Math skills?

[Dave Bagwill]

Ok - an idle question re: sloping of the rim from soundhole to rim at headblock.

The top of the rim is flat to begin with, then we, by whatever machinations, slope the rim from approx soundhole to the rim at the headblock. One way or another, we have sloped that portion of the rim at a central angle of about 1.3*.

The rim at the headblock is now about 1/16" lower than the lower bout.

I cannot make the math work. Out of curiosity , where am I going wrong I'm using 5" as an approx radius.

[Dave Bagwill]

Another view

[Herman]

Where you went off track, is that your angle is not 1.3 degree but about 0.68 degree.
The closer to the rim, where the slope begins, the steeper the angle.
So 5 inches is quite far to start. The soundhole does not need to be in the slope area.
The edge of the fingerboard, where the slope begins, is mostly about 3,5 inches from the body edge.
If you sand from there, your angle would be now 1 degree ( sin x = 0.06" / 3.5" )

Good luck
Herman

[Dave Bagwill]

H - thanks, I think! ...I'll have to read that a few times to see if I can understand it.

G'day mate!

[John Parchem]

I am with Herman. Your second post with the right triangle is the best of the two models to use. Your first using arcs is not correct, mainly your choice of 5" for a radius. I think the 5" or as Herman said maybe 3.5" would be part of a chord of an arc. As Herman said the angle is .68- to 1 degree for a true flat top. If you work with radiused X braces you would need to increase the angle a bit as the saddle location is not longer in plane.

[Dave Bagwill]

Ok.
We do want that 1.3* slope of the upper bout, correct? As complementary to the neck angle.

I think there is an arc in principle. Don't we elevate the upper bout, not along a straight line, but what is an arc, admittedly very small, in order to get the correct slope?

[Dave Bagwill]

Ok, I did a re-calc using 2 legs at 3" each and an inside curve of 1.3*, and sure enough the answer came out to .060" approx, or 1/16".
Thanks.