Stewart, I’ve got a project coming up where I’ll be needing to cut tapered sliding dovetails on the ends of a 20’ L x 8.5’ bookshelves. As far as I know, I don’t think there is an upcut spiral dovetail bit, so I will be jigging up to use two routers. One will be a straight upcut, followed by the dovetail using the same template while it is set.
One thing I’ve found in the past, is to take care to sand or cut the raised or bent up fiber along the edge of the slot made by the first pass. If not, it raises the router base off the wood a little, this will then cause the joint to be out of tolerance.
On smaller parts than I’ll be using on this upcoming projects, I usually make the first rough out cut with a dado blade on the TS which is a lot faster especially if there are lots to cut.
Something else that just occurred to me that you didn’t ask about, but I’ll be trying is the amount of taper from front to back. In the past, I’ve normally just chosen something like 1/16”-1/8” arbitrarily. However, in thinking about it, each time you double the front to back taper, you half the distance of starting to squeeze, before it stops sliding any further. I’m starting to like the idea of going wider. Do any of you have experience with this? What about you mathematicians, what say ye?