It’s been a busy week and I have just now found a minute to continue this series on my Roubo workbench. I left off the first post with a pile of S6S leg and stretcher blanks. I now needed to create mortises in the legs to receive the stretchers. The proto-legs and the drawing with mortise dimensions are shown above.
The first step is to figure out which faces will be facing out, and which legs go in front, left, right, etc. The construction-grade douglas fir boards I selected are pretty decent actually and with a bit of thought I can get most of the show faces to be knot-free.
With the legs marked for their positions, I can start laying out the mortise walls. Actually, I need to lay out the tenons on the stretchers, and then transfer these to the legs.
Here I am setting my Veritas dual marking gauge to 1/2″, which is the width of the tenon shoulders on all sides.
And marked around the end of a stretcher:
The left side is unmarked on this one because it is a top stretcher, and I want a slightly shorter mortise so as to leave some meat at the top of the leg, below the leg-to-benchtop tenons. All the other stretchers will get an even 1/2″ shoulder on all sides.
With the stretcher ends marked, I can take the marking gauge and set both cutters, in preparation for marking the mortises.
The Veritas gauge is perfect for this kind of thing. The thin cutting wheels slip definitively into the initial knife lines, and then each rod can be locked into place. With that positioning preserved, the second lock knob (the one closer to my hand) is secured, which maintains a set distance between the two cutting wheels. Now the main lock wheels can be loosened, and the gauge fence moved until the pair of cutters is aligned for the mortise.
That works perfectly for the side walls of the mortises. For the short walls, I take a leg and measure for the bottom wall of the mortise, and mark it. The top wall will be located by using a story stick derived from the actual tenon the mortise is to receive:
Above is how the stick is created. After this the stick can be placed alongside the leg, and one mark aligned with the existing mark for the bottom wall of the mortise. Then the other mark can be transferred with a square to define the top wall, which with any luck will be exactly the same distance from the bottom wall as the tenon cheeks are.
All of that ends up producing this:
The pencil lines are not important; they are only to give me an idea of where to place my gauge lines along the length of the leg.
The top and bottom marks were first made on a single leg. I then set up a transferring rig, which is just two battens attached to the bench top at a 90 degree angle.
This allows me to align the ends of two legs and transfer the mortise wall marks from the first leg to all of the others. This should ensure some consistency in the markings from leg to leg by avoiding measurement errors.
Enough marking! Time to actually make these mortises. I’ll be using a router to excavate 95% of the mortise, then finishing the walls. Most of these mortises are about 2″ deep, so I’ll use a 3/8″ end mill.
Below is my homemade setup for large router mortises. The plywood base plate has a fence on either side, and each fence sets the maximum extent of the mortise wall on the opposite side. The jig is clamped to the work, and the two cross-pieces on the jig determine the length of the mortises. I set these fences by eye, using the edge of the bit and the knife lines, leaving just a hair for trimming. Once everything is set, the jig just needs to be re positioned for the next mortise- the mortise size will have already been set.
This process went pretty well (again, the pencil lines are irrelevant at this point)
However, I did have one screw-up. I was setting up the jig and got interrupted by a phone call. After coming back to the shop, the jig looked ready to go, so I started. I kept noticing the bit grabbing on the top mortise wall…unusual. Finally I realized something was not right. Turns out I had forgotten one of the F-clamps, so the jig was moving a bit each time the router plate hit the jig’s cross piece, extending the mortise well past the line:
Whoops! Luckily the wall didn’t move out past where the tenon’s shoulder will be, so it won’t be visible. I think I will cut a patch later to fill in the empty space.
Here’s all 12 undercarriage mortises routed:
Now to trim them up, and make them into proper rectangular mortises. I am using a paring block for this, to help keep the mortise walls perpendicular.
Now to trim with a sharp chisel. This is fun on the side walls, not as much on the ends. But one by one they get done:
Finally:
Now, onto the tenons in part III! Thanks for reading.
Robert Hazelwood Woodworking 