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I have posted a three part tutorial on making a deflection measurement jig, calibrating it and making actual measurements in the Tutorials section of this website.

Next will be examples of top deflection measurement forms, normalizing information for comparison and examples of instrument deflection maps.

aloha,

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David C. Hurd
http://www.ukuleles.com

Alice laughed: "There's no use trying," she said; "one can't believe impossible things." "I daresay you haven't had much practice," said the Queen. Through the Looking Glass by L. Carroll

"Since my house has burned down, I have a much better view of the moon"

David, thank you for taking the time to do this. I've been wondering about my archtops & hope to use your methods to get some comparative data. Much ppreciated!

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Dave
Milton, ON

Hi David,

In your part 3 tutorial, should the sound box be supported at the rim instead of lying on its back? Couldn't your dial indicator be measuring the stack up or additive effect of the top AND the back's deflection? Should the dial indicator support bar's feet be resting on the rim to get a more accurate measurement too? If the one foot was resting near the sound hole couldn't the sound hole be deflecting as weight is applied to the bridge area? Nice work and pictorials BTW.

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tim...
http://www.mcknightguitars.com

Dave, thanks for the positive remarks.

Tim, those are all good questions and right on the mark.

Don MacRostie in American Lutherie #94 had a very nice article on doing measurements for mandolins which should be directly applicable to archtops.

Yes, the dial indicator frame should rest on the rims and those braces which are essentially non-deflecting. In my experience the cross-brace immediately below the soundhole, if properly made, will be very stiff and not contribute to the top deflection measurement, especially if the measurement is some distance away from the brace. And that's why the dial indicator frame should be somewhat longer than the longest lower bout dimension - so that the little movable "legs" can be put on the rims/stiffest braces.

The wedges for the back - to keep the instrument steady - should be at the rims. Whether the back deflection affects the top measurement is a function of how stiff the back is. If it's very stiff, I don't think it's a problem. I usually make these measurements during the building process before the back has been glued on so the "back issue" doesn't enter into the argument. Even a back which has been tuned to the front is still pretty stiff. But rather than try to argue a point away, it would be best to do comparison measurements with and without the back touching the resting surface.

There are many interesting issues to concern us with this process.

For example how much actual load does the dial indicator put on the top? Probably little in comparison with the weight but it's still a good idea to try to have the total indicator deflection be in essentially the same range all the time.

What if the lever arm isn't exactly level each time? How does the effect weight value change?

What effects do the day-to-day differences in temperature and humidity have on the stiffness measurements? Clearly the stiffness of the plate will change but to what degree?

I suggest that the top deflection approach is not a "silver bullet" but rather a powerful tool to be used in conjunction with other approaches. It can give both beginner and intermediate makers a real leg up on the optimization process.

I also suggest that the important thing is to make a collective good start, begin to collect comparable measurements as a group, discuss them as a group, refine our methodologies and continue to move forward.

aloha,

_________________
David C. Hurd
http://www.ukuleles.com

Alice laughed: "There's no use trying," she said; "one can't believe impossible things." "I daresay you haven't had much practice," said the Queen. Through the Looking Glass by L. Carroll

"Since my house has burned down, I have a much better view of the moon"

Great information David, thanks!

_________________
Steve Smith
"Music is what feelings sound like"