Belly bulge on F-5 top

[GTison]

Are tops that the peak is behind the bridge weaker? And/or can the mandolin top graduations be compensated for this.
I see some mandolins that have the peak behind the bridge even by the big name company. This brings me to a question about CNC tops with the peak behind the bridge. How does this happen if it is a CNC top? Doesn't that machine make the top the same each time?

[HoGo]

Are tops that the peak is behind the bridge weaker? And/or can the mandolin top graduations be compensated for this.

Yes and yes.
For given thickness the arch that is closer to straigth line from bridge to tailpiece will be stronger. But none are actually straigth so the shape defines flexibility, IMO, even more than the thickness in this area. It's all decisions of luthier how he carves the top...
The Griffith A-5 is said to have very distinctive tone which can be because of the position of bridge higher on the body resulting in apex of arch well behind the bridge and the wide long arches area behind bridge. I didn't particularly love the tone I heard on the few recordings to my ears it has some of the flavor of (too) thin toppped mandolins.

I see some mandolins that have the peak behind the bridge even by the big name company. This brings me to a question about CNC tops with the peak behind the bridge. How does this happen if it is a CNC top? Doesn't that machine make the top the same each time?

This is very simple, they use the outline of body to generate 3D shape of arch using one or two simple crossection profiles in their CAD software and result is that the apex is in the widest part of top which is approx 1/2"-3/4" behind bridge.
Even if you start with top with apex under bridge the tension of the strings will tend to deform top and bulge the part behind bridge up so after some time every (normal) top will show this to soem extent. So all is question where your top starts and how far your graduations will let it deform. If it goes too far the joint opens and voila...
Simple reglue won't hold in these cases (been there). Repaired joint is always weaker than original no matter what you do, and the forces in bulging top cause bad leverage at the outside of the joint, often the inside is not even open so cleats on the inside won't help (unless they are more of small cross braces). In most cases you won't be able to close that crack satisfactorily without large force against the bulging arch risking cracking the top. Simply put, these tops lack strength along the arch and the only way to do long term repair is supply what is missing.

[fscotte]

Bryan Kimsey, among many others, have cut backs off and repaired tone bars, etc.. Reattached the back. Im not certain why folks see this as difficult. Ive done it and I have limited repair experience.

As John Hamlett once said, its just wood.

[William Smith]

Bryan Kimsey, among many others, have cut backs off and repaired tone bars, etc.. Reattached the back. Im not certain why folks see this as difficult. Ive done it and I have limited repair experience.

As John Hamlett once said, its just wood.

I was going to say that, that is what Randy Wood does for re-graduations, tone bar shaving etc on mandolins...Pry top or back center seam issues? It is tricky to get the body points right, that are dovetailed into the wood though and I have an F-7 that will show that boo-boo!

[HoGo]

It's not difficult but will leave scars.
Removal of top or back on mandolin will always show unless you refinish at least ribs. You can do touch up but good luthier will always see it.
You can saw the back off a'la Randy Wood and glue it back but the scar will be visible. You can painfully and slowly open the joint with spatula but chance that no wood or finish will chip are damned near zero. And gluing it back seamlessly after all the built-in stresses are released is mission impossible.
Even on violins where weaker glue is used for tops and backs, there is typically some chipped wood along the joint (painlessly glued back where it belongs) and after years of maintenance the valuable instruments have to receive new edging or half edging. When neck is removed from valuable violin, the restorers will sacrifice neck and cut through neck wood not to remove any wood from back button. They add new wood to neck for reset.
Gluing back top on violin is simple because the overhang hides any imperfections.

[NRMiller1958]

Thanks to all for the great feedback. I'm travelling at the moment, but when i get a chance I'll do a more thorough exam inside the instrument.

Adrian, I see your Schofield post from 2015, but no pictures. I'd love to know more about your "chest patch."

Neil

[HoGo]

Adrian, I see your Schofield post from 2015, but no pictures. I'd love to know more about your "chest patch."

Neil

Here is picture where I'm gluing in the patch. I decided to glue bent strips a bit over 1" wide 1/16" thick fitted together so they create one large patch. The bent strips will be a bit stronger than carved piece of wood and assure better bond as gluing such large surface with HHG in one step is nearly impossible. I'm not sure if I have mor epics of the work...
I worked against cast that I used to cordct the arch before the patch and I added strips right o the edge of f holes. There are four or five strips already in the pic, I started with one in center and then added two more at sides each day. After thorough drying I regraduated the top and glued new tonebars.

[fscotte]

And dont fret a patch. Theres a pic around here of a Gibson that had the nastiest patch installed while it was being built...

[Bernie Daniel]

As did Roland White's Randy Wood mandolin. He had someone reinforce the underside of the top with (If I remember correctly) carbon fiber cloth.

Often wondered if one could not use Tyvek (spun high-density polyethylene fiber sheet) for that same purpose i.e., splint top cracks in spruce? It is also extremely light weight and strong and could be bonded to wood with epoxy or with cyanoacrylacte I suppose.

[Bernie Daniel]

Bryan Kimsey, among many others, have cut backs off and repaired tone bars, etc.. Reattached the back. Im not certain why folks see this as difficult. Ive done it and I have limited repair experience.

As John Hamlett once said, its just wood.

I agree! With almost no real formal repair training I took the back of a 1937 K-1 mandocello, did a "Frank Ford mandocello repair" on it and glued it back together and most people would have a hard time proving the back was ever off (there are a few clues) so I say it it can be done with patience and some wood working experience.


Added note: I suppose it would be much harder with an F-5 style then the simpler A-style mandocello that I opened up.

[HoGo]

Often wondered if one could not use Tyvek (spun high-density polyethylene fiber sheet) for that same purpose i.e., splint top cracks in spruce? It is also extremely light weight and strong and could be bonded to wood with epoxy or with cyanoacrylacte I suppose.

I doubt thin layer of anything would provide the necessary support. Even one layer of CF would add more weigth than strength, you would have to add it on the outside of the plate to work (the tensile strength of CF layer would hold the bulge under control). Just imagine the geometry of the problem...
Polyethylene is pretty much impossible to glue with any conventional glues. You can encase it in thick layer of epoxy but that adds too much weight and would do no much good as well.
Simple spruce patch to thicken up the area after arch correction is the way.

[HoGo]

Those old instruments are often easier to disassemble than modern, old hide glue can be brittle and sometimes you can find a seam already open to start the job (especially on larger instruments). Modern instruments glued with Titebond are nightmare to open cleanly....

[rcc56]

I've pulled a few backs in my time. Until recently, all had been older instruments assembled with hide glue. With a lot of patience, they came off without too much collateral damage.

But my latest job is a recent issue Gibson J-45, on the bench now. The owner had fallen on the guitar, and the back cracks were really bad, and pieces of a couple of the braces were missing. Back replacement seemed to be the most practical way to go. So I gave the owner a quote, and he said to go ahead.

When I tried to cut through the thick finish to expose the binding joint, I knew this one was going to be trouble. Finish falling apart, etc. When I finally got to the binding joint, it wouldn't release without splintering the side. Half an inch into the job, it was obvious that I needed to cease and desist, and resort to a different approach. I got the Dremel and router base out and routed most of the way through the binding. The results were much better.

I wanted to at least make an attempt to salvage the old back, but one look at the glue joint and a couple of pokes with a sharp tool was enough to convince me that I would be wasting my time. Hours of sawing and/or trying to work through it with hot knives was not going to yield good results, and the old back was already in bad shape. I ordered wood for a new back, saved enough of the bracing to reuse two of the braces and make a template for arched clamping cauls; and sacrificed the old back.

The new back is on and I'm starting the finishing process. The old back is in the scrap bin for finish matching and later use as shim stock. The missing splinter has been grafted and everything looks reasonably good.

The moral of this story is that while the chances of successfully removing a back on an old hide glue instrument are fairly good with patience, the right tools, a thoughtful approach, and a bit of luck, all bets are off on a new instrument with thick finish and modern glues. Anyone who wants to try it on a modern instrument is in for a tough ride with a high risk of failure.


It's easy to suggest pulling a back on an instrument that's a thousand miles away from the safety of an armchair and a computer keyboard. Getting down in the trenches and doing the job is a horse of a different color.

[HoGo]

I pulled back off Gibson F-9 few yeas ago and had to heat the putty knife really hot to release the glue joint, scorched the wood in few places. I didn't care because I ws going to reefinish it and remove substantial amount of wood anyway but I cannot imagine doing a clean job of cimply removal and glue back. The cleanest seems to be the sawing off.
On guitars I found a technique where the top aor back is routed away without touching the binding and new plate is made using the guitar as a template so it fits nicely back into the binding. I don't remember who but it was one of well known guitar makers/restorers (this time not Frank Ford).
Found it:
http://www.grevenguitars.com/retopping-demo.html

[Ivan Kelsall]

From Adrian - "...you would have to add it on the outside of the plate to work ." Why ? - if the CF on the inside spans the weak area,then when the bridge presses down on it,it'll still be under 'tension'. As the bridge pressure tries to 'spread' the weak area,the CF will hold it & ''pull it back'',
Ivan

[HoGo]

From Adrian - "...you would have to add it on the outside of the plate to work ." Why ? - if the CF on the inside spans the weak area,then when the bridge presses down on it,it'll still be under 'tension'. As the bridge pressure tries to 'spread' the weak area,the CF will hold it & ''pull it back'',
Ivan

Ivan, it's not the pressure of the bridge against top that causes this. The bridge pressure just moves the whole action into area between bridge and tailpiece. Take long thin (bamboo) stick and press it against wall, it will bulge. If you put something against the bulge somewhere in bridge position to prevent more bulging at the point and push a bit more, the weaker half of the stick will bulge even more between the obstacle (bridge) and the end and the other half will actually flatten a little by the leverage against the obstacle. So the whole force is equivalent to pressure from inside towards the top halfway between bridge and tailpiece. any reinforcing of type of cf cloth or fiberglass is effective on the outside of the bend only, on the inside it will do nothing at all.
There can be other forms of top failures like "harmonica fold" under tailpiece when recurve area is weak e.g. too deep/wide/thin and allows the bototom block to rotate under tension of tailpiece or central top sinkage under bridge when general arch is good but the top is too weak (thin, too small tonebars) in the center. in these two cases layer of CF would help, but I would still opt for a layer of wood if there is access to damaged area.

[Ivan Kelsall]

Adrian - My misunderstanding !. I'm thinking of a lateral '' left to right'' situation rather than a longitudinal, ''neck to tailpiece'' situation. Of course the bulge is operating along the longitudinal axis - apologies,
Ivan