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Is my top caving in?
https://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=41354		 Page 1 of 2
Author:  klooker [ Tue Sep 10, 2013 8:39 am ]
Post subject:  Is my top caving in?
This is number 3, a Padauk OM.

It hasn't got much attention lately since I finished my 13 fret L-00 Nick Lucas/Santa Cruz inspired build.

I got out the OM the other night & noticed that the action was really high on the upper frets. I took it to the bench & discovered this. eek
Attachment:
TopSink-PadaukOM.JPG


It has a bolt on neck & I never glued down the fingerboard extension because I figured that I'd do some more tweaking after it settled in.

Is this a result of not securing the extension? The top still has plenty of arch side to side, it hasn't sunk in the lower bout.

Thanks,
Kevin Looker
Author:  Chris Ensor [ Tue Sep 10, 2013 9:00 am ]
Post subject:  Re: Is my top caving in?
Has it raised behind the bridge? It could be caused by the rotation of the bridge. But if that is the case, you should see a hump behind the bridge.
Author:  DennisK [ Tue Sep 10, 2013 9:01 am ]
Post subject:  Re: Is my top caving in?
Do you have a shot of the bracing? Particularly the upper bout structure. Looks questionable, but not dead.

Is the humidity lower than when you glued the upper transverse brace? My spruce toppers change pretty significantly throughout the year. But I build in very low humidity, so it's more troublesome the opposite way, developing a ski jump in the summer if I don't compensate in the fingerboard leveling.
Author:  TonyKarol [ Tue Sep 10, 2013 9:47 am ]
Post subject:  Re: Is my top caving in?
You're in Ohio .. I doubt they have a low humidity problem there in hte summer months ... was your UTB flat when glued on ?? You say there is radius side to side, meaning you used some sort of radius in your X brace, and possibly tone bars too - but radius behind a bridge is easy to maintain, as the bridge pulls it up anyway. I would say your upper bout braces are undersized, and it collapsed some .. it might be stable right there for good, you just need to do whatever to get it in playing shape again - setup, neck reset even ...
Author:  klooker [ Tue Sep 10, 2013 9:50 am ]
Post subject:  Re: Is my top caving in?
It looks good behind the bridge.
Attachment:
BellyArea.JPG


It also looks good across the UTB.
Attachment:
UTB-Area1.JPG


I don't have a shot of the bracing but I followed the Grellier plans and there is no popsicle stick brace. I've marked on the top with tape how the X and the UTB are oriented.
Attachment:
UTB-Area2.JPG


Kevin Looker
Author:  DennisK [ Tue Sep 10, 2013 10:16 am ]
Post subject:  Re: Is my top caving in?
So there's just open space between the headblock and UTB?

I can't see anything wrong in the new shots, but I agree with Todd. Just get it playable again and see if it holds. Definitely glue down the fingerboard extension this time. It acts as a sort of wimpy brace in that style. Still inadequate IMO, but apparently it tends to hold up long enough that people still build and post plans with no A-frame or headblock extension or buttresses. Although the headblock extension style can go badly as well, if the UTB is inadequate to handle the focused force on it. Probably A-frame too. And the buttress style probably could fail if the back is inadequately braced. Just have to follow the paths of force and see if there are any spans of plate unbraced or heavily scalloped being exposed to high strain :)
Author:  Rodger Knox [ Tue Sep 10, 2013 10:56 am ]
Post subject:  Re: Is my top caving in?
How far is it from the bridge to the intersection of the X? Closer is more likely to rotate.
If it looked like that with string tension, I'd say everything is OK, but with it unstrung I'm not so sure. How much does the bridge rotate when it's brought up to tension? There was a discussion a while back about techniques for measuring it, they're all pretty straightforward. Attach some kind of long, thin pointer to the bridge with the guitar in a fixed position, and measure the movement of the end of the pointer when tension is applied (or removed). It should be about 2°. If it's much more than that, you have a problem. If it's 2° or less, you're probably good to go.
Author:  Michael Lloyd [ Tue Sep 10, 2013 11:16 am ]
Post subject:  Re: Is my top caving in?
From the photos I'm not sure if you have any additional finger bracing between the x brace and the UB brace running beside the sound hole. I usually add these for the additional strenght.
Author:  theguitarwhisperer [ Tue Sep 10, 2013 11:31 am ]
Post subject:  Re: Is my top caving in?
Very likely that it's a result of not gluing the fingerboard extenstion.

In the future, you can simply tack the first quarter inch of the end if the fingerboard, that way you can disassemble it in the future for more tweaking if necessary but it will still perform it's function of countering rotational forces.

Frankly, I'd just glue it down. When you relax the strings, it will likely spring back to where it was.

The only real reason to do as I described is if you plan on leaving it in the white for a bit and want to play it in the meantime before you laquer it.
Author:  Rodger Knox [ Tue Sep 10, 2013 11:51 am ]
Post subject:  Re: Is my top caving in?
theguitarwhisperer wrote:
Very likely that it's a result of not gluing the fingerboard extenstion.

In the future, you can simply tack the first quarter inch of the end if the fingerboard, that way you can disassemble it in the future for more tweaking if necessary but it will still perform it's function of countering rotational forces.

Frankly, I'd just glue it down. When you relax the strings, it will likely spring back to where it was.

The only real reason to do as I described is if you plan on leaving it in the white for a bit and want to play it in the meantime before you laquer it.


Take another look. The strings are off and the finish is on.
Author:  klooker [ Tue Sep 10, 2013 2:02 pm ]
Post subject:  Re: Is my top caving in?
There is nothing between the neck block & the UTB - just like the drawing.

The neck is off & it's still sunken.

I won't be able to take any more measurements until this evening.

Kevin Looker
Author:  theguitarwhisperer [ Tue Sep 10, 2013 2:12 pm ]
Post subject:  Re: Is my top caving in?
Rodger Knox wrote:
theguitarwhisperer wrote:
Very likely that it's a result of not gluing the fingerboard extenstion.

In the future, you can simply tack the first quarter inch of the end if the fingerboard, that way you can disassemble it in the future for more tweaking if necessary but it will still perform it's function of countering rotational forces.

Frankly, I'd just glue it down. When you relax the strings, it will likely spring back to where it was.

The only real reason to do as I described is if you plan on leaving it in the white for a bit and want to play it in the meantime before you laquer it.


Take another look. The strings are off and the finish is on.


You're right, I said "Likely", LOL! In this case it didn't. Damage done, maybe?

Also, the finish being on makes me wonder why he didn't glue the tongue down, as the only reason I can think of not to is the one I stated.

But point taken....
Author:  theguitarwhisperer [ Tue Sep 10, 2013 2:15 pm ]
Post subject:  Re: Is my top caving in?
klooker wrote:
There is nothing between the neck block & the UTB - just like the drawing.

The neck is off & it's still sunken.

I won't be able to take any more measurements until this evening.

Kevin Looker


You can probably shape and glue in some soundhole braces, and likely correct the sinking top as well as provide some support for the future.
Author:  Tom West [ Tue Sep 10, 2013 2:40 pm ]
Post subject:  Re: Is my top caving in?
Kevin: Just a few comments without any conclusions. The picture of the belly behind the bridge looks like there is space under the straight edge. The would be out of line based on my experience. The UTB looks like it is quite substantial which to me is much better then too small. It would be interesting to see a picture closer to the C/L of the guitar rather then to the side as you are showing with the 18" scale. I don't think a lack of popsicle brace is of much concern in terms of your problem. I have build a number of guitars without gluing down the extension of the fingerboard. A few have been like that for at least 7 or 8 years and once the guitar settles in they appear to be quite stable. I'm sure gluing down the extension must add something to the guitars resistance to distort but I think it's like Dr. Livingstone's demise...........!! Wish I could add more.
Tom
Author:  klooker [ Tue Sep 10, 2013 9:03 pm ]
Post subject:  Re: Is my top caving in?
The neck has been off for at least 5 hours.

I took this photo with the straight edge about 1/4" off center because it would fall into the mortise if it was on center, but it is running parallel to CL. The gap is about 0.045".
Attachment:
TopSinkCL.JPG


The distance from the center of the X to the bridge pins is about 2-3/8".

The only reinforcement around the sound hole is little strips of spruce about 1/8" thick so no real bracing.

The upper finger braces were shaved down a lot - sorry I can't better quantify that, but I don't think they're offering much structural support.

The gap between the straight edge & the top in the area below the bridge is only about 0.007".

Thanks for all the replies.
Kevin Looker
Author:  crich [ Tue Sep 10, 2013 9:57 pm ]
Post subject:  Re: Is my top caving in?
When you put a straight edge parallel , right over the UTB, does it sag? Or is it straight? Is it in tack (compromised)? It's been fairly humid here in Ohio which makes this problem all the more perplexing to me. It almost looks like you used wet braces when building then they dried out?? Clinton
Author:  klooker [ Tue Sep 10, 2013 10:14 pm ]
Post subject:  Re: Is my top caving in?
crich wrote:
When you put a straight edge parallel , right over the UTB, does it sag? Or is it straight? Is it in tack (compromised)? It's been fairly humid here in Ohio which makes this problem all the more perplexing to me. It almost looks like you used wet braces when building then they dried out?? Clinton


Fairly humid is an understatement but the guitar has been kept in my shop around 50% RH.

I you look at one of my earlier pics, I put the straight edge directly over the UTB & you can see that it still has some convex shape to it - there is light peeking through at the outside edges.

The UTB is in tact & sound.

The only other thing that's come to mind is that I didn't build this one on a radius dish - I used cauls. As a result, I suspect that the top didn't wind up as a perfect sphere so maybe there was some initial "distortion" but the fact that the action has risen on the upper frets tells me that something has moved. The fingerboard is straight, but's its angle to the body has definitely changed.

A straight edge across the frets used to land abut 1/16" above the bridge, not it dives about 1/32 to 1/16 under the top edge of the bridge.

Kevin Looker
Author:  Trevor Gore [ Wed Sep 11, 2013 4:18 am ]
Post subject:  Re: Is my top caving in?
Kevin, what you might have may be an example of what I explain in this sometimes somewhat tetchy exchange on another forum here, especially as you weren't building in a dish. It's a bit of a long read, but at least get to the "sketches" I drew and all should become clear. Your photos and explanations seem to bear most of this explanation out, but it seems you may also have some neck sag. This may be because of weak sound hole bracing or a combination of weak sound hole bracing and not filling all the gaps between the neck block and UTB with solid wood. With the design you have used, not gluing down the fretboard extension may well have contributed to the problem.
Author:  ChuckB [ Wed Sep 11, 2013 5:43 am ]
Post subject:  Re: Is my top caving in?
Kevin,

Since you did not use a dish, how did you taper the sides to match a sphere? If you used no taper on the sides, a flat UTB, and radiused bracing other wise, the result of gluing the top on would be similar to what you have.....with sides sanded in a dish, the waist area would be higher than the rest. If it is not, the top would be sunken in .

Chuck
Author:  klooker [ Wed Sep 11, 2013 6:05 am ]
Post subject:  Re: Is my top caving in?
ChuckB wrote:

Since you did not use a dish, how did you taper the sides to match a sphere? ...


I tapered the sides using a curved sanding stick - think of a cross sectional slice from a radius dish. I've since invested in radius dishes. The sides are tapered but maybe not as nicely as if they were done in a dish or bowl sander.

Kevin Looker
Author:  Terence Kennedy [ Wed Sep 11, 2013 9:19 am ]
Post subject:  Re: Is my top caving in?
Looks to me like you got some head bock rotation and as evidenced by the low saddle there may not have been enough of a neck set initially to compensate for the settling in. The upper bout geometry may have been a little off as well.

Gluing the fretboard extension will definitely help stabilize the head bock and upper bout. I'd probably re-set the neck to give extra clearance over the bridge. (1/16" ?) Bolt on the neck and support the headstock and put 6lbs of weight on each shoulder of the upper bout to mimic string tension. See if a straightedge still clears the bridge easily. If so, glue the extension and expect a little less movement than before you glued it.

Doing this might produce some weirdness on the fretboard at the neck-body that might have to be re leveled.

Getting a feel for upper bout behavior and settling in characteristics of your guitars and neck attachment system over time seems to be an ongoing process and at least for me has resulted in continued modification of building techniques as experience is gained. I'm probably pretty slow but at almost 60 guitars I am just now starting to feel that I'm getting close.
Author:  Tom West [ Wed Sep 11, 2013 10:00 am ]
Post subject:  Re: Is my top caving in?
Kevin: Wonder if you checked these things before string -up. The more I see and hear leads me to think this was built into the top. A substantial UTB that does not appear to be distorted. The tops of the sides were not done in a dish. Neck block rotation tends to pivot at the 14th fret and move outward at the back attachment point basically from a bit of flattening of the longitudinal arch in the back. While personally I do use 1/4" braces at the sound hole, there are tons of guitars out there with flat braces like yours. So I think part of your problem is built in and part is box distort as above which has raised your strings and can be off set by a bit of reset and thus not affect your string to top distance at the bridge. I think you will find that the guitar will be happy and not move any further. But what the heck do I know. You can take that with a grain of salt and $1.50 will get you a cup of coffee most anywhere.
Tom
Author:  theguitarwhisperer [ Wed Sep 11, 2013 10:32 am ]
Post subject:  Re: Is my top caving in?
I'm taking the liberty of re-posting your picture from the other forum, Trevor. It seems these things are always resolved when some form of illustration is added.... :D
I could easily see this as being the case, exacerbated by neck block rotation.
If so, likely not a "problem" per se, once the tongue is glued, simply a fact of geometry.
Author:  lespaul123 [ Wed Sep 11, 2013 11:01 am ]
Post subject:  Re: Is my top caving in?
I have seen this before on one my guitars that I have since retopped. I believe my issue came from the sanding dish/ glue up. What I think happened is, as I was sanding with dish the waist was not secured very well. As I was sanding I was pushing a little hard down at the waist. This caused the waist to be sanded a little extra. I noticed it before I started to shoot clear, but from what I have seen in your pictures looks identical to what I saw. At that time of building, I wasnt flattening the upper bout, which could have resolved this issue. I would also check to see how square the top is to the sides, in the waist area.
Author:  klooker [ Wed Sep 11, 2013 2:23 pm ]
Post subject:  Re: Is my top caving in?
Thanks everyone for the replies [clap]

It's good to see people put thought into replies about guitar building.

Yes, my top looks exactly like the diagram that came from Trevor's link.

I've gone ahead & reset the neck.

I also glued a wedge to the end of the fingerboard extension to fill in the gap & prevent a major ski jump. I still have to do final shaping on it before I bolt on the neck & glue down the extension.

I've learned a lot from this little episode.

Thanks again,
Kevin Looker
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