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  #1  
Old 01-28-2015, 07:14 AM
Dan of SC Dan of SC is offline
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Default fingerboard extension

I've built 3 guitars from a book "Build Your Own Acoustic Guitar" that came with a full size set of plans based on a Marin OM. The only problem common to all 3 is the fingerboard extension sticks up above the top and has to be bent down about 1/16" to 1/8". I've been radiusing the top braces at 15 feet. But I'm not sure about the big brace just above the sound hole. It seems as though the radius on this brace might be key to having the figerboard extension lie flatter on the top. Does this sound logical or is there another reason for this problem? Thanks, Danny Gray
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Old 01-28-2015, 08:25 AM
marioed marioed is offline
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Hi Danny,
Your 15' radius on the top may be a factor. Most tops have a radius of 25' to 40'. I use a 25' radius for the top and a 15' radius for the back on most of my builds. I also usually try to keep the area of the top under the fretboard extension fairly flat.
Regards,
Ed
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Old 01-28-2015, 08:26 AM
redir redir is offline
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Yes a radius should be imparted in the UTB and it will help in this regards. The other technique which I use in addition to a radiused UTB is to cant the rims from just at the tip of where a tangent line touches the very top of the sound hole projected to the rims and forward to the heal. Take off about 1/16th inch there and you get a better angle.

The other simple solution to this problem is to make a wedge out of spruce or what ever your top material is or what ever your FB material is, or what ever you want, to take up that slack.
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Old 01-28-2015, 09:04 AM
Tom West Tom West is offline
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Danny: The 15' radius is the main part of your problem. At about 25' to 30' feet or so and this will generally work out close to perfect. I'm surprised that at 15' radius and the quoted clearance of 1/16" to 1/8" that you can get anything close to a decent action with proper string to top clearance(approx. 1/2") at the bridge. I would think you would be quite a bit higher then 1/2". The 15' radius is a good radius for the back with the 25' to 30' a more suitable one for the top. Not a big problem.............it gets fixed in the next guitar. Good luck and take care.
Tom
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Old 01-28-2015, 10:33 AM
charles Tauber charles Tauber is offline
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The question to ask is the geometry of the curvature of your top. If you provide the curvature in a curved dish, the geometry of the top, particularly the upper bout, is a portion of a sphere. If you provide the curvature by other means, the curvature of the top, particularly in the upper bout, is a cylinder. If it is cylindrical, it doesn't really matter what radius you use as the centreline of the top will lie on a single straight line. In that case, what matters is the angle of the neck relative to that centreline. If the angle of the neck tilts backwards - the head lower than the top - the fingerboard extension will need to be bent at the juncture of the body since it lies on a different plane that the centreline of the top. This is what happens in a neck reset - either the fingerboard extension is bent downwards, or a wedge is placed beneath it to maintain the plane of the fingerboard. (Classical guitars are often built with a neck angle in which the head is higher than the top and the fingerboard extension must be planed wedge-shaped - thinning from the neck juncture towards the soundhole - to allow the top of the fingerboard to line in a single plane.)

If you are using a spherical geometry, you will still have the same issue - the angle of the neck must lie on the same "plane" as the centreline of the top in the upper bout. Trevor Gore, in his books, provides a very detailed mathematical analysis of the geometry, and then simply says, "Oh, just flatten the upper bout with a sanding block", thus creating a flat plane, at an angle slightly larger than 90 degrees to the sides, to which the fingerboard surface of the neck must be coplanar. He also suggests using a shallower curve for the transverse braces of the upper bout, but that is another story.

In either case, look to the neck angle that you are using. On the finished instruments, what vertical distance do you have from the top to the bottom of the strings, at the bridge?

Another possible cause is that the neck is vertically misaligned from the top and sits proud. In this case, the fingerboard would need to be bent downwards to attach to the top and there would be a gap between the top and the bottom of the fingerboard adjacent to the neck joint.

Last edited by charles Tauber; 01-28-2015 at 11:13 AM.
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Old 01-28-2015, 05:13 PM
murrmac123 murrmac123 is offline
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I have never read Kinkeads's book, but I suspect that the 15' radius in the OP is a typo and that he actually meant 25'. I cannot believe that the author advocated bracing the top with a 15' spherical radius.

If he did in fact build the top with a 15' (spherical) radius than there is no way he would have a gap of 1/8" at the end of the fretboard extension unless the neck was set at a radically steeper angle than the conventional 88.5 degrees.
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Old 01-31-2015, 08:22 PM
LouieAtienza LouieAtienza is offline
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I'd say something is amiss. I always check my neck joint angle so that the fretboard stays coplanar with the top at that area. It's a lot easier to adjust the tenon shoulders first than to try to bandage things later.
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