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Hello my friends. ok. I am doing a Dreadnought Koa B/S with sinker top. just glued on the top to the body, and I worry that the deflection when pressing the lower bout is to deflective so to speak . is there a good guideline to go by here ? lets say I put on a 2 kg weight or 5 kg weight, then how much would I want the top to deflect ( once the top is glued on ! . is like 5mm to much when pressing by hand ? .

All the best Lars.

Recheckt the deflection with a correct instrument, and it deflects about 1,5 - 2 mm or 0.06 inch. when using about 5 kg pressure.

Lars

Lars: Excellant question,and I certainly don't know. The best I can find are vague answers about a bit of pressure should give you a bit of movement. Kent Evertt mentions it in his video,sent him an e-mail asking about defined numbers and did not get an answer. It seems to me that if we MEASURE deflection of raw tops that we should be MEASURING the braced tops on the guitars. To me it may be the more important time to be measuring in terms of feedback. Lars, if you or anyone else comes up with any info it would be appreciated if you could let us know.Good luck.
Tom

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A person who has never made a mistake has never made anything!!!

I look for about 1/32" or .03". with 5 pounds of pressure on the bridge.

Chuck

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"To live a creative life, we must lose our fear of being wrong"
- Joseph Chilton Pearce

Two possible conditions------------1. bridge on,or 2. bridge off.

Tom

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A person who has never made a mistake has never made anything!!!

Thanks chuck. well 5 lbs = 2.3 kg. I checkt and then I am right on spot. .. if there are other inputs on this, then let me know.

Lars

Suprized this is not getting much input...????
Tom

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A person who has never made a mistake has never made anything!!!

I have seen this thinking before with respect to archtops, and that makes some sense to me why they would do this (since the arch is what creates much of the plate stiffness and they can influence it with the recurve).

But on a flat top, what is the purpose of measuring deflection after the top is braced and on the rim? Unless you are doing this prior to putting the back on and then adjusting the bracing I think the horse has probably left the stable by that point. And if you ARE doing this with the back off, how is measuring deflection better than just tap tuning it?

I am not being a nay-sayer, this might provide some valuable insight. I just don't understand what is to be gained by measuring deflection on the top after it is braced and attached.

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Brock Poling
Columbus, Ohio
http://www.polingguitars.com

I'm with Tom and have never taken it beyond the old thumb below the bridge test where I press and want to see some deflection. How hard I press and what deflection I see has never been measured.

Sounds like you are onto something Lars that may be a good way to control the process and help in getting more predictable results. Sorry I can't be of more help...

on my 10th guitar, and still testing different bracing. this one I made the main X braces real thin at certain points ( where you most normally scalloup them ) and after I tap tuned it to what my ears liked to hear, I got a little nervous, to the eyes it looked very thin, but felt stiff enough to the feel. So therefor I made the judgement to glue it on. after I glued it on, it was a little to flexible as I pressed the thumb on the lower bout ( as Hesh mentioned)....
Thats why I was asking this question.
wanted to know if I should put the bridge on once done, or not. So it would not rise to much once the strings are on etc....
thats why I needed to see what the normal deflection was on a bridgeless top glued on.

But I still think its a valuable thing to measure , - do a deflection test on your best guitars and see if they are similar in deflection !

A good part of David Hurd's book: "Left Brain Lutherie", is concerned with deflection measurements. He doesn't give as many examples as one would like, and most of those are classical guitars and ukes, so you might not get as much from it that was as you'd like. OTOH, it's a very useful book in setting out procedures and talking about why this stuff could be useful.

Dave's using this method to fine-tune bracing at the end. He checked out the top on the infamous 'corker', and I seem to be getting about the same results by use of Chladni patterns. There are lots of good ways to do this stuff.

Deflection & tapping to hear the resonance.
That's what I do .
I use my thumb(high tech)for deflection.
And first finger for the tapping.
I'm listening for a good but not long sustain.
The note the body produces lets me know if I'm underbuild ,over built or close to what I was shooting for.
Depending on body depth & width & bracing & top & back thickness etc...
A steel will be totaly different than a nylon top in the amount it moves;
And the tap tone produced.
Mike

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Mike Collins

Thanks Alan Mike and all of you for the inputs. these last 3 guitars i´ve been doing I have been scalloping tha braces in a certain way to ly liking, as the tap tone really comes to live. After putting the body together with the bindings on , the tap tone is really hugh, all of these are having the similar deflection so hopefully I am getting some similar results. Now I just need to finish them so I can hear what they sound like with strings on .
below is what I am using to check the deflection



Still