The Acoustic Guitar Forum

The Acoustic Guitar Forum (https://www.acousticguitarforum.com/forums/index.php)
-   Build and Repair (https://www.acousticguitarforum.com/forums/forumdisplay.php?f=44)
-   -   Gluing Top Braces (https://www.acousticguitarforum.com/forums/showthread.php?t=604284)

Victory Pete 01-18-2021 11:53 AM

Gluing Top Braces
 
Do you glue top braces to a flat or radius surface when using a Go Bar deck?

charles Tauber 01-18-2021 12:14 PM

That depends on whether you want a flat top or an arched top. If flat, glue braces on a flat surface: if arched, glue braces on an arched surface. That's regardless of how you chose to clamp them while the glue dries.

Seems pretty self-evident. Am I misunderstanding your question?

Victory Pete 01-18-2021 01:10 PM

Quote:

Originally Posted by charles Tauber (Post 6607904)
That depends on whether you want a flat top or an arched top. If flat, glue braces on a flat surface: if arched, glue braces on an arched surface. That's regardless of how you chose to clamp them while the glue dries.

Seems pretty self-evident. Am I misunderstanding your question?

The braces for a Dreadnought are radiused. If you clamp to a flat surface some of the radius of the braces will be reduced. If you clamp to a radius dish the radius will remain.

charles Tauber 01-18-2021 01:47 PM

Quote:

Originally Posted by Victory Pete (Post 6607934)
The braces for a Dreadnought are radiused.

Most are, yes.

Quote:

If you clamp to a flat surface some of the radius of the braces will be reduced.
True. Is that the result you want? If so, do that. If not, don't do that.


Quote:

If you clamp to a radius dish the radius will remain.
Also true. Ideally one would use a radius dish that matches the radius one wants to end up with (i.e. the same as the radius on the braces).

I still don't understand what is your question. It seems like you already know the answer. Glue radiuses braces using a radiused dish: glue flat braces using a flat surface.

Victory Pete 01-18-2021 02:16 PM

Quote:

Originally Posted by charles Tauber (Post 6607968)
Most are, yes.



True. Is that the result you want? If so, do that. If not, don't do that.




Also true. Ideally one would use a radius dish that matches the radius one wants to end up with (i.e. the same as the radius on the braces).

I still don't understand what is your question. It seems like you already know the answer. Glue radiuses braces using a radiused dish: glue flat braces using a flat surface.

My first build was a StewMac kit and the instructions were to glue the braces, which were radiused, on a flat board. That guitar was very loud. I have since bought a radius dish and have been using that but wonder if the first guitar was so loud because of the conflict of the radiused braces and the flat gluing surface, It seems to me that could make the top that much stiffer.

Alan Carruth 01-18-2021 02:46 PM

There are lots of anecdotes on this, one way and the other, but I'm not sure there's much actual data. Even 'identical' guitars made from 'the same' wood sound different, simply because it's a natural material and varies. I would hesitate to attribute a change in sound to some particular variable on the basis of a sample of two instruments. And, since all of us tend to hear what we expect to hear you'd have to do 'blind' tests.

I use arched bracing and radius dish throughout, but not for sound. Up here in New England the humidity and temperature changes so fast and so often, and swings so widely, that top and back cracks are the repairman's bread and butter. An arched top is less likely to split than a flat one, so using the radius dish is one way to keep the customers from coming back for the wrong reasons. ;)

Victory Pete 01-18-2021 05:48 PM

Quote:

Originally Posted by Alan Carruth (Post 6608041)
There are lots of anecdotes on this, one way and the other, but I'm not sure there's much actual data. Even 'identical' guitars made from 'the same' wood sound different, simply because it's a natural material and varies. I would hesitate to attribute a change in sound to some particular variable on the basis of a sample of two instruments. And, since all of us tend to hear what we expect to hear you'd have to do 'blind' tests.

I use arched bracing and radius dish throughout, but not for sound. Up here in New England the humidity and temperature changes so fast and so often, and swings so widely, that top and back cracks are the repairman's bread and butter. An arched top is less likely to split than a flat one, so using the radius dish is one way to keep the customers from coming back for the wrong reasons. ;)

My dish has a 28' radius, so it doesn't match the braces exactly which forces them down giving me some of that stiffness I want.

https://i.imgur.com/Si7bi7f.jpg

redir 01-18-2021 06:35 PM

I think I read once that at Martin Guitars they either do, or used to, glue radiused braces by clamping them down flat so that when they released the clamps the top would spring into the radius of the braces. I've never tried this though and I wonder what the purpose of doing so would be for. It seems to me that as mentioned it would reduce the radius but I can't quite mentally picture that in my mind.

charles Tauber 01-18-2021 08:13 PM

Gluing arched braces using a form of different radius won’t increase stiffness.

Victory Pete 01-19-2021 05:23 AM

Quote:

Originally Posted by redir (Post 6608266)
I think I read once that at Martin Guitars they either do, or used to, glue radiused braces by clamping them down flat so that when they released the clamps the top would spring into the radius of the braces. I've never tried this though and I wonder what the purpose of doing so would be for. It seems to me that as mentioned it would reduce the radius but I can't quite mentally picture that in my mind.

That puts stress and stiffness into the top. I see people doing different things online. I see flat braces glued using a radius dish which seems to do the same thing, create stress and stiffness in the top. So it seems to me you should not do both, radiused braces and a radius dish for clamping, You get no internal stress and stiffness.

Victory Pete 01-19-2021 05:24 AM

Quote:

Originally Posted by charles Tauber (Post 6608350)
Gluing arched braces using a form of different radius won’t increase stiffness.

I think it will, because the surfaces don't meet up perfectly, they will fight each other making stiffness.

Glen H 01-19-2021 10:39 AM

I think you’re confusing tension with stiffness

Victory Pete 01-19-2021 10:59 AM

Quote:

Originally Posted by Glen H (Post 6608717)
I think you’re confusing tension with stiffness

Not really, they are related. I have glued my radiused braces to a flat surface, like my first build, which has great volume.

https://i.imgur.com/bHxVwX9.jpg

John Arnold 01-19-2021 01:11 PM

Quote:

I think I read once that at Martin Guitars they either do, or used to, glue radiused braces by clamping them down flat so that when they released the clamps the top would spring into the radius of the braces.
Spring back is less than complete, so the result is less radius than what was cut into the braces.

In the old days at Martin, there was more spring back when gluing to a flat surface because the braces were not pre-scalloped. With pre-shaped bracing (CNC), I believe Martin uses radius dishes these days.
I glue uncarved (rectangular) braces to a radiused surface, but not a dish. I cut the radius into a 2X4, and use those 1 1/2" wide pieces to back up the top in the go-bar deck. This is a simple and effective system, particularly useful since I employ a compound radius on my tops.

Victory Pete 01-19-2021 01:31 PM

Quote:

Originally Posted by John Arnold (Post 6608869)
Spring back is less than complete, so the result is less radius than what was cut into the braces.

In the old days at Martin, there was more spring back when gluing to a flat surface because the braces were not pre-scalloped. With pre-shaped bracing (CNC), I believe Martin uses radius dishes these days.
I glue uncarved (rectangular) braces to a radiused surface, but not a dish. I cut the radius into a 2X4, and use those 1 1/2" wide pieces to back up the top in the go-bar deck. This is a simple and effective system, particularly useful since I employ a compound radius on my tops.

Are your braces radiused?

Mark Hatcher 01-19-2021 04:34 PM

I don't believe it is wise to build any more tension into the top than you need to. I think you are right when you say you gain some stiffness like that. The problem is you may voice the top to that stiffness and with time the top settles into its new position and you loose the tension and your top deadens.

This is also why I don't use the top and back to hold the sides in position.

charles Tauber 01-19-2021 05:54 PM

Quote:

Originally Posted by Mark Hatcher (Post 6609076)
I think you are right when you say you gain some stiffness like that.

Okay, I'll bite, now that two people have asserted that.

By what mechanism does gluing arched braces to a flat top using a gluing caul that is flatter than the braces increase stiffness?

While doing so will increase the tension and compression of the components being glued, how does that relate to increasing stiffness?

yellowesty 01-19-2021 08:08 PM

As long as all the elements (the braces and the soundboard, in this case) are operating in their linear mode (stress proportional to strain), then gluing a contoured brace to the soundboard using a flat gluing caul won't change the overall stiffness.

The resulting soundboard (and contoured brace) shape will be that which equalizes the strains in the two elements. That will result in a shape which is less pronounced than the original brace shape and will lock opposing strains into both elements.

It sounds like a bad idea. Any environmental change (temperature, humidity, aging of the materials, etc.) that changes the stiffness of one element more than the other will rebalance the strains, resulting in a changed shape to the soundboard. That's unlikely to improve playability.

And we most certainly want to construct our musical instruments such that the components function in a linear mode. Otherwise, the instrument will, itself, create anharmonic (aka dissonant) sounds (don't call them "harmonics," they won't be).

On a similar topic, this is a reason to contour the back of the bridge and the top surface of the bridgeplate to match to soundboard contour. There is no upside in locking unnecessary strains into our builds.

charles Tauber 01-19-2021 08:57 PM

Quote:

Originally Posted by yellowesty (Post 6609233)
On a similar topic, this is a reason to contour the back of the bridge and the top surface of the bridgeplate to match to soundboard contour. There is no upside in locking unnecessary strains into our builds.

And what is the proven downside to not contouring bridges and bridge plates? If there is no upside and no downside, there is no compelling reason to do one or the other.

As I alluded to previously, one can make rational academic arguments on both sides of the issue. Empirical proof of either is more difficult to establish.

Victory Pete 01-20-2021 03:54 AM

Quote:

Originally Posted by charles Tauber (Post 6609133)
Okay, I'll bite, now that two people have asserted that.

By what mechanism does gluing arched braces to a flat top using a gluing caul that is flatter than the braces increase stiffness?

While doing so will increase the tension and compression of the components being glued, how does that relate to increasing stiffness?

It is like keeping a truss rod very tight with a straight neck, it makes the neck more stiff and improves volume, sustain and tone.

JonWint 01-20-2021 07:54 AM

Quote:

Originally Posted by Victory Pete (Post 6609378)
It is like keeping a truss rod very tight with a straight neck, it makes the neck more stiff and improves volume, sustain and tone.

"Stiffness" is only determined by the structural member dimensions and its Young's modulus (E).

Keeping a truss rod "very tight" will change the relief. The stress in the truss rod will not affect stiffness.

You can build a prestressed top system but it will not increase stiffness.

Victory Pete 01-20-2021 08:25 AM

Quote:

Originally Posted by JonWint (Post 6609479)
"Stiffness" is only determined by the structural member dimensions and its Young's modulus (E).

Keeping a truss rod "very tight" will change the relief. The stress in the truss rod will not affect stiffness.

You can build a prestressed top system but it will not increase stiffness.

I disagree. If you tighten trussrod as much as possible counteracting the pull from the strings, the neck will be stiffer just as a neck with no strings and a loose trussrod will be less stiff.

Alan Carruth 01-20-2021 08:37 AM

Victory Pete wrote:
"It is like keeping a truss rod very tight with a straight neck, it makes the neck more stiff and improves volume, sustain and tone."

Are you sure?

Tightening the truss rod puts the neck itself in compression. Loading a column in compression lowers it's resonant frequency, which goes to zero at the stress that causes the column to buckle. This relationship has been used as a reliable method of non-destructive testing in aircraft truss structures.

I suspect that, in most cases, the load of a tightened truss rod is nowhere near the buckling stress of the neck, but without data it's hard to say. Some of those electric guitar necks are pretty long and skinny. ;)

Also germane is the question of how you know this. How are you measuring volume, sustain and tone? Two of those, volume (or power, at least), and sustain are amenable to some sort of objective measurement, which should settle the whole question relatively easily. Have those measurements been made? We all hear what we expect to hear.

Victory Pete 01-20-2021 09:06 AM

Quote:

Originally Posted by Alan Carruth (Post 6609505)
Victory Pete wrote:
"It is like keeping a truss rod very tight with a straight neck, it makes the neck more stiff and improves volume, sustain and tone."

Are you sure?

Tightening the truss rod puts the neck itself in compression. Loading a column in compression lowers it's resonant frequency, which goes to zero at the stress that causes the column to buckle. This relationship has been used as a reliable method of non-destructive testing in aircraft truss structures.

I suspect that, in most cases, the load of a tightened truss rod is nowhere near the buckling stress of the neck, but without data it's hard to say. Some of those electric guitar necks are pretty long and skinny. ;)

Also germane is the question of how you know this. How are you measuring volume, sustain and tone? Two of those, volume (or power, at least), and sustain are amenable to some sort of objective measurement, which should settle the whole question relatively easily. Have those measurements been made? We all hear what we expect to hear.

Even Neil Young agrees, Larry Cragg explained it in an interview.

redir 01-20-2021 10:32 AM

It sounds like it's sliding in into an argument of technical terms. I'm not an engineer but it sounds like the term 'stiffness' might be part of the engineers manual and that it is an actual property of in this case the top of the wood.

So if you measure the top for stiffness you get a number. Then no matter how you brace it you are not changing the number or the measured properties of the top. However of course you are also bracing the top which makes it stronger for the sake of handling over 100 pounds of string tension.

So if you suspend a floppy unbraced top across two supports at each end and put a 5 pound weight in the middle it will deflect say 1/2 inch for example. Now brace the top and do the same test. It only deflects .03 inches now so its 'stiffer' in the layman's sense of the word but the Young's Modulus of the top has not changed.

The engineers can correct me if I'm wrong.

charles Tauber 01-20-2021 12:10 PM

Quote:

Originally Posted by redir (Post 6609597)
It sounds like it's sliding in into an argument of technical terms. I'm not an engineer but it sounds like the term 'stiffness' might be part of the engineers manual and that it is an actual property of in this case the top of the wood.

So if you measure the top for stiffness you get a number. Then no matter how you brace it you are not changing the number or the measured properties of the top. However of course you are also bracing the top which makes it stronger for the sake of handling over 100 pounds of string tension.

So if you suspend a floppy unbraced top across two supports at each end and put a 5 pound weight in the middle it will deflect say 1/2 inch for example. Now brace the top and do the same test. It only deflects .03 inches now so its 'stiffer' in the layman's sense of the word but the Young's Modulus of the top has not changed.

The engineers can correct me if I'm wrong.

Quote:

Young’s modulus, numerical constant, named for the 18th-century English physician and physicist Thomas Young, that describes the elastic properties of a solid undergoing tension or compression in only one direction, as in the case of a metal rod that after being stretched or compressed lengthwise returns to its original length. Young’s modulus is a measure of the ability of a material to withstand changes in length when under lengthwise tension or compression. Sometimes referred to as the modulus of elasticity, Young’s modulus is equal to the longitudinal stress divided by the strain. Stress and strain may be described as follows in the case of a metal bar under tension.
https://www.britannica.com/science/Youngs-modulus

Quote:

The bending stiffness (K) is the resistance of a member against bending deformation. It is a function of the Young's modulus E, the area moment of inertia I, of the beam cross-section about the axis of interest, length of the beam and beam boundary condition.
https://en.wikipedia.org/wiki/Bendin...by%20a%20force.


The top material has its modulus of elasticity (Young's modulus), the bracing has its own. The top material has its own bending stiffness, the bracing has its own. When you join them - such as by gluing them together - the bending stiffness of the "assembly" is a combination of its parts.

As is often stated here, the area moment of inertia, and hence, bending stiffness, of a beam of rectangular cross section is proportional to the cube of its height and directly proportional to its width. That is, if you double the width of that "brace", it is twice as resistant to bending, but weighs twice as much. If you double its height, it is eight times as resistant to bending, but weighs twice as much.

Similarly, the bending stiffness - a function of the geometry of the "beam" - of a guitar top, for example, is heavily influenced by the thickness of the top. The top's modulus of elasticity - a property of the material - is not affected.

To be clear the "stiffness" we are discussing is its resistance to bending, aka "bending stiffness". This discussion has to do with mechanical structure.


Quote:

Originally Posted by Victory Pete (Post 6609378)
It is like keeping a truss rod very tight with a straight neck, it makes the neck more stiff and improves volume, sustain and tone.

Victory Pete is asserting two things. First that tightening a truss rod increases the (bending) stiffness of a neck. Second, that the increase in stiffness changes the response of the guitar to increase its "volume, sustain and tone".

The second assertion has to do with changes in response based on assumed changes in mechanical structure.

I'm not going to attempt to dissuade him from those unsubstantiated assertions.

Victory Pete 01-20-2021 01:17 PM

Quote:

Originally Posted by redir (Post 6609597)
It sounds like it's sliding in into an argument of technical terms. I'm not an engineer but it sounds like the term 'stiffness' might be part of the engineers manual and that it is an actual property of in this case the top of the wood.

So if you measure the top for stiffness you get a number. Then no matter how you brace it you are not changing the number or the measured properties of the top. However of course you are also bracing the top which makes it stronger for the sake of handling over 100 pounds of string tension.

So if you suspend a floppy unbraced top across two supports at each end and put a 5 pound weight in the middle it will deflect say 1/2 inch for example. Now brace the top and do the same test. It only deflects .03 inches now so its 'stiffer' in the layman's sense of the word but the Young's Modulus of the top has not changed.

The engineers can correct me if I'm wrong.

You are right, thanks for thinking outside the box.

Gordon Currie 01-20-2021 08:24 PM

Quote:

Originally Posted by Victory Pete (Post 6609529)
Even Neil Young agrees, Larry Cragg explained it in an interview.

If renowned luthier Neil Young says it, it must be true. :rolleyes:

Alan Carruth 01-21-2021 10:02 AM

So how did Neil Young measure it?

KingCavalier 01-21-2021 12:29 PM

I think he starts the process "Down by the river"


All times are GMT -6. The time now is 12:05 AM.

Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2021, vBulletin Solutions Inc.
Copyright ©2000 - 2021, The Acoustic Guitar Forum

vB Ad Management by =RedTyger=