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Thread: Tenor Koa top thickness?

  1. #1
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    Default Tenor Koa top thickness?

    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
    "Why is it that you never have time to do it right the first time, but you always have time to do it right the second time??"

  2. #2
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    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.
    My Real name is Terry Harris

  3. #3
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    Excellent information for me!! Thanks for steering me to that website!
    "Why is it that you never have time to do it right the first time, but you always have time to do it right the second time??"

  4. #4
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    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.
    Chuck Moore
    Moore Bettah Ukuleles
    http://www.moorebettahukes.com

  5. #5
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    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!
    "Why is it that you never have time to do it right the first time, but you always have time to do it right the second time??"

  6. #6
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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.

  7. #7
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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 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.
    Last edited by Beau Hannam Ukuleles; 02-17-2020 at 08:08 AM.

  8. #8
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    Quote Originally Posted by Beau Hannam Ukuleles View Post
    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!
    "Why is it that you never have time to do it right the first time, but you always have time to do it right the second time??"

  9. #9
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    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!!
    Last edited by saltytri; 02-17-2020 at 09:04 AM.

  10. #10
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    Quote Originally Posted by saltytri View Post
    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

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