Official Luthiers Forum!

I just bought a dial indicator and I'm about to start measuring stiffness. I'm thinking of using a couple of dowels and a board with notches to place the dowels at about 6", 8" and 12" apart would be a good start. I'm also thinking that in addition to measuring plates and braces, measuring assembled tops and backs is a good idea.
So, I'm looking for so advice on how to measure - what to do and what to avoid.
Any pictures would be greatly appreciated.
Thanks!

_________________
"An adventure is only an inconvenience rightly considered. An inconvenience is an adventure wrongly considered." G. K. Chesterton.

[QUOTE=SteveS] I just bought a dial indicator and I'm about to start measuring stiffness.[/QUOTE]

Steve,

Given your post title and all of the spam viagra e-mails I seem to be getting, you had me wondering for a moment there bro

_________________
Dave White
De Faoite Stringed Instruments
". . . the one thing a machine just can't do is give you character and personalities and sometimes that comes with flaws, but it always comes with humanity" Monty Don talking about hand weaving, "Mastercrafts", Weaving, BBC March 2010

For braces I would use 12" spacing on the support rods, as most of your critical brace lengths on Om-dreads will be in the 14"-18" finished length. this way your measuring close to the beam length you are actually using. Whatever means of deflection weight you use needs to remain constant in both mass and area so that you always compare apples to apples. Also I would rig my test fixture to measure on the bottom edge as opposed to the top edge so that you can measure dead on the apex of the deflection. If you measure from the top you have move of the apex to clear the weights foot print.

Steve,
The key is consistency. Make sure your dowels are always in the same place
and the weight you use is the same for every measurement of a certain kind.
I have my 'dowels' (actually metal tubes from the scrap yard) set at 16" apart
for measuring top plate deflection. The weight I use is a big blue bolt (scrap
yard). I have no idea how much it weighs, but I always use the same bolt so
my numbers are consistent. Here's my set up:

duuh. Why do you need this thing?

_________________
"Preoccupation with an effect gives it power and enhances the error"
from "Your Owner's Manual" by Burt Hotchkiss.

To verify deflection in tops, backs or braces stock to use as a relative baseline comparison of stiffness properties

[QUOTE=nickton] duuh. Why do you need this thing?[/QUOTE]


You want to measure longitudinal stiffness. Depending on what methodology you are using this helps to determine target thickness of your plate.

_________________
Brock Poling
Columbus, Ohio
http://www.polingguitars.com

Very nice looking jig/setup Dave.

_________________
Tickle your guitar daily, and it'll tickle you back.

Dave Hurd's 'Left Brain Lutherie' book goes into excruciating detail on this, at all stages of construction. He includes equations for finding things like the Young's modulus from your deflection data, which makes it possible for you to compare your results with other folks, and thus learn faster.

[QUOTE=MichaelP] .....I would rig my test fixture to measure on the bottom edge as opposed to the top edge so that you can measure dead on the apex of the deflection. [/QUOTE] Thanks,
Point taken. That shouldn't bee too hard to do.

_________________
"An adventure is only an inconvenience rightly considered. An inconvenience is an adventure wrongly considered." G. K. Chesterton.

So how thick does a spruce top generally need to be? Do you need this device to figure it out every time? I also wonder how this affects bracing size and placement. I have much to learn obviously. Someday I may get that book...I'm just learning to fly by the seat of my pants and duplicating what's on my other guitars.

_________________
"Preoccupation with an effect gives it power and enhances the error"
from "Your Owner's Manual" by Burt Hotchkiss.

Steve! Your a noob? No way - I've seen your work!

What are the main findings of stiffness? You want high/low/in between. Intuitiveness, sounds as if it may be unreliable in this case.(Now that is a noob question)

Billy Dean Thomas

_________________
Billy Dean Thomas
Covina, CA

"Multi famam, conscientiam, pauci verentur."
(Many fear their reputation, few their conscience)

[QUOTE=nickton]   So how thick does a spruce top generally need to be? Do you need this device to figure it out every time? I also wonder how this affects bracing size and placement. I have much to learn obviously. Someday I may get that book...I'm just learning to fly by the seat of my pants and duplicating what's on my other guitars.[/QUOTE]

Again, it depends on your philosophy. Some build to target thickness. Once it hits .XXX" it is time to stop.

Others build to target deflections. Put the plate on the deflection rig, test it, sand, retest it, etc. Until you hit a given deflection. In this situation every plate is different. Some might be .125" others might be .068" (as the crowd feints... ). This methodology takes into consideration that every piece of wood is different.

Brock Poling38995.0489814815

_________________
Brock Poling
Columbus, Ohio
http://www.polingguitars.com

Stiffness _ratio_ counts too. With a top that has higher crosswise stiffness you can rune the braces more 'along' the grain, and make the top a little lighter. It's a game of inches, or, maybe, grams.