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What could be going wrong here? Top distortion behind bridge
http://mowrystrings.luthiersforum.com/forum/viewtopic.php?f=10101&t=51336		 Page 1 of 4
Author:  jfmckenna [ Fri Dec 14, 2018 12:37 pm ]
Post subject:  What could be going wrong here? Top distortion behind bridge
I'm struggling to figure this out but I need to make this a learning experience and move on. Here are the details:

1) Top is Sitka
2) X-Brace at 96deg and 1/4in bracing Sitka
3) Bridge patch: Flat Sawn Osage Orange laminated to QS Spruce (grain direction of spruce same as top)
4) Bridge is BRW and the x-Brace extend through the lower corner of bridge wings
5) straight bracing, not scalloped

None the less the top is extremely distorted at the bridge corners. I use deflection testing for the top and this top I have made stiffer then most. In fact the last guitar I built deflected twice as much and has no problems and is of the same design.

I double and triple checked and the bracing is definitely not loose.

The only thing I can think of is that the X is twisting. The distortion is right on top of the X-Bracing.

Here is a pic of the bass side:

Image

Here is the treble side, It's harder to see here because it's not as bad:

Image

Here is a shot of the bracing:

Image

This is extremely disappointing and I'm struggling to figure out what the problem here is, any input would be greatly appreciated. I have no other choice but to retop this guitar. But I want to learn from this and never let it happen again.

Regards.
Author:  meddlingfool [ Fri Dec 14, 2018 12:51 pm ]
Post subject:  Re: What could be going wrong here? Top distortion behind br
Wish I had an answer for you.

Why do you feel it needs to be retopped?
Author:  johnparchem [ Fri Dec 14, 2018 1:00 pm ]
Post subject:  Re: What could be going wrong here? Top distortion behind br
Was the top plate thickness in your normal range? The braces can make the top stiff as read by a deflection test, while a thin top can telegraph the braces and\or stress points around the bridge. I had to retop my 8 string nylon crossover because the top was too thin. It did sound nice though, I did not want it going to a client that way.
Author:  DennisK [ Fri Dec 14, 2018 1:09 pm ]
Post subject:  Re: What could be going wrong here? Top distortion behind br
Flat bridge glued to radiused soundboard with brute force clamping pressure?
Author:  jfmckenna [ Fri Dec 14, 2018 1:39 pm ]
Post subject:  Re: What could be going wrong here? Top distortion behind br
johnparchem wrote:
Was the top plate thickness in your normal range? The braces can make the top stiff as read by a deflection test, while a thin top can telegraph the braces and\or stress points around the bridge. I had to retop my 8 string nylon crossover because the top was too thin. It did sound nice though, I did not want it going to a client that way.

That's exactly the position I am in. This is not deliverable. I don't even really measure thickness. Well, I do and I enter it in my book, but none the less I hold deflection as the more valuable measurement when designing the top. And that's why it's confusing because this top was stiffer then what I usually go for because I was going for a more treble tone.

Ed, The pics don't do it justice. If you run your finger over this you can really feel it. I usually go for a bit of bridge rotation and top belly but in this case there is something else going on.

Dennis, I forgot to mention, both the bridge and the bridge plate were arched to accept the top. The bridge and plate were glued down with an arched caul as well.
Author:  meddlingfool [ Fri Dec 14, 2018 1:45 pm ]
Post subject:  Re: What could be going wrong here? Top distortion behind br
Yes, I could see that being a problem if it's for a client. Just out of curiosity, did you measure bridge rotation? Is a laminated bridge plate your usual practice?
Author:  jfmckenna [ Fri Dec 14, 2018 1:49 pm ]
Post subject:  Re: What could be going wrong here? Top distortion behind br
meddlingfool wrote:
Yes, I could see that being a problem if it's for a client. Just out of curiosity, did you measure bridge rotation? Is a laminated bridge plate your usual practice?

No it's not a usual practice but I have done it before including the last one which was essentially the same as this one only the top was twice less stiff... Hence my confusion.
Author:  DennisK [ Fri Dec 14, 2018 1:53 pm ]
Post subject:  Re: What could be going wrong here? Top distortion behind br
Hmm, this really is a stumper. How long has it been strung up? How much distortion remains when the strings are loosened?
Author:  LarryH [ Fri Dec 14, 2018 2:16 pm ]
Post subject:  Re: What could be going wrong here? Top distortion behind br
That's a real stumper. Can you please let us know what you discover?
Author:  jfmckenna [ Fri Dec 14, 2018 2:32 pm ]
Post subject:  Re: What could be going wrong here? Top distortion behind br
DennisK wrote:
Hmm, this really is a stumper. How long has it been strung up? How much distortion remains when the strings are loosened?

Good question. I strung it up for a week now and it slowly progressed to this point and stopped. The distortion almost goes away when the strings are off but it's still there.
Author:  DanKirkland [ Fri Dec 14, 2018 3:10 pm ]
Post subject:  Re: What could be going wrong here? Top distortion behind br
Some reason I can't see the pics?
Author:  SteveSmith [ Fri Dec 14, 2018 3:20 pm ]
Post subject:  Re: What could be going wrong here? Top distortion behind br
DanKirkland wrote:
Some reason I can't see the pics?
Me either.
Author:  Michaeldc [ Fri Dec 14, 2018 3:38 pm ]
Post subject:  Re: What could be going wrong here? Top distortion behind br
Me either
Author:  Herr Dalbergia [ Fri Dec 14, 2018 4:06 pm ]
Post subject:  Re: What could be going wrong here? Top distortion behind br
I also can not see the pictures!
Author:  J De Rocher [ Fri Dec 14, 2018 4:57 pm ]
Post subject:  Re: What could be going wrong here? Top distortion behind br
I can't see them either, but they were visible this morning.
Author:  StevenWheeler [ Fri Dec 14, 2018 5:07 pm ]
Post subject:  Re: What could be going wrong here? Top distortion behind br
Since I can't see the pics either, I'm going to take a complete stab in the dark.
Is your bridge patch in the correct position? With a hardwood patch I shoot for 1/8" in front and behind the bridge, when doing a softwood laminate I add another 1/8" in either direction.

Steve
Author:  jfmckenna [ Fri Dec 14, 2018 5:29 pm ]
Post subject:  Re: What could be going wrong here? Top distortion behind br
I hate Google Photo's!

I re-posted the images. Hopefully you can see now.
Author:  Michaeldc [ Fri Dec 14, 2018 5:40 pm ]
Post subject:  Re: What could be going wrong here? Top distortion behind br
jfmckenna wrote:
I hate Google Photo's!

I re-posted the images. Hopefully you can see now.


I can see the first two. The bracing shot is still missing on my feed anyway.
Author:  DennisK [ Fri Dec 14, 2018 5:55 pm ]
Post subject:  Re: What could be going wrong here? Top distortion behind br
Yep, the two outside photos are showing again now, but not the bracing.
StevenWheeler wrote:
Since I can't see the pics either, I'm going to take a complete stab in the dark.
Is your bridge patch in the correct position? With a hardwood patch I shoot for 1/8" in front and behind the bridge, when doing a softwood laminate I add another 1/8" in either direction.

Steve

I saw the bracing picture earlier, and the bridge patch extends plenty far behind the bridge. The distortion is happening right on the bridge patch, which is part of why it's so puzzling. It shouldn't be possible for such a thick sandwich of material to deflect so much.

jfmckenna, check again that the lower corners of the bridge patch are still glued down. See if you can stick one hand inside the box, hook your finger around to get your nail at the potential gap, and then push down on the soundboard from outside to try and flex it open.
Author:  bionta [ Fri Dec 14, 2018 6:03 pm ]
Post subject:  Re: What could be going wrong here? Top distortion behind br
Might there be a void in the bridge patch lamination? Layers separating?
Author:  Joe Beaver [ Fri Dec 14, 2018 6:18 pm ]
Post subject:  Re: What could be going wrong here? Top distortion behind br
That is strange. The deflection appears to be right over, or at least close to the x, but it doesn't seem to be influenced by the x. Strange indeed.
Author:  DanKirkland [ Fri Dec 14, 2018 6:30 pm ]
Post subject:  Re: What could be going wrong here? Top distortion behind br
So some thoughts and I'm just going to throw these out for discussion sake.

That kind of distorion almost always happens when a brace isn't glued well enough. I've fixed it on numerous guitars. I'm not trying to sound accusatory but are you 150% certain that the braces are glued well enough?

On the bridge. Possible that your glue joint is stronger on the wings than the middle?

On the bridge plate. What's the grain orientation of the osage piece?
Author:  Clay S. [ Fri Dec 14, 2018 6:35 pm ]
Post subject:  Re: What could be going wrong here? Top distortion behind br
Do you do perimeter sanding? The top might be thin around the edge. If the top regains it's shape when you loosen the strings you could add a tall finger brace (a.k.a. side tone bar) where the deformation is occurring and see if that helps.
Author:  Haans [ Fri Dec 14, 2018 7:38 pm ]
Post subject:  Re: What could be going wrong here? Top distortion behind br
Can't see any but 3 photos JF...
Author:  jfmckenna [ Fri Dec 14, 2018 7:40 pm ]
Post subject:  Re: What could be going wrong here? Top distortion behind br
I tried posting the pic again.

I know what you are thinking because I had the same thoughts but the Bridge patch is absolutely 100% not coming apart. The braces are not loose either.

I do normally thin the edges but not on this one :D

The osage is flat sawn, spruce is QS along the lines of the top. Nice stiff and light plate it was.

I'm thinking that the X is actually twisting and that a 1/4in X-Brace at 96deg angle is too much. Thoughts?
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