Now I have this marvellous depth stop thing, but how do I actually set the depth correctly to any value I need, i.e. how do I make sure both blade guides are exactly in parallel and at the right depth? Well, I made another jig out of plywood and little pieces of veneer of known thickness:
Using it is rather simple. First, make sure the nuts are not tightened yet, so everything can move freely. Select two identical strips of veneer of the desired thickness (or, if you want to go deeper than the thickest veneer you have, use more than one layer) and put them into the frames on the horizontal bit of the jig. Hold one side of the sawblade (or rather: the depth stop on one side of the sawblade) against the vertical bit of the jig to make sure everything is at right angles, let the teeth of the blade rest on the wood between the strips of veneer, and the lenghts of beech on the veneer either side of the blade. Make sure everything rests on the ground along the entire length, then tighten the bolts. You can put your thumb on the nuts (that is what the 3.5 cm holes cut into the vertical bit of the jig are for) and use the other hand to tighten the bolts with a screwdriver.
If you want to make sure nothing moves out of position you can paint the inside of the blade guides with something rubbery (e.g. gummi arabicum glue used for putting photographs into albums; many contact adhesives such as Pattex Classic form a slightly elastic rubbery film, so if you put a very thin coat of that on, nothing can slide after the screws are tight), then you don’t need to tighten the bolts all that much to get a good grip.
Now you can cut your fret slots without having to worry about going too deep. Later, after radiussing the fingerboard, you can put the saw through the slots again for the final depth, and this time your slots will even follow the radius. The 10mm guides either side of the blade will make it easier to keep the saw at right angles to the surface of the fingerboard – if you veer too far from 90° you will notice.
Instead of veneer, you can use plywood. Some arts supply shops sell extremely thin birch plywood (in German “Flugzeugsperrholz”): 0.2 mm, 0.4 mm, 0.6 mm, 0.8 mm etc., often used for building architectural models, or sheet plastic. Sheet metal would work as well, but you risk hitting the metal with the saw’s teeth and that could dull or otherwise damage the saw.
By the way, I’ve heard measure, measure, cut a lot, and it is a very sensible principle – measure, go back and make sure you measured correctly, then cut. What they often forget to mention is the second half: swear, repeat.
I had measured everything very nicely, so measuring a second time didn’t reveal any errors. Everything was reasonably accurate. The error had occurred earlier. I had made the space for the outside strip of veneer (i.e. not the one along the vertical bit of the jig but the other one on the “open” side of the jig) too narrow. The depth stop of the saw needs to touch the vertical bit of the jig to make sure the saw is at a right angle to the base of the jig. But both sides of the depth stop need to rest on the veneer. The one strip of veneer must be narrower than the depth stop to make sure the teeth of the saw don’t rest on the veneer but on the base plate of the jig. The other strip needs to be wider so that the depth stop on that side doesn’t rest on the frame I built to hold the veneer.
I had, by mistake, made both sides the same width, because of which the depth stop facing away from the vertical part of the jig rested on the frame and wouldn’t go as deep as necessary (because, obviously, the frame is deeper than the deepest fret slot anyone would ever want to cut). So I had to make that part of the frame wider. Because I had glued the piece of wood to the base plate, I had to remove a few millimetres of material using a chisel. Took me a while, but it was a good excercise, and it isn’t even all that noticeable either. And anyway, it needs to work properly, looking good is not all that important. In a jig, that is…
Sawdust Glue and Wire 