View Full Version : Structural failure is upon us. A learning opportunity perhaps
There is no getting around it - the mandolin which I had built from the iv kit failed structurally. I originally bought an A kit with no holes and decided to turn it into an oval hole mandolin. The top is collapsing along the long axis of the oval hole. It's quite pronounced.
I knew this was an unorthodox thing to do as the top differs from a Gibson A oval hole instrument, as well as where the neck is attached. However, I decided to go for it and all seemed well enough. In fact, a few others on this forum tried to do a similar thing around the same time.
You can see a picture of the inside right after I cut the hole, with the X braces marked and then a picture of the damage as it is today. The braces seem to be holding up the top well enough and then the crease begins right where the braces end.
As much work as I had put into it, I'm looking at this as a learning opportunity rather than be upset.
The questions that pop into my mind are - was the hole in a bad position? Was the top too thin - I know that the traditional oval hole mandolins have a much thicker area around the hold and the are above it where the neck extension is traditionally supported. Are the X braces in the wrong place? Have any of you seen this type of failure before? For the iv-builders who ventured this way - how are your oval hole hybrids mandolins holding up?
Just to be clear - this is not a complaint about the iv kit at all! I know I deviated from the intended design but I was adventurous.
Now it's time to learn and move on and maybe help other budding builders avoid this.
Avi---great attitude--learn and then try to do better next time. I have found that the oval hole is attractive and I favor it but have had to also reinforce the area around the hole. Perhaps a method of doing this might be to leave more material in the vicinity of the hole and/or to also extend the X brace a bit higher up, even to the rim of the body. I still have to remember that the mando has 8 strings pulling with approximately 20 pounds for each string along the longitudinal axis of the instrument and then also consider that there is a downward pressure being exerted upon the bridge.
Is it possible to salvage the top by removing your bars ad replacing after truing the top up again?
There are 2 critical points that come to my mind in this regard, the thickness of the top (how thick is it?), and the arching of the top (does the arch drop down to the level of the neck block before it reaches the block?).
To maximize the strength of the structure the long arch should fall from the bridge to the neck block before it levels out. The cross section of the long arch should minimize any "S" curves. This means no re-curve near the neck. Top thickness in this area is usually more than 3mm (.120") and often considerably more. Stiffer and stronger wood can be fairly thin but softer wood needs to be significantly thicker.
The questions that pop into my mind are - was the hole in a bad position? Was the top too thin - I know that the traditional oval hole mandolins have a much thicker area around the hold and the are above it where the neck extension is traditionally supported. Are the X braces in the wrong place? Have any of you seen this type of failure before?...
1. Yes. The hole in an oval hole mandolin, guitar, or any other instrument with a hole in the string line has the hole in "the wrong place" if structure is the primary concern. The structural deficiency is compensated for with bracing, and that's why we get away with it.
2. Yes. The structural failure means the top was too thin for the design.
3. Not really, but they probably ended too soon. If they extended to the linings, or very nearly to the linings north of the bridge the collapse might have been avoided.
4. Yes. I've seen it in old Gibsons and in oval hole mandolins by other makers, as well as in guitars.
Can the mandolin be salvaged? Could you pull the back and re-brace the existing top? Or replace the top?
Two things strike me from the photos. The area under the fingerboard out to the edges is way to thin, and the neck looks like it is sitting very high off the body making for a tall bridge. That would put a lot of angle into the strings at the bridge, making for a lot of pressure on the top. Can it be salvaged? Anything can, but unless you can get the neck and top off, forget it.
Thanks for all the feedback so far!
pelone and sunburst - both of you suggested extending the x-braces higher - maybe to the rim. This makes a lot of sense to me now that I see where it failed in relation to the braces. My inside photo shows the braces stopping at the long axis of the hole. In reality the braces are carved down to nothing by the time they reach this point. I can definitely see that maintaining more brace material in that area could have helped. I had originally used this photo (http://www.petercoombe.com/Construction.htm) (the first one) as a general guide. It's clear that Peter's braces extend much more than mine. The only explanation I can have is that I used the braces that came with the kit and originally intended for tone bars and I now realize that they were not long enough. I had not considered enough the importance of the brace around the hole especially in light of the fact that old Gibson mandolins had minimal bracing. However - they did have much more thickness to support the hole. I can see this now
Michael - Thanks for the guidelines. Unfortunately I was working with a pre-carved top plate in the kit which started out very close to (even slightly under) the typical graduation for an f-hole A mandolin.
Hans - interesting comment about the neck angle. My experience was almost opposite. I had a hard time achieving low action to the point where I had to shave down the bridge below the min setting.
Rob and others - I would have been ok to keep on playing it like this but I think it's unstable and does not hold the pitch well enough. I may try to take it apart some day but for now I think I'll devote my (precious little) time to building something new and better
If any other IV kit builders have pictures of the bracing inside their instruments and would care to post and comments on how the mandolins have been holding up - that would be great
I really appreciate the collective expertise in this forum and the willingness for people to post constructive comments. It's a Good place to be
I applaud your attitude, a failure is a great learning opportunity, and you'll learn more from it than 10 successes, ok, maybe 3.
But above all it is an impetus to build again, and that's where you want to be. Some would want to jump right away and repair this one, some would just leave it and start new, that is up to you, but don't just sit.
I build a round hole, flat-top mandolin, and my tops are maybe thinner than typical Gibson-style A's. I place a longitudinal brace on each side and running past the soundhole.
Extra bracing in the hole area should not hamper the sound producing qualities of the top too much.
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I agree, the top is too thin. Around the soundhole mine are around 6mm in the centre and around 4.5mm on the sides of the soundhole and really thick north of the soundhole and under the fingerboard. The one I have here is 6mm in the centre, 5mm on the edges of the soundhole and 8mm under the fingerboard. The braces don't extend into the linings, but it is quite thick there (4-4.5mm) so is not necessary. With your's, if you had extended the X braces into the linings and glued a cross brace north of the soundhole under tjhe fingerboard (as in Guitars and my flattop mandolins) it probably would have survived.
My own oval hole Saga kit build uses a Gibson stye horizontal brace under the hole. Mine collapsed (or began to collapse) exacly like yours. I removed the back and inserted baces from close to the ends of the horizontal brace to close to the corners of the heel where it meets the sides. The braces come within roughly a quarter inch from the hole. I used maple braces. When strung up again I was surprised that the tone remained the same, or very close to it.
Structurally yours, and mine, collapsed at exactly the weakest points on the top,and the area taking the greatest strain.. I think you could experiment with the bracing and correct the problem. I suspect you will be as pleased with the tone as I was.
Been there, done that<G>
You can fix this problem by cross laminating a reinforcing patch between the soundhole and the headblock on the inside of the top. There is no need to remove the top, because you should be able to work through the soundhole. Below is a photo of an F4 I built years ago which suffered the same problem. The cross laminated "patch" extends from the headblock and wraps around the the top and sides of the soundhole. This repair did not affect the tone of the instrument. After years of being flogged by its present owner, it has never collapsed nor shown signs of failure. As I recall the patch consists of three thin slices of top material all "cross laminated." The glue used was just good 'ol Titebond.
What you're basically doing is adding timber inside the top that most builders carve into the external portion of the plate below the freboard with an oval hole mandolin. I still use this reinforcing method when I build a carved-topped mandolin with a round or oval soundhole. I much prefer the look of the F5 style fretboard extension with its "cleaner" external top configuration.
The questions that pop into my mind are - was the hole in a bad position? Was the top too thin - I know that the traditional oval hole mandolins have a much thicker area around the hold and the are above it where the neck extension is traditionally supported. Are the X braces in the wrong place?
A question: Are the braces still glued, or coming off of the top in that area? Check with a mirror.
Observations: Graduations and arching could certainly be part of the problem, but I think the position of the X could be problematic too. If it's sinking by the soundhole, I suspect it also bulges under the afterlength (south of the bridge). Besides needing a little thicker graduation than other "minimum" areas (being in the string axis), this area of the top under the afterlength needs special "protection" on X-braced instruments which are naturally less longitudinally. Yours looks a bit vulnerable in this area.
Though I don't have experience with the IV arching, if it is low (I've seen this on the SM kit), with a deep recurve, it may be inherently weak, compounding the problem.
It's intuitive to have the braces pass under the middle of the feet of the bridge, but if you move it back a bit, you can narrow the X slightly, support the entire arch more fully, and achieve a more evenly divided and stable structure. Combine this with beefier grads in the upper and lower bout, and you should be fine.
If this top really is toast, it might be the opportunity to remove it and carve a new one...though this could get complicated, and most likely involve removing the FB first.
I just noticed that some additional comments were added to this older posting of mine. Thanks so much for the words of experience!! The top continued to distort to the point of not being repairable. I decided, rather than invest a lot of time in a new top, to start on a scratch build -my first. It's also an oval hole but with a properly designed top. It's going slow but I'm enjoying the process and learning a lot. In the meantime, the kit instrument has become my testing/experimenting vehicle. I recently installed an internal pickup in my main mandolin and used the broken kit to test my endpin drilling plan for jack installation. I use it for fretting experiments etc. It's good to have it around as a reminder of all the things that went well as well as the mistakes. But I'm moving on. Thanks again for the enlightening and thoughtful remarks