Deflection testing

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phillycyberjoe
Posts: 4
Joined: Fri Apr 19, 2013 9:57 am

Deflection testing

Post by phillycyberjoe »

Hello All,
I just joined this forum the other day and have been really enjoying reading through all the valuable user contributed content. I am a born and raised Philly guy and I now live a bit NW of the city in Horsham, Pa.

I have built a couple of kit guitars from StewMac and was pretty happy and proud of them. I tried building a Tele style electric from scratch and I must say that I am thrilled with how well it came out and how well it plays.

Today, I guess I can say that I officially started a scratch build of an acoustic since I jointed two really nice Carpathian Spruce tops. Since I've never done this before, I've been reading a bunch about finding the desired thicknesses of various tops. While reading, I stumbled upon what John had mentioned about deflection testing and would like to learn more about it. It would seem to me that testing deflection would reduce a significant amount of variability while trying to find the optimum thickness.

It would be great it someone could share some specifics about how to build a simple deflection testing jig and how best to use it.

Thanks in advance for any help!!!

Joe
tippie53
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Location: Hegins, Pa
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Re: Deflection testing

Post by tippie53 »

the deflection test allows you to use a ratio of pre and post deflections . If you use a given weight at the post pre brace and post brace , you can find a relationship between the 2. When one is divided by the other you can find a math relationship.
It is up to you to find the one that makes your ears happy. I use a standard thickness top of .117 on forward shift and .105 for rear brace. What the ratio you want is tricky. I like to measure the movement pre brace. After brace with the weight I want to see the deflection at the point of the saddle.
I adjust the braces to the point of movement. For me about a 2lb weight.
John Hall
Blues Creek Guitars Inc
Authorized CF Martin Repair Center
president of Association of Stringed Instrument Artisans
http://www.bluescreekguitars.com
B. Howard
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Re: Deflection testing

Post by B. Howard »

I use a2.5 kilo test weight. I set my plate thicknesses by deflection. They tend to range from about.95" to.115" thick. I then do another set of deflection tests to tune my braced top to get the movement I want, these tests are conducted in both directions and must fall into a very tight set of parameters.
You never know what you are capable of until you actually try....

Brian Howard
www.brianhowardguitars.com
Taylor authorized service
Custom finishing services

Brian howard's guitar building & repair blog
http://www.brianhowardguitars.com
tippie53
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Re: Deflection testing

Post by tippie53 »

the key here is that you have to find the ratio that give you the sound you are looking for. You need to understand that this isn't about making the top floppy , if that were the case we would be bracing paper.
When you start with a raw top and do deflection you have to be consistent in your methods. What those methods are , you decide.

I use a set distance between a support and a given weight placed to the center.Once braced I apply the weight and I want to see movement upon shaping braces.

I will admit that I am mostly trusting my eyes and hands to my patterns . A too stiff top can actually make the guitar feel harder to play.
John Hall
Blues Creek Guitars Inc
Authorized CF Martin Repair Center
president of Association of Stringed Instrument Artisans
http://www.bluescreekguitars.com
B. Howard
Posts: 709
Joined: Wed Jan 11, 2012 9:42 pm
Location: Hummelstown, PA
Contact:

Re: Deflection testing

Post by B. Howard »

tippie53 wrote: When you start with a raw top and do deflection you have to be consistent in your methods. What those methods are , you decide.
That sums it up very nicely. There are no standards for this, everyone who does deflection testing is working by heir own formulas and with their own methods.
You never know what you are capable of until you actually try....

Brian Howard
www.brianhowardguitars.com
Taylor authorized service
Custom finishing services

Brian howard's guitar building & repair blog
http://www.brianhowardguitars.com
phillycyberjoe
Posts: 4
Joined: Fri Apr 19, 2013 9:57 am

Re: Deflection testing

Post by phillycyberjoe »

Thank you gentlemen......I think I've got it.

If I consistently use a fixed amount of weight on an unbraced plate and reach a certain amount of deflection and the resulting sound is pleasing to me, then I should keep sanding future tops and backs until I reach the same deflection regardless of thickness.....within reason. Obviously, this has everything to do with the stiffness of the wood species being used.

I could then apply that same methodology to pre and post braced plates to find the ratio and then do my thickness sanding to that target ratio.

If I understand this correctly, this should help with consistency of thickness sanding. Did I get it right??

Thanks again,
Joe
phillycyberjoe
Posts: 4
Joined: Fri Apr 19, 2013 9:57 am

Re: Deflection testing

Post by phillycyberjoe »

Gentlemen,
I crafted a very carefully worded reply that was fit for a Pulitzer, to let you know that I understand and appreciate your explanations. Somehow it didn't get posted.

It would hurt my brain to try to do that again, so let me say that I thank you for deflection explanation and I really do get it....it's all relative!!

Thanks again,
Joe
 

Re: Deflection testing

Post by   »

phillycyberjoe wrote:Gentlemen,
I crafted a very carefully worded reply that was fit for a Pulitzer, to let you know that I understand and appreciate your explanations. Somehow it didn't get posted.

It would hurt my brain to try to do that again, so let me say that I thank you for deflection explanation and I really do get it....it's all relative!!

Thanks again,
Joe
Joe,
Since that was only your second post on the forum it needed to be approved. All noobs are subject to approval on their first two posts. The post was approved so you're a veteran now and no longer a rookie.
As for Pulitzers, we don't give out no steenkin' Pulitzers. We give out 'TIPPIE' awards infrequently. Very infrequently.
Now.......please put your modesty aside and post the links to your CNC machine photos. I know they're out there............
-tommy
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