[Edit: I made a video that includes this technique in action.]
The mortises in the new bench/whatever-it’s-called are pretty big, and I can’t do them like my usual ones, because I don’t have a mortise chisel that large. So I’m using the old “bore holes to get rid of most of the waste and chop down all sides” method.
This got me thinking that there are a few extra things I do for these that might be useful to someone. So let’s go through them. First, when marking out the mortise, I scribe a line along the center of the mortise:
This is useful for the next step, boring out most of the waste with brace, because I know exactly where to put the lead screw of the auger bit:
Notice that I’m using a square to help keep the bit parallel to the long sides of the mortise direction, and a piece of painter’s tape on the bit to mark the depth. I liked these particular mortises a lot more than the ones in the top, because there was plenty of room to spare when boring down. Going a bit beyond your tenon depth with the brace makes it a lot easier to get out the required amount of waste. (And through mortises are cheating.)
Another note on the holes is that I’m staying a bit away from the ends by, say, 5mm or something. This makes it easier to chop the ends later on.
Next it’s time to get most of the waste off the sides. I’m using an old W. Butcher, uhhh, I guess it’s a firmer chisel, for the sole reason that it’s beefy:
You don’t want to go all the way to the side yet, and while you’re doing this, pay attention to the grain on your initial cuts. Because the wall of the mortise is longer than the chisel, the wood will split in the grain direction past the end of the chisel. If it splits in the wrong direction, you could split it out past your marks for the wall of the mortise.
Now that you’ve figured out the grain direction, mark the “low corners” of the mortise. What I mean by this is that the corners where, if you start splitting the grain at that point, the split will extend into the waste part of the mortise.
And of course, here I am, marking an incorrect corner:
In this board, the correct ones are the top left and bottom right. You’ll see this mark “move” later on in this post after I figured out what was going on.
If it’s just a normal, single piece of wood that you’re mortising, these “low” spots should be at opposite corners. Your grain could reverse or do some other horrible thing, though. And the piece above has the mortise spanning two laminated boards, so it’s possible that it wouldn’t follow that rule, either.
In any case, once you’ve got this done and enough of the walls wasted away, you’re ready to work into those “low” corners. Start with cross-grain chopping on the ends, but don’t go quite all the way to the side, and keep some wood left on the ends (remember that I’m ignoring that first erroneous mark here):
Then, when you’re close, chop down the side with a narrow chisel:
Notice how the waste is splintering out to the waste side of the mortise wall. You might need to do a little back-and-forth to get to your line on the side, but it shouldn’t be too bad with a narrow, sharp chisel.
You also might need to be a little cautious if your wood is uneven. This is (southern) yellow pine, and the darker latewood in this stuff can really shove your tools around. Douglas-fir can do that, as well.
When you’re all the way to the bottom of that corner, grab a wider chisel and chop the wall right next to the corner:
Work your way to the other side in this direction. The wood will continue splitting into the waste, as you can see above. When you reach the end, you can square it off like the “low” corner; when chopping the wall over there, it should not split because you can overlap the stuff that you’ve already done; the chisel will slice instead of tearing out at the border of where you already worked if you keep it flush to the wall.
Then you can go all the way to the ends. I don’t have any photos or anything of this; just use the standard practice of working your way back to the ends until you have enough left that you can cleanly chop it without bruising the wood. Though it’s not so relevant here, it really matters on a through mortise.
You’ll probably need to tune the walls a little due to the way that chisels tend to cut. One nice thing about these big mortises is that it’s really easy to stick a square down there to check to see if it’s complete:
Notice how the “low spot” mark on the near wall has migrated from the left to the right in this picture. Sigh.
So maybe this is helpful. I don’t know. Mortising can be time-consuming. I really understand the reason that hollow-chisel mortising machines exist. And I have to admit that they are kind of tempting.
In any case, the joint that I made in this post was the last one for the new auxiliary bench:
It’s ready for glue-up.