Official Luthiers Forum! :: View topic - Any theories as to why? Cupping wavey sides

Not sure what happened there, but it doesn’t look like more than 10-20 mils. I’d sand it flat. Since the tailblock is there, I wouldn’t worry too much about getting a little thin there. I’d be more concerned about thinning sides elsewhere. Honestly, though, many of those great prewar D-18’s have paper-thin sides — .060 and less. I know, cause I’ve fixed holes in a bunch...

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Flat sawn wood's got a well founded rep. for cupping.
Looks sand-able to flat at the block.
For elsewhere I've used lotsa 0.275" square side struts, scalloped after gluing, clamping into the sides to pull things flat against a flat caul first before sanding.

Colin North wrote:

I might look into doing something like that. I'm not sure how to go about clamping them in place but perhaps a 'go-bar' to the other side of the guitar would work.

I should mention that these sides did not show cupping before building the box. Though I can't say I really looked for it either.

How certain are you that they were .080 across the entire width?

If it were me, I would just sand it flat and move on. No way will you sand through the sides if that's your max cuppage.

As to why exactly, who knows? I wouldn't dwell on it...

Is your block flat on the inside?

Is the back now attached qs well? If not you could use side supports a little stiffer than normal placed at or
close to where the low spots are to help flatten things out. Supports with a little crown might even be better.
A little extra stiffness in the sides shouldn't negatively affect the finished tone too much, imo.
Flat grain, one way or another, is the probable cause.

Dead flat sure helps if you're going glossy...

in 20 years I seldom see dead flat sides so that is why you sand and scrape flat. On your block I wonder about your glue process. Did you use a flexible caul when clamping? I use plexiglass and clamps so that the plexi rolls with the tail and neck block.

Did you use fish glue perchance?

Author:	jfmckenna [ Thu Apr 09, 2020 7:26 am ]
Post subject:	Any theories as to why? Cupping wavey sides
The shot below is at the tail block too. That is what baffles me because I know for a fact the tail block was dead flat and clamped with flat cauls. It's 3-ply mahogany block tapered to the thickness of the linings but there's just no way it could bend like that. The waviness is in other locations around the guitar too. I'm afraid to just sand it dead flat becasue the sides were already bent pretty thin at .08in. The ONLY thing I can think of is that the sides having been bent with water expanded, was glued to the guitar blocks and top (I build off the top) then the back joined closing the sound box and then the sides shrank back. The problem with this hypothesis is that I waited a full 8 days before gluing the sides to the top, using peones. I would have thought that was plenty of time for the sides to dry out. I have certainly done it much sooner in the past. And two, like I said that block was dead flat! And I can tell by looking at it that it is glued across the whole length... I just don't get it! And it's very disappointing. Oh one more potential hint. This is flat sawn wood. This is the very first time I have ever used flat sawn wood. Hard lesson learned?

Author:	Ken Jones [ Thu Apr 09, 2020 9:01 am ]
Post subject:	Any theories as to why? Cupping wavey sides
Not sure what happened there, but it doesn’t look like more than 10-20 mils. I’d sand it flat. Since the tailblock is there, I wouldn’t worry too much about getting a little thin there. I’d be more concerned about thinning sides elsewhere. Honestly, though, many of those great prewar D-18’s have paper-thin sides — .060 and less. I know, cause I’ve fixed holes in a bunch... Sent from my iPhone using Tapatalk Pro

Author:	jfmckenna [ Thu Apr 09, 2020 10:27 am ]
Post subject:	Re: Any theories as to why? Cupping wavey sides
Ken Jones wrote: Not sure what happened there, but it doesn’t look like more than 10-20 mils. I’d sand it flat. Since the tailblock is there, I wouldn’t worry too much about getting a little thin there. I’d be more concerned about thinning sides elsewhere. Honestly, though, many of those great prewar D-18’s have paper-thin sides — .060 and less. I know, cause I’ve fixed holes in a bunch... Sent from my iPhone using Tapatalk Pro Yeah elsewhere is what I am worried about. I suppose I could cut the binding channels first to keep an eye on how thin it's getting then recut the channels after thinning.

Author:	Ken Lewis [ Thu Apr 09, 2020 10:40 am ]
Post subject:	Re: Any theories as to why? Cupping wavey sides
Is the back now attached qs well? If not you could use side supports a little stiffer than normal placed at or close to where the low spots are to help flatten things out. Supports with a little crown might even be better. A little extra stiffness in the sides shouldn't negatively affect the finished tone too much, imo. Flat grain, one way or another, is the probable cause.

Author:	Colin North [ Thu Apr 09, 2020 10:44 am ]
Post subject:	Re: Any theories as to why? Cupping wavey sides
Flat sawn wood's got a well founded rep. for cupping. Looks sand-able to flat at the block. For elsewhere I've used lotsa 0.275" square side struts, scalloped after gluing, clamping into the sides to pull things flat against a flat caul first before sanding.

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Author:	jfmckenna [ Thu Apr 09, 2020 11:05 am ]
Post subject:	Re: Any theories as to why? Cupping wavey sides
Colin North wrote: Flat sawn wood's got a well founded rep. for cupping. Looks sand-able to flat at the block. For elsewhere I've used lotsa 0.275" square side struts, scalloped after gluing, clamping into the sides to pull things flat against a flat caul first before sanding. I might look into doing something like that. I'm not sure how to go about clamping them in place but perhaps a 'go-bar' to the other side of the guitar would work. I should mention that these sides did not show cupping before building the box. Though I can't say I really looked for it either.

Author:	meddlingfool [ Thu Apr 09, 2020 11:39 am ]
Post subject:	Re: Any theories as to why? Cupping wavey sides
How certain are you that they were .080 across the entire width? If it were me, I would just sand it flat and move on. No way will you sand through the sides if that's your max cuppage. As to why exactly, who knows? I wouldn't dwell on it... Is your block flat on the inside?

Author:	Colin North [ Thu Apr 09, 2020 11:46 am ]
Post subject:	Re: Any theories as to why? Cupping wavey sides
jfmckenna wrote: Colin North wrote: Flat sawn wood's got a well founded rep. for cupping. Looks sand-able to flat at the block. For elsewhere I've used lotsa 0.275" square side struts, scalloped after gluing, clamping into the sides to pull things flat against a flat caul first before sanding. I might look into doing something like that. I'm not sure how to go about clamping them in place but perhaps a 'go-bar' to the other side of the guitar would work. I should mention that these sides did not show cupping before building the box. Though I can't say I really looked for it either. FWIW, I removed the rims from the mould and used 3/4" x 3/4" pieces of ply, thin cork faced, outside, them largish spring clamps to pull the side struts in to sandwich the side against the caul. Clamp the end block(s) in a soft lined vice to hold the rims.

Author:	jfmckenna [ Thu Apr 09, 2020 1:47 pm ]
Post subject:	Re: Any theories as to why? Cupping wavey sides
meddlingfool wrote: How certain are you that they were .080 across the entire width? If it were me, I would just sand it flat and move on. No way will you sand through the sides if that's your max cuppage. As to why exactly, who knows? I wouldn't dwell on it... Is your block flat on the inside? That thought had occurred to me too but I'm reasonably sure that it's .08in across. I do only measure the edges with calipers but I also use a thickness sander too. The block is flat on the inside with angled/rounded off edges.

Author:	Michael.N. [ Thu Apr 09, 2020 1:51 pm ]
Post subject:	Re: Any theories as to why? Cupping wavey sides
That doesn't look too bad. I've had it far worse than that. I'm not sure it's necessary to have it ruler flat either. May just be my taste but things can look a bit stale and rigid if things get too 'perfect', then again I'm not fond of severe cupping either.

Author:	meddlingfool [ Thu Apr 09, 2020 1:57 pm ]
Post subject:	Re: Any theories as to why? Cupping wavey sides
Dead flat sure helps if you're going glossy...

Author:	Michael.N. [ Thu Apr 09, 2020 2:13 pm ]
Post subject:	Re: Any theories as to why? Cupping wavey sides
Not many backs are dead flat, soundboards either. We usually get some telegraphing of braces through the top. There are many imperfections. It's all a matter of degree. Even if things are dead flat it's unlikely that they will stay that way for too long. Time and string tension see to that.

Author:	Bruce Sexauer [ Thu Apr 09, 2020 2:54 pm ]
Post subject:	Re: Any theories as to why? Cupping wavey sides
I'd let it go as is. Even thickness is FAR more important to me that the appearance of flatness. When energy passes through changing thickness the energy itself becomes distorted in dimension and speed. Yeah, I know, but that's what I think. You could set a wet sponge on it and see what happens.

Author:	Clay S. [ Thu Apr 09, 2020 3:25 pm ]
Post subject:	Re: Any theories as to why? Cupping wavey sides
I like Ken Lewis's idea of using side reinforcements to flatten and add stiffness to the sides. According to some, the mass added to the sides might actually improve the sound of the instrument.

Author:	phavriluk [ Fri Apr 10, 2020 5:22 pm ]
Post subject:	Re: Any theories as to why? Cupping wavey sides
Everybody's fleas feel like gazelles, I know, but the cupping in the picture seems to me to be quite within the realm of ordinary. At least for me. I've built three with sides cupped far worse than the picture and they turned out just fine (and flat). A sanding block wide enough to bridge the full width of the side works wonders. And I've used stiff side struts to force the sides to flatten. Worked out fine. Got to remember about the convex arch in the side struts. That would have saved me many hours of scraping and sanding. Extra work, sure. But we are dealing with wood, not metal castings. The stuff can have a perverse mind of its own.

Author:	Terence Kennedy [ Sat Apr 11, 2020 3:50 pm ]
Post subject:	Re: Any theories as to why? Cupping wavey sides
I wonder if hide is the best choice for gluing end blocks. It’s a curved joint and without a perfect radius match and perfect clamping cauls it would seem difficult to get the good fit required for hide. Ever consider a glue with some gap filling properties for head and tail blocks? You’ll never have to take them apart. I’ve always used Tightbond for them but have recently used Smith’s All wood epoxy. It has the advantage of no water.

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Author:	jfmckenna [ Thu Apr 09, 2020 3:50 pm ]
Post subject:	Re: Any theories as to why? Cupping wavey sides
Ken Lewis wrote: Is the back now attached qs well? If not you could use side supports a little stiffer than normal placed at or close to where the low spots are to help flatten things out. Supports with a little crown might even be better. A little extra stiffness in the sides shouldn't negatively affect the finished tone too much, imo. Flat grain, one way or another, is the probable cause. Oh I missed this post, yes the sound boxes are closed. After that, that's when I noticed it. This is how I mount the back and it gave me something to think about too though I have done this on 65 guitars now and this is the first I noticed. Could the pressure crush the sides? But then if so wouldn't they just come back?

Author:	Clay S. [ Thu Apr 09, 2020 9:08 pm ]
Post subject:	Re: Any theories as to why? Cupping wavey sides
meddlingfool wrote: Dead flat sure helps if you're going glossy... And Dead Flat finish helps if you are going with slightly cupped sides. https://www.mohawk-finishing.com/produc ... sh-clears/ about $7 a can Or you can buy it from Amazon for $21.58 with free shipping

Author:	meddlingfool [ Thu Apr 09, 2020 10:31 pm ]
Post subject:	Re: Any theories as to why? Cupping wavey sides
Lols...

Author:	bluescreek [ Fri Apr 10, 2020 5:37 am ]
Post subject:	Re: Any theories as to why? Cupping wavey sides
in 20 years I seldom see dead flat sides so that is why you sand and scrape flat. On your block I wonder about your glue process. Did you use a flexible caul when clamping? I use plexiglass and clamps so that the plexi rolls with the tail and neck block.

Author:	jfmckenna [ Fri Apr 10, 2020 10:20 am ]
Post subject:	Re: Any theories as to why? Cupping wavey sides
bluescreek wrote: in 20 years I seldom see dead flat sides so that is why you sand and scrape flat. On your block I wonder about your glue process. Did you use a flexible caul when clamping? I use plexiglass and clamps so that the plexi rolls with the tail and neck block. That is the way I normally do it too. When I would glue up both sides at the same time. This time I did things a bit different using peones and gluing up one side at a time with HHG. I used a cork lined caul. But because I glued up one at a time it also allowed me to see that the joints were made well. That's what's so puzzling to me. Everything was nice and flush. Today I'll rout out the end graft and will be able to get another close look at it.

Author:	SnowManSnow [ Fri Apr 10, 2020 4:33 pm ]
Post subject:	Re: Any theories as to why? Cupping wavey sides
Rolling pin sander for the win:) Sent from my iPhone using Tapatalk

Author:	TRein [ Sat Apr 11, 2020 11:36 am ]
Post subject:	Re: Any theories as to why? Cupping wavey sides
Did you use fish glue perchance?

Author:	jfmckenna [ Sat Apr 11, 2020 11:48 am ]
Post subject:	Re: Any theories as to why? Cupping wavey sides
TRein wrote: Did you use fish glue perchance? No. I use HHG for jointing the peones to the top and the sides to the head and tail block, LMI yellow glue to join the linings to the back. --- I should have taken a better pic. I wanted to show a pic of that tail block in particular because I don't see how a 3-ply mahogany block could warp like that. But in the upper bout lap side of the guitar I measured the gap with feeler gauges and it's about .026 in. So sides of of .08 would come down to .054. that's pretty darn thin though it is only in one spot. My new hypothesis is since I used Super Soft 2 I perhaps crushed the sides during clamping and they have no memory to some back flat. So at this point I have no choice but to do some creative sanding, some compromise, and just blend it in best as possible.