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 Post subject: Neck, bridge or braces?
PostPosted: Fri Jun 01, 2018 7:14 pm 
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Mahogany
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Hi all,

I completed my third acoustic in January and delivered it to the friend I was building it for interstate. Between then and now the action has increased notably and I just wanted to pick your collective brains about what you feel is the most likely culprit.

The guitar is a Taylor 814CE style with a torrefied spruce top and quarter sawn spruce bracing. The top is circa 1/8 thick. The bridge plate is walnut and 3/32 thick. Bridge is ebony.

Known potential issues:

The braces are definitely quite scalloped including above the X which I subsequently learned is bad practice. That being said there is still reasonable height on them in the lowest sections, though others may disagree! While I am concerned about the scalloping above the X the top has not caved in above the bridge. Hard to tell from the attached photos that is a bit of a rotation of the bridge due to string tension and a slight belly behind the bridge.

Hindsight etc etc, but the other issue I am aware of is that only really the front corners of the bridge intersect the lower arms of the X brace, and even then only just. You’ll also note that I tapered the edges of the brace in the opposite direction to the taper of the braces, meaning potentially even less support, and a narrower platform on which the rotational tension from the strings is exerted?

So, these are just some potential concerns.

Am I best:

1. Reinforcing the bridge plate (maybe with CF), maybe even filling in the triangle between it and the X

2. Adding additional bracing parallel to the arms of the X

3. Replacing the bridge with a wider one shaped to straddle the braces. Should a bridge always intersect the lower arms of the X brace? I notice in Trevor Gore’s book (bought in part to seek some answers on this matter) that all 4 major braces with his falcette bracing run under the bridge.

4. Reset the neck?

My concern with 2 is that the top felt plenty stuff to me and I wasn’t that blown away by the tone. The tap tone sounded a bit dead to me. Possibly left the soundboard too thick.

During these repairs I plan to reduce the thickness of the lacquer on the top (currently circa 1/64 thick) to see if that improves things. If not I’ll potentially try reducing the thickness of the soundboard, but only if I’m confident it’s not going to exacerbate the bellying.
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PostPosted: Fri Jun 01, 2018 7:49 pm 
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My main suspects would be the scalloping of the upper X legs and removal of one soundhole brace for the pickup control. The whole soundbox tends to fold at the waist due to that big hole cut in it :) But the soundboard should be concave beside the soundhole if that is the case. Or at least flat.

To check the bracing stiffness, measure the bridge rotation under tension (Design book, page 4-34)



These users thanked the author DennisK for the post: Bosco Birdswood (Fri Jun 01, 2018 7:57 pm)
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PostPosted: Fri Jun 01, 2018 7:59 pm 
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Mahogany
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DennisK wrote:
My main suspects would be the scalloping of the upper X legs and removal of one soundhole brace for the pickup control. The whole soundbox tends to fold at the waist due to that big hole cut in it :) But the soundboard should be concave beside the soundhole if that is the case. Or at least flat.

To check the bracing stiffness, measure the bridge rotation under tension (Design book, page 4-34)

Yeah annoyingly the only way to fit the LR Baggs Anthem preamp and controls is to flatten that brace right down. Incredible pickup but very annoying! I did have concerns about the upper braces but there is no evidence at all of a shift above the X, which makes me think the issue is elsewhere.


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PostPosted: Fri Jun 01, 2018 8:19 pm 
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Do you have a shot of a straight edge on the top starting at the neck intersection down to the bridge to see if the top is still domed or now concave?

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PostPosted: Fri Jun 01, 2018 8:27 pm 
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klooker wrote:
Do you have a shot of a straight edge on the top starting at the neck intersection down to the bridge to see if the top is still domed or now concave?

Not just yet sadly as I don’t have the guitar back. I’ll have it in a few weeks.


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PostPosted: Fri Jun 01, 2018 9:42 pm 
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Could be lots of things, including simple ones, like not enough truss rod tension...

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PostPosted: Fri Jun 01, 2018 10:29 pm 
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Trevor Gore wrote:
Could be lots of things, including simple ones, like not enough truss rod tension...

Alas, I tried that when I briefly had access to the guitar (no tools though). Neck was straight as a die.

I notice, Trevor that not only do all major braces intersect the bridge with your falcate bracing design but your bridge plate is notably longitudinally longer than the one on my guitar. This makes me wonder if you had designed your guitars in such a way as to address the exact issue I am having.

Interested to hear your thoughts on the Bridge Doctor. Immediate thoughts from reading parts of your book so far is that it adds notable weight to the soundboard, and potentially increases the losses to the sides of the guitar?


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PostPosted: Sat Jun 02, 2018 12:17 am 
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Do you have numbers on the action? What was it before delivery and what is it now? Also, what was the humidity in your shop when you built it and what is the humidity in its new home?

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PostPosted: Sat Jun 02, 2018 2:19 am 
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J De Rocher wrote:
Do you have numbers on the action? What was it before delivery and what is it now? Also, what was the humidity in your shop when you built it and what is the humidity in its new home?

Humidity is definitely a consideration. My “shop” is a tin shed but it was built during summer in Melbourne, Australia which is very dry and warm. It is now in Sydney which is notably more humid. So it could just be as simple as that I suppose?


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PostPosted: Sat Jun 02, 2018 9:38 am 
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Bosco Birdswood wrote:
I notice, Trevor that not only do all major braces intersect the bridge with your falcate bracing design but your bridge plate is notably longitudinally longer than the one on my guitar. This makes me wonder if you had designed your guitars in such a way as to address the exact issue I am having.

I use the 2 degrees of bridge rotation rule of thumb. Check it out, Design, p. 4-33. Remember that falcates have CF above and below. The shape of the bridge plate is designed to avoid stress concentrations and locally high curvatures at the back of the bridge in order to prevent bridge-peel types of failure. (I've never had one, so it probably works!) Going from the pics, the bridge doesn't look over-rotated, but pics can be deceptive.
Bosco Birdswood wrote:
Interested to hear your thoughts on the Bridge Doctor.

I've not seen/measured enough of them to form a proper opinion. So I won't add to the hearsay.
Bosco Birdswood wrote:
Humidity is definitely a consideration.

Definitely. But Sydney's been pretty low humidity for the last few weeks. I think you're going to have to wait until you have your hands on it and can figure which part moved.

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PostPosted: Sat Jun 02, 2018 9:50 am 
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Rh is one thing to consider but scalloping the upper braces is a no no. That is a structure area that needs to be stiff to handle the coupled forces of the neck block and bridge. This allows the top to dip and the bridge to roll.
Also walnut is not a good plate material. I tried it one and had issues. When you get this back you want to do 2 things.

document the top under the stress of the string load. Then watch what happens as you remove that stress , If the top remains high and constant with the string load you may suspect RH
If you see the top move as the stress is removed you should be able to ascertain where your weak point is

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These users thanked the author bluescreek for the post: Bosco Birdswood (Sat Jun 02, 2018 3:58 pm)
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PostPosted: Sat Jun 02, 2018 3:50 pm 
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Just to make sure... the comment about scalloping the upper x, we are talking about not scalloping the leg itself like the lower is, but still scalloping the end as it heads into the lining, correct? Otherwise I’m going to have some issues down the road! :D


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PostPosted: Sat Jun 02, 2018 4:20 pm 
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Trevor Gore wrote:
I use the 2 degrees of bridge rotation rule of thumb. Check it out, Design, p. 4-33. Remember that falcates have CF above and below. The shape of the bridge plate is designed to avoid stress concentrations and locally high curvatures at the back of the bridge in order to prevent bridge-peel types of failure. (I've never had one, so it probably works!) Going from the pics, the bridge doesn't look over-rotated, but pics can be deceptive.


Thanks, Trevor. I’ll check it out shortly. Does it mention in your book somewhere the type of CF tow / mat you use? If you could recommend an Australian supplier that would be great.

Trevor Gore wrote:
I've not seen/measured enough of them to form a proper opinion. So I won't add to the hearsay.


Spoken like a true scientist! Thanks, Trevor. Really appreciate the advice. I’ve been very fortunate to have a direct line to the authors of both technical books I have recently tackled. The other being The Art of Sound Reproduction by John Watkinson, who just happens to be my best friend’s dad :)




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PostPosted: Sat Jun 02, 2018 4:52 pm 
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What is the neck attachment system? Are both the tenon and fretboard bolted, the tenon only with a glued extension, or a glued dovetail? A bolt on, especially with a bolted extension can move more than a glued extension.

I have had owners that were humidity freaks and really ballooned their guitars with too many case humidifiers or left them out in hot humid conditions. They never fully came back and required a slight tweak of the neck set.

John’s advice was good. See where a straightedge hits the bridge with and without string tension. That will be helpful in seeing if the neck angle has settled. I’d let the guitar sit at a reasonable humidity for a couple of weeks, do the straightedge test and if necessary reset the neck. It may have found it’s sweet spot as far as settling and be just fine after that.

Your specs and pictures don’t look all that bad except for a little more scalloping of the upper transverse brace than I like and scalloping of the upper X which I see no reason to do.

Have your earlier guitars settled this much?

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PostPosted: Sat Jun 02, 2018 6:37 pm 
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Bosco Birdswood wrote:
Does it mention in your book somewhere the type of CF tow / mat you use?

Yes, p 11.41.
Bosco Birdswood wrote:
If you could recommend an Australian supplier that would be great.

I get mine from a mob that used to be called FGI, (now Nuplex) in Brookvale, NSW. Search carbon fibre Australia for other suppliers.

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PostPosted: Sat Jun 02, 2018 6:42 pm 
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Terence Kennedy wrote:
What is the neck attachment system? Are both the tenon and fretboard bolted, the tenon only with a glued extension, or a glued dovetail? A bolt on, especially with a bolted extension can move more than a glued extension.

I have had owners that were humidity freaks and really ballooned their guitars with too many case humidifiers or left them out in hot humid conditions. They never fully came back and required a slight tweak of the neck set.

John’s advice was good. See where a straightedge hits the bridge with and without string tension. That will be helpful in seeing if the neck angle has settled. I’d let the guitar sit at a reasonable humidity for a couple of weeks, do the straightedge test and if necessary reset the neck. It may have found it’s sweet spot as far as settling and be just fine after that.

Your specs and pictures don’t look all that bad except for a little more scalloping of the upper transverse brace than I like and scalloping of the upper X which I see no reason to do.

Have your earlier guitars settled this much?

Hi Terence,

The neck is bolted on with the fretboard extension lightly glued. If I did a reset would I just gently conform the extension to the upper bout or would I need to plane the section of the upper bout that sits under the fretboard so that it is parallels to the run of the fretboard?

It’s tricky with the other guitars because they are both mine and I did multiple adjustments, setups, fret levels re-crowns etc over time on them as I honed my skills, so I couldn’t say for sure that none of them shifted like this. It does look like the dreadnought I built has a similar tip on the bridge but the soundboard on that one is bunya pine and ended up being a lot thinner than the soundboard on this guitar. I did scallop the X though, but only the lower arms. The reason I scalloped the upper arms of the X brace on this was because I felt that it was very stiff in the upper bout, but I suppose it’s probably supposed to be! It is still pretty stiff I would say. In retrospect finning the soundboard probably would have been a better approach but I will be the 1st to say that I have no idea what I am doing when it comes to building guitars!


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PostPosted: Sun Jun 03, 2018 8:20 am 
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I suppose if a big correction of the neck is needed you might have to shim the extension but I can live with a slight drop off. Good luck and post your findings when you get the guitar back :)

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These users thanked the author Terence Kennedy for the post: Bosco Birdswood (Sun Jun 03, 2018 11:54 am)
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PostPosted: Sun Jun 03, 2018 11:55 am 
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Terence Kennedy wrote:
I suppose if a big correction of the neck is needed you might have to shim the extension but I can live with a slight drop off. Good luck and post your findings when you get the guitar back :)

Will do. Itching to get her back now!


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PostPosted: Sat Jun 23, 2018 8:04 am 
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OK, so she’s back in “the shop” (my bedroom) and there are a few interesting revelations;

The action was caused by the truss rod being too slack. I’ve tightened it up and the action is now good. Massive relief!

I put a straight edge across the lower bout and without tension there’s circa 1.25 mm gap either side between straight edge and the soundboard, and 1.85 under tension. The angle on the bridge under tension is less than 2°. From bridge to edge along the centre seem there’s almost no change, and zero movement that I can see in the upper bout. It’s very straight longitudinally with no evidence of caving in due to the scalloped upper X brace arms. Also a relief.

The bridge is slightly off centre as it seems I didn’t line the neck up perfectly, so it doesn’t sit centrally on the X brace arms, and I also initially worried it wasn’t supported enough. Having mapped out the x brace with magnets though it’s not as bad as feared.

Do you guys think this is sufficiently supported looking at the green brace traces?

The owner (a friend for whom I built the guitar at cost) is very happy with it, and since tightening the rod it plays well and sounds better (unexpectedly). But... I have this niggling thing where I know I shot too much lacquer on the soundboard (circa .35mm as discovered when scraping back for the bridge). I want the guitar to sound as good as it can be without making major changes given it’s a finished instrument.

So, do I leave it as a perfectly acceptable 3rd guitar with which the owner is happy, or do I remove the bridge (titebond), thin / strip and re-apply the lacquer and then maybe replace the bridge with one that straddles the x brace better as per the other picture?

I just don’t think I can reduce the thickness of the lacquer convincingly without removing the bridge but am open to suggestions. ImageImage


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PostPosted: Sat Jun 23, 2018 8:36 am 
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The thicker lacquer may eventually craze and give it that Vintage vibe, but if the guitar sounds and plays good I would call it done and not mess with it. The enemy of "good" is "better". Redo's often become a can of worms. Ask your buddy what he thinks - his is the most important opinion.


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