Join Date: Apr 2010
Location: Near Boston, Massachusetts
My experience cutting mortises in douglas fir is that it just plain sucks. Either the wood is wet, in which case is comes away in stringy chunks, or it's dry, in which case it shatters. Here's how I finally made it work on some fairly dry posts:
1) Mark the mortise well. If it's a through mortice, mark both sides.
2) Pre-drill. Use a drill press, bit brace, whatever.
3) do a rough cleanout with a medium sized chisel. The ones I were cutting were almost nine inches long by one inch wide, and I used a 3/4" chisel for the first round of cleaning out. Don't worry about getting the walls straight, just get the big chunks out. If it's a through mortise, go halfway from one side, then flip the piece over and work from the other side.
4) Clean up with the widest chisel you can fit. I used a 1" for the ends and a 1 1/2" for the sides. I tried to clean out from the outside to the center sloping up, meaning the board that was a perfect fit when it entered the board wouldn't fit at the center. Once I had the edges of the mortise clean and straight, I used the wide chisel to slow pare away the excess inside, working on a diagonal. I finally put a mark all the way around the edge with a pencil to make sure I wasn't going to cut the edge, and that let me speed things up a little.
If your final work will be in something other than douglas fir, I recommend trying it in that. You may find it's a lot easier.