Official Luthiers Forum!

Kyle and I want to start doing deflection testing - and I sure could use some help on coming up with the items we will need to purchase to make this happen.

Also - while I'm at it - what do you use for a gram scale? I see an inexpensive one at HF - but I want accuracy over price (to a certain point)

thanks

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P A U L B U R N E R
Burner Guitars

I use our kitchen scale. Any digital scale from Sir La Table or another similar store should work.

The key item is a dial or digital indicator. Both the HF-level ones have been fine for me. I prefer a dial as I can use it faster, but others like the digital for the zero button. 1" travel is plenty.

Also 1-3 weights. For the longitudinal (stiffer) direction, a steel bar around 14-17" long, with weight around 5 lb works well. It helps if it's narrow (~1") and doesn't roll. For the lateral direction, I use a a 0.9 lb piece of angle-iron. A 0.110" Sitka top will deflect in the neighborhood of 0.12" - 0.20" with those weights and typical spans. If testing half-panels, change the 5 lb to ~2.5, and the 0.9 to quite a bit more. I also use the 0.9 lb weight as a 'tare' weight for longitudinal testing, to remove twist-warp in the panel before testing with the 5.

The rest is wood and fasteners. Particle board, MDF, or another stable wood helps.

There are lots of good, low-$ digital scales on ebay. I've not tried the HF, but the others I've tested have all been very accurate, around 0.1%. The button-feel quality varies by model. Digiweigh is one brand I like (1000BX and postage models... I've not tried their others, though).

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David Malicky

I do the same as David, but I put the indicator underneath and use a weight that rolls so it is self centering. You do want the weight to be at least as wide as the board you're measuring. And the pre-weighting helps with consistency.

Mike

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Mike Lindstrom

Can you explain this pre-weighting please.

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P A U L B U R N E R
Burner Guitars

If you're trying to measure the deflection of a top, you'll lay it across the jig with a measuring device in the middle. Say you're going to see how much it deflects when a 5 pound weight is placed on it. Pre-weighting is putting a small weight (say a half pound) on the top and zeroing your gauge there. It takes up any slack in your system and gives you a more consistent starting point for your measurement. Then you measure by adding your standard weight in addition to the

I don't know if it is valid from an engineering standpoint to produce real, accurate data, but measuring for consistence from top to top, it seems to work. It at least gets me in the ballpark.

Mike

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Mike Lindstrom

Yes, it is valid from the engineering standpoint because you've got a linear elastic system (unless you load the panel beyond its elastic limit...).

Below, you can see how Brian Burns does things (scroll down to near the bottom of the page):

http://www.lessonsinlutherie.com/woodtestingandvoichirez.html

You might want to consider tap testing, too. Just as accurate, much quicker. Then have a think about how you're going to use the data.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Links to dial or digital indicators for deflection testing would be helpful please.

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P A U L B U R N E R
Burner Guitars

We mostly use Enco's Economy 1" indicators and they've been fine:
http://www.use-enco.com/CGI/INPDFF?PMPAGE=331

HF's are probably similar:
http://www.harborfreight.com/1-inch-tra ... r-623.html
Or ebay:
http://www.ebay.com/itm/1-DIAL-INDICATO ... 0465053594

I've used 1 HF digital--it's been good, too:
http://www.harborfreight.com/1-inch-sae ... 93295.html

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David Malicky

David Hurd also has a lot of good information on his site. Here's the info on static
deflection measurements, but look at his other information as well.

http://www.ukuleles.com/Technology/statmeas1.html

I use HF digital indicators and scales, and haven't found them to be inaccurate, at
least in the measurement ranges I'm using. The technology is pretty cheap to
buy these days.

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Gene

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