit has spiraling up the shaft that carry waste out of the whole to prevent clogging. Forstner bits are good for shallow holes but not so good when it comes to deep holes.
I recommend getting a set of Jennings bits if you can. I found a complete set of Russell Jennings auger bits for sale at an antique show for $48. I think I ended up bundling some stuff and getting the whole set for $40. All of the bits were there, sharp and ready to go! Of course, you don’t need all these bits. You really only need a few sizes. You’ll need a bit to drill peg holes, and a bit to drill out mortises. That’s it. Peg holes are often 1/2″, 3/4″, 1″. Mortises are often 1.5″ and 2″.
If you’re doing this by hand using a brace like the one pictured above, you probably won’t be able to drill a 2″ hole through 4″ of wood. It takes a lot of muscle. A better way to drill a big hole by hand is to use a T-Brace. These drill bits have a wooden handle at 90 degrees to the bit. The handles provide leverage so you can really torgue the bit into the wood. It’s slow but effective. Another alternative to the power drill is the boring machine (pictured below).
A boring machine is simply a hand-powered drill press you set on the timber. Millers Falls was the predominate manufacturer of these machines but there are other brands out there as well. Your auger bits probably won’t fit a boring machine without some jerry-rigging, the shaft of the drill bits tend to be too wide for the chuck on a hand brace.
T-Auger bits are simply auger bits with a 90 degree handle on it. These types of augers are useful if you don’t have a boring machine and you still need to bore relatively large holes. A hand brace has a hard time drilling anything larger than 1″. You need more leverage than a regular brace can handle. A t-auger bit allows you to use both hands to turn the auger bit.