Dip in ukulele top will interfere with fret board mounting

Hobo

Hobo

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Everything was going along pretty well with my StewMac Tenor Ukulele until I noticed that the top has a 3/32 inch dip that's going to interfere with mounting the fretboard. The fretboard at the sound hole is too high to lie flat and will need to be forced down (producing a curve at the at the highest frets) or I could insert a small wedge from underneath to keep the finger board flat. I have two questions: 1. How would you address this problem and 2. Why did the top develop a dip in the center? The flat along the neck aligns with both ends of the top... it's the center of the top that dips slightly. These photos should give you a visual of the problem:

stewmactop3.jpgstewmactop2.jpgstewmactop1.jpg
 
Without actually looking at the uke, I would guess that the sides of the uke were not flat on the top surface before the top plate was glued on. If you took the uke and laid it on a flat surface, like sheet of glass or a table saw bed, you would have probably seen the gap then. That would have been the time to correct this problem. You can remove the top now and sand the sides so then rest evenly on a flat surface, but that probably is not what you want to do. That is what most luthiers would do if this happened. I suppose also that the top braces could have a reverse bend to them causing the top plate to sink once it was glued on, but that is unlikely. A slight dome is preferred these days by most luthiers but you probably didn't check this out before gluing. This is how you learn. You can do a dry run by calculating how high the bridge will need to be by laying a straight edge on the fretboard and measuring the distance from the straight edge to the top plate where the bridge will be. If it measures more than 1/2", you need to do something different. The bridge is going to be way too thick to try to compensate for the added height. Do not do that, that will be making the problem worse. One way to bring the fingerboard flat to the top plate surface as it is now, is to reset the neck angle so the fingerboard lays flat to the top. If the neck is already glued on, that won't work either. I would remove the neck and do it over.

Is the neck already glued on?
 
BlackBearUkes said:
Is the neck already glued on?
Click to expand...
Yes, it's glued with Titebond II and I used a bolt as well. I sanded both the top and bottom on a large 12" steel sanding disc laid flat on the workbench as well as on a piece of glass with sandpaper attached to the surface. I didn't notice any problem prior to glue-up. Both sides appeared to lay very flat on a flat surface after sanding. A straight edge laid across the top – touches at the neck joint and at the bottom edge of the top, meaning there is a slight inward dip with deepest part being 3/32" at the sound hole. The flat of the neck (minus the fret board) is inline with both the top and bottom edge of the top plate... the neck is not misaligned, the top has a slight concave dip.... and speaking of dips, I feel like one.:(
 
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Here is a builder (ie non-luthier answer). I've produced this problem, a dip around the soundhole. Probably caused by the waist at the back being wider than the waist at the top.

First, with the fretboard lying on the neck as if glued there, check the bridge height. Place a straight edge with its end at the place where the nut/zero fret will be, put a spacer at the 12th fret which is the action you want (something around 2.5-3mm - drill bits are good). Measure from the straight edge to the top at the bridge position. If your top dips around the soundhole but rises again toward the tail, this may be about right.

If the spacing is too high for the bridge, then you probably need to remove the neck and reset the angle.

If it's about right you can proceed to disguise the problem.

Clamp the fretboard to the neck (at around 1st and 12th fret). If it's already glued on, no clamping is required. Using finger and thumb, try to clamp the fretboard extension to the top. Will they meet? If so, you can simply glue the extension to the top with appropriate clamping. This will raise the top slightly at that point, but you already have a non-flat top so who cares? It will also bend down the fretboard a little, but again why worry? The notes will be playable, though the action there will be a fraction higher than it should be, but these are your least used frets.

I can tell you from experience that no-one other than a luthier will notice unless you point it out to them.

If my diagosis of the reason is correct, which you can check by measuring across the waist top and back, the other cure is to remove the back, pinch in the waistat the back until the top lies flat, then glue the back on again and trim off the overhang. On the assumption that this is the first of many I'd reject this option, leaving the dished top as a reminder for future builds.
 
ProfChris said:
Here is a builder (ie non-luthier answer). I've produced this problem, a dip around the soundhole. Probably caused by the waist at the back being wider than the waist at the top.

First, with the fretboard lying on the neck as if glued there, check the bridge height. Place a straight edge with its end at the place where the nut/zero fret will be, put a spacer at the 12th fret which is the action you want (something around 2.5-3mm - drill bits are good). Measure from the straight edge to the top at the bridge position. If your top dips around the soundhole but rises again toward the tail, this may be about right.

If the spacing is too high for the bridge, then you probably need to remove the neck and reset the angle.

If it's about right you can proceed to disguise the problem.

Clamp the fretboard to the neck (at around 1st and 12th fret). If it's already glued on, no clamping is required. Using finger and thumb, try to clamp the fretboard extension to the top. Will they meet? If so, you can simply glue the extension to the top with appropriate clamping. This will raise the top slightly at that point, but you already have a non-flat top so who cares? It will also bend down the fretboard a little, but again why worry? The notes will be playable, though the action there will be a fraction higher than it should be, but these are your least used frets.

I can tell you from experience that no-one other than a luthier will notice unless you point it out to them.

If my diagosis of the reason is correct, which you can check by measuring across the waist top and back, the other cure is to remove the back, pinch in the waistat the back until the top lies flat, then glue the back on again and trim off the overhang. On the assumption that this is the first of many I'd reject this option, leaving the dished top as a reminder for future builds.
Click to expand...
YUP! you've explained that well Prof..I think!:eek:..I've given away a few ukes like that..What you need is a good accurate mould to build em' in..another thing that causes a similar problem is clamping radiused edged sides down onto a flat surface...thats why all my tops are now flat
with a slight rise in the centre of the lower bout only...guess who I got that from?:eek:
 
Problem #2 is Titebond II...a glue that is unsuitable for instrument making. Too rubbery...too much cold creep. If you have to use Titbond, use the original formula.
 
True flat tops (ie. not domed) always seem to go concave, made worse by humidity issues. They never seem to fully recover even after pushing the humidity up.
I guess that's why makers started doming the soundboards.
That dip seems excessive though. Somethings amiss.
 
Just fill with glue and clamp hard together... the top will come up a bit and the tail extension will go down a bit..It won't effect playing at all..and it will look ok to every one but you :)
 
ProfChris said:
I've produced this problem, a dip around the soundhole. Probably caused by the waist at the back being wider than the waist at the top.
Click to expand...

Thanks to you and the other who have commented. The width across the waist at top and bottom are identical. Apparently, there's another reason for the dip. The kit instructions called for a slightly domed back and provided braces that were shaped to produce the shape. After assembly, I see no dome on the bottom, but rather a slight dip on the front at the soundhole. Anyway, the fret board will bend down to meet the top with no problem... I was wondering if shimming from underneath the fretboard would allow it to remain flat and still allow gluing it to the top. Thanks again!
 
That's not quite true. There's nothing magical about 45% humidity. It's simply an average that Luthiers work to. It's the change in humidity (a drop) that causes a Top to go concave. If you build at 45% and the Humidity drops to 30% it will still result in a concave soundboard.
 
Michael N. said:
There's nothing magical about 45% humidity. If you build at 45% and the Humidity drops to 30% it will still result in a concave soundboard.
Click to expand...

Yep, if you glue up at 70% humidity and then it drops to 30% it certainly won't be magic that splits your soundboard.

45-50% gives the top a chance to go up or down with the humidity swings without splitting.

If you really want an instrument that can travel around a bit and doesn't need to be stored in a glass humidor and you don't want it to be concave, build a 2-3mm convex dish into the top at 45-50%. The worst it'll do then is dish up more when the humidity is up and go flat when the humidity goes down.
 
Which is why I wrote that Luthiers work to an 'average'. It's a compromise but in itself there is nothing that states that you must work to 45%. If you lived and played an instrument in a hot humid climate, 45% might not be such a good idea. In fact it will positively be a bad idea. Again, if you lived in a dry arid climate the 'average' humidity may not be such a great idea.
The point I was really making is that it's the drop in humidity that causes the concavity of the soundboard and not the humidity level in which the instrument was built.
You won't stop true flat Top instruments from going concave. Trust me, 80% of the instruments I build have a flat Top and flat Back. My workshop is humidity controlled. All of them show varying degrees of concavity even after a few weeks. You have little chance of getting them back to Flat either. It doesn't matter how much you try to humidify. Every 19 th century Guitar that I've examined (the flat Tops) show concavity.
 
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Given all the reports of Kamaka troubles, I'd say that building in a humidity controlled environment of 45% to 50% is a good idea no matter whether you think the instrument will stay in your area or go elsewhere. There have been enough comments re cracks and seam separation for concern. I would imagine that a huge percentage of Hawaiian-made ukes...ones made on humid sides of the islands...wind up in the continental US and in average humidity of 20% or lower than the ambient humidity of the workshop. We have to consider our ukes to be world travelers and build accordingly.

I cannot imagine why one would build knowing that tops will go concave even if it is "historically correct". Seems to me that we have the benefit of being able to see what has gone wrong with traditional construction and not build that way. Just because something is traditional does not mean it's correct or good.

One viable alternative to building in a concave dish is to sand or carve an arc into the glue surface of braces and glue then on springing the ends down. That will pop a bit of a dome into the top even if you build on a flat workboard or glue to a flat surface. It's a pretty common technique among Spanish guitar builders.
 
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