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Anyone doing deflection testing on their tops after they are braced? How are you doing it and how much weight are you using? What type of deflection are you aiming for?
Thanks.

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Brian

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

https://www.howardguitarsdelaware.com/

Not to change the subject somewhat but I would be interested to hear how folks are doing deflection testing on tops before bracing.

Mike

ok folks, I finally weoke up , did a quick search and found a plethora of information on deflection testing. I now understand Brians question on deflection testing after bracing. Good morning.

Mike

I guess it would be difficult since the flexibility of a braced soundboard is not going to be even throughout the entire soundboard. For example, the periferal will be more flexible while the bridge area will be less.

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Alain Moisan
Former full time builder of Acoustics, Classicals and Flamencos.
(Now building just for fun!)

Here's mine, you can test in various places on the top as Alain points out.

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Jim Watts
http://jameswattsguitars.com

I use something similar, but less elegant, than Jim's and put the weight on a bridge. On a finished guitar, I check in front and behind the bridge before and after stringing. One interesting thing I've found is that downward deflection at the bridge is not necessarily a good predictor of what will happen with torque. In other words, 2 similarly braced guitars with the same downward deflection at the bridge may deflect quite differently from each other under string torque.

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http://www.chassonguitars.com

Jim, could you possibly indicate (a) your span of the apparatus feet, from the closest edge to edge (b) the weight (2.5 lbs?) and (c) some rough idea of deflection values that constitute a soft top, a normal top, and an inordinately stiff top.

Jim,
I am sure this sounds like a dumb question, but I don't understand the working of your deflection measurement tool. It looks like the weight is acting down through the gage. How are the gage and weight working?
Ed

I have made similar test apparatuses for other purposes, so I will answer just to keep Jim's workload low, to keep him fresh to provide a comprehensive answer to my questions : -)

First, one positions the apparatus on the guitar, unloaded. Then, one zeroes out the dial indicator. Next, you load the device with the reference weight, and then record the new reading on the dial indicator. The difference in thousandth's of an inch is the deflection under load.

In general, as long as you always use the same weight, the greater the deflection under load, the softer the top. Very stiff tops will show little deflection, and soft tops will show larger deflections. The measurement take place via the indicator, but the "data" it provides is relative, not absolute. It is only useful for comparisons. One might build a similar looking device to measure the relative stiffness of trampolines or fishing rods for that matter, and they work, provided you only use them for their specialized purpose.

Now at the risk of complicating the explanation -yet for the sake of completeness- it is possible to calculate the absolute stiffness or rather the Modulus of Elasticity (or MOE) of materials with such a device, provided several conditions are strictly met, e.g., measuring uniform beams of known dimension, with a reference mass, and a known span of supports, and then calculating the various values using a specific mathematical equation that can be found in the book Understanding Wood, by Bruce Hoadley. I have built devices just for this special purpose, but again, Jim's device is for simple comparison of one guitar top to another. As Kent noted, it will not necessarily reflect strain under torque, which is a different kind of stress than simple top loading.

The very reason I asked the question I did, is so I can perhaps use his empirical observations rather than having to build a differently calibrated device and having to sample many guitars before having some idea of what to look for. As long as I build a device with the same span and weight, I should get similar results. The weight of the board or the spring in the dial indicator is negligible, since the dial indicator gets zeroed out with these things taken into consideration before the weight is added.

In reality, it would make sense for builders to settle on a reference span and weight, so that everyone can compare notes. I would submit that Jim's device is as good as any for a universal reference, once he tells us what he used : -)

Ed , jm gave a very good explanation of the use of the jig. The jig is indeed loaded through the indicator stem, placing the load only at the indicator tip (.75 Dia cork lined disk to avoid dents).

Jm, this a fairly new jig for me and I am using it but I don't feel like I have any definitive answers for you unfortunately. David Hurds book "left brain lutherie" has a lot of information about this, particularly the way deflections relate to the torque of the strings/bridge. I wish I'd made this years ago.
With my jig I'm using a 2.5 lb weight. This provides a 3 lb load with the wieght placed directly over the indicator stem. The load was measured by appying it against a postal scale.
The span of the jig is irrelevant as the span is set by the guitar top not the jig but mine is 12 inches.
Happy to answer any other question you might have, assuming I know the answer

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Jim Watts
http://jameswattsguitars.com

I agree. Definitely something interesting to try. I'd think one mesure every square inch or so should give an accurate 'deflection graph' (if I may call it so...).

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Alain Moisan
Former full time builder of Acoustics, Classicals and Flamencos.
(Now building just for fun!)

Since I already have Stewmac's neck relief gauge, I could probably use that on the underside of the top, or rig it up somehow with a weight, right? I'd rather use it on the underside so I don't have to build a jig for the top, just put weight on it.

John, that should work.

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Jim Watts
http://jameswattsguitars.com

That's what I was thinking when I asked about this. Have any of you currently doing this type of testing taken any measurements from factory made guitars of similar design for comparison? I was actually thinking of testing the tops after bracing, but before gluing the rims. And testing them upside down from the way pictured, with the weight applied to the bridge-patch. Mainly because I figured the uplift force of the string tension would be much greater than the downward force created by the saddle. Any thoughts on this approach?

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Brian

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

https://www.howardguitarsdelaware.com/

Neglecting static force vectors, the deflection test should be able to give some idea of top compliance when subject to strong vibration. I would have no qualms about using the same span and mass on a range of guitars that were not necessarily identical, but roughly of the same top area.

I think the degree to which the device might prove useful is only limited by the attention of the operator, and his or her ability to make correlations and draw inferences.

You mean in the engineering of these?

http://www.poundland.co.uk/images/206/original/aero.jpg

: -)

Gee Todd, with the edit, you deprived me of my harmless and good spirited attempt at humour. No matter, one could tell from your posts that some serious academic and engineering training lurks in your background : -)

I don't have a degree in aeronautics, but I spent the better part of two decades mentally working on a design for the last unaccomplished feat of aviation, that being human powered "vertical" flight. Imagine my disappointment when I finally googled it (not quite sure why I had not to that point) and discovered batteries of university teams not only working on the problem, but making good progress.

Actually, I know why it took me so long to google it. I had never once heard the idea even proposed, in any context of feats of aviation. By every reckoning, they had all been completed. This is one of the pitfalls of working alone and in complete isolation. Now I suppose I should put some thought toward the World's first flying dreadnaught

Human powered vertical flight? Pshaww...
1) approach edge of cliff/tower/yawning chasm
2) step off
3) ta-da!

Stipulation number one: Must be repeatable

Well....I'm pretty sure it'll work for anybody...

Not the landing, but the sudden stop!

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Waddy

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