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14th fret hump, neck angle, body design
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Author:  Andy Zimmerman [ Sun Jan 15, 2006 1:14 pm ]
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I am a bit confused!!!! Shouldn't you NEVER have a hump or valley at the
14th fret. If so, the neck angle doesn't align up with the body to give a
straight path for the fretboard.

Shouldn't you build the guitar so that the neck and therefore the
fretboard, follows a straight line over the top. AND build it with this
straight line to give you the proper height at the bridge. If the body isn't
built with the correct angle, then the fretboard will never be straight.
(Unless you sand it straight with bottom of the fretboard being angled at
the 14th fret)

So much is said about adjusting neck angle for proper bridge height, but
isn't the body more important if you want both the neck and the body to
be in a straight line???
I may be rambling.....


Author:  Dickey [ Sun Jan 15, 2006 1:53 pm ]
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I like the way FFord 'splains it on Frets

Look under refrets of old martins, his specialty, he puts several layers of masking tape up the fretboard a bit, grabs his trusty No.5 Plane body wrapped in sandpaper and makes a smooth transition from soundhole to past the body join. It's in the article, that if I weren't so lazy, I'd go find for you. So sick'em.

Now, others attack this problem from underneath the fretboard before gluing on the neck. Lance has a sanding arm thingy somewhere, Lance! It's pretty cool, and I need a look at it anyway myself, LANCE! Where is that picture is it in the Jigs and Tools? I may have a pic of it somewhere.....Dickey38732.9546064815

Author:  Sylvan [ Sun Jan 15, 2006 3:26 pm ]
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OK. This is one of the more serious problems in building an acoustic
guitar. Here is what I do: First off my tops are flat from the soundhole to
the neck block. All of my doming is behind the soundhole. My
soundboards also slope downward from the soundhole to the neck block
approximately 1/32". I cut the mortise and tenon on the neck at an angle
- I don't know what it is but it is very slight, probably about 1 to 1 1/2
degrees off of 90 degrees. As a result of the two procedures, the neck
angle meets the top at the perfect angle so that both the top and the
neck/fretboard mate with no space. However, almost every guitar will
take some finessing but, in my opinion, you must build into the top the
slant of the fretboard so the geometry will be right. I start the process by
sanding the rims and block before kerfing to get the downward slant. I
know that Jim Olson and Charlie Hoffman do the slant with a jig and a
router. I use the sanding discs, To each his own. This is followed by
another sanding with the disc when the kerfing is installed.
I suspect that, depending on your building methods, you can figure out
the angles that work best for you. But, unless you take the time to figure
it out, the fretboard will never lay flat on the top. If there is any interest
beyond this explanation, the next time I go through this I'll try to take
some photos and give a better pictorial explanation.Sylvan38732.9781365741

Author:  Rod True [ Sun Jan 15, 2006 3:31 pm ]
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Sylvan, the reason Jim Olson and Charlie Hoffman use a ramp is because they both build a totally flat top. No dome.

I use the ramp and router like the two mentioned above and of the 4 I've built, I have never had a hump problem.

Author:  Don A [ Sun Jan 15, 2006 3:41 pm ]
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Sylvan, I think I speak for all of us, YES. The description above is clear, but pictures are always appreciated. It would be a good addition to the library and your own site as well.     

Author:  Serge Poirier [ Sun Jan 15, 2006 3:48 pm ]
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Andy, If i may, i'd like to know if sylvan puts that angle inside of the head block before glueing it inside the soundbox? And Yes, Sylvan, i'm up for pics as well.

Thanks Serge

Author:  Andy Zimmerman [ Sun Jan 15, 2006 4:16 pm ]
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When I build my guitars, I do dome the top. (25') I keep the braces flat
above the soundhole and I DO put an angle on my neck block, fretboard
extension block prior to gluing up. For me it is 91.5 degrees for my 14 fret
OM. This, with straight neck to body joint. No angle change, and a flat
fretboard, I get the correct height at the saddle.


Author:  Pwoolson [ Sun Jan 15, 2006 11:43 pm ]
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Andy, I got your PM but wanted to respond here so everyone could read it and perhaps comment further.
As Sylvan said, you need to design this into your guitars. It's an often overlooked detail and I think that reasoning might be because often times people just get lucky and it all works.
this is what some of us are doing. I know Lance uses a similar jig to flatten his tops at the right angle.

Basically the white piece of UHMW is the thickness of the bridge (plus the thickness of the plexi on the end) giving the perfect slope of the top. I just draw witness lines from the top of the soundhole to the neck joint and start sanding until they are done.
That said, I don't really like this system. I'm not too crazy about thinning the top in this area. So my next design will be for a router to ride on a set of rails that will be at the perfect angle, which you'll have to figure out what the perfect angle is in your particular situation. Anyway, the router will take the dome off of the linings above the soundhole and the neckblock and give them a straight angle instead.
There are several fixes to a hump or dip problem. The one that I've found to be the least obvious is to remove the frets, force the extension to make the hump or dip, glue it onto the top, and sand that fingerboard flat again. Once the fingerboard is flat and true, refret. You won't be taking off much material so it won't be noticable.

Author:  Dave White [ Mon Jan 16, 2006 12:27 am ]
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Then again you can also build the neck with a fingerboard extension support and have a free floating extension.

Then you can even add in an adjustable neck, and with flying buttress carbon fibre braces supporting the neck block you can arch and lightly brace the upper bout area to give maximum tonal response, have the neck doing it's job of keeping straight with the strings at the right playing angle in relation to the bridge - and the two don't "fight" each other along a glued in fingerboard extention.

Sorted!! Dave White38733.3606365741

Author:  Colin S [ Mon Jan 16, 2006 12:54 am ]
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Like Dave said.


Author:  Serge Poirier [ Mon Jan 16, 2006 1:07 am ]
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Wow Dave, love your method, is there any documentation on how to build that extension or a link i could go to, to build this way ?


Author:  Pwoolson [ Mon Jan 16, 2006 1:32 am ]
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Dave, I've been looking into that option as well. I'm curious, do you see a gap between the fingerboard and the top? (I can't make it out in the pics). Or does it all sit flush. If it sits flush, how do you go about calculating that?
I'm looking at John Mayes method pretty closely. It's a fingerboard extension that gets bolted into an extension of the neck block.

Author:  Michael Dale Payne [ Mon Jan 16, 2006 1:36 am ]
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This one of the reasons that lead me to not level fretboards till attached to the body. I have since changed the way I set my necks but I still level the FB after attaching the neck to the body. I also feel this gives you some fine tuning on set angle, in relation to the bridge ht.

But in general I have always felt that the possibility of the hump was a good reason to level the FB after attachment.

Author:  Dave White [ Mon Jan 16, 2006 2:14 am ]
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There is a slight gap, but not so as you would notice and it doesn't really bother me that much. If it "touched" but not firmly there could be issues with buzzing. For an adjustable neck you probably want leeway both ways anyway to set higher or lower actions - granted over time it probably will need to be adjusted upwards more than down. Here's a closer photo (a bit hard to get in focus and it shows up my "naff" joinery at the back of the bound fingerboard ):

If you thought hard enough and did the sums you can get it very close - same technique as if you were going to glue the extension down. The reason I don't do a boltdown like John Mayes is that I only want the neck to be locked to the neckblock on the vertical plane. The neck geometry and "physics" are all about the neck construction and the neck block within the box construction. The top can then do it's own thing quite happily. If you are trying to fix the neck ( and fingerboard) both on the vertical AND horizontal planes (i.e top bolt down) I don't see a win win as to my mind expecting the top/neck in vertical and horizontal planes to dance in "perfect harmony" is just not on. A lot of the upper bout traditional top bracing is to give structural support around the glued fingerboard which if they did move harmoniously you wouldn't have to do.

Keeping the problems seperate means you can brace the upper bout for tone as much as for strength. It is said that the upper bout is tonally dead - well it will be if you have to brace the **** out of it

Just my philosophy for building and as they say ymmv.

Author:  TonyKarol [ Mon Jan 16, 2006 2:42 am ]
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Hi Dave .. to get rid of the joint at the fretboard binding, you need to either mitre it, or do the end of the FB first, which is what I do - at least then if its not dead on, its almost invisible to the player.

Author:  Dave White [ Mon Jan 16, 2006 3:09 am ]
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Thanks, I have this one sussed now.

Author:  Andy Zimmerman [ Mon Jan 16, 2006 3:27 am ]
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Based on your above reply, I take it you build the box so that the slope of
the top above the soundhole allows for a perfect position of the bridge
(height). THEN using your neck jig, you can easily make the neck angle
match this slope.



Author:  Pwoolson [ Mon Jan 16, 2006 4:23 am ]
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Andy, yes you are correct. In theory, it's designed that way. Sometimes they don't work out in practice as well as I would like. But I see this as one of those "big picture" things. If you only concentrate on one issue, you'll neglect others.
Dave, by no means am I picking on your guitar. But when I look at custom guitars and attempt to judge "quality" I look for things like gaps and voids. The fb extension is the second place I look, after the neck joint. One thing you could do to make your neck appear more intentional (because it is indeed intentional) is to make the mahogany extension the full width of the neck. Then instead of seeing a void, you would see a little bit of the mahogany from the extension. Not only would that NOT look bad, but I think it would look quite nice. You'd have to be pretty careful in the planning of it to make sure it fits just right.

Author:  Dave White [ Mon Jan 16, 2006 5:02 am ]
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I suppose this would be possible but as the extension sits in a pocket in the top you would have to address the issue of a visible line looking down from the top. You would have to have a perfectly seamless join that would still allow the neck extension to move up and down.

As I said ymmv - for me it's tone first, cosmetics second, but then I am largely a dilletante maker thank god and don't have to rely on sales to feed the family.

Author:  Andy Zimmerman [ Mon Jan 16, 2006 5:55 am ]
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Thanks Paul

Author:  Pwoolson [ Mon Jan 16, 2006 7:15 am ]
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Terry, thanks for the correction, I indeed "double added" the thickness. So, yes, the UHMW is the thickness of the bridge and the plexi that holds the sandpaper is 1/4" which is the thickness of the fingerboard. Thanks.

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