Tenor Koa top thickness?

ChuckBarnett

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I'm baaaack... But this go 'round will be much, much easier!

Tenor #2 has been started. And I must say a very special thank you to all of you who helped me with that first one! I took a full year off but am back at it, building a Koa tenor for a granddaughter. :)

I've got this top down to about 0.075" thick and am trying to remember what y'all have said about thickness. I think this is a good place to stop as there will be some sanding later. The top of the top went through the drum sander to clean up the rosette once I got as close as my scraper skills could take me. So it's in need of more finish sanding.

Any tips?

I know Chuck Moore uses a deflection method to settle in the final thickness. I think it must be a trial and error method to figure out how much weight to set on the several points of the top to compare deflection? Or is there a plan that somebody has made that is transferable?

Once again, Thanks!

20200215_073625.jpg
 
Check out Dr. David Hurd's website
ukuleles.com
He has a whole section on his site about compliance testing and it has been a great help to me. There is lots of other very useful information on his site including his book Left Brain Lutherie.
After a bunch of instruments I'm being able to tell when to stop as I'm sanding a top to thickness. I still use the deflection testing tor the final brace shaving tho.
 
Chuck, I learned the top deflection method from David Hurd. Too bad you didn’t visit him when you were in Hilo. It is a method used to repeat past successful results. But there’s the rub, you need to have had some good past results to begin with! It won’t tell you what the deflection should be unless you’ve got a good place to start. Like Terry said, this comes from building lots of instruments, good and bad. But just as a starting point, the weight on my jig is somewhere around 550 grams and my deflection near the bridge is about .08” to .09”. The specific amount of weight on the jig doesn’t matter but comparing one set of numbers in a past good result and comparing them to a present set of numbers, does. After a while you get a feel for it and I rarely use my deflection jig anymore, only when I’m using a wood that’s foreign to me and I suspect something may be off. Using your thumbs to press and test for deflection is good practice but David Hurd is a scientist and as such likes more precise tools and methods to measure things.
 
Thank you, sir! That is great help for the rookie!! Terry's post pointed me to David Hurd and that will be helpful, as well. Last November someone could have told me David Hurd was a well-known punk drummer from the 90's and I'd have bit.

Yeah, I lack the experience to get too cranked up about some of this stuff but the forum has been there for me in big ways!
 
It's been years since i've looked at David's website and i can't remember if he gives target numbers.
But i will say this:
Quoting deflection testing without target numbers to new builders is simply not helpful.
A new builder needs numbers to work with.
Not giving numbers is like handing a person a wrench and telling them to build a car- just not helpful without years of experience.
 
It's been years since i've looked at David's website and i can't remember if he gives target numbers for top deflections.

But i will say this:
Suggesting deflection testing to new builders without target numbers to work with is simply not helpful.

A new builder needs numbers to work with.

Not giving numbers is like handing a person a wrench and telling them to build a car- just not helpful without years of experience.

Deflection testing is great with knowledge of target numbers, but it takes years to develop that, unless you are given data.
 
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It's been years since i've looked at David's website and i can't remember if he gives target numbers.
But i will say this:
Quoting deflection testing without target numbers to new builders is simply not helpful.
A new builder needs numbers to work with.
Not giving numbers is like handing a person a wrench and telling them to build a car- just not helpful without years of experience.

Thanks, Beau, for the perspective. That's one of the great things about this forum: you hear stuff and you can ask the forum about it and get good info! :)
 
I've been doing deflection testing for just long enough to have an appreciation for how long it's going to take to accumulate enough data to make it useful. It's not likely to do you any good on your second build.

I do have as-built data for dozens of tenors, the minority of which have hardwood tops. Based on my experience with these, most of which are koa, mahogany, myrtle and cherry tops, I'm going to stick my neck out and say that at .075 you've gone far enough for now. This isn't a bad guess, which is about all you can do in the absence of the feel that you get for wood and for the process by building a bunch of similar instruments. You'll lose some more thickness in final sanding and will have a pretty decent chance of ending up with a good and durable instrument provided you get the bracing about right. As in, "not too heavy and not too light." I can already hear the next question!! :)
 
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I'm going to stick my neck out and say that at .075 you've gone far enough for now. You'll lose some more thickness in final sanding and have a pretty decent chance of ending up with a good instrument provided you get the bracing about right. As in, "not to heavy and not too light." I can already hear the next question!! :)

I would agree that ~ .075 is about right for a hard top. But it is really more about bracing than top thickness (although that is certainly important) that gives the ukulele a good sound. I've attached a picture of one of my braced tops for reference (spruce not koa). Unfortunately I don't have the numbers because I'm too lazy to look it up. The uke sounded good. As for all these fancy deflection devices and methods, I agree it is overkill for a beginner. It ain't rocket science here.

brace.jpg
 
Sequoia,

Thank you for the bracing photo. That raises 3 questions:
1. What is the little block under the center brace?
2. How thick is the bridge plate you've used?
3. There looks to be quite a lot of space between the 3 long lower braces and the lateral one below the sound hole. My only experience is from the LMI tenor plan and they're much closer in that plan.
 
It's an unanswerable question really. Koa can vary from almost an iron wood density to a spruce density. Grain flame and curl all figure in. I look for stiff but light weight for tops. That's before we even wander into the bracing scheme. If you had a fantastic sounding instrument laying around I would tell you to take the bridge off and try to get about the same deflection. Whatever you decide to do measure the stiffness before bracing even if it's just flexing it with your hands so you can begin to develop a reference. Most new builders tend to over brace.
 
Sequoia,

Thank you for the bracing photo. That raises 3 questions:
1. What is the little block under the center brace?
2. How thick is the bridge plate you've used?
3. There looks to be quite a lot of space between the 3 long lower braces and the lateral one below the sound hole. My only experience is from the LMI tenor plan and they're much closer in that plan.

1. Ah yes, my maybe silly little invention (?) which I call the "bite me" patch. The patch goes under the central fan which has been notched. The idea is to distribute the upward force on the top by the central fan as the bridge wants to rotate forward. It also distributes the pressure on that vulnerable top join between the heel block and the brace which can possibly cause splits (cracks?) down
the line.

brace1.jpg

2. The bridge plate is ~0.1 inches more or less. I don't get too hung up on bridge plate thickness as long as the grain is perpendicular (90 degrees) to the grain of the top.

3. I believe that the fan braces should NOT be tied off (butt joined) to the lower transverse brace. Strong structurally but acoustically deadening. Just my opinion. However the distance between the lower transverse brace and the fan braces is probably not that big a deal. Close but not too close.
 
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Sounds like good advice, Mike. But I'm sure I'll be scratching my head as I wonder what on earth what I'm doing means. Nonetheless, I will be flexing away. :)
 
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