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| Repairing Top Surface For Bridge Replacement http://mowrystrings.luthiersforum.com/forum/viewtopic.php?f=10101&t=55896 |
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| Author: | Rick Cowan [ Tue Sep 12, 2023 10:17 am ] |
| Post subject: | Repairing Top Surface For Bridge Replacement |
Hello All, I am getting back to a bridge replacement I started some time ago. FYI, this is the same bridge job I referenced in a recent post regarding a possible neck reset (http://luthiersforum.com/forum/viewtopic.php?f=10101&t=55880). The bridge had lifted quite severely and the bridge plate was split from the torque induced by the string tension and pulling the bridge pins forward. Once I got the bridge off I could see that someone had previously repaired a section of the top where E, A and D bridge pins are located. There was also a fair bit of damage to the surface from the bridge removal (some may have been me; some from the previous repair): Attachment: IMG_3246-cropped-reduced.png My bigger concern (and main question), is related to the surface damage (tearing) to the top resulting from the removal of the bridge. I want to ensure that I have a solid bond between the new bridge and the top. How do you folks suggest approaching prepping/repairing the the deep tears in the grain so there is a smooth surface to which the bridge can adhere? Some of the fissures are deep enough that I feel sanding them out will weaken the integrity of the top where the string tension will be highest: Attachment: Bridge Removed-reduced.png As always, thanks for your shared insights folks! Rick |
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| Author: | mountain whimsy [ Tue Sep 12, 2023 11:07 am ] |
| Post subject: | Re: Repairing Top Surface For Bridge Replacement |
I'm certainly no expert on this, but I have run across this on a few bridge reglues or replacements. Plywood tops are terrible for getting a clean bridge removal. That top veneer is super thin and I suspect the glue used for the ply weakens with heat. I also find that that glues used on this level of guitar does not help with getting a bridge off easily. I usually have to use a lot more heat for a lot longer time than if it was glued with HHG. What I've done is first try and secure any loose plies that have detached during the removal process - glue and clamp. Next I use my dremel with base to level out and clean up the surface. I've had to take it down to the next ply when it's really bad. But I'd rather have a clean surface to work with. I then create a small ledge in the rim of the bridge that is just deeper than the ledge in the guitar top to make sure I am getting good contact between the bridge and top. It's then a standard glue up with HHG and clamps. Cheers! |
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| Author: | Ken Nagy [ Tue Sep 12, 2023 11:16 am ] |
| Post subject: | Re: Repairing Top Surface For Bridge Replacement |
I saw in a presentation given at a Zoom MVA meeting that some violin repair people are using scans, and 3D printing to address just this sort of thing. Replacing lost wood with something that won't be seen, and not taking away any more wood. Probably geared for a high end restoration, but it was cool. If you have all the stuff, and know how to use it you can print a perfectly shaped piece to glue in place. Then you plane, sand it level with the top, and have a full, flat surface to glue on. He also talked about using the scan to mill a wooden surface to fit in the place that needs to be built back. I would be MUCH happier to do that. It seems that SOME of the ridges could be smoothed out before scanning, But milling using a scan would be a lot easier that chalk fitting something like that. Without scans, 3D printing, and CNC mills; that I don't have; I would have to smooth some, fill some, and then tackle chalk fit doubling with spruce, little sections at a time. That tear is strange how it didn't really get outside the bridge area at all. Oh. I see it is a plywood top? Plywood doesn't seem to have much strength vertically, does it. |
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| Author: | Barry Daniels [ Tue Sep 12, 2023 11:23 am ] |
| Post subject: | Re: Repairing Top Surface For Bridge Replacement |
3D printing wood? Haven't heard of that. Can you provide details? |
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| Author: | Ken Nagy [ Tue Sep 12, 2023 11:58 am ] |
| Post subject: | Re: Repairing Top Surface For Bridge Replacement |
It wasn't 3D printing wood. It was using the scan, and using that to program a mill to cut the reverse image so it fits. At one shop we scanned car parts to make dies to form them. Same thing on a small scale. |
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| Author: | dofthesea [ Tue Sep 12, 2023 1:17 pm ] |
| Post subject: | Re: Repairing Top Surface For Bridge Replacement |
I took a class with Joe Glaser at Northwoods. And they have a system where they CNC the bottom of bridge so it matches the irregularities of bridge footprint. A pretty neat repair system. |
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| Author: | Clay S. [ Tue Sep 12, 2023 2:04 pm ] |
| Post subject: | Re: Repairing Top Surface For Bridge Replacement |
A glue that is not usually recommended for gluing bridges, but has wonderful gap filling properties is a slow set high strength epoxy. The slow setting will allow the epoxy to soak in and under the loose fibers between the plys, and the gap filling properties will fill the voids between the bridge and the top. Make sure you put a tape "dam" on the inside of the top to block the pin holes (which you will have to drill the epoxy out of after the glue sets). It is not an ideal solution, but again, on a value limited instrument it may be a pragmatic way to get the job done. Is that one of the "newer" Chinese made maple bodied Craviola guitars? |
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| Author: | Rick Cowan [ Tue Sep 12, 2023 2:40 pm ] |
| Post subject: | Re: Repairing Top Surface For Bridge Replacement |
Clay S. wrote: A glue that is not usually recommended for gluing bridges, but has wonderful gap filling properties is a slow set high strength epoxy. The slow setting will allow the epoxy to soak in and under the loose fibers between the plys, and the gap filling properties will fill the voids between the bridge and the top. Make sure you put a tape "dam" on the inside of the top to block the pin holes (which you will have to drill the epoxy out of after the glue sets). It is not an ideal solution, but again, on a value limited instrument it may be a pragmatic way to get the job done. Is that one of the "newer" Chinese made maple bodied Craviola guitars? Hi Clay - The guitar is the Craviola design, Model # AWKS 6 and was made in Brazil in April 1975. Don't quote me, but from what I can tell from my searches online, these guitars were constructed with Brazilian Rosewood sides and backs. I have seen references to spruce tops, but this one is definitely plywood. Regards Rick |
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| Author: | Rick Cowan [ Tue Sep 12, 2023 2:46 pm ] |
| Post subject: | Re: Repairing Top Surface For Bridge Replacement |
mountain whimsy wrote: What I've done is first try and secure any loose plies that have detached during the removal process - glue and clamp. Next I use my dremel with base to level out and clean up the surface. I've had to take it down to the next ply when it's really bad. But I'd rather have a clean surface to work with. I then create a small ledge in the rim of the bridge that is just deeper than the ledge in the guitar top to make sure I am getting good contact between the bridge and top. It's then a standard glue up with HHG and clamps. Thanks Tony. I think this is the best approach for me. I am familiar with the ledge treatment. Appreciate you insights. Cheers! Rick |
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| Author: | Rick Cowan [ Tue Sep 12, 2023 2:46 pm ] |
| Post subject: | Re: Repairing Top Surface For Bridge Replacement |
Thanks everyone for your suggestions! Regards Rick |
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| Author: | Barry Daniels [ Tue Sep 12, 2023 2:48 pm ] |
| Post subject: | Re: Repairing Top Surface For Bridge Replacement |
Plywood top on hopefully solid brazilian rosewood? Sounds like a good candidate for a retop. |
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| Author: | Clay S. [ Tue Sep 12, 2023 3:19 pm ] |
| Post subject: | Re: Repairing Top Surface For Bridge Replacement |
The back and sides are also plywood on most models. I've read (but never seen) that some of the "high end" Giannini guitars had solid spruce tops and solid Bolivian rosewood (pau ferro) back and sides. Someone on the AGF did retop a Craviola with a solid spruce top and was quite happy with the results. My first guitar was a Giannini classical - I still have it. A Christmas gift from my parents it wears the scars from back packing trips and campfire gatherings. The back and sides were "Brazilian walnut" (Ipe?) plywood. Giannini used a lot of plywood for it's stability. They shipped guitars internationally to all sorts of climatic conditions. It has remained playable for over half a century. |
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| Author: | Hesh [ Tue Sep 12, 2023 8:31 pm ] |
| Post subject: | Re: Repairing Top Surface For Bridge Replacement |
It is a candidate for epoxy in keeping with the ethic of the repair being appropriate for the instrument. |
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