Cutting a mortise with a registered mortise chisel
When making mortises by hand there is one way which is supreme and that's using a registered mortise chisel (these are ground with parallel sides and are taller than they are wide to help keep them straight in the cut). I needed an 8mm or 5/16" chisel for this project as I had designed it that way. I bought a Narex chisel as I'd heard good reports about this brand. My existing mortise chisels are English made Crown brand made in Sheffield.
First of all if you have a mortise gauge use the chisel to set it's width and lock it off. You then only need to move the fence to set the position of the mortise.
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| Setting the mortise gauge width |
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| Setting the mortise fence |
Then mark the position with a knife to sever the across the grain and the mortise gauge to mark with the grain. The defined perimeter is then undercut slightly at the ends to create a knife wall.
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| Using a knife to mark cross grain |
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| Marking the mortise |
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| Mortise perimeter defined |
Using the chisel position it around 3/32" from the end of the mortise within the perimeter. Make sure the bevel is facing away from the end of the mortise and
lightly chop.
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| Initial mortise chisel chop |
Move the chisel on about 1/8" and again lightly chop. Go all the way to the other end of the mortise stopping short by 3/32" or so and make sure to turn the chisel through 180 degrees before you reach the end (the bevel facing the other end of the mortise).
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| Series of light chops |
You can then break up the wood with a bench chisel turned at 90 degrees to the mortise chops and carefully remove the waste.
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| Breaking up the waste |
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| Remove the waste with a narrow bench chisel |
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Using the chisel bevel down to clean up the
mortise bottom |
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| Undercutting the knife lines |
You will have gone down about 1/8" into the wood and then created a wall either side that you can then use to register the sides of the chisel against.
Proceed with chopping the mortise out using the same procedure as before but now as you get deeper you can chop harder and remove more in one pass.
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| Chopping to depth |
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| Measuring the depth |
Once you have got to depth you can then remove the ends with one or two vertical chops. This will leave vertical walls at the ends. Clean up the bottom with a regular bench chisel scraping out any waste (it doesn't have to be perfect), remembering not to lever waste out by leaning on the edge of the mortise. You can also use a swan neck mortise chisel to clean out the bottom if you desire.
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| Chopping the ends of the mortise |
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| Full depth |
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| Chopping the other end |
The end results I've found are more accurate than using a router and you don't end up with rounded ends that either need squaring off or the tenon needs its corners clipping off.
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| Mortise complete |
You can also cut mortises by hand when you've already part glued up the sub-assemblies as I had. There was not way possible for me using either a router nor my dedicated mortiser to do this as I just couldn't hold the workpiece. The one disadvantage of cutting mortises by hand is that it is slower than cutting by machine but on small volumes might actually be quicker than having to set a machine up.
The Narex mortise chisel by the way is superb and as good as any of the others I have in my arsenal.
If you do want to cut mortises entirely using a machine then the moral of the story is to get the design right BEFORE you start cutting wood. I didn't!
