Official Luthiers Forum!

Hey folks,

Just closed up #6 and going to do #7 next (doing two at a time), which is a moment for reflection. A few questions, comments are appreciated.
1) Are there real benefits to reverse kerfing? Seems there is some sentiment this helps tone. Think I'll try this, if for nothing else than having the experience.
2) Notches for braces - I've been going through the sides and covering with binding. Is there a benefit to pocketing the lining only instead or is this a stylistic choice? (Then of course the question of braces that stop short of the lining altogether).

On a different topic, thinking about trying out sapele and khaya for the necks, again to just try it out. No real question here, but interested in comments. Wondering if the heavier, denser sapele will contribute to tone / sustain in any noticeable way.. or will just make a annoyingly heavy neck.

I hope you all are having good holidays!
Bill

Reverse kerf: Probably doesn't matter.

Notches: If you go through the sides, it limits the height of your brace ends to what will be covered by binding. If that's what you want anyway, then it's not an issue.

It might be a good idea to chisel a tiny bit off the endgrain of the braces after routing for binding, to provide clearance for back shrinkage in low humidity. Otherwise the braces will be trying to push the binding outward, and can sometimes succeed.

Notches are necessary. In low humidity, the back tries to turn inside out, which is comparable to laying the guitar on its face and placing a heavy weight on the back. The braces carry the stress out to their ends, where it gets concentrated and peels them up. Alternatively, if the brace ends are feathered down to nothing then the brace will flex along with the back and spread the stress over a larger area so they're less likely to peel (and almost certainly won't on something with low humidity expansion like quartersawn rosewood, but may still on something with high expansion like flatsawn maple).

As for neck woods, you can probably counteract any balance change by using lighter weight tuners. Being out on the end, they have more leverage than the neck wood itself, so smaller changes in mass make a bigger difference.

I doubt the higher neck mass will be enough to make a noticeable difference in tone, but you never know.

I don't think there is any way to know for sure if reverse kerfed lining improves tone or not. I use it because I like its appearance, because you can always get clean notching of brace ends into the lining (on standard lining, brace ends sometimes fall on the gaps between the dentils), and it makes the rims way more rigid than they are with standard lining. I don't know if that extra rigidity affects the overall tone or not (I doubt it), but the increased strength appeals to me in principle.

The brace ends on my guitars are tucked into the linings and stop short of the sides. I don't run the notches through the sides because I don't want to end up with the brace ends pushing against the binding from the inside, which could happen with changes in humidity. Probably doesn't matter in practice, but that's my reasoning.

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

Reverse kerf lining can be really helpful depending on your building style. I use solid/laminated linings and it really makes for a stiff rim. I like that because I like to glue the top and back on, out of the mold. Once my linings are in, it holds it’s shape out of the mold. My guess is, once the plates are on, the stiffness of the rim is about the same no matter what lining you use.

I tuck all my back braces ant top braces above the soundhole into the linings. All top braces below the soundhole get feathered out to nothing.

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Bryan Bear PMoMC

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

This is once of the things that I really like about reverse kerfed linings. Since the web is on the outside, once the tabs are glued to the sides it forms a very rigid edge. I also find them easier to bend around the the funky curves on the back. Since the web is narrower, it can be more easily coaxed to fit the compound curves near the waist. I even use a small razor saw to slice into the web a little bit where I need to get a little extra flexibility.

And most importantly, I really like the way they look. Just seems to come out cleaner on the inside.

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Tony Thatcher
Bozeman, Montana

Solid linings and reverse curf lining (mimicking solid linings I beam style) provide a higher impedance interface between the top and the rims (rims are stiffer) thus more of the tops vibration is reflected back into the top. It is hard to compare on the same guitar but I suspect the node lines of the mono pole would move closer to edge of the guitar. Good or bad I will not say, but I do think it is different than normal kerfed linings.

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http://www.Harvestmoonguitars.com

I’m far from an engineer but I wonder how much stiffer the rim is once the plate is glued on. I suspect it is a bit stiffer but not much. Maybe one of the engineers can weigh in.

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Bryan Bear PMoMC

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

I am an engineer! Computer engineer, but more of it is the same than one might expect. I have seen and heard a noticeable difference in just bolting on some big washers to each side (ala Gore). I was able to move the top resonance 4 hz and the chlaldi mono node pattern moved toward the edge. The extra mass also increases the impedance mismatch between the top and sides. My only point is that there is a sensitivity. I suspect it does not matter for a steel string guitar but there may be a consideration for a concert classical guitar.

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http://www.Harvestmoonguitars.com

Well that didn’t take long

I forgot about the added mass since reverse meted lining have quite a bit more wood. I could see that helping quite a bit. I’m starting two boxes right now and am using a slightly denser wood for my laminated linings and will make them a little taller for the added mass.

I’m not a skilled enough maker to really reap the benefits but there’s nothing to lose either.

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Bryan Bear PMoMC

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

Has anyone actually measured rim deflection before linings were applied, after, and after closing the box (with solid, kerfed, and reverse kerfed linings)? Lots of variety of opinion on several sites, including the boys in the shop (aerospace engineer, etc.), but I have looked for published test results and have come up dry thus far. Opinions are interesting, but data would be useful.

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We have become a civilization that elevates idiots, prostitutes, and clowns. Am I still to defend it? Yes, for its principles. Yes, for what it was. Yes, for what it still may be.

-Mark Helprin, The Oceans and the Stars: A Sea Story, A War Story, A Love Story (A Novel)

Thanks all, these are insightful answers! I appreciate it.
Good practical reasons.

Happy New Year, folks!
Bill

I agree with Woodie: I'd like to see some data. Reverse kerf liners are a tad heavier than regular ones, but I suspect the difference is not all that great. The added stiffness of the inside web is noticeable when the top and back or not on, but once those have been glued up I suspect it's not. The different section of reverse kerf liners should make them stiffer further down from the top and back edges, and that might make a difference. Given the variability in wood and other things I suspect it would be very difficult to measure any consistent differences that could be reliably attributed to the liners. It would be a bigger project than it seems.

One advantage of reverse kerf linings is that you can make them. I don't like the look though.

New username, same Pat Mac

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Pat

Somagyi makes a case for reverse kerfing making the sides more rigid due to the portion of the kerfing is facing the inside of the box. Kind of made sense so that's what I use- he certainly knows a lot more than me! My sides are laminated (will be adding CF as well next round) so they are already very stiff. Every little bit helps!

As far as tone- I certainly don't think anyone would notice the difference between reverse kerfed vs kerfed, for me it's just peace of mind to know that I've made my sides and rim as stiff as possible.

But does it? I'm not sure. In other words, I don't know that something that provides additional rigidity to the rims before they are glued to the sides adds anything of value to the rigidity of the closed box. The closed box has a lot of rigidity because it is a closed box. Does installing any specific sort of lining appreciably increase that rigidity? I hesitate to say yes to that.

I think there are very valid reasons to use reverse kerf linings or laminated linings that have nothing to do with the rigidity of the closed box. They look a certain way (aesthetics count), and they can be used to add mass to the sides at the top/sides glue joint, which can be useful (per Gore/Gilet). And I can see how a more rigid set of rims can be easier to work with before the plates are glued on. But do they add rigidity to the closed box? I struggle with that.

If somebody has some science on this, I will be happy to defer to that scientific analysis. Until then, I remain skeptical of the whole added rigidity thing.

I don't think it's a question of whether reverse kerfed linings add rigidity to the closed box or not. There's no question that they greatly increase the stiffness of the rims compared to standard kerfed lining. If you take a set of rims with standard kerfed lining (no top or back on yet) and hold it vertically by the head or tail block and then rapidly raise and lower it vertically, the rims stretch and shrink lengthwise like a spring. If you grab both the head and tail block and twist in opposite directions, the rims will twist. With reverse kerfed lining, neither of those movements happen.

It seems to me that the stiffness added by the individual components is additive. So it's really a question of how large the stiffness added to the rims by reverse kerfed lining is relative to the stiffness added by standard kerfed lining. It could be that the difference is relatively small compared to stiffness added by gluing on the top and back, but someone would have to do actual measurements to determine the relative contributions.

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

Everyone agrees that without the plates attached reverse kerf linings make the rim much stiffer. Once the plates are on, there seems to be two ways of looking at it. You look at it as the added stiffness of reverse kerf linings may be small compared to the gains in stiffness gluing the plates. I look at it as gluing the plates canceling out much (all?) of the extra stiffness from reverse kerf linings. I'm not sure if I said that well or which way is correct.

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Bryan Bear PMoMC

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

I guess I don't see how gluing on the top and back could cancel out the added stiffness provided by the reverse kerf lining. It would still be there. I could see though how the stiffness added by the top and back could greatly exceed the effect of the reverse kerf lining if that was relatively small compared to the effect of closing the box.

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

Correct. Mostly.

The special case is Smallman style classical guitars which have a very stiff rim assembly (colloquially known as the "toilet seat") because the tops are very thin and fragile and don't contribute much to the stiffness of the box.

The majority of the effect of reverse kerf linings (or "solid" linings) is due to their extra mass, which increases the impedance mismatch between top and sides, keeping more of the bending wave in the top to produce sound and changing the momentum balance between the moving part of the top and the rest of the guitar. And for those wanting data on that, all the measurements and modeling of the effect and how to achieve it are in that good set of books.

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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

It hadn't occurred to me that the reverse kerfed linings would be heavier than standard lining since they both feel pretty light in the hand. So I weighed some 32" pieces of both types in mahogany. Standard kerfed linings = 22 g and reverse kerfed linings = 33 g, so 50% heavier. After fitting to one of my dreadnought bodies, that would add about 18 g to each side. I don't have a thicknessed side lying about to weigh, but I'm guessing that would not be an insignificant increase in weight.

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

I didn’t say it well at all. Not really canceling out the stiffness of the linings as defining the max stiffness. I guess I’m looking at it as, once the plates are on it is now at the total stiffness regardless of what kind of lining are on there. Like when I winch up my belt, it doesn’t matter if I am still wearing suspenders. So the BENEFIT of the added stiffness of solid linings is cancelled out by gluing on the plates. . . I think. . . What were we talking about again. . .

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Bryan Bear PMoMC

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

Since work was closed today due to snow, I had some time and thought I'd take a quick stab at some data of sorts, not real data mind you
I put together a simplified Finite Element Analysis (FEA) study to see the general trend various linings have on the closed box as far as the side stiffness is concerned.
Please do not take these numbers as absolute, this model was only put together to get a sense of the general trend of the effect of various linings.
My study used a classical shaped rim section and I constrained the neck block and heel block so they couldn't move under load. I also used isotropic material with a modulus in line with wood. Not totally a real world situation but should work for a trend analysis.
The load was 2 pounds of force along the shown axis distributed over a 1/2" diameter, approximately mid-way up the side. I also only modeled half of the body.
The first illustration shows the original shape.
The 2nd and 3rd illustration is the load placed on the sides without any ling at all, you can see the distortion of the sides when compared to the original shape. Please note that the deflections are greatly exaggerated for visualization purposes. This condition deflects about .250". This seems to pass the "haha" test to me.
The 4th illustration shows the sides with kerfed linings, they deflect approximately .036"
The 5th illustration shows the sides with solid linings. They deflect approximately .0065", much stiffer.
The 6th illustration is with kerfed linings with a front and back plate. Its deflection is much more local in the shape of a local depression with amount of deflection being minimal in the .001"range. This image is looking at the inside of the guitar to show the linings.
And finally, solid linings with plates, which are indeed stiffer by a very very small amount, about .0001".
According to this model, IMO there's no practical difference as far as stiffness goes between the various linings once the box is together.

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Jim Watts
http://jameswattsguitars.com

Can you run one with solid linings and the plates on to see how it compares to kerffed lining?

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Bryan Bear PMoMC

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

Bryan, I was in the middle of editing doing just that when you posted.

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Jim Watts
http://jameswattsguitars.com

Interesting...I wonder what bracing and plates that would behave more like actual components would do? Even without, it appears as though linings have very little impact on deformations of the body, with something like 1/10,000" difference between solid (stiffest) and kerfed (less stiff). Great stuff!

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We have become a civilization that elevates idiots, prostitutes, and clowns. Am I still to defend it? Yes, for its principles. Yes, for what it was. Yes, for what it still may be.

-Mark Helprin, The Oceans and the Stars: A Sea Story, A War Story, A Love Story (A Novel)