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Disaster strikes a sentimental guitar top
http://w-ww.luthiersforum.com/forum/viewtopic.php?f=10102&t=12807		 Page 1 of 1
Author:  luthier24 [ Thu Jul 12, 2007 1:53 am ]
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I built this guitar for my son about a year age and evidently made the top a bit too thin This is particularly disappointing because my son and I had a father-son weekend last year and we flew up to Williamsburg and visited Steve at Colonial Tonewoods where my son picked out the woods for his guitar. He loved the Sitka top and Ziricote back and sides. When I originally built the guitar, I had to redo the top finish so the result was thinner than designed. More recently, the bridge was pulling up and the top was arching significantly. I added a transverse brace under the arching area and re-glued the bridge (again after some additional sanding). Well you can see the disastrous result.



My son really wants this guitar with a new top. Are there any suggestions how to best remove the top and what to be aware of? I have used a steam source to loosen PVA glue in the past and think that may be the best approach.
Author:  joe white(old) [ Thu Jul 12, 2007 2:14 am ]
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I can't offer any technical advise but I can't tell you how shocking it was to look at that photo! I hope it can be salvaged and returned to it's former condition better then before.
Author:  WaddyThomson [ Thu Jul 12, 2007 2:41 am ]
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Sorta gives one a sick feeling inside to see that.  I don't have any advice either, but I'd bet someone will. 
Author:  JB Dickey [ Thu Jul 12, 2007 2:48 am ]
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Mark, We feel your pain on this guitar.  It's a sad end to a great effort to build that special instrument for your son.  But, there must be a silver lining to the effort too.  That's what you are looking for, and sharing this with those of us on the OLF will serve to instruct other new luthiers.


I've not had to replace a top, but others who've discussed it here have used a clothes iron to heat the joint and a palette knife to slip along and seperate the joint.  Of course, you are faced with removing the fingerboard or the entire neck too.


One thing I'd like to see is a shot of your bridge face on.  What is it's shape?  How large is it in relation to a standard Martin style bridge.  The reason I mention that, is quantity of glue surface must be considered.


I will have to say, the spruce looks thin in the pictures.  I try to get close to the .110 standard thickness, could you get a caliper on there and tell us your actual thickness?  That would help us understand the failure of the top.


Eventual outcome, a new top, and refinishing, and a happy father and son with the NEW and IMPROVED guitar.  Please share this rebuild on this thread.  We're all eyes and ears.  Best of luck, bd
Author:  Pwoolson [ Thu Jul 12, 2007 3:12 am ]
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Re topping is not that huge of an ordeal. Before you try any steam on heat, you need to get the binding off. (Or route inside it like John Greven does but that's just insane) Run the top through the binding/routing process again. Now you can see the edge and get in there with a hot knife. One thing I've done in the past is to set the router to cut a very deep channel. Then set the depth the same as the thickness of the top. Route around stopping at the neck and tail blocks. You don't want to try to have the entire thing come free while routing. Then hot knife the block areas off. Good luck!
Author:  John How [ Thu Jul 12, 2007 3:49 am ]
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[QUOTE=Pwoolson] You don't want to try to have the entire thing come free while routing. Then hot knife the block areas off. Good luck![/QUOTE]

Ye, this is very important because if you rout too deeply then the top will loose it's support, sag and you'll gouge into the sides & linings too deeply. I'd rout about 3/4 into your lining width and then use the hot knife method as Paul described.
Author:  Don Williams [ Thu Jul 12, 2007 4:26 am ]
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Exactly as Paul and John describe it. Deal with the bindings first, then carefully work at getting the top off.

I have to second Bruce's request to see the shape of the bridge. A lot of folks don't realize the the shape and size of a bridge is important as to how good a glue joint you will get, and how it will mechanically resist the pulling action of the strings. Too pointy in the back middle and you create a stress riser that can cause a failure.
Author:  Michael Dale Payne [ Thu Jul 12, 2007 4:39 am ]
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I have two questions.

First is does the X-brace support the wings of the bridge? I can't tell from the picture. If not this is an important issue.

Two, was there a bridge plate installed? I assume there was but again I'm not sure I see signs of either in the picture signs of it in the picture.

The reason I mention this is that this is exactly the failure I would expect if the bridge plate was not installed or if the bridge plate was installed but the X-brace did not cross under a good portion of the bridge wings
Author:  Steve Saville [ Thu Jul 12, 2007 4:40 am ]
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I disagree about the top being too thin. I know of builders that go a lot less than .110" with great long term stability. How thick was that top?
It looks to me like the bridge is a bit small, I can't tell with out a top view of it.
The most obvious thing I see is what looks like a very small bridge plate.
It also looks like the saddle is pretty high, creating a lot of torque on the saddle.
Just a few guesses about the core issue. Best wishes with the repair.
Author:  TonyKarol [ Thu Jul 12, 2007 4:43 am ]
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I feel the pain as well .. thats gotta hurt.

Another thing to consider when re-topping - it looks to me like you have a fairly high (thick) bridge, and a lot of saddle showing - this combo adds a HUGE torgue to the top at the bridge, and IMO is too high (maybe you cold measure and tell us what that measures ...

I would consider either arching the top a touch more or less - hard to tell (cant tell if you used a radius bowl to build this one) and /or possibly re adjusting the neck angle - ideally you want he strings to be about 1/2 inch above the plane of the top at the saddle.
Author:  luthier24 [ Thu Jul 12, 2007 5:56 am ]
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What excellent points you OLFers bring up. The saddle is high and the string break point angle is steep. To answer your questions the bridge, bridge plate, and bracing pattern are remarkably similar to James Goodall's Grand Concert (sorry James but I own and love your RGCC model).

The bridge is 1-1/2" by 6" by 3/8" tall. The bridge plate extends 1/8" beyond the bride and touches the X braces. The X bracing is directly beneath the corners (pointy wings - similar to a Taylor bridge) of the bridge.

The top was originally sized to be 120/1000" in the center/bridge area, graduating down to 90/1000" around the perimeter. Because of issues I mentioned in my original post, the center is signifcantly thinner than planned! I am not exactly sure what the top radius is but I planned for 25'. Does this give anybody any more clues?
Author:  Barry Daniels [ Thu Jul 12, 2007 5:59 am ]
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It looks like the bridge plate ends at the back edge of the saddle which would have created a significant stress riser at that point. This is further confirmed by the location of the fracture. On the new top, I would recommend that you extend the bridge plate at least 1/4" past the bridge.
Author:  burbank [ Thu Jul 12, 2007 6:07 am ]
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Too bad.

The top doesn't look all that thin to me either, but the saddle is a bit tall, which might have put the strings a bit far above the top.


There' a tutorial that's been posted here recently where the top is taken off with the bindings left on the body, the new top is cut to fit so that the bindings and side finish are not disturbed. It's very slick. I'll see if I can dig it up.
Author:  Pwoolson [ Thu Jul 12, 2007 6:22 am ]
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[QUOTE=burbank] Too bad.

The top doesn't look all that thin to me either, but the saddle is a bit tall, which might have put the strings a bit far above the top.


There' a tutorial that's been posted here recently where the top is taken off with the bindings left on the body, the new top is cut to fit so that the bindings and side finish are not disturbed. It's very slick. I'll see if I can dig it up.[/QUOTE]
It's John Greven if that helps your search.
Author:  burbank [ Thu Jul 12, 2007 6:24 am ]
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Here's a link to John Greven's site.

Paul, can you elaborate on your take on Greven's method? do you see undue risk?

Author:  James Ringelspaugh [ Thu Jul 12, 2007 6:26 am ]
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For a guitar where I wanted to save the purfling I cut the top off at the bottom of the bindings with a simple jig consisting of a few blocks of wood and an exacto-type blade.



I simply took it around the guitar in many many passes:



When I got to the heel and tail blocks I very carefully sawed through them with a blade from a dozuki saw. I wore a leather glove for a little stability and to keep from getting cut:



I had to take off an equivalent amount off the neck at the heelcap as well. If I didn't want to save the pearl in the bindings I would have just routed it off.
Author:  Pwoolson [ Thu Jul 12, 2007 6:30 am ]
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[QUOTE=burbank] Here's a link to John Greven's site.

Paul, can you elaborate on your take on Greven's method? do you see undue risk?

[/QUOTE]
My take...undue risk? No I don't see it that way. I see it as an amazing amount of craftsmanship to be able to pull it off. I'm not sure I could pull it off. But it's a cool technique nonetheless. The next time I have to re top something, I might give it a whirl.
Author:  Don Williams [ Thu Jul 12, 2007 6:47 am ]
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John Greven's method is brilliant, but requires very careful attention to the setup of tooling, not to mention a good amount of patience and skill to pull it off. I agree with Paul completely. Undo risk? No. The top is already wrecked, and if you slip up and have to rebind it, then so be it. But if you do it right, that's a slick repair job. Very very clever.
Author:  treblemaker [ Thu Jul 12, 2007 6:48 am ]
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Many issues at work here....

That top does indeed look too thin in the bridge area. Also, as Barry alluded to, the plate seems to run short of the rear of the bridge. Not good.

But look at something else. Why didn't the bridge plate come up with the bridge? See, if solid, un-slotted pins would have properly fitted, the strings' relentless "pull" would have been spread out over a larger area, wit the bridge/top/bridge plate creating a mechanical "sandwich", and this top would likely still be holding up fine. What happened here is that the glue was holding everything due to poorly fitted pins, and when some of the glue joint failed(the area under the bass side), the area where the glue held, simply ripped the poor top apart because all the 'pull' was spread over a small area, less than half the bridge.

The above was compounded by a tall bridge, and even taller saddle, which multiplied the strings' 'pull' greatly.

She was doomed from the start, but y'all can learn from this.

To remove the top, pull the neck off, then if the bindings are plastic, remove them(they'll pull off easily), if they're wood, continue as such: put a 1/2" rabbet bit in your router(meaning, a rabbet set wit a bearing that will allow a 1/2" cut), and set the depth to just shy of the thickness of the top. Now route the entire perimeter. You should avoid cutting the linings at any time, but you should be able to see them through the paper-thin spruce. The top will still be held at the blocks, but not for long: grab the top at the soundhole, ad give it a solid tug! You'll have the top in your hand, and the boat is ready for its new top. Takes but a minute, and is very safe, unlike knives, saws, and steam. I couldn't let you saw away at that guitar...

Sorry this had to happen on this guitar, but there's a good lesson there for everyone. The details are what make "it", and what break "it", and always for a good reason.



Author:  JohnAbercrombie [ Thu Jul 12, 2007 6:50 am ]
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Mark-
Sorry to see your 'bad luck' but I'm learning a lot from the expert responses you're getting. The OLF is a wonderful place, with the experts who give so freely of their experience. (Thanks!)

I'd agree (and I'm no expert) that looking at the bridge plate size/location is an important issue here.
Another thing I thought about was whether you had scored around the bridge before removing finish from the top, prior to gluing. I don't know if that is the construction sequence you used, but it can cause problems. I recently took a class with Sergei deJonge and he insisted that we score the finish with a pin (sewing pin), not a knife. He recounted instances where builders had cut into the top wood using a marking knife. A knife cut into the top wood, combined with a bridge plate which didn't span the bridge edge, could provide the starting conditions for the failure shown in your photo.

Please keep us posted with your progress on the top replacement job. That's an excellent close-up photo, BTW- thanks!

Cheers

John
Author:  Michael Dale Payne [ Thu Jul 12, 2007 8:03 am ]
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I can't speak for James Goodall but I think you will find most successful designs have the X- brace passing well inside the outer edges of the bridge wings. To me this is both structural and tonal. I agree that you bridge looks in this photo too stubby both too high and Too narrow. After looking at the picture carefully I see the bridge plate. but the X-brace to the out side of the bridge wings is an issue in my humble opinion.
Author:  Rod True [ Thu Jul 12, 2007 8:46 am ]
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I'll add a couple more thing in the mix here.

Looking at the picture, specifically under the bridge and between the top and bridge plate.



First between the top and bridge plate. No offence but there doesn't appear to have been enough glue when you glued the bridge plate on. Otherwise that joint wouldn't have failed like it shows in the picture.

Also, when you went to glue the bridge on, did you have to remove the finish first? It looks like the top might have been scored enough for the bridge to rip off the top. Look at the left side at the bottom of the bridge, you see the fibres from the spruce.

If the top was scored when scoring the finish for removal and the bridge plate not adhering to the top from the joint being starved of glue, I could see this happening.

As far as removal of the top goes, forget about the binding if they are wood and do what treblemaker says, it's the easiest by far.

[quote=treblemaker]To remove the top, pull the neck off, then if the bindings are plastic, remove them(they'll pull off easily), if they're wood, continue as such: put a 1/2" rabbet bit in your router(meaning, a rabbet set wit a bearing that will allow a 1/2" cut), and set the depth to just shy of the thickness of the top. Now route the entire perimeter. You should avoid cutting the linings at any time, but you should be able to see them through the paper-thin spruce. The top will still be held at the blocks, but not for long: grab the top at the soundhole, ad give it a solid tug! You'll have the top in your hand, and the boat is ready for its new top. Takes but a minute, and is very safe, unlike knives, saws, and steam. I couldn't let you saw away at that guitar... [/quote]

You might be able to save the paua rosette there too once the top is off.
Author:  PaulB [ Thu Jul 12, 2007 10:23 am ]
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Yeah, you should be able to save that rosette, been there done that.

I'm not glad this happened to you. But I am glad that you've shown the pics and gotten a critique on what happened and ideas as to why it happened.

I love the OLF, but IMHO we don't see enough structural failures. Do they hardly ever occur? Or, are people reluctant to show where they went wrong?. We can all learn as much from structural failures (and the reasons for them) as we can from building technique tutorials.

Looking forward to seeing the reair, good luck!
Author:  Colonial Tonwds [ Thu Jul 12, 2007 10:31 am ]
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Mark,
Call me..I can work magic
Author:  Kent Chasson [ Thu Jul 12, 2007 12:20 pm ]
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In case 2 more cents are needed to make a dollar...


By looking at the photo I'd say that Rod mentioned a big factor, scoring the top when scoring around the bridge.  High saddle, thin top certainly contribute but look at the left side of the bridge.  There is a nice layer of spruce attached to the bridge and absolutely no damage to the top behind the bridge.  The top was almost certainly scored at least .010" deep and the top peeled.


Another issue is runout.  Look at the contrast between the left and right side of the bridge.  The grain is going down on the right side and presumably up on the left.  The break is totally different.


Best of luck putting on a new top.  As someone said, it's not such a huge job.
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