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  #16  
Old 05-04-2017, 01:59 PM
Halcyon/Tinker Halcyon/Tinker is offline
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That's what I'm saying. Thin a Sitka top to .110, measure the deflection with your rig, build a dread (likely the most referenced acoustic guitar sound), bam, you have your first reference point. Adjudicate and adjust.
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  #17  
Old 05-04-2017, 02:01 PM
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Originally Posted by Halcyon/Tinker View Post
That's what I'm saying. Thin a Sitka top to .110, measure the deflection with your rig, build a dread (likely the most referenced acoustic guitar sound), bam, you have your first reference point. Adjudicate and adjust.
Okie dokie.
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  #18  
Old 05-04-2017, 02:39 PM
dekutree64 dekutree64 is offline
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Originally Posted by TEK View Post
What kind of mass do you shoot for on say a OM soundboard with braces installed and tuned? I have been weighing the last several and was thinking ( I don't have my notes with me) mine were coming in around 260 grams??
I have not been checking mass on the soundboards without braces. I usually stop thicknessing at a certain deflection.
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Max 300 grams including bridge/pins/saddle. Less is better.
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  #19  
Old 05-04-2017, 03:08 PM
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Originally Posted by redir View Post

Do you have like some sort of standard dimension to your braces before you test them to deflection and then glue them up and carve them out or do you actually shape the brace fully then do deflection and glue them up?
We have a set dimension that we deflect all of our bracing stock - 5/16" x 3/4" x 19". That's just an arbitrary size we built a deflection jig for.

For the first hundred guitars or so, we would deflect each brace with a given weight and record the number. Then we would cut the height down to 11/16" and re-deflect. We would use the two numbers to see if the braces were becoming floppier with material removed. Short answer is yes some braces do occasionally become much less stiff the thinner they get, but most of our bracing stock generally loosens proportionally to height. So now we only do one deflection test on braces.
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  #20  
Old 05-04-2017, 04:18 PM
TEK TEK is offline
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Rog,
I am no expert but have seen that a good deflection starting point on just the top without braces is, top blocked at 18" apart and a 5 pound 5 oz. weight in the middle you want 1/4" of top deflection. The video I saw was with the top pieces joined but the shape of the guitar not cut out.
When doing this a lot of the Sitka tops I have checked start to end up around
.110" - .115" give or take with 1/4" of deflection.
So set up two 3/4" tick boards 18" apart and get one 1/2" thick and put it in the middle. Keep taking a little off the top and keep checking until it touches the center board with the weight in the middle of the top.
Of coarse you can make a much more elaborate set up with a dial indicator but you can get it done this way.
That is at least a starting point. If you do it this way at least you have some sort of consistency.
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  #21  
Old 05-04-2017, 05:41 PM
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It's funny because I went through the same thing that frog did a couple years ago and was frustrated but this (Halcyon says below) is exactly what you have to do.

I don't even know what the brick I use weighs actually, I jsut know that it is the brick that I will use till the day I build my last guitar.

Quote:
Originally Posted by Halcyon/Tinker View Post
That's what I'm saying. Thin a Sitka top to .110, measure the deflection with your rig, build a dread (likely the most referenced acoustic guitar sound), bam, you have your first reference point. Adjudicate and adjust.
This is how I do it. I will literally keep that brick, that dial gauge and those two pieces of angle trim used as a stand for ever and continue to do the exact same thing every time. I think I only have about ten instruments done so far so I have a long way to go. I'd have to check but that deflection was about .4 inches IIRC. It's a lot more then I ever thought it would be actually.


Last edited by redir; 05-04-2017 at 06:57 PM.
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  #22  
Old 05-04-2017, 05:43 PM
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Originally Posted by nacluth View Post
We have a set dimension that we deflect all of our bracing stock - 5/16" x 3/4" x 19". That's just an arbitrary size we built a deflection jig for.

For the first hundred guitars or so, we would deflect each brace with a given weight and record the number. Then we would cut the height down to 11/16" and re-deflect. We would use the two numbers to see if the braces were becoming floppier with material removed. Short answer is yes some braces do occasionally become much less stiff the thinner they get, but most of our bracing stock generally loosens proportionally to height. So now we only do one deflection test on braces.
Very cool. Sounds like you got a lot of data too.
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  #23  
Old 05-04-2017, 05:59 PM
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Originally Posted by TEK View Post
Rog,
I am no expert but have seen that a good deflection starting point on just the top without braces is, top blocked at 18" apart and a 5 pound 5 oz. weight in the middle you want 1/4" of top deflection. The video I saw was with the top pieces joined but the shape of the guitar not cut out.
When doing this a lot of the Sitka tops I have checked start to end up around
.110" - .115" give or take with 1/4" of deflection.
So set up two 3/4" tick boards 18" apart and get one 1/2" thick and put it in the middle. Keep taking a little off the top and keep checking until it touches the center board with the weight in the middle of the top.
Of coarse you can make a much more elaborate set up with a dial indicator but you can get it done this way.
That is at least a starting point. If you do it this way at least you have some sort of consistency.
Thanks, very helpful!
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  #24  
Old 05-05-2017, 11:56 AM
Alan Carruth Alan Carruth is offline
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One advantage of getting numbers on things like the mass of the brick and the spacing of the supports is that you can convert your idiosyncratic deflection numbers into standard units, such as Young's modulus. Then you can trade information with other folks who have done the same, even if they use different methods to find the numbers.

Another advantage is that you don't actually have to keep checking until you get the 'correct' thickness; you can crank the numbers into formulae (which can be pretty simple) and find the proper thickness after one measurement.

There are several sources that can tell you how to do this. One is David Hurd's book and the Gore/Gilet books are another. If you want to dig around a bit there are older articles in places like the 'Journal' of the Catgut Acoustical Society and the old 'Journal of Guitar Acoustics' that get into this as well. Material testing is a pretty standard discipline in mechanical engineering, and there's lots of information out there.
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  #25  
Old 05-05-2017, 04:43 PM
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rogthefrog rogthefrog is offline
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Originally Posted by Alan Carruth View Post
One advantage of getting numbers on things like the mass of the brick and the spacing of the supports is that you can convert your idiosyncratic deflection numbers into standard units, such as Young's modulus. Then you can trade information with other folks who have done the same, even if they use different methods to find the numbers.

Another advantage is that you don't actually have to keep checking until you get the 'correct' thickness; you can crank the numbers into formulae (which can be pretty simple) and find the proper thickness after one measurement.

There are several sources that can tell you how to do this. One is David Hurd's book and the Gore/Gilet books are another. If you want to dig around a bit there are older articles in places like the 'Journal' of the Catgut Acoustical Society and the old 'Journal of Guitar Acoustics' that get into this as well. Material testing is a pretty standard discipline in mechanical engineering, and there's lots of information out there.
Thanks, Alan, that makes sense. I'll be measuring things this weekend.
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  #26  
Old 05-06-2017, 05:07 PM
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Quote:
Originally Posted by TEK View Post
Rog,
I am no expert but have seen that a good deflection starting point on just the top without braces is, top blocked at 18" apart and a 5 pound 5 oz. weight in the middle you want 1/4" of top deflection. The video I saw was with the top pieces joined but the shape of the guitar not cut out.
When doing this a lot of the Sitka tops I have checked start to end up around
.110" - .115" give or take with 1/4" of deflection.
I did my deflection test today, after thinning the top a bit more than I had left it (to ~.105), and the results were consistent with this ballpark. My weight was a little less (4 lb 8 oz), and the supports only 16" apart (so I'd expect a stiffer area), and the deflection was about 3/8". So it looks like this top is on the right track.

Thanks everyone for the info!
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Last edited by rogthefrog; 05-06-2017 at 05:12 PM.
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  #27  
Old 05-06-2017, 05:30 PM
dekutree64 dekutree64 is offline
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Originally Posted by rogthefrog View Post
I did my deflection test today, after thinning the top a bit more than I had left it (to ~.105), and the results were consistent with this ballpark. My weight was a little less (4 lb 8 oz), and the supports only 16" apart (so I'd expect a stiffer area), and the deflection was about 3/8". So it looks like this top is on the right track.

Thanks everyone for the info!
Did you mean to write 3/16"? Because 3/8" deflection on a shorter span would be very flexible.
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  #28  
Old 05-06-2017, 07:53 PM
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Originally Posted by dekutree64 View Post
Did you mean to write 3/16"? Because 3/8" deflection on a shorter span would be very flexible.
Neither ;_;

The deflection is a hair under 0.315, so about 5/16.
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  #29  
Old 05-06-2017, 08:42 PM
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Well there you go. Now write that down and begin the very slow process of changing things up and recording everything you do and the changes they make. Then in 10 or 15 years you should have a pretty good idea what works best.
Pretty simple huh? This is all in good fun...
Have fun with your build.
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  #30  
Old 05-06-2017, 09:55 PM
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Originally Posted by TEK View Post
Well there you go. Now write that down and begin the very slow process of changing things up and recording everything you do and the changes they make. Then in 10 or 15 years you should have a pretty good idea what works best.
Pretty simple huh? This is all in good fun...
Have fun with your build.
Thanks. Fun is being had. Today I beheaded my one-piece neck blank because I had botched up the headstock thicknessing. Then I cut a decent scarf joint, but failed to glue the headstock onto the neck twice. Things are sliding around too much. I may put in a couple of registration pins and try again.
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