The angle measured between the surface and the face of the miter will be different at any given point along the curve. First sort out how you will make the cut. I would plan to make a fixture to hold the parts to cut them at the bandsaw so you can use a simple 45 degree cut. I would also plan to finish up the cut with a hand plane. You could also make a fixture to hold the parts with a guide for a hand saw.
You could rough cut it with a jigsaw and then use a hand plane to put in the rolling bevels. Boat builders have planed rolling bevels by hand for thousands of years and it’s still a viable way. Or as I wrote create a fixture to support it with a guide for a handsaw.
I doodled this up from your model. Basically a sled to hold the piece. I was thinking of doing it at the bandsaw with the table tilted to 45°
Unfortunately, it’s like many joints on a wooden boat, where the 3D curve twists when the surface it flattened. If you cut it when flattened it will have to be a “rolling bevel” with a constantly changing angle. Feasible, but an advanced technique.
Unfortunately I do not have acces to a band saw, only a jigsaw and a table saw.
And I do not really trust my planing skills enough to try a rolling bevel.
But - if I understand your example correct, Dave - if I cut the edge in a 45 deg while the piece is laying flat on the ground, then the faces would meet on the outside, but on the inside the joint would be more and more open towards the bottom?
I think I could check that by taking a measurement at the bottom, and then if it is more than 45deg, then I would be safe (?).
How did you set up your measurements for the above example of where you have shown the angles? I tried to recreate it, but I’m not sure what axes to go for. Would I select a start point on the inner edge - draw a line from the point and out parallel to the red axis - and then, from the same start point, draw a line in the angle of the miter so that I hit the outer edge - like on this picture? If that is correct, how do I get the line to lock on the angle? Did you manually write the degrees in the text box or can you make the label appear automatically when you use the protractor tool?
If you can hold the saw at the right angle you could get close but I expect you’ll want to cut from the front since the outside dimension is the critical one and with a jig saw you won’t be able to tilt the shoe far enough so you’ll still have a bunch of wor to remove material toward the inside of the hood pieces and there will be more to remove toward the bottom of the hood. Another possibility might be to rig up a holder for a belt sander.
To measure the angles I used the midpoint of segments along the outside of the curve and set up a guideline on the front face at 90° to the edge. Then aligned the Protractor along the edge to measure between the front face and the edge of the miter. Clumsy gif of it.
Remember that this curve is only an approximation of the real thing. My spot measurements are not exact but they are close enough to give the idea of what happens as you proceed along the join line.
If the miter is problematic for you to cut perfectly, maybe conside getting it as close as you can and then glue in a round bead to cover any gaps. If you’re going to paint this thing, epoxy or some other filler can hide a multitude of sins.
Ask any boat builder who uses modern techniques! For example, the “stitch and glue” method uses loops of wire to hold more joints or less closed without even bothering with a bevel and then puts a fillet of epoxy across the resulting gap. The wires are trimmed or removed after the epoxy sets and the holes filled with more epoxy.
To visualize this - when looking in plan view the plumb cut will be 45°. The saw angle will change relative the the surface, and because it is curved it will change dynamically as you roll up the piece.
Dave - thanks for the animation!
I’ve used the last hour trying to replicate and understand what you do and I think that I now got it. The concept is basically that you just ‘extend’ the faces with guides and then measure the angle between them? Like in this picture?
Bmike - thanks a lot for that plugin suggestion! I found out that I could use it in the 2017 desktop version, so now I can check the measurements from the two methods up against each other - great tool!
So - to conclude - does this look right with the angles?
Am i right that if the bottom piece gives an 54 deg. angle, then if I lay my plywood flat and do a 45 degree cut, then it would just open up a little on the back - I can fill that with epoxy - but it would still close tight together on the front side?
If you look at the attached image, you can see that the bevel angle is always sharper at the top than at the bottom of a board. The difference is typically around a degree. So, if you bevel the end of each board to the upper angle while it is laying flat, that will leave just a tiny gap when they are assembled into the curve. Easily filled with epoxy after assembly. I think, though, that you will want to make jigs to keep the boards at the proper curves while you assemble them and to align the front and sides for assembly. There will be nowhere that you will get a tight joint that will work with ordinary glue.
In my case it seems like it would be approx 9 deg. difference from top to bottom - if I use the tool correctly (?). I’m sure that can be filled with epoxy, but it sounds like a good idea to use a jig on the assembly, since I guess that is the only way that I can make sure that they are aligned correctly.
The thing about ordinary glue, is that because the gap would simply be too big for that to hold up?
In your image you have a measurement of 58,4 deg at the bottom. I have a measurement of 54,4 deg. Is your measurement from the actual file (so that my measurement is not correct)?