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Let's see some pictures and ideas on how to make adjustable necks! Clay posted some photos here, although we can only guess at the internal structure. And I described an idea in the post before that, which I've made a crude model of in Blender.

The adjuster would be a lag screw ground flat on the tip, with a squiggly knob glued onto the head so you can turn it easily with your finger through the soundhole. Threaded directly into the upper transverse brace, which may be hardwood for more durable threads. This will provide greater friction than metal threads, reducing the chance of it vibrating its way out of adjustment.

If the hinge is placed on the outside as in all but the last image, the hinge pin could be pulled to remove the neck entirely for easy travel storage. Obviously the hinge will be a prominent visual feature and felt by your thumb on the neck, so it will need to be crafted with care. One option would be to make it out of carbon fiber, by wrapping fabric around a rod inside a teflon tube. Then the rest could be form-fit to the neck and glued so it doesn't have any screws to feel, and no cold feel of metal in general.

The stealth version with the hinge inside the box would have much less movement range, but that's kind of a good thing so the neck doesn't flop all over if you take all the strings off. It only needs a few degrees of adjustability to be useful. The main disadvantage is the lack of removability. But it will be great for harp guitars, where neck removal wouldn't help anything anyway, and it will allow a beautiful continuous curve all the way from the harp headstock to the cutaway point.

I built my first acoustic with an adjustable neck based on Mike Doolin’s design. It works very well.

Some disadvantages:
1. Original design adjusted on the neck block from inside the box. I see he has changed his design to adjust at the heel and this would be my preference too.
2. Fretboard extension floated on two 1/4” graphite rods. That was not enough support and notes above the body join suffered as you might expect. A more substantial support is needed.

Here’s a link to Doolin’s site.

http://www.doolinguitars.com/articles/adjnecksys/

Edit: I would use the Doolin method over a hinge because the two upper pivot points (setscrews riding on brass inserts) are adjustable which allows a precise adjustment of the neck centerline. Again, moving access to the neck bolts to the outside of the heel would make it relatively easy to remove the neck. For a non-travel guitar only the adjustment bolt would need to be accessed from the outside of the heel.

Some photos:

You can see the setscrews for adjusting the centerline on the upper sides of the neck mortise. Neck bolts are captive in the headblock and Bellevue washers provide tension on the neck bolts.


1/4" graphite rods are not sufficient support for the fretboard extension and notes above the 14th fret are obviously damped. If I do it again I would put a tenon underneath the extension to get some mass under there.


You can see the brass inserts for the setscrews. I have never used the one on the bottom of the tenon as the bottom neck bolt holds everything just fine.


The neck is inlet into the body about 0.10" so there is no gap when viewed from the side.

_________________
Steve Smith
"Music is what feelings sound like"

I think you could eliminate that top bolt as well. As with the bottom brass insert+set screw, the only useful function it performs is keeping the neck from flopping if the strings are all loosened. On regular bolt-ons it can also prevent a small gap from opening due to the heel flexing inbetween the top bearing surface and bottom bolt, but that's not an issue here since this has a gap to begin with and is inlet into the body so you can't see it.

Good point on the side-to-side adjustment. My long and narrow hinge would also be weak to side impacts to the headstock, so I've widened it out some. And instead of screwing to the headblock, it now slides down into a slot in it, and is pinned by a dog-tipped screw. Side-to-side adjustment would be done at construction time by sanding material off the hinge. The headblock will need some dowels drilled and glued through it to prevent splitting, and may need an insert of hard material for the bearing surface near the top.

I've also fattened up the A frames and worked the one on the cutaway side into a sort of buttress to bridge that skinny spot in the structure.

EDIT: Better idea for side-to-side angle adjustment: now that the hinge is screwed to a flat section of the neck (unlike the external hinge design), I can just make the screw holes in the hinge slightly oversize and use pan heads instead of countersunk.

What’s the thinking behind having the joint be mechanically adjustable vs adjustable via some more inert method like swappable shims? To me it looks like stability is being somewhat sacrificed vs say a bolted joint w/ shims but I’m not sure what benefit is being gained in return?

I own a Hofner flattop with a mechanically adjustable neck and they are common here so I have them through the workshop frequently for setup or repair. In theory, you can change neck angle between songs (maybe even mid-song) with this joint. In practice, why would you? I’ve never encountered anyone who uses the joint in this way so it becomes more of a point of failure as the adjustment hardware seizes, strips if it is not maintained.

The benefit is increased comfort and reduced cost. I wouldn't expect it to be adjusted more than twice a year for small seasonal movement, but I wouldn't expect a bolt-on to be shimmed until it's been uncomfortable to play for a while. And like a truss rod you can adjust it under tension until it buzzes and then back off, whereas each test of a bolt-on or saddle shaving requires a moderate amount of work (and in the case of saddle shaving, a lot of work if you go too far and have to start over).

My hinge design may turn out to be a failure. It was just an idea, and you're watching the real-time evolution of that idea to its logical conclusion. The Doolin design Steve posted does have some advantages (particularly the ability to adjust the nut-saddle distance in addition to the neck angle), but it also doesn't look significantly easier to adjust than a shimmed bolt-on. And in my case, I want something that I can use on harp guitars without having to make a flat spot perpendicular to the neck for it to butt up against.

Do you have any photos of the Hofner style? From an image search it looks like a heel-less neck, but I can't find any detail of the adjustment mechanism.

Actually the Doolin neck joint is quite easy to adjust because I don't use the bottom setscrew at all and the top neck bolt and top setscrews are a one time adjustment. Being able to move the intonation back and forth about 50 thou is a definite advantage but again, that's a one time adjustment. I have a 1/4" ratchet set up with a 2" hex bit on it so I can adjust action height with the bottom bolt through the sound hole with the strings on. If the adjustment was on the outside of the heel it would be super simple. At 0.050" per turn on the bottom bolt you have a lot of very fine control over the action height.

I think the hinge idea could work if you can develop a way to adjust the centerline and if the hinge is very precise with no slop.

One other big thing for me is that there needs to be enough wood (or??) under the fretboard extension so that the notes above the body join sound good.

_________________
Steve Smith
"Music is what feelings sound like"

I don’t have any internal shots of the mechanism to hand but it’s a conventional-looking heel with a Stauffer style adjustment through the heel via a square drive tool (eg a clock key/winder)


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I once built an adjustable neck where the adjustment bolt was captured in the heel underneath one of those end pin jacks I had sawn off really short. So, the end pin jack served as a strap button, and the hex head furniture bolt underneath could be adjusted by sticking an Allen wrench through the hole in the end pin jack. This was a good way for the adjustment hole and bolt head to be camouflaged. It still works great. I might do that again someday.

As mentioned Martin is doing an adjustable neck joint that is so very adjustable no shims are required for a player... a player to adjust their own neck angle even at a gig and experience success.

Seems to me it's a pretty good idea when you are attempting to build a better mouse trap to know what mouse traps are already out here and have sold tens of thousands of iterations. But that's just me....

As to Josh's question do you recall the wars... wars over a bolt-on not sounding as good as a dovetail?

We go to a great deal of trouble when resetting a dovetail to have as much wood-to-wood contact in the joint as possible. We do this with bridge glues too. When we shim a Fender style bolt on neck we and other quality shops match the material of the body or neck and make a shim that has contact with the neck and body the full length of the pocket.

Why? Vibrational transfer and vibrational transfer is basically on the most basic level what we, Loofiers... should be all about.

From the Hauser that I've worked on to countless Taylors to Ken Parkers there are lots of implementations of adjustable necks already out here and let's not forget Fender with tens of millions of "micro-tilt" adjustable necks in the wild.

Some work OK as intended but you still have the bolt-on vs. one piece construction conflict. Some have vibration being transferred with only a set screw as a point of contact.... Some like Taylor have full wood to wood contact and other than the shims being proprietary it's a decent design.

Anyway if it were me I would be reviewing these, what's already in service too it has a lot of bearing on the goal here.

Here's some stuff on the Martin SC series and I've worked on several of these for Jeff Daniels the actor and his son Ben of the Ben Daniels Band. These models are a big hit with gigging musicians and can be action adjusted on the fly in the field by a player, wow.

https://www.martinguitar.com/blog-categories/from-the-factory/blog-052621-what-makes-martins-sc13e-so-revolutionary.html

It's an interesting joint if someone can find a break down of it that is more of an engineering drawing please post. I have this but it's proprietary to Martin and service centers like us not for public display so I can't post what I have.

Yep, that is the primary purpose of this thread, to gather the information in one place because it's hard to find what has been done if you don't already know, and hard to guess what the actual mechanisms are if you only have external pictures to look at (such as that Martin SC).

The majority I've seen are the same as what Steve posted. Two pivots near the top and a screw near the back which pulls the heel as a lever. Sometimes screw head inside, sometimes outside, sometimes neck inlet into the body to hide the gap, sometimes visibly "floating" on the pivots. But all the same fundamental mechanism.

It looks like Ken Parker's necks are normally floating on a single post, and the adjustable version is floating on two posts. Judging by the location of the screw on the back, the one at the end of the neck is the pivot, and the one closer to the edge of the body changes height to tilt the neck (class 3 lever).

Pivoting at the upper transverse brace is an interesting idea. Playing around with it, I could eliminate all the metal except the adjustment screw, greatly reducing the weight. Mill a U slot in the brace, and carve a mating cylindrical protrusion on the end of the neck. And the last bit of neck overhanging the UTB doesn't need to be so strong now, so I can make it thinner and sink the neck deeper into the body, which I prefer. Makes the funky buttress even funkier, though. EDIT: Wait a minute, the buttress/A frame may not even be necessary anymore since the longitudinal neck force is dumped entirely on the UTB instead of the headblock.

Nice to see a desire to eliminate the metal that would be my choice too for an acoustic guitar. Or at the very least use stainless.

A recurring problem in guitar design that we see in the repair side is corrosion of metal parts. It can be a big deal too as in a seized truss rod and be behind the need for invasive, expensive repairs.

Additionally I really hate seeing any acoustic guitar with a neck teetering on something metal such as a set screw or bolt. I greatly prefer as much wood-to-wood contact as possible.

I messed around with my own design for an adjustable neck angle neck back in 2007 and never built it but I did draw it out. My idea was unique and unlike anything else I have ever seen.

If you have ever see the the neck cradle devices for holding a guitar in place on a bench. They are half round that rocks on the bench to fit different height guitars and flat on top with a groove for the neck to sit in. StewMac may sell, still a big version of this half circle with the neck resting on it.

Make the half circle 4 - 5" and have it be the top of a two piece neck block with a matching curve on that the half round rests in. The two halves can rotate at will but whenever they do they still have FULL wood-to-wood contact no mater what the neck angle.

That's what I wanted to achieve in my design spec, full wood-to-wood contact. If I could draw worth a dang I would post a pic of it but I can't draw.

Something like this? Loosen the screws, tap the neck with a mallet to shift it, and re-tighten. But with such a large rotation radius, it would change the nut-to-saddle distance by a relatively large amount... I probably should have made the neck the convex side, because the ideal is to have the center of rotation on the fret plane. As it is, the thickness of the neck adds to the radius, whereas the other way it would subtract.


EDIT: Oh, or maybe like this, so the fingerboard extension rises up quickly when adjusted, solving your action problem with minimal angular motion. And the screws are in better position this way.

Wow, that looks really interesting!


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Yeah it does and good going Dennis for illustrating this concept so quickly.

What I had in my mind's eye was a bit different. A very low radius with the radius being on the top of the neck block and a matching radius on the back of the neck.

What you drew could work too maybe even better. The goal for me back then was lots of wood-to-wood contact are so the curve on the back of the rides in the matching curve of the neck block.

Regarding changing scale length when a neck needs to be reset the scale length as already changed to be less so bending the neck back only restores the original or something closer to it intended scale length in Dennis's example or what I intended with the radius on the top of the neck block and bottom of the neck.

The heel rides in a pocket on the face of the rim like a few other implementations that exist now.

Very cool Dennis, thanks for doing this!

Good point, so it's a feature, not a bug
Certainly an interesting family of joint designs to explore. Thanks for sharing! I hate when good ideas are lost to history.

The Sliding Circle Joint shown in side drawing is remarkably similar to the neck attachment used on vintage Kay Kraft Venetian guitars.
My first adjustable neck joint design used a cove cut in the end of the neck and a corresponding knuckle attached to a mortise in the body. The axis of rotation was about 1/2" below the surface of the soundboard. Adjusting the angle caused the string length to change ever so slightly which noticably affected intonation.
It is probably worth noting that the Kay Kraft guitars had floating bridges which could be tweaked once action was set.
I find adjustable neck joints a requirement. The other day I was playing guitar and my wrist started hurting. We've had a good amount of rain as well as slightly cooler temps which meant the a/c wasn't running as much. As we know, higher humidity causes the action to raise. Out came the adjustment wrench and I was able to lower the action just enough to eliminate wrist pain. This was done while up to tension and took less than 1 minute.
It took me a considerable amount of time to work out the kinks in my 2.0 adjustable neck joint version and I'm not quite ready to make it open sourced.

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That's real value Tom when you can make an adjustment and stop hurting. Sounds very promising and I understand not making it open source. I just got back from a hip x-ray, we aren't getting any younger my friend....

Here is my usual crude drawing for the "adjustable/ removable" neck design I use for travel guitars. The neck is designed to be removable and store inside the body. The action adjustment feature is somewhat of an after thought but can create a guitar neck that never needs a reset.
The neck pocket is cut at an angle - deeper toward the back of the guitar. If a uniform depth pocket is preferred then the angle could be cut on the heel. I have used flat metal plates installed into the heel for the cap screw to bear against, but I think using the softwood insert with the broken thread is a simpler and just as effective solution. A single 10-24 socket head cap screw provides plenty of strength to resist the forces placed on the neck. I couple of small metal "dowels" hold the neck in place under the fingerboard which for travel guitars is cut (at an angle) between the 12th and 13th fret to allow the neck to come off without any overhang of the fretboard (which might otherwise be subject to possible damage) For a guitar with a non-removable neck and glued down fingerboard extension relieving the underside of the fretboard between frets spanning the neck pocket (12-13 or 14-15) might allow it to bend without loosening the frets.
The principal advantage I see to this design is the simple construction and low cost and easy replaceability of the hardware. It is also easy to make adjustments to the action.

P.S. I tried a stainless socket head cap screw and found it to be problematic - adjusting it under tension seemed to make it gall and lock up. Lubrication may have helped, but after struggling to remove it I went back to the typical black type.

The current issue of American Luthiery has a fantastic and very in depth article on Ken Parker. His current adjustable neck design on his archtops using a single post which only adjusts for height, and not angle.

A fundamental premise of his design is his extensive use of carbon fibre within his necks and neck-posts, to the extent that the truss rods in his prototypes were unable to change the neck curvature. So he doesn't have truss rods in them anymore.

I was kind of thinking the same thing when messing with Hesh's circle joint As the circle radius approaches infinity, you end up just sliding flat surfaces vertically.

I knew I was missing something obvious on the hinge design, and I finally realized what it is. The headblock hinge can be replaced with two little hooks permanently anchored into the headblock. This way the neck comes off without having to pull the hinge pin, and the headblock can be nice and solid.

Interesting, no worries about getting a precise fit like with the pin in a standard type hinge. Still need a way to set the centerline.


Steve

_________________
Steve Smith
"Music is what feelings sound like"

It's done while screwing the hinge to the neck. The holes will have a little wiggle room for adjustment.

Saw this video today on a different take on adjustable necks and thought it might keep the conversation going. He shows the neck at around the 1:55 minute mark. I am pondering a neck system that would use something like a bass tuning machine with an Allen screw adjustment but can't get my head around it.

https://youtu.be/hzPR0Jez-E8?si=rUtYTXBNIZ0FwlnP

Thanks for that link Paul.

Fascinating builder!

Here’s one that YouTube threw me…

https://youtu.be/akHdBRnK-qE?si=rGn5EFQlmGediAxC