I'm relatively new to hand-cutting mortises. I just bought myself a set of Sorby mortise chisels and set to work on a hunk of walnut for a project I'm working on (a piano bench)
I've got twin haunched tennons on the rails going into the legs. Looks kind of like this:
---------------+ | +------+ | 3/4" x 1-1/4" deep | +---+ | 3/4" x 3/8" deep | +---+ | 3/4" x 1-1/4" deep | +------+ | ---------------+
The rail is 3/4" thick and the tennons are 3/8" thick.
The question is, how best to cut the mortise? Would you chop out the full 2-1/4" width down to a 3/8" depth, then dig two deeper recesses for the tennons into the bottom of that? Or would you dig the full-depth recesses for the tennons individually, then dig out the shallow haunch in between?
My initial idea was the first way. The problem is that after I dug out the whole 3/8" deep by 2-1/4" long trough, I realized I didn't have any good layout lines on the rough bottom of this to start the inner walls of the deeper parts. Doing it in the reverse order solves that problem, but seems like it will be a lot more complicated to clear chips, etc.
Which way would you do it? Is there a right or wrong way, or just personal preference? The plunge router beckons, but for now I'm trying to go the traditional route and do it by hand.
You need your layout lines, so dig out the full depth mortices first, treating them as two separate mortices. You'l save yourself a lot of potential grief of you don't cut the legs to finished size at this stage - leave an extra length of 1 - 2" on the tops of the legs. This will allow you to get some serious leverage without the thin portion above the mortice breaking out, and will also give you an area to clamp onto without marring the project.
You'll also do yourself a favour if you arrange for the width of the mortice to be exactly the same as that of your mortice chisel - pick the chisel that's nearest to 1/3rd the thickness of the stock. To this end it's best to select the chisel you're going to use first, use it to set your mortice gauge, dig your mortices, then use the same gauge to mark your tenons. Cut the tenons last, then fine trim them for an accurate fit with a shoulder plane.
Clamp all 4 legs together, then mark off all horizontal lines as one unit. Unclamp them then use a mortice gauge to mark the vertical lines, and another marking gauge to mark off the vertical centre line of each mortice. Normally it's good practice to knife in the horizontal limits of the mortice, but this line is apt to break out under heavy leverage, so don't do that in this case.
Clamp the legs back together in register (this will support the side walls of the mortices), and clamp the whole assembly at 90 deg to the bench, with the top of the legs towards you - working from the end like this will help to keep the mortices truly vertical.
You're right about chip clearance - such narrow mortices don't leave much room. So what you want to do is to clear out the majority of the waste from the deep elements by boring it out. Use a drill that's the finished width of the mortice, and mark off the depth of the mortice with a bit of tape around the bit. Auger-type bits remove the waste best, but use a slow drill speed or a hand-brace and watch the leadscrew doesn't break out on the far side. If you can, try to arrange things so that you leave around 1/16 -
3/32" waste at each end of the deep mortices. This will give to an area to lever against with your chisel Remember to tape your chisel to depth as well.
When the deep mortices have been cleaned down to depth ( an airline can come in handy here for clearing waste), you can then knife in the horizontals and, holding the chisel absolutely vertical side-side and fore-aft, with a mallet trim the 1/16-3/32 waste at each end of the mortice to the finished line.
The area between the tenons can then be tackled. Drill, chisel, then clean up to finished depth with a router plane (Record Stanley 71) if you have one. If not, then clean it up by chiselling along its length with the chisel bevel down, working with the grain.
Once you've glued up and allowed to cure, cut off the extra horns at the tops of the legs.
I would do a practice piece to work out my technique. I would chop two separate mortises and then chisel out the slot in between. Have you seen the Frank Klausz video?
In my mind, there is no reason to drill out the mortises. I've ended up spending more time cleaning up the mortises with a chisel than it takes to chop them with a chisel from scratch.
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