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Hi all! I don't often ask questions here since most questions seem to already have been answered, but I am confused about Hesh's tutorial for flattening the upper bout to match the neck angle. http://www.luthiersforum.com/forum/viewtopic.php?f=10117&t=25931

Everything is very clear, except Hesh is using a 25' radius top and mine will be 28'. Will this make enough of a difference to have to change the size of the 1/8 " shim? Also, does the body length of the guitar factor in here? I guess the angle will be different for a shorter guitar.

The only solution I can think of is to put a shim across the guitar body at the bridge position. It would have to be the amount that the soundboard is arched in the center of the body plus the height of the saddle ( 1/8"?) But maybe I am over thinking this. Can you offer any insight? Thanks.

Adam

No, I've done it with 28' and other body shapes as well and think that I say that somewhere in the toot but it's been few years so I could be wrong abut that.

It works well with the same shim for 28'.

I suspect that you are thinking that flattening the upper is setting the neck angle, two different operations and the flattening the upper bout process does not have to be overly precise. We are simply removing the dome from the top in the fret board extension region.

Thanks Hesh. I was figuring that the upper bout had to be at the same angle as the neck, because diverging neck angle and soundboard (the angle of which is set by the flattening of the upper bout) created a gap. Sounds like I am getting a bit too worried about it.

No problem Adam. When we fret the extension it wants to take a dive downward as well from fret tang compression. The amount of fall-away is not critical and you can tweak it with the frets too if need be.


Flattening the upper bout is related to another toot I did about setting neck angles. The neck angle one is where we take dynamic measurements and have to be far more precise.

I am sure sanding with a shim on the heel block works great as you gain experience with it. I know Sylvan Wells does that too.

I actually do use a bar across the body as a pivot point. It is located to correspond with the front of the bridge and is roughly the same height as the bridge. The sanding block has a piece of 1/4" plexiglass attached to mimic the fretboard with 80G stuck to it.

Fine tuning is with the Fox paddle with the pivot point about bridge height but can vary based on your construction techniques and movement after settling in.

I use a Bourgeois type double tenon bolt on which moves a little more under tension compared to a glued extension and glued dovetail. By trial and error I have found that having the plane of the upper bout such that with no tension the fretboard plane of the extension would just hit below the top of the bridge works well for me. I overset the neck to give a good 1/16" clearance without frets and when strung up and settled in the neck set is usually good and the angle of the upper bout winds up giving about the right drop off of the extension.

I usually don't have to sand the fretboard or frets for drop off much these days but did before I figured out the optimal upper bout angle for my neck attachment and upper bout bracing system.

I think the optimal upper bout angle will vary with each builders construction techniques and depend on how much the guitar settles in over time. It's good to try to see as many of your guitars as possible back in the first year or two to see what they are doing once they leave home and modify construction techniques as needed. One thing none of us need is a rising extension one or two years out.

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