Top Deflection rig

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DarwinStrings
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Re: Top Deflection rig

Post by DarwinStrings » Wed Nov 12, 2014 1:52 pm

Well yous have made this job a easy one for me, thanks for sharing.

Jim
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DarwinStrings
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Re: Top Deflection rig

Post by DarwinStrings » Fri Nov 14, 2014 6:13 pm

So with help from here I built a rig, took the measurements and did the maths. As I am not confident mathematician I sent my measurements to Jeff Highland and he did that maths for me to check if I was getting it right, we got the same results and I am happy to announce that I have a guitar with a monopole mobility of 15 x 10^-3 s/Kg. Thanks again Trevor.

Jim
Life is good when you are amongst the wood.
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Edzard
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Re: Top Deflection rig

Post by Edzard » Fri Nov 06, 2015 10:00 pm

Hello All,

Please tell me/confirm to me that the spindle of the (analoge) gauge your all using goes "through" the housing of the gauge...

Other wise I'm going nuts how this works with the above jigs!

I'm asking because (a) I want to build one of my own and (b) I have a few digital gauges and there the spindle on top of the housing is fixed.



Thanks,


Edzard

Craig Bumgarner
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Re: Top Deflection rig

Post by Craig Bumgarner » Sat Nov 07, 2015 9:03 am

Yes, the dial indicator I use goes all the way through the dial part on the housing. The load is applied to the spindle protruding from the top of the gauge and applies pressure to the top by way of the spindle coming out the bottom of the gauge. The top deflects and the gauge measures.

Many dial indicators have a cap on the top that protects the top of the spindle, but this can be removed by unscrewing it. All the dial indicators I have seen have the spindle protruding from both sides, otherwise it is much more complicated to support the spindle within the dial body.
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Edzard
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Re: Top Deflection rig

Post by Edzard » Sat Nov 07, 2015 10:05 am

Thanks Craig!

Found this neat construction. It's from Michael Cone ( http://www.coneguitar.com/acoustics/a02.html)
Compliance_testerL.jpg



Edzard

Craig Bumgarner
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Re: Top Deflection rig

Post by Craig Bumgarner » Tue Nov 10, 2015 3:59 am

That's a nice looking jig, but it has a major flaw. The way it is set up with the base on the table, it will measure not only the top deflection, but also the guitar side deflection, the deflection (compression) of whatever the guitar is sitting on, plus any play in between. These will be considerable and likely affect the measurement substantially. (I know because this is how I made my first jigs). To get rid of all this, the jig needs to rest on the top of the guitar like some of the pictures earlier in this thread. Also, watch out for that pretty brass weight if you happen to bobble it :o
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jeffhigh
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Re: Top Deflection rig

Post by jeffhigh » Tue Nov 10, 2015 12:29 pm

Yes Craig , that is the problem and why I designed mine to sit on the edge of the soundboard.
It does not then matter whether the guitar is sitting on a towel, foam blocks, or surgically enhanced breasts.....

JamesO
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Re: Top Deflection rig

Post by JamesO » Tue Nov 29, 2016 12:30 pm

I'm trying to understand why and how we test top mobility, but I'm having trouble finding it in the table of contents in Design and Build. Can someone point me to it? Thanks. :oops:

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Trevor Gore
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Re: Top Deflection rig

Post by Trevor Gore » Tue Nov 29, 2016 6:39 pm

There are plenty of listings under mobility and monopole mobility in the Design index.

In Contents, Sections 1.7.1 and 1.7.2. The 2nd edition also has an Appendix which goes into the measurement process in a bit more detail (which is mostly covered in this thread).

Monopole mobility is important because it gives a measure of the sensitivity and responsiveness of a guitar, i.e. its potential to be able to produce a good deal of sound.

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Trevor Gore
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Re: Top Deflection rig

Post by Trevor Gore » Wed Jun 28, 2017 11:51 pm

The traveling 3 day modal tuning course goes far and wide, which means hoping the gear at the destination works OK, or taking your own. The problem is my gear is all made of heavy hardwoods so easily exceeds a baggage allowance, so I thought I would re-do my top deflection rig to a lower mass version and make it multi-purpose so I can use it on a three point bending rig as well.

Here's the new top deflection rig:
DSCF0004s.jpg
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This is all pretty self explanatory if you've read through to early part of this thread. The cross beam is now spruce, so less than half the mass of its Tassy oak predecessor. The gauge has gone low mass digital. It's basically a tyre tread depth gauge by iGaging (as used on the LMII string height jig). The loading is through the shaft as discussed above and the load is 1kg of lead shot in a plastic bag in the "velvet" bag (which is the lining material of Hiscox cases).

The loading platform is a push fit on the gauge shaft:

DSCF0033s.jpg
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The gauge has all the usual digital benefits of "zero" and "hold" buttons and reads in mm, decimal inch and fractional inch for those who refuse to use SI units!

The shoe that sits on the guitar (see first pic) now has an extra bit of wood glued on the outside, so the cross beam can be used on my new low mass, knock-down, 3 point bending rig:
DSCF0016s.jpg
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Cross grain stiffness can be measured as well by setting the beam diagonally across the test panel:

DSCF0013s.jpg
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The top loading and top reading indicator is much easier to set up and use than my old rig, which was a troublesome top loading arrangement with a difficult to read dial indicator beneath.

The last bit of new gear is a low mass Chladni sig gen, but I'll start another thread for that.

johnparchem
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Re: Top Deflection rig

Post by johnparchem » Thu Jun 29, 2017 7:19 am

Trevor, thanks for the post. I am going to convert mine to an igaging digital probe. Half the price of the modified LMI version for string action measurement.

Dave M
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Re: Top Deflection rig

Post by Dave M » Fri Jun 30, 2017 9:01 am

Trevor thanks for the description of your updated design. Your jigs always look so neat compared to ones own rather scruffy efforts.

It does still bother me that you would rely on a single measurement of the deflection of the top with an applied mass. A number of measurements with a series of increasing masses is surely a better experimental technique...?

You show a mid point deflection arrangement, presumably to measure Young's modulus of a half top. Does that imply that you are using this technique as well as the resonant frequency method for getting at this value?

All the best Dave M
------------------
Dave

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Trevor Gore
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Re: Top Deflection rig

Post by Trevor Gore » Fri Jun 30, 2017 9:26 am

Dave M wrote:
Fri Jun 30, 2017 9:01 am
It does still bother me that you would rely on a single measurement of the deflection of the top with an applied mass. A number of measurements with a series of increasing masses is surely a better experimental technique...?
Sure! I have three shot bags, but I also have very little problem with repeatability.
Dave M wrote:
Fri Jun 30, 2017 9:01 am
You show a mid point deflection arrangement, presumably to measure Young's modulus of a half top. Does that imply that you are using this technique as well as the resonant frequency method for getting at this value?
I nearly always use the tap test method. However, occasionally I have wood that can't be reduced to a tappable rectangle (think Euro spruce with corners cut off, or a Braz back that is "coffin" shaped). The 3 point bend comes into its own then because it is usually possible to isolate a rectangular shaped span and get a reading. Stuff that is cupped or twisted can be measured, too, by pre-loading to flatten the cupping/twisting and taking measurements with the cup/twist facing up then down (and taking multiple readings). Probably not as accurate as tapping flat stock, but it gets you a reasonable number to work with.

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