You will need to get the joint closer than that. The amount of pressure needed to draw the joint together will be the amount of pressure that will be trying to pull the joint apart. It can cause the wood to split in the joint or elsewhere.
Not the answer I wanted, but I guess it’s the answer I needed to hear. So maybe y’all can help me figure out the cause of the problem. I’m a baffled by what’s happening.
I did the first two edges using an 8’ x 1” square aluminum tube as a fence. I clamped it to the board at each end, and could tell there was a slight flex in the middle. But that should have resulted in a concave curve (cutting slightly deeper in the middle as the fence flexed away from the bearing on the pattern-maker’s bit). Instead, I have a convex curve, where the boards meet in the center but not at the ends.
For the second and third pairs of edges, I used the edge of a Formica countertop as my straight edge. This also generated a convex curve, which I find even more baffling. There’s obviously no flex to the countertop, so the curve must already exist. But I can’t find it. I ran a three-foot level along the edge of the counter which I know is perfectly straight, and there’s no gap to be found.
Here’s my best hypothesis. I don’t think either straight edge is the problem. The problem is that the boards are not actually perfectly flat, and the router is following the curve of the surface, and translating it to the edge. Does that make sense?
Since I don’t have a jointer, but I do have a planer, I milled both faces of these boards with the planer. I know that this creates a problem where if one face is curved or twisted, that curve gets transferred to the other face, creating two parallel but not flat faces.
So might that be the source of the problem? A slight curve or twist along the length of the face could translate to a convex curve on the edge, right?
But that also doesn’t make much sense, because I would then expect edges not to be perfectly perpendicular along the whole length, and that doesn’t seem to be the case. There they meet, they meet perfectly; where they don’t meet, they have a perfectly parallel gap, not one that tapers.