Official Luthiers Forum!

If I were to summarize a lot of these posts, and offer a suggestion based on my experience... someone who started by doing the mortise and tenon manually before building the body and ended up switching methods. I've also experimented with different radii, as well has using a solara to create a flat upper bought and domed lower.

1) the flatter your top, the less trouble you will have, period. 25' radius is more of a curve than I would consider for a top, and it's probably forcing your neck angle to be too steep to ride that curve gracefully. On Dread-sized bodies, we use a 40' radius for the top.

2) build or buy a jig that allows you to cut the mortise and tenon in the finished body and neck (before the fretboard goes on). This approach is very common for custom builders that do several guitars a year (based on what I've seen and heard). We use the Lmi jig, paired with a high-quality plunge router and it was worth the money.

Build on!

I am not. I had a batch of custom CNC necks made for me a few years ago - and before that i used Chris at Hanalei Moon. Loved his work.

_________________
P A U L B U R N E R
Burner Guitars

John, I have been pondering this exact thing - and this sounds like it will give me a way to check things and adjust if off.

Thanks!

_________________
P A U L B U R N E R
Burner Guitars

[quote="TerrenceMitchell"]1) the flatter your top, the less trouble you will have, period. 25' radius is more of a curve than I would consider for a top, and it's probably forcing your neck angle to be too steep to ride that curve gracefully. On Dread-sized bodies, we use a 40' radius for the top./quote]

This is interesting and I'm open to considering this. 25' is how I was "taught" - but that doesn't mean the person who taught me was right (grin). I'd be curious to find out what others are using for the radius on their tops.

Thanks!

_________________
P A U L B U R N E R
Burner Guitars

That was my first impression, and was why I said that it's a pretty big ask to expect those relationships to remain constant throughout assembly. Even tiny shifts in the neck block alignment can result in significant changes to the neck angle, both back and forth and side to side.

I view setting the neck angle as two discrete processes.

The first is creating proper geometry in the body to assure that the fingerboard plane will meet necessary parameters as far as having acceptable alignment to the desired string height, which for me, means making sure that there is no upward bump at the body join. I specifically want fallaway at the body join, which thankfully happens automatically for me by putting 28' radius braces onto a flat rim set.

Once the geometry of the body is set, I create the geometry of the neck to specifically match the geometry of the particular body the neck belongs to, and it's different every time. You could not take a neck from #200 and expect it to work on #201.

That's why I think having pre-matched parts makes this step actually more difficult. I think that most neck angle jigs like the LMI one are designed to adapt to what the body geometry actually is, and allow you to account for it in the creation of the initial steps of the neck building process. If the body is a bit like this, then the neck goes a bit like that...

Anyway, you've gotten a lot of very good advice from other people so far. The two things that have stood out most for me are making sure your block is square by screwing it into the mold, and to look at whether you can flatten your UTB radius to create the geometry that works for you radius.

there are many ways to do this. While I use a 28 ft radius I only radius from the top of the sound hole to the neck joint. This keeps the top flat in the bridge area. Not a fan of domed tops.

_________________
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

Welcome back, Paul!

I use a 30’ radius and don’t do anything special to the rim.

I use a neck angle jig. The LuthierTOOL and LMI jigs work from the same concept of setting the neck angle based on a measurement from the saddle area. You butt the soundboard up against an arm. This arm is adjusted to a target gap at the saddle location (I like 3/32”). The movement of the arm adjust the plane of the neck, and you rout. The perfect neck angle for the neck is built into each neck. Robbie O’Brien has a video demoing the LMI jig on YouTube, and LuthierTOOL has on as well. Seeing how they work makes so much more sense to me than what I just wrote. It’s really pretty basic and brilliantly effective.

I don’t have gaps under the fretboard on my current two guitars, but I have had very minute gaps in the past. They sand out easily.


Sent from my iPhone using Tapatalk

I like to use a fully domed top, on the belief that it increases stability. This necessarily means that the projection at the bridge will be too high if the fingerboard end is flat on the top. You can either taper the fingerboard thickness or set the neck a bit 'proud' and use a wedge under the fingerboard. There was something of a vogue for raised fingerboards in the Classical world a while back, with the originator, Humphrey, making big claims about improvement in the tone. In the end that aspect didn't turn out to be a valid selling point, but it does make high fret access a bit easier, and since Classical players eschew cutaways it is still fairly popular.

Note that raising the neck this way does not require using a tall bridge. You just set the neck angle so that the fingerboard projection ends up the correct height off the top for the bridge height you want. That's the really important variable here. The string height off the top at the bridge affects the tone and also the loading on the top, so you want to get that 'right'. The neck angle is just the way you do that; it's not important in itself in the range of plausible angles for a guitar neck.

We radius the top of the rim at 28' as well as the X, fingers, tone bars, and bridge plate. The UTB is radiused at 60'. The 28' radius bracing and bridge plate goes on in the 28' dish, and the 60' UTB, soundhole bracing, and flat UTG gord on the top on a flat caul.

The lower bout will pull up a bit under string tension and the final longitudinal radius will be a bit smaller than 28', while the upper bout above the soundhole ends up flat (but not concave as often seen with flat UTBs or flattening the rim. This works with anything between Size 5 12 fret guitars and SJ-200-sized 14 fretters.

As for setting the neck angle after the body is closed, I believe I have covered the math in detail, as well as the neck setting gauge we use in the shop...check the archives.

Last day here at Greenridge for the foreseeable future...Todd is back to full duty after his hand surgery, and I have to get back to work on other projects. Filippo should be back from Guatemala soon, so time to head down the road!

_________________
If you're really successful at (BS'ing), it means you're not hanging around enough people smarter than you.

Neil deGrasse Tyson

Woodie, I just want to confirm that you are not altering the 28ft r on the rim north of the waist. You are saying you radius the whole rim to a constant 28ft. radius then glue in the 60ft UTB on a flat caul and glue the whole thing onto the rim set correct?

_________________
Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

I think part of Humphrey's design was the way the strings pulled on the bridge like a harp too.

I have built true flat tops and with a radius. In either case I always arch the UTB and in the case of radius tops I impart that canted top angle in the body of the guitar. That in combination of the arched UTB sets the ramp of the FB extension to the bridge.

I love nothing more than to see a perfectly flat transition of the FB over the body joint. I don't do fall away but if I did it would be easy enough to just sand it in.

Yes...that is correct. The 28' radius on the rim and 60' radius on the UTB (but glued flat) ends up giving us just the right offsets to keep the upper bout from going concave over time, the correct angle to take the fretboard extension without wedge or taper, and avoids a hump or hollow under the fretboard extension.

_________________
If you're really successful at (BS'ing), it means you're not hanging around enough people smarter than you.

Neil deGrasse Tyson

I think OP might discover too many cooks in the kitchen. I suggest that OP pick one source and ignore the others. These varying techniques are not variations on a theme, they're differing paths to the same goal and mixing them together just won't work.

_________________
Peter Havriluk

the Humphreys design was for classical not steel

_________________
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 agree. There are many paths that all work, but finding one that seems logical to you and following it is the way to go. Another few guitars up the road, if that one didn't suit you, try another or vary what you tried the first time.

For me, at least for now, I sand the rim to 28' radius, and radius the top to match, transverse and all. I cut the neck tenon on an LMI style jig and then match the fingerboard extension to the curvature of the top as it's built as I would on an archtop. I've done it differently in times past.

When I first was learning guitar building from Charles Fox many years back, he was doming the tops from the center of the X brace down towards the lower bout and leaving the area above the soundhole flat. He was building Spanish style with a slipper foot at the time. That worked fine, but after having done some archtops and using the LMI style jig, I'm comfortable with adjusting the neck angle and fine fitting the extension to match what I've actually built into the body.

But I'm not the greatest cook.

Dave

Good call. Thanks for pointing that out. You cannot just mix and match methods.

_________________
Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

jfmcenna wote:
"I think part of Humphrey's design was the way the strings pulled on the bridge like a harp too."

That was the idea, which he said came to him in a dream. Harps do tend to sound really nice unless they are badly strung, so as an idea it's pretty good. The problem is getting enough of an up angle of the strings off the soundboard. Humphrey made his with about a 5 degree up angle, through a combination of putting the taper on the top instead of the back and raising the neck angle. This is simply not enough to gain the sonic benefit, but it does put extra stress on the top. He ended up putting in low cross braces along with longitudinal or fan braces to fight this. The tops probably ended up heavier and/or stiffer than a normal fan brace top, which worked against any increase in efficiency.

Back when I was researching string forces I looked at the way a small harp works (building lots of different instruments can be handy). The strings pull upward at about a 40 degree angle, so the tension change force of the vibrating string can act directly pulling the top upward.

If you pluck a string exactly in the middle of it's length you should only get the odd-order partials in the transverse force; the fundamental and frequencies at 3, 5, and 7 times that, and so on. The tension change signal is frequency doubled relative to the transverse, so it would have frequencies at 2-4-6... times the fundamental. On a flat top or classical guitar plucking the string exactly in the center so that it moves 'perpendicular' to the soundboard gives a signal that is mostly odd-order partials, with a little bit of the even-order ones from the tension change. On an arch top there is almost no even order energy; it's so low it's hard to see in a spectrum analysis. On a harp plucking the string in the middle produces all of the partials, but the even and odd orders die out at different rates because they're coupled differently to the sound board. I have a classical 'test mule' with a removable neck that allows for varying the string angle, although it doesn't preserve the action in the process, so it can't be played with the neck up. You can pluck the strings,though. I set it up with the strings pulling upward on the soundboard at 5 degrees, and got no detectable increase in the even-order partials with a center pluck. As my Indian-born calculus professor used to say: "As the djangle goes to djero, the sine of the djangle also goes to djero..."