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Have those of you who thickness your tops to a certain deflection noticed any general patterns between a given deflection measurement and the corresponding range of thicknesses it tends to produce?

Every set is different. Some will be extra stiff, some will be on the floppy side, and so forth, but have you noticed any general connections between a given deflection and the range of thicknesses it tends to produce?

For example a deflection of .25 tends to produce tops in the range of .xx-.xx" thick in the european spruces.

I have my deflection jig ready to go, but I haven't had an opportunity to use it yet. I'm planning to pick up a bundle of tops from High Mountain and running them through to get some ideas, but it also seems like a question that would make for interesting discussion.

I use 0.25" with 5 lbs and have had tops go from the mid 90's to about 110. I measure both sides of the top and stop when one of then deflects 0.25". If I flex the top and it feels stiff it will probably end up thinner, no matter what the species. I built a fixture based on David Hurd's book and have done a few tops that way but have ended up with a simple long-grain fixture with about 17" spacing (IIRC).

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Steve Smith
"Music is what feelings sound like"

Cool. Thanks, Steve.

I forgot to mention I'll be doing the 5 lb. weight with 18" spacing. I think that seems to be pretty standard.

Not 100% sure I understand the question.

I've noticed my tops are between .090 - .110 for spruce, maybe a little thicker for the hardwoods.

I measure it and document it, but don't really care about it, as the deflection is what I care about. The thickness is an artefact only. That, to me, is the point of doing deflection testing.

That's my jig. 5lb at 18". Top is just a random blank, not thicknessed.

Meddlinfool- Is that with the top joined or each half by itself?

Do you guys that are doing this, use a consistent width plate?

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Pat

Stiffness is linear with the width of the plate, I.e. A plate that deflects .125" @16" wide will deflect .250" @ 8" wide. I have found that as long as your plates are roughly the same width, they will deflect about the same give or take a few thou. If you want to be really sure, calculate the material properties

It is a joined panel ready for a rosette.

I'm with uvhsam. The top may vary between 16-17", but the percentile's are close enough to be good enough for me.

Simply wondering if we can see a general relationship between thickness range and deflection. In other words, wondering how disparate different boards from a given specie tend to be. Thanks for sharing what you've found so far.

I would answer your question by saying each piece of top wood is different. It is possible to have two pieces of a particular species to be the same stiffness (deflection) but vastly different thicknesses. This is why a builder would choose to "deflect" their tops, so as to be able to thickness a top to a standard stiffness not an arbitrary thickness measurement

I understand and have acknowledged this. Again, looking to see if there are general relationships.

My Spruce tops tend to be about .090" to as thick as .115" . It just depends on the piece. I have not found any relationship. In fact, I am always surprised at the variability within species. I counter this by measureing the material properties of the plates and choosing plates that meet a criteria of both stiffness and mass.

I suppose the answer would be yes, if you used wide enough parameters. For instance I could say all my top fall between .090- .125, no matter what species, spruce, mahogany, or primavera, or cedar, and it would be true.

If you wanted to find out something like, 'all lutz falls between .090-.098, and all sitka falls between .095- .115', I don't think that correlation exists. I've certainly not found it to be so. I've also found it unnecessary to leave hardwood tops substantially thicker than spruce, in fact sometimes to get to the same deflection numbers, they've sometimes been thinner than spruce sometimes has been.

To me this is the value of deflection testing. The actual thickness becomes a byproduct of the stiffness you're after, instead of stiffness being the fluctuating byproduct of a target thickness.

I've been able to get great results first try with untried species using deflection testing.

uvhsam, are you the Sam that teaches the Advanced Voicing course at Galloup?

Yeah

I've been doing deflection for about four years. Joined top, rosette installed, Soundhole cut out. I've been preweighting with 1lb and adding 1lb. Same support points and measurement point.



I've got a small group of great pro players to critique the builds at the store where I sell.

I have tried to correlate the deflection with the enthusiasm exhibited by that group and of course my own feelings.

For me the magic number for an OM with a spruce top is around 0.043 to 0.048. This has resulted in tops from 0.112-0.120 for the same species. A bit thicker than I had been doing in the past.

I do voice after the box is closed before binding by thinning the edges of the lower bout and listening and feeling the top so it's not all deflection.

Deflection has been an extremely useful tool for me but after reading stuff being done Trevor Gore, Alan Carruth, and Sam I feel I am still at a medieval level.

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It's not what you don't know that hurts you, it's what you do know that's wrong.

Terry, if you feel that you are on a medieval level, I must be a luddite. I never have done anything but taps and flexes by hand, mind and eye. Not a numbers guy.
Not looking for the "Son of Irvin" tone though...more of a son of Larson, Stella, Holzapfel.

I test my tops after they are joined and rough cut to shape, no rosette or soundhole. I test both directions and usually see a slight variance in the numbers from one side to the other. I find most spruces are very close to each other and the numbers overlap quite a bit. I am less concerned with the species of the top and more concerned how it reacts to load.

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Brian

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

https://www.howardguitarsdelaware.com/

I want to get into this.
I can see how the jig is set-up.
Please help me here: With a 18" spacing and a 5lb weight (over the bridge area?) I'm looking for about a 0.25" deflection (with the dial under the bridge area?)? Is 0.25" deflection a good number to aim for as a beginning?
What would be a good number with braces on?

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We are all in the gutter but some of us are looking at guitars.

After seeing, playing, and hearing your guitars Haans I'd say don't change a thing. Maybe you could measure the defection of the grin on Dakota Dave's face when he plays one of your babies.

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It's not what you don't know that hurts you, it's what you do know that's wrong.

Terrence, you mentioned that your OMs top thickness has increased. Have you found they have increased in bass or treble with thickness? Any specific tone characteristic change with the thickness? Thanks


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Joe,

Place the weight in the center of the panel. The weight I use makes it almost the full width (empty Glengoyne tube fille with pennies).

The only way to find out if .25 is a good number for you is to build one, and modify accordingly...

I changed some other things around the same time such as using BZ more for bridges instead of Ebony and gluing frets with hide glue but improved clarity as well as tonal separation and evenness were commonly mentioned. That was my feeling too, less muddy.

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It's not what you don't know that hurts you, it's what you do know that's wrong.

Thanks for reply. I understand your answer and that is what I plan to do, but...
Maybe I should have asked: with a 5lb weight and 18" spread is 0.25" deflection a BALL PARK figure?

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We are all in the gutter but some of us are looking at guitars.

Joe, if you're using a full size joined spruce plate with no rosette/soundhole I think that's a good ball park figure. Keep in mind I'm still learning but I've done about 7 tops that way. You're asking the same question I asked a few years ago and getting about the same answers too I started with 5lb of mixed weights in the middle and 0.25" deflection and I'm still doing the same thing except I've changed to a piece of solid steel rod about 12" long wrapped in plastic wrap for the weight. I usually thin the edges of the top a bit after the box is done to loosen it up some.

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Steve Smith
"Music is what feelings sound like"

Excellent, thanks Steve.

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We are all in the gutter but some of us are looking at guitars.