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Or at least paper thin spruce. I was working on a Seagul Dred yesterday and the spruce cap must have been a full .10in thick or more.

Jay De Rocher wrote:

For the top in the photo, I took deflection measurements before and after carving the braces:

Uncarved braces = 0.040"
With cap added to x-brace joint = 0.027"
Fully carved braces = 0.049"

I was impressed by how big a difference there was between the uncapped and capped x-brace joint numbers.


I am surprised by how much difference capping the X made but still it is only about the thickness of a high e string. The difference between uncapped, uncarved braces and finished braces is only .009 thousandths - again equal to a very thin guitar string!
How tight are y'all getting your numbers? And what does perimeter sander do to them?

In my case, those numbers are from three or four measurements for each one. The 5 lb weight is heavy enough that the needle on the dial gauge doesn't flutter around at all when the top is loaded, and the numbers are quite repeatable.

The 0.009" difference between the uncarved, uncapped braces and the fully carved braces is indeed small, but the difference between the uncarved, capped braces and the fully carved braces is 0.022". What that indicates to me is that capping and then carving the braces redistributed the stiffness around the top. Adding the cap to the uncarved braces significantly reduced the weakness of the x-brace joint making the top stiffer overall. Fully carving the braces after capping the x-joint then loosened the top back up but in different locations on the top, the scalloped lower x-brace arms likely being the biggest contributors.

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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

Capping definitely makes a difference in how the top vibrates, and, of course, it reduces the likelihood of a split brace by a lot, so you should do it just for that.

David Hurd's book 'Left Brain Lutherie goes into great detail on deflection measurements. He fine tunes the top that way after assembly, too. He puts a 2# weight (iirc) at the bridge location and measures the deflection at a grid of points around the lower bout. He trims the braces to get smooth contours. It's a good method if you are able to work in through the sound hole. I suppose it would work with the back off, too.

I would not put a weight on a braced top without a cap. If you do, and you get a little extra deflection then you most likely have broken the top part of the X-brace joint where the end grain of the top half is glued to the side of the other X-brace.

I have no idea what a cap is. I found that Martin glues cloth over the x brace. Is that a cap?

I wondered how my wood for the small baroque guitar would fare. I used a 2.6 pound box of sharpening stones, and the 3mm thick 100 year old Eastern Red Cedar moved about .06 inch. I thought 2.5 would be good because it will only have about 70 pounds of string, but looking online I see that steel strings can go from about 80-180 pounds. I was thinking something like 130-140.

Looking at that I wondered. Do you guys change the strings up or down in tension if you find that you overbuilt, or went too far in the response category?

For fun I tested a violin I haven't glued up; slightly shorter than the guitar. The walnut back moved .015 inch. The Alaskan Yellow Cedar belly moved .025 inch; it has a massive bass bar as a test. Bot are only 2+ mm thick in the bouts. The guitar will only get 2 braces, something like 3/8 X 5/8.

Deflection sounds like something that is easy to test, and keep notes on.
And then tune from the outside after gluing, and throw the numbers out the window!

Then again, tuning like Alan suggested; building on the belly in the Spanish style; you could tune, and test deflection.

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Why be normal?

You can use deflection testing to get you closer to the target so there is less tuning to do...

Yes, though many of us prefer a thin (think 1/32”) sliver of spruce for a more solid cap. I’m not sure if the cloth has the same structural benefit.


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The cloth patch works pretty well. The idea of the cap is to take up the tension load on the open side of the cross lap joint, and a piece of cloth does that about as wel as a piece of wood, so long as it's glued down well.

So, to play the devil’s advocate, while a patch or cap is probably effective to “take up” said tension load, does this actually occur when a top is in service? I know at least a couple of highly regarded builders who regard the cap as completely unnecessary, unless someone uses the guitar as a cushion under their butt.
By the same token, perhaps deflection testing should apply the load to the underside of the top?
Discuss, please.


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I test both sides and use the average. But I don’t test braced tops...

If my thinking is correct then as string tension causes the bridge to rotate it would put a downward force on the center of the X there by trying to open that joint up. So IOW the top side, opposite side of what is glued down, of the brace would be under tension. A cap that spans that gap would even out the tension across it.

The string tension is using the guitar top as a cushion under it's butt. One reason the X brace is so effective is that it concentrates "strength" at the point where much of the force is applied. The little slip of wood that caps the X adds a lot of strength for very little weight.
There have been some highly regarded builders who once believed the back graft was superfluous. This much less highly regarded builder doesn't.

What jfmckenna said.

Back in the days when I did more repairs I had several guitars come in with X braces that were split out from the center of the lap joint. If there was any run out to the brace (and it's not uncommon in even good production guitars) the brace would split down to the soundboard and the top would sag in front of the bridge. Nobody would confess to sitting on the top, and I believed them. The lap joint is a big stress riser due to the sudden change in section and stiffness, and once a split starts it will run to it's logical conclusion. I never saw that sort of failure on a guitar with cloth patch, and never had any further such problems on one where I put a cloth patch on. As far as I'm concerned, reinforcing that area is simply the right way to do it, and it's certainly simple enough.

Then IMO you are wasting your time. Having done this on quite a few tops I can tell you that the sweet spot is very narrow, +/- .003"-.004" with .095'-.105" being the general range for sitka. ( Now you can reverse engineer that if you like...)

Deflection tuning the top plate is an enhancement to tuning the braces but on it's own will likely make very little difference if you are tuning your braces another way. This procedure relies on the concept that the top itself is a unit with sub-components and is tested and tuned as it is assembled.

The top plate's job is to vibrate and undulate freely enough to produce good sound without being soft enough to run away and either wick off energy or reintroduce (re-couple) unwanted out of sync frequencies and phase cancellations. The braces do the bulk of the work as they both support and control string pressure and vibrations but they also are the structure that disseminates and distributes the vibrations to the top plate.

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Brian

You never know what you are capable of until you actually try.

https://www.howardguitarsdelaware.com/

What's interesting to me is that a 'free' top, not mounted on a guitar, vibrates in pretty similar ways whether it's braced or not. This suggests that you could make an unbraced top thick enough to take the bridge torque and neck loads and it would sound pretty much the same. The main problem would be that you'd have to make it a lot thicker, and it would weigh too much to produce much sound. Bracing adds stiffness without adding too much weight.

Every study I've seen about bracing suggests that there is no 'magical' system; any bracing scheme can be made to work pretty well. The key is to get it 'right' whatever that is. I find it interesting, for example, that the main lines of the Taylor 'V-brace' system are quite similar to the Gibson 'Mark' bracing that was inspired (supposedly) by the work of Kasha. Gibson's attempt was a disaster, but there are folks who really like the V-brace setup. Gibson could screw up boiling water, but Taylor got it more or less right.

My feeling is that 'getting it right' means getting the bracing to the point where it does it's job of adding stiffness in the right places without interfering too much with the way the top vibrates. The resonant modes on an unbraced top tend to show smooth and well-formed Chladni patterns, where a badly braced top will have patterns that are not well formed, or even missing, because the braving is too 'lumpy' in the wrong places. 'Tuning' a top, however you do it, is not so much a process of getting the 'right' pitches as just getting the bracing out of the way. In David Hurd's deflection maps of soundboards he tries to get smooth contours. When he checked out a guitar I'd tuned using Chladni methods he found it was just what he was looking for. Different routes to the same solution.

I'm gonna try some deflection tests on my next (third) acoustic, so been reading up on it. I don't understand preloading. I know what it means in a mechanical assembly, but for an open top plate I'm not at all sure. Can someone either explain the process or point me to a reference as applied to a guitar top? Thanks, Roy

Check out Chris Paulick's YouTube on thicknessing tops. It show Everett's method on deflection along with some reference numbers (weight, deflection numbers, support spacing). https://www.youtube.com/watch?v=xDDJVAaD9n8

Since someone else has reopened this thread...

If I am remembering my physics classes correctly then a better experimental method for this would be to measure the deflection using a series of increasing weights. Plotting deflection against weight (mass) and taking the gradient does the averaging out that people doing multiple tests are getting. You only need a few points, perhaps five.

The reason I suggest this is that any bad measurements will show up as being far from the straight line. This may be due to something shifting, perhaps a badly zeroed dial gauge or whatever. Whereas using only a single weight you won't pick up on a problem measurement. And it doesn't take much longer to do.

As people above have said I use the G&G method normally but wood suppliers are sometimes naughty and send tops with the corners chopped off - I'm never sure why, in which case you have to go back to deflection testing.

Please don't take this as wanting to start a whole debate - just my two cents worth.

Dave

I can't give you the theory** of why you need to preload, but I can tell you what I have found.
I calculate Elong only, preloading with about 850 or 1250g and then add to that either 1250 or 1650 g to obtain an extra deflection measurement of around 3 -4 mm (depends on stiffness and thickness of course)
I do this in both directions.
The deflection from the preload in the 2 directions can actually vary a lot, 19-20% not being uncommon, zeroing the measurement each time for each direction.
Using the deflection due to the extra weights my results vary by a maximum of around 1% over 8 measurements, 4 in each direction.
If you included the variation in the preload deflections in the calculations, I just checked and found it could throw the calculated values out by as much as 15%.......
** - On reflection, preload may be required because the board being tested is slightly warped or bowed. Preloading overcomes these "anomalies"

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

I have some amazing pieces of Red Spruce mostly that are cut with the corners off as Dave mentioned and so you cannot do any reasonable scientific tests on them other then braced deflection tests. So that is why I do those. The top is cut to shape, braced and then I do the deflection test.

But what I have found is the the 'sheet metal' test works pretty amazingly well. When I first started building there was not a lot of talk about testing tops, it was more about building to spec and 'feeling' the wood. It took an apprenticeship and many years before a luthier could know how to thickness and brace a top to get the sound they were after. I have no doubt that is true. But then I came across a Youtube video in the very early days of Youtube and I wish I could remember the luthiers name to give him credit but he would thin a top and then hold it on it's edges and shake it. Once it sounded like the warble affect that you hear with a piece of sheet metal wop wop wop wop wop, then it was done, or to some degree there of.

At some point I got into deflection testing and since there really is no starting point for that what I did was use the sheet metal test, do a deflection and record the results and move on. Then I got the G&G books and learned about the acoustic testing. What I found? Was that the G&G results coincided with the sheet metal test so well it kind of blew me away. It also matched my target deflections to a tee.

Long story short. The sheet metal test is an old school method for finding Youngs Modulus by feel and by ear. And it seems to work well even with wood that has cut off corners.

Yep, that's what Chris does on the YouTube link above. Shake that thing!

Years ago I took Kent Everett's voicing class and he taught us to use the 'sheet metal' test. I've been using it ever since. It's only failed me once; I was testing a torrefied red spruce top and it split while I was shaking it. I think the top had a defect but I was sure unhappy about it at the time-ha.

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Steve Smith
"Music is what feelings sound like"

Kent Everett! That was it, thanks. I always forget.

Wood 'cold creeps'; the longer you keep a given load on it, the more it deflects. You need to keep the interval between loading and measurement as uniform as possible. The best way to do that is to put the load on, zero the gauge, and then remove the load, noting the deflection immediately.

David Hurd goes into deflection testing deeply in his book 'Left Brain Lutherie'.