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PostPosted: Mon Jul 10, 2017 5:15 pm 
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From what I understand, neither the bridge or the bridge plate are radiused. The issue is shrinkage of the top. I know that of the old guitars that I'm working on, all are almost dead flat. I have 3 bridges, one taken off of a 1944 Martin, and 2 from 1953 Martin's. I checked, and indeed, all 3 are flat as a pancake with no radiuses.

It makes sense when you think of what happens when guitars meet low humidity, or age. The cells condense, and shrinkage occurs. The only thing creating the top radius are radiused braces, which are made of wood comparable to your top wood. They give. Plates and especially bridges, are made of much denser woods. So, if the top shrinks along with the braces, you end up with a flat top, or in many cases, a sunken top, while the dense woods maintains their original radiuses..

So, which gives first, a spruce top or ebony, under these circumstances? Like everything else, I'm sure that other build practices apply and may cause variation in this scenario.

It makes enough sense to me that I'm no longer going to radius my plates or bridges.


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PostPosted: Mon Jul 10, 2017 6:02 pm 
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Joined: Sat May 28, 2016 8:48 am
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Location: Savannah, GA
This is a very interesting and thought provoking discussion. I've only made 4 so far, but I haven't radiused any of the plates. However, on #1, the wings of the bridge started to immediately separate from the top, and I took it off, radiused it and re-glued it and it is holding strong. So...I just put a lot of effort into doing the same prior to gluing up my last few bridges. Am I getting myself into trouble here?

V/R
Ken


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PostPosted: Mon Jul 10, 2017 6:27 pm 
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Location: Hegins, Pa
2 questions
A how did you glue it
B are you sure the winds didn't warp

usually bridges lift for a bad glue joint or warpage.

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Blues Creek Guitars Inc
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PostPosted: Mon Jul 10, 2017 6:36 pm 
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Location: Savannah, GA
John,

I glued it with titebond. But...I noticed when I put the clamps on the first one that they had to force down the wings of the bridge. They were dead flat when I put it on, but the top had the radius and they didn't sit flush. Bill Corey's book also said to sand the bottom of the bridge so it matched the top...and I did that and it held. So...I just did it again...3 times :-/

V/R
Ken


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PostPosted: Mon Jul 10, 2017 7:20 pm 
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This is really interesting, and perhaps an illustration of how counter-intuitive some things can be.

I've radiused my bridges, the goal being a better glue joint, Ordinarily, that's the thing to strive for. But John, I guess you're saying to look out 50 or 75 years in the future and to consider what is going to happen to the top as it ages.

Now, will there be a problem of cracking only if the top dries out? Would there be a problem if one were able to keep the guitar at 50% humidity for 75 years? The odds of keeping the guitar at ideal humidity for 75 years are pretty slim, of course. But assuming one did...would the top still shrink and crack just from the effects of aging that occur in a piece of wood? How old would a piece of wood have to be before you could make a top from it and not have it shrink any more from aging?

Are torrified tops more stable?

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PostPosted: Mon Jul 10, 2017 8:09 pm 
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if you choose to radius a bridge you are more than welcome to do that but be sure you also radius the bridge plate.

I just don't see an advantage to it. There are many interweb falicies out there . If you look at a top that just came out of a dish that is 28 ft rad I bet it isn't even 22

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Blues Creek Guitars Inc
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http://www.bluescreekguitars.com


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PostPosted: Wed Jul 12, 2017 10:48 pm 
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Location: Pittsburgh PA suburbs
Hmm. I didn't radius the bridge plate on my first and only build. I did put a sheet of sandpaper on the top of the guitar body to sand the underside of the bridge though. Is that technically radiusing the bridge?

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PostPosted: Wed Jul 12, 2017 11:30 pm 
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Location: Visalia, CA
I have to admit that this a confusing subject. If I glue a flat bridge to a radiused top, and the wood moves; as in swelling, shrinking, etc, how is it better or diffetent than the two surfaces mating to each other? If the two unmated wood surfaces, one or the other, isn't it just as likely that there can or will be a failure of the joint, or cracking, breaking......
I'm sorry I just don't get it. Not for argument sake, it just isn't making sense to me.


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PostPosted: Thu Jul 13, 2017 6:38 am 
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Location: Chadds Ford, PA
This seems like an opportunity to over-engineer a guitar. I didn't even remember that tops were arched 4 years ago (I had done this thru Irving Sloane's book when I made guitars 1975-1977) and certainly was pre-contemplative until going to Blues Creek 1.5 yrs ago. Consequently, I don't have any experience with radiusing bridgeplates, but for the bridges like nkwak I conform the bridge gluing surface with sand paper on the finished top. I have noticed that at the vacuum glue-up stations at Martin that the bridgeplates are not radiused as far as I can tell and from the seconds I buy there are generally flexible enough to take on the top shape when glued. And also the 28' curvature in those 6 inches has to be tiny anyhow.


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PostPosted: Thu Jul 13, 2017 6:48 am 
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Joined: Fri Mar 03, 2006 7:09 pm
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Location: Hegins, Pa
if you radius a bridge you now have a downward radius.

under low RH the top will then shrink and this downward stress applied to the top will cause it to focus a force at the ends of the bridge.

a flatter bridge won't apply as much of a focused stress riser. Can you radius a bridge ? sure but of the many builders I know , not many do it.

Wayne Henderson doesn't
John Arnold doesn't
David Nichols doesn't


you may if you wish.

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Blues Creek Guitars Inc
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Board of Directors of Association of Stringed Instrument Artisans
http://www.bluescreekguitars.com


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