Neck alignment and gap under fingerboard
 

Please could somebody explain how to avoid a wedge shaped gap under the fingerboard extension. If for example I am aiming for 0.400 above the bridge with a straight edge on a fretted fingerboard how do I avoid a gap between fingerboard and the top?
I am getting close to assembling my sides, neck block and tail block and can't get my head around this issue.
My top has a 25' arch longitudinally only.

Thanks

That wedged shaped gap is the product of neck angle and top geometry, and can be eliminated by manipulating one or both. It's also acceptable to put a wedge under the fingerboard, some builders use neck angle/top geometry that require it. Do you have a set of plans you are using as a guide? That's where the solution to your problem lies, it should be determined in the planning stage.

Edit: I meant to say my top has a lateral arch of 25' not longitudinal.

Thanks Rodger, so if I wish to avoid this gap do I need to taper my sides (rims) slightly from the waist to the neck before I join the top? In other words creating the correct angle to match the neck angle whilst providing the correct elevation in a straight line above the bridge. I hope that makes sense.

Correct. This is how I do it anyway.

Also, please could you explain how the gap could be eliminated using only the neck angle?

Lord knows i am not a luthier ( although having aspirations (amateur) in that direction ) but one ability i do have is the facility for 3- D geometric visualization .

In that respect, i would have thought that the least complicated method of dealing with the neck angle issue would be leaving the top of the sides totally flat (ie in a flat plane ...no tapering ) and contouring the UTB to give the desired neck angle.

There's nearly as many ways to do this as there are people building guitars. I've built the upper bout flat, and transition to a 15' sphere for the lower bout. With 1/4" thick fingerboard sitting on the flat upper bout, the curvature below the waist give me 1/2" string height off the top at the bridge. I've also built with a 12' transverse (lateral) arch (Larsen brothers). On that I sanded the top under the fingerboard to the angle necessary to get the 1/2" at the bridge. That leaves the top a little thinner under the fingerboard, but it's less than 1/16" that needs to be removed. It's also pretty common to use a 25' sphere and flatten the top under the fingerboard by using a 60' radius on the UTB.

I draw precise plans using drafting software that allows me to work out how everything fits together before starting construction.

Simplest way is just glue the fretboard on, it will conform to the different heights, then level and fret, there will be no gap.

Steve

There are two categories of solutions. The first is to have the surface of the neck to which the underside of the fingerboard is glued be 180 degrees from the surface of the top to which the underside of the fingerboard is glued. There are numerous ways to achieve that.

The second is what Steve describes, below. Glue the fingerboard down tight, bending at the body juncture to accommodate a non-180 degree angle, then just level the top surface of the fingerboard. This will result in a tapered thickness of the fingerboard, a practice common in classical guitars that have a neck angle the opposite of steel string guitars (i.e. negative angle instead of positive).

Both will work.

I've had nothing but problems if I just force fit and glue the extension down. Maybe I'm doing something wrong but instead I sat down one day and figured out how to solve this annoying problem and that was to cant the top from the sound hole forward and use an arched UTB to set the angle to the bridge.

Otherwise a wedge shaped shim is perfectly acceptable and the better solution imho.

Measure the angle between the top and heel area of the side. Then make the angle of the top of the neck and heel the complementary angle.

The way I've done it normally, is pre-adjust the neck angle beforehand, by slightly tapering the rim from the UTB to heel with a wide sanding board. It ends up being 1 deg. or less. That give the angle of top to heel area at 91deg. The heel then should be the complementary angle or 89deg. I cut this angle FIRST before doing my neck tenon. Also this is a lot easier if you use a straight tenon; if you're doing a dovetail you have to nut it (or come very close).

The other way I've done it is to build my neck with the neck wood running completely through to the end of the fretboard. I then would do 1) a raised fretboard with a slight negative angle, 2) for a bolt-on, pocket the entire tongue area into the top which allows me to do whatever I want with the neck angle, or 3) trim the tongue back slightly and make a pocket for the remaining, allowing the fretboard to lay flat. With either way, I'd position the neck where I want it, then scribe the heel to the side and/or floss the heel to make a tight fit. Again, this is done with a straight tenon, not a dovetail.

There is some danger of the tail wagging the dog if one alters the UTB to suit the fingerboard extension. I can think of more than one good reason to keep some arch in the upper bout. I am surprised to hear than so many address this as a problem since I have never noticed any issue. Perhaps it is because I use a relatively flat top, but I use double the arch in my UTB than I do in my other top braces specifically to tighten the upper bout. IMO this is quite important, if anything is.

I mostly build dead flat unless otherwise warranted and have a similar approach. For any given guitar I build a quick and easy work board with a Balsa shim that I glaze with epoxy to harden up. Picture worth 1000 words but in essence what I do is arch the UTB as well as cant the top which can be seen with the balsa shim.

This approach has saved me a lot of head aches with getting the neck angle right.

Wedge shim to cant the top above the sound hole:
https://lh3.googleusercontent.com/8v...=w1280-h720-no

Arched UTB:
https://lh3.googleusercontent.com/2N...=w1280-h720-no

Another thing that can help is to draw it out side view.

Get some wrapping paper and draw it out on the white backing. Otherwise - you can get a big piece of cardboard out of the trash and it will work fine.. Just use a yard/meter stick to line it all up.. And you will see how the alignment works....

And there is no shame in using a wedge shaped shim... Make one out of scrap mahogany and off you go... The trick is to make it look like you planned it as a feature rather than letting it look like a mistake/afterthought..