Measuring Elastic Constraints - Lookover

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John Greenlaw
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Measuring Elastic Constraints - Lookover

Post by John Greenlaw » Sun Nov 11, 2018 2:31 pm

Hello everyone!

Can someone look over my data and see if I'm doing everything right. I have attached photos of the board (#2 Honduran Mahogany) the FRC's for the frequencies and the excel sheet. I am not sure if I am reading the FRCs correctly. Basically I am just referencing the elastic constraints excel sheet and finding the peaks that match around that frequency. Any comments would be greatly appreciated!


Cheers,

John from Canada!
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Trevor Gore
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Re: Measuring Elastic Constraints - Lookover

Post by Trevor Gore » Sun Nov 11, 2018 10:58 pm

John,

Your frequency measurements look about right. Check out Table 6.1 in the white book. Mahogany will fall between spruce and rosewood.

Your elastic constants are incorrect, though. Your numbers for E long (for example) should be ~ 10 GPa, which is 1.0E+10, not 1.xxE+16 as you have. Check your zero count, e.g. millimeters are 10^-3 meters etc. Mass should be in kgs not grams. Target thickness in SI units would be in meters but are x 1000 to give millimeters in Table 4.5-3.

John Greenlaw
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Re: Measuring Elastic Constraints - Lookover

Post by John Greenlaw » Tue Nov 13, 2018 5:27 am

Hi Trevor!

Thankyou for looking over these calculations! I changed the grams to kilograms. I attempted to change the calculations for F long but when I reduce the power to equal 1.0E+10 it changes the target thickness. How did you set up the formula in excel to calculate F long? I have attached a screenshot of the formula that I used on "Example" for top woods.
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Trevor Gore
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Re: Measuring Elastic Constraints - Lookover

Post by Trevor Gore » Tue Nov 13, 2018 9:05 am

John,

Carefully check Table 4.5-3 in the black book, especially the density numbers. You're showing 0.395 Kg/m^3. Should be 1000 times that.

Also check out these threads:

viewtopic.php?f=33&t=4019

viewtopic.php?f=33&t=3626

The 2nd edition of the books has everything fixed.

John Greenlaw
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Re: Measuring Elastic Constraints - Lookover

Post by John Greenlaw » Thu Nov 15, 2018 10:16 am

Ok, Thanks Trevor! I've read through the links once and doing a second reading just now!

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kiwigeo
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Re: Measuring Elastic Constraints - Lookover

Post by kiwigeo » Thu Nov 15, 2018 5:18 pm

You'll get there John. Maths isn't my best subject so it took me a while to get my units and zeros sorted out. Trevor at first seems a bit cruel when he doesn't always give you the easy answers but by not doing so he's prompting you to go back and understand the equations.
John Greenlaw wrote:
Thu Nov 15, 2018 10:16 am
Ok, Thanks Trevor! I've read through the links once and doing a second reading just now!
Martin

John Greenlaw
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Re: Measuring Elastic Constraints - Lookover

Post by John Greenlaw » Fri Nov 16, 2018 10:53 am

Oh yes, totally! I enjoy figuring it all out and really appreciate that Trevor points out areas that need work. I'm hoping to "get it" in the next couple of weeks because I would like to get this guitar done for the customer for Christmas 2018! LOL....

John Greenlaw
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Re: Measuring Elastic Constraints - Lookover

Post by John Greenlaw » Sun Nov 18, 2018 7:55 am

Hello everyone!

I think I got the equation! It was mostly keeping all the numbers in meters and kg. Can someone confirm with me that this all looks correct? Now to calculate the predicted mass! Exciting!!!! Yeahoo!!!

John from Canada!!! eh!!!!
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Trevor Gore
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Re: Measuring Elastic Constraints - Lookover

Post by Trevor Gore » Sun Nov 18, 2018 3:01 pm

That looks about right, John. You seem to have replicated the Table 4.5-3 results.

Feeling good? :D

Well done!

John Greenlaw
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Re: Measuring Elastic Constraints - Lookover

Post by John Greenlaw » Mon Nov 19, 2018 2:48 am

Oh yes, it is so Good! Thank you very much Trevor for the links and the help!

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