Thread: Obtuse join angles

Hi Paul. Since I don't see any answers yet, I'll take a stab at getting you some help. First, let me make a couple of assumptions and you can correct me if I'm wrong.

Let's assume you mean to let the sides have the dovetails and the back wall have the trenches. That would lead me to make the trenches in the back wall just as if I was going to make a sliding dovetail at a right angle to the back wall. I would be careful to make the trenches of a size not to exceed the width of the sides.

After making the trenches, I would have to make the dovetails for the sides at an angle to the sides so to speak, so they meet the back wall at a right angle. To lay out the dovetails, I think I would hold one of the sides up to the back wall and draw the trench onto the side, if you know what I mean. That is I wouldn't try to measure anything but instead use the existing trench to size the dovetails.

I hope this is clear enough to at least give you some ideas about how to approach this. If I got the assumptions wrong, let us know and maybe we can give you some better advice.

Regards,
Jim

If you're comfortable with dovetails in a square box or an octagonal box, the 105-degree corner is in between, with regard to extensions of the insides of the walls. But simpler finger joints might be easier for the rest of us, and pretty much just as strong if properly glued. With finger joints, it doesn't matter which is the dovetail, and which is the trench. Draw a plan view of the wall intersections to establish the depth of cuts. I'd make the fingers (or dovetails) slightly longer than needed, for final cutoff later. Draw all three conditions (90*, 105*, & 135*) at larger than full size, for insight.

Cheers,
Joe

Thanks for the replies. The dimensions are:

Wall 1000mm
Board thickness: 15mm
Wall Angle: 105 degrees

Possible Dovetail bevel angle: 15 degrees

I hadn't actually considered a sliding dovetail and must confess I'm somewhat daunted by the thought of such a long joint, but also feeling pleasantly challenged.

I'm using hand tools and don't have a dovetail plane. I understand that I can make a bevelled template to cut the angle with a back saw, then chisel out the waste from the trench, all of which makes the trench seem the easier part.

If I go with this method it seems the male part would be biggest challenge. I would proceed by ripping the end of the board at 15 degrees. Which would make the inside angle now 105 degrees, and the outer 75 degrees. This means I only need to make a single bevel, the inside, for the male part at 15 degrees to the face of the board.

So now that I arrive at this point I realise that the failing of this method is that there is the overhang of the rear wall being the distance that the trench is placed inward from the edge, but this could be ornamental (see below). I also worry about the strength of this join running with the grain and only allowing a depth of about 6mm (using Japanese Cedar)

Finger joints seem like a good option provided I can manage a uniform angle using saw and chisel. I suppose once joined I could decide whether the protruding fingers provide an interesting ornament or whether to trim them flush.
best-Paul

Hi again Paul. Now that I see better what we're talking about, I have to agree with the finger joint suggestion. It would be rather simple even for a hand tool only joint. I would make the fingers the width of your smallest chisel. By the way, with more fingers than dovetails, the finger joints will give you the stronger joint. Anyway, just make the joint as if it was going to be a right angle joint but leave the "outies" longer than you normally would. You can just estimate but err on the longer side if you know what I mean or, make a sample first. Then, take a chisel and use it to make the "innies" angled to 105 and you're done. To ensure they're all the same angle and tight, just create a chisel backer board that has the angle you want and flush it up to the "innies" so the chisel is at the same angle over and over.