Because folks are a bit tentative making these cuts, in last year's class it was something that seemed to take a lot longer than I thought it would. The other problem was that after the cut was made, students then used a rasp to "tune-up" the fit. As often as not, they would take material from the wrong end of the bevel which made the fit worse, not better - even after being instructed the proper way to fit the stems - multiple times. To solve these issues and to get a good cut the first time out, I decided to develop a jig to help the students achieve repeatable results.
The jig consists of two parts and a five clamps.
In this case, we're looking at a jig designed to work for the bow stem of Jeff Horton's Stonefly canoe design. There are jigs for both ends of the canoe and we've also made similar jigs for Dave Gentry's C15 kayak.
The first part of the jig is a bracket that attaches to the frame nearest to the bow's stem form. (Seen below as the green-colored part.) Note: click on the images to open them at a larger size.
The first element of the jig is a horizontal member. Attached to the horizontal member are blocks which fit into the inwale notches to set the location and height. The horizontal member of the bracket is clamped to the two "ears" on the form to hold it in place. You'll also notice a second member - a "tongue" that sticks out at a 90° angle to the first piece and is centered above the keel. Underneath are some blocks - better viewed in the image below:
These blocks keep the two long horizontal pieces at a 90° angle to each other and provides a slot of sorts for the second part of the fixture.
The second piece (Seen below as the blue-colored part.) of the fixture is basically a copy of the stem form, mostly, but with some added feature. Because it is a copy, it is the same thickness as the plywood being used for the stems - in this case, 1/2".
The features include some slots for the stringers to pass through at the height where they will normally go - this takes a bit of checking to make sure that the stringer's curve is fair. The depth of the slots are such that the outermost part of the stringer passes through at the line which is offset from the forward edge by 1/2". The width of the slots is taken by scribing the stringers high and low points onto the form. The extended tail fits into the notches on the bracket described above and is clamped there. The bottom of the form is clamped to the end of the keel to keep it from sliding side-to-side.
With the stringers installed, they pass through the slots. (The stringers are shown in the color red and modeled as if they've been cut off.)
The stringers get clamped to the blue colored piece of the jig at the back. (Out of the way of the saw blade!) The tool used to make the cut is a Japanese Bakuma style saw - which doesn't have a spine or "set" to the teeth. To make the cut, you simply lay the flat of the blade against the face of the jig and cut straight down along that surface. When all the stringers have been cut flush to the blue colored piece of the jig, you can then remove the jig and install the bow stem form.
Because the jig simulates having the stem in place, the stringers mate perfectly with the stem:
While this may not be a great use of time for someone building a one-off boat, we're building three copies of some designs and the time spent building the jig has saved more time in the cutting process than we lost building the jig - a good trade-off. The other consideration is that this method of making the cuts is the best one that I've found to date.