Official Luthiers Forum!

So I think I finally know what the dreaded 12th (or 14th) fret hump is all about now. I'm building a stewmac 000 kit with one potentially major mod which is that I used a 25' dish to profile the sides and brace up the top.

I just finished installing the fretboard onto the neck and did some quick checks with a straight edge. BTW I used an arbor press and one of them 16" radius cauls to fret and it worked out perfectly - in like buttah and they all seem pretty tight. I had to fret to the 14th fret prior to dry fitting: because of the tenon, I wouldn't be able to get to the 13th and 14th fret after gluing the FB to the neck. I checked with my straightedge and the neck with FB seems very straight as far as I can tell. No gaps at any fret between 1st and 14th.

So anyhow, I put the neck into the mortise and laid it such that the FB extension is laying flat on the top. This introduced quite a gap in the cheeks on the "anterior" or top side. when I laid a straightedge on the FB, it was oh, 1/8 to 3/16" over the bridge (this was just eyeballing). Basically it looked way too high.

If I tighten the bolt a bit and adjust so the neck so that the cheeks are flush with the sides and the FB is just touching the top at the soundhole, I get almost a .050" gap under the FB where the neck meets the box and the straight edge is around 1/16" or less over the bridge. If I could slide the neck "backward" it would be about what I hear is perfect.

I imagine that if I tighten it up so that the straight edge is flush with the top of the bridge, the top will be bending the FB extension up quite a bit (almost 50 thou) wrecking the action. Is this the 12th fret hump you guys all talk about?

I'm thinking that the solution is to plane down the top with a hand plane & sand paper to make it match the angle I'm looking for. This would require planing down about .050 at the soundhole and almost nothing where the neck meets the body.

Is this the right train of thought or is there something else I should be trying?

_________________
http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

Hi Andy - yeah it sounds like you have the 12th fret hump but not to worry because you can't know the extent of it, for better or worse, until you fit the neck better and set the proper neck angle. Once you do that you will know how much of the fret board AND upper bout will need to be trued up so that the fret board extension sits flush and flat. Remember too that some drop off in the fret board extension from the 12th to the sound hole is desirable. On a 14 fret I shoot for .015 drop off at the last fret at the sound hole.

Once you fit the neck better and set the neck angle so that you comes in just over the bridge then you can true up the top and fret board extension for a better fit.

I would approach it like this:

1) Set the neck angle correctly and observe what you have left that needs to fit better in respect to the upper bout and fret board extension. Here is a toot that may be helpful in setting the neck angle in respect to the bridge: http://www.luthiersforum.com/forum/viewtopic.php?f=10102&t=15022%22

2) Once the neck angle is set truing up the upper bout, and there are methods that use a pivoting sanding surface, would be my next step. If indeed it works out that you still have a .050 gap, but lets see first after setting the neck angle, .050 is too much IMHO to remove from the top's upper bout. I would then flatten and true up the upper bout to some degree but not as much as would remove .050 from the top.

3) Relieve the bottom of the fret board extension so that it fits flat or with the mentioned drop off to get the last part of the fit right.

Some builders will build in a flat in the upper bout area. This is what I do and to do so I use an upper transverse brace that is not radisued. With a 25' dome on the top and a flat upper bout I don't get the hump. Another method is to use less of a radius on the upper transverse brace or just flatten the middle of the brace where the fret board extension will sit.

Here is a link to another method of truing up the upper bout: http://luthiersforum.3element.com/pages/jig_tools_tech/dome_leveling_jig.htm

Thanks Hesh, that seems like a very doable plan. I think that as you suggest, taking some off the upper bout and some of the FBE will do the trick. Those jigs don't look too complicated either.

It definitely makes sense to try and "build out" some of the dome in the upper bout by using a flat transverse brace. I just used what came in the kit which was already domed. However, the kit uses a flat rim with a sorta domed top so had I built it as intended, the hump probably wouldn't have been as bad...but then the dome on the top would have probably been miniscule as well.

I'm off to K-zoo and GR for the next couple of days but will probably have some time Saturday and next week to play around some more.

_________________
http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com