Neck angle for Octave mandolin (teardrop shape)

[hugh153]

Moving forward with my first octave mandolin build due to very good advice from members. The top will be slightly arched (15ft radius). Have seen 4degrees and up to 8 degrees suggested as the angle at which the heel meets the body. Can anyone suggest what angle should go for?
many thanks,
hugh

[Dusepo]

I previously shared some octave mandolin plans in this thread which may be helpful: https://www.mandolincafe.com/forum/t...ans-and-Advice

[Jupiter Ukulele]

I just finished my first octave mandolin, also with a 15' radius to the top. I don't know what the neck angle is (don't really care for a number). I set the neck angle so that the fingerboard lays flat on the top which, because of the 15' radius, angles the neck back slightly. Worked out well, and no fretboard hump or funny shims under the board against the top.

[RobH]

Make a jig to determine the neck angle. This is what I used on a carved top Octave Mandolin. I wanted a 7/8 to 1" bridge height and a 3 degree angle worked for my build. It takes all the guesswork out of the neck angle.

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pic attached

[Bill McCall]

Can you draw a full size mock-up and just measure the drawing? I’ve always thought it’s easiest to build parts to a full size drawing, rather than work it out as you go along.

Ymmv.

[rcc56]

On a mandolin with a moveable bridge, set the neck so that the bottom corner of an accurate straightedge laid on top of the fretted neck butts into the bridge ~8/64" [1/8"] below the top of the bridge. This will produce a string height of 4/64" [1/16"] above the 12th fret when the instrument is assembled and tuned to pitch.

--But--

For an octave mandolin with a moveable bridge, we will need a higher string clearance at the 12th fret to accomodate the longer scale. To accomplish this, the straightedge should butt into the bridge 12/64" [3/16"] below the top of the bridge. This will produce a string height of 6/64" at the 12th fret, which should be about right for an instrument with a 20 to 22 inch scale.

The procedure is different on flat-top guitars with a fixed bridge and removeable saddle. On those, we generally want the straightedge to just contact the top of the bridge or butt into the bridge no more than 1/32" or so, depending on the flexibility of the top and the neck. We would go about the same on an octave mandolin with fixed bridge and removeable saddle.

Fretted instrument geometry 101: A change in bridge height above the plane of a fretted fingerboard produces half that amount of change in the distance in string height above the 12th fret.

[Tavy]

Draw a plan!

The angle doesn't matter as such - what matters is bridge height and the break angle over the bridge - get those right and everything else will follow.

[rcc56]

Ditto Tavy.

Those of us who set mandolin and guitar necks don't measure the neck angle in degrees. We use the straightedge on the fingerboard, and carve until we get the desired string heights at the bridge and 12th fret.

For whatever it's worth, those who build open back banjos with dowel sticks generally rough their neck angle out at 3 degrees, sometimes a shade more. But banjos are a different animal, and I would not expect the angle to be the same on mandolins or guitars.

[peter.coombe]

On octave mandolins I use a neck angle of 1.5deg, and the bridge height is set by the custom made Brekke bridge. What height you set the bridge is up to you. The neck join is a dovetail and I set the dovetail so the string height will be correct for the bridge height. A wedge is fitted under the fingerboard, the fingerboard is not glued to the top but is raised, and is gives better access to the higher frets. That is one way of doing it, you can also glue the fingerboard to the top so long as the top has an induced arch of around 15ft, but that will give you a smaller bridge height. Is up to you to decide how you want to do it, but a good start is to work out the desired bridge height and stick to it. There is nothing set in stone for octave mandolins.