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I wish I understood this better. So excuse my ignorance here. I like to install frets in FB then glue to neck. Before that, I dry fit the neck. Martin suggests that a neck w/o a FB, properly set should straightedge to bridge location with 1/16" rise over sound board. But, with fretted FB attached? What is it? Is the bridge roughly a planar extension of the FB?

Lets start with the idea of a fretted FB attached to the neck. A straightedge along the fretted FB should intercept the bridge how? Dead on to bridge surface? Above? Below?

Mike

I like to give 1/16" gap between top of fret plain and the top of the bridge.

The neck will pull up slightly more than the top which will reduce that gap just a bit. It seems to leave about 1/8" of saddle over the bridge and I tend to make my bridges 3/8" high, thus giving a string height of 1/2" which seems optimal. This is with a 1/4" high fretboard.

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Add the thickness of the fretboard and the height of the frets to 1/16th of an inch. You will likely end up with something between 5/16ths and 3/8ths.

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In the end I want a straight edge running on top of the frets to just clear the bridge. That ends up making a 1/8-3/32" saddle with normal relief.

That said, the most important thing is making sure the height of the strings above the soundboard at the saddle is where you want it. That is the first consideration in order to calculate neck angle or fingerboard thickness/taper.

You want the neck angle to be such that your fingerboard, with the frets, overshoots the height of the bridge itself at its leading edge (not the saddle) by 1/16th (your mileage may vary) to account for the top pulling in under tension. For example, assuming a .250" fingerboard and a .375" thick bridge at its center, and assuming medium fret wire with a crown of .039, you want to your straightedge to hit a point at the leading edge of where the bridge will be of .375 +.024 (amount necessary to get 1/16" overset) = .399" above the soundboard. Notice that my measurement is without the crown height (which you'll get once its fretted), but with an additional .024" that when combined with the .039" fret crown will get you to the 1/16" overset you need to account for the top pulling in.

Thanks guys. Great info. You confirmed in my mind what I thought was the case, and then some!

Mike

Todd, how are you getting .050" for the frets? Standard Martin medium fretwire is .039" high. Also curious about why you specified a .350" bridge thickness. Not that there's anything inherently wrong with it, but generally bridge thicknesses are .312 or .375."

This starts at sides. You need to create an angle at the neck block. Think of it this way , The end result would be a 3/8 bridge and 1/8 saddle so that you have a total of 1/2 inch string height at the bride. The top should measurement should be .094 but if you are over a 1/16 and under an 1/8 in you should be ok.
I use the radius disks so I can precisely establish this point. I want the area from the neck block to the top of the sound hole to have a slight angle to allow the neck angle and make the fretboard come onto the top at this angle, You are trying to match a fret plane, string line and the action height , so you are matching these 3 geometries .
Once you master this you will have a much better playing and sounding guitar.
There are a few points to consider about this. 1 what radius do you plan on using. Most Martin style builders will use a 1 1/2 degree. I use this and I do it this way.
http://www.youtube.com/watch?v=L3YJh9OT4mw
Here I do not radius the entire top , my main concentration is the angle created by the dish as it trues up the rim. I can tell you when I am done with this I will have about 1/4 to 3/8 clearance off the tail block when I check the created angle. The one variable you have to figure in , is how much the top will rise under a string load , and this is influenced by the brace scheme,
In this method I create 3 areas of facet on the top , the angle , a flat area from the top of the sound hole to behind the bridge and a slight radiused area in the lower bout. By shape the design does increase the stiffness of the top. Once I set the neck, I like to check the neck angle with the fret board off. I am looking at this point for a 1/16 in to 1/8 in clearance at the point of the saddle.
With the fretboard on , I will shoot for 1/16 off the top of the bridge . I will take 3/32 as the max. Again this can be different depending on your bridge design. I use a martin style pyramid or belly with a matching radius and not a non radius bridge. I also do not want to see more than .180 saddle on a drop in bridge nor more than .150 on a glued in saddle.
In short you need to think at the the lines on the neck , FRET PLANE, the action height of the strings on the 12th fret and the relief all to line up to hit that 1/2 in point. Also one point to bring up is not to over set a neck. This can cause undue stress and over rotate the bridge, This brings up point 2
Compensation for the guitar.
There is no 1 number for this ,as compensation is individual to the guitar. We all know the scale length but you have to add a few variables and understand them for the best intonation. I will be giving a presentation on this subject at this years ASIA symposium. In a nut shell , you have to think about the use of the guitar and ad a bit for bridge rotation. As a rule I will set up the guitar so that on initial set up I may be about 5 cents flat. I also use a .110 saddle and plane for the compensation length to be about 1/4 into the saddle slot. that way as the guitar moves in a week to 10days I can compensate the saddle for the best intonation after the action and nut adjustments are all finalized.
Hope this helps.

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John Hall
blues creek guitars
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As John Hall has pointed out,the time to get this straight is when the box is being built,if left till your doing the neck,problems can be made evident. A lot of folks doing their first guitar are is a real hurry to get started and build the box. I don't think they have an understanding of the neck plane,the body top and the height at the bridge location until after the body is constructed. Problems can be corrected but nice not to have problems. John Hall has been preaching this for quite a while. Good job John.
Tom

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thanks for noticing Tom
We are here to help

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

I use John's basic technique (learned from his videos, thanks John!)
I work off the 1.5 degree angle also. I also shoot for 1/16" to 1/8" rise in the plane of the board.

This should not matter if you have the board glued on, or fretted or not. What you want to see is the DIFFERENCE in height, between where the neck hits the body and the saddle location. I just place a straight-edge (during pre-set, onto a bare neck, no FB) and if the straignt edge is 1/8" off the soundboard at the neck joint, and 1/4" off the soundboard at the saddle location (marked with a bit ot tape) then I know my angle is looking good, and I need to come straight down.

Once you've fully set the neck down, without a fingerboard, you'd want to see basically zero height at the neck joint, and between 1/16 and 1/8" at the saddle location.
If you have a board glued on, or frets installed, just add these heights to that 1/16-1/8".

One difference in my technique is that I do not use a radius dish to sand the top rim (although I built a few this way.) What I do do is put a 1.5 degree chamfer on the neck block (top side) tail block and kerfing. I use a sanding stick for this. The rim is still flat, only the kerfing and blocks are angled. I build the top in a 28' radius dish, and glue it to a flat rim. Martins are built this way, you can sight down the neck and across the top and see that the rim is flat. The top buldges, but the rim is flat. You get that 1.5 degree slope at the upper bout and the fingerboard extension lays right on top.

I would add that when you are building the proper angle into the upper bout consider that there will be some movement of the neck and possibly the headblock with string up so if the upper bout angle is exactly co-planar with the neck angle without tension you may be looking at a rising fretboard extension under tension. Try to figure the amount of extension drop off you want when putting the angle in the upper bout. If you don't do that you will have to sand the drop off into your fretboard. It took me a while to figure that out.

Different neck/body joints move differently. With the glued dovetail there is a lot of top still glued to the headblock as well as the fretboard extension all of which limit headblock rotation under tension. With a bolt-on with a glued extension it's still pretty stable. If you do a double tenon neck in the manner of Bourgeois/Wells/Mayes the mortise for the fretboard tenon removes most of the top that is glued to the headblock (almost all of it in the case of a cutaway). That can make for a potentially unstable headblock that will rotate a small amount under load until it gets comfortable. That can happen slowly over the first 6-12 months of life. That was a problem for me when I started using that joint and took me a while to figure out what was going on. I'd see guitars at a year that had higher action and a slight rise to the fretboard extension. It was easy to fix by doing a small neck reset to maintain saddle height but should not have happened in the first place.

I discussed the problem with a well known builder that also uses that joint and he had seen that issue as well. For the record this is what we came up with. It keeps a lot more of the top glued to the headblock and along with some other mods I think it will help. So far it seems to be working but I don't have any out there long enough to be sure. My understanding is that Dana was aware of this problem and approached it in a lightly different but effective manner.



Anyway, when calculating the angle you want for your upper bout consider the type of neck joint you are using.

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I totally agree. I spend a lot of time working the box set up to achieve the correct side to back & top alignment. In fact, on this set of builds, after dry fitting the neck assembly (described above) and measuring to bridge, my straightedge was just touching the leading edge (top) of the bridge. According to what I'm reading here, I have a little work to do to raise it up about 1/16". I will say, I measured a unstressed J40 and I got the exact same result as I am seeing in my builds. I also measured a Taylor 810, same thing. So I am not 100% sure I should do anything more.

But I will read through all of these comments tonight and do some more thinking.

Thanks!!

Mike

I asked the IRS for a clear answer on this question and this is the answer I got.

1. Scale length.
2. Fret height.
3. Bridge height.
4. Body width at widest point.
5. Add line 2 and 3.
6. Subtract line 5 from line 1.
7. If line 6 is less than 0, go to line 14.
8. If line 6 is greater than 0, subtract line 6 from line 4.
9. Multiply the square root of line 8 by the reciprocal of line 2.
10. If line 9 is less than 0, go to line 14.
11. If line 9 is more than 0, go to line 13.
12. If line 9 is 0, go to line 13.
13. The straight edge, laying on the frets, should be just a skosh above the top of the bridge
14. You have the worlds smallest Ukelele.

Now that is funny.

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George :-)

Thanks George. We must be the only ones on here that fill out their own tax returns.

Frets on or off?

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If you mean the 810 and the J40, on. Both are excellent condition guitars.

Mike

Has anybody ever considered the possibilty that maybe ... just maybe ..the whole concept of gluing the fretboard tongue to the soundboard is basically flawed ...??? With the availability of CF in varying profiles it isn't rocket surgery to design a totally rigid fretboard which would cantilever over the soundboard and which would be immune to the stress/ humidity problems which inevitably attend the traditional Martin style construction.

Got to say, I've never been totally enamoured with that design, for lots of reasons, hence coming up with this one.


Well, nothing is totally rigid. What's required at the (say) 19th fret is an impedance mismatch with the string when it is fretted. Both stiffness and mass help in achieving this and this is how the tilt neck designs work (thinking Stauffer style, here). If you thin down the fretboard upper bout overhang (i.e. less neck wood in the overhang of a Stauffer design), you rapidly get too flexible and loose tone. You're best off using either brass or steel as the extra stiffness reinforcement under the fretboard as the mass helps a lot in getting that impedance mismatch. A number of luthiers (some on this forum) seem to have independently ended up with a solution similar to that described.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Trevor, that's very similar looking to the design I was taught by George Morris at Vermont Instruments.

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I've not seen George's. Any pics?

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Hey Trevor, with your design it removes most of the top that is glued to the head block. Other than having your mortise block tight against the top brace do you do anything else to prevent head block rotation?

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It's not what you don't know that hurts you, it's what you do know that's wrong.

It's a non-standard head block made from two parts, so the remaining gluing surface is probably larger than you get on a standard head block. Plus butted to the UTB, plus sound hole doubling patch, plus real bracing round the sound hole, plus CF on most of the bracing.

It's going nowhere fast.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Thanks Trevor.

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It's not what you don't know that hurts you, it's what you do know that's wrong.