Step 7: Chop the Mortise & Fit the Joint
Now it’s time to use your mortise chisel (or “mortice chisel” for my British friends) for chopping your mortise. I’ve seen several methods for chopping a mortise, but I’ll sharing my favorite method. I like to setup my mortise chopping using a traditional method from England:
The mortise board is held stable between the clamp and a board (that I placed in a vice). The mortise board also sits on top of another board. Again, the squigly lines are where you’ll be chopping with your mortise chisel:
The width of your mortise lines should perfectly match your mortise chisel’s width. The sides of the mortise chisel will be barely touching your layout lines:
STOP! You do not want the chisel to initially chop on the end lines or else the force and expansion will lengthen your mortise opening, which would prevent a perfect fit for your tenon. So start chopping with your mortise chisel about 1/4″ inch away from each end of your mortise lines (see above). By “chop” I mean use your wooden mallet to hit the top of the mortise chisel. Chop straight downward. The angle of your mortise chisel’s bevel will cause the mortise chisel to go downward at an angle.
Keep chopping until you feel a lot of resistance, then pull the chisel straight out of the hole and “walk the chisel” down the edge of the board a little bit and chop downward again. Repeat this until you get within 1/4″ of the other end of your mortise and stop. I said stop! Just like the beginning, don’t chop on this line either…at least not yet. Flip the chisel around and repeat the chopping process going the other way, trying to start the chisel’s bevel on an unchopped spot. Your hole will get deeper and deeper.
Warnings: I try not to wiggle the chisel sideways when pulling it out of the mortise because I don’t want to alter the width of the hole. I also try not to use my mortise chisel to pry loose chips/chunks out of the mortise. Just keep chopping through those chips. If they really be come cumbersome, use an awl to remove the chips, but don’t ruin your mortise edges when prying. When you’ve marched back & forth down the mortise a couple times (and relieved the pressure) you can go ahead and place the mortise chisel just shy of each line and drive straight down to create a clean mortise wall.
How do you know when to stop chopping? I use my combination square and adjust it’s length to match my tenon’s length:
Presto! You now have a perfect depth gauge…just stick it in the mortise to see how close you are to your final mortise depth:
When your combination square bottoms out, then it’s time to stop chopping.
Note: If you are chopping a through-tenon you would just repeat this marking & chopping process on the other side, and meet in the middle. You shouldn’t try to exit through one side…it’ll give a nasty exit wound. Alright, back to normal mortises:
Now grab your 1/4″ bench chisel (or smaller) and pare/trim the inside edges of the mortise until they look vertical.
But don’t pare too much or the joint may become too loose.
Try dry fitting your tenon first to see how tight it is. Don’t force the tenon into the mortise with a heavy mallet blow, or the mortise could split. I prefer using just my hands:
If your joint feels too tight, but your mortise walls look nice and vertical, then don’t pare the mortise anymore. Turn your attention to trimming the tenon instead. You can trim it using a shoulder plane or a wood chisel. Just take small cuts then retest the fit.
But don’t let all this trimming & paring scare you. If you followed this tutorial, then you shouldn’t have to do much trimming, if any at all. Hand pressure, or a few light taps with a mallet should seat the tenon in tightly.
The more mortise and tenon joints you make the faster you’ll get at it!