For timber framing, you’ll need at least one chisel. Most mortises in a timber frame are 1.5″ or 2″, so if you only own one chisel, I would make it a 1.5″ framing chisel. With this one chisel, you could cut an entire timber frame. Other size chisels just make the work easier and faster, but it’s not necessary to have a whole set of chisels. Most woodworkers are familiar with a bench chisel or a pairing chisel. These are smaller chisels with beveled sides used in fine or finish woodworking. For timber framing, we need big, thick, long mortising chisels. They have square sides instead of beveled sides. They’re a lot thicker than a regular bench chisel. They’re also a lot longer than a bench chisel so they can reach the bottom of deep mortises.
The two chisels pictured left are both 1″ T.H. Witherby socket chisels. The one in back is made specifically for mortising, you can see it’s a lot thicker than the chisel in front. Both chisels are sturdy enough to be used in timber framing. If you’re planning on cutting 1″ mortises without pre-drilling, get a really thick mortising chisel like the one pictured here so you can really hit and lever it hard. If you’re pre-drilling your mortises, you may prefer a thinner chisel like the framing chisel in the front of the picture.
Chisels come in two styles, tang and socket. A tang chisel is a chisel with a long pointed metal end that the handle fits around. A socket chisel has a hollow end that the handle fits into. For timber framing we need something we can really whale on when cleaning out a mortise. I only buy socket chisels because I fear a tang chisel just won’t stand up to the abuse.
The handle should be made of a durable hardwood that can take a beating. Ash, Hickory, and Oak are classic examples. The handle of the chisel is often turned on the lathe. An alternative handle shape is an octogonal shape. The octagon maps to the joints and angles in your finger joints when you grip the chisel. It also prevents the chisel from rolling off the beam you’re working on.