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Bridge causing top deformation
http://mowrystrings.luthiersforum.com/forum/viewtopic.php?f=10101&t=53273		 Page 1 of 2
Author:  bcombs510 [ Sat Jun 06, 2020 11:20 am ]
Post subject:  Bridge causing top deformation
Hello,

I just strung up a new build and the top is deforming a bit on the bass side above the bridge. I suspect that the caul I used when clamping the bridge wasn’t as good of a fit as it should have been.

I normally use vacuum with titebond but wanted to use hide on this one. I clamped using a caul against the bridge plate along with the SM bridge clamping caul on top. The one with the screw down feet that clamp the wings. The caul on the inside wasn’t the full width of the bridge plate, so that could be it too, but I think it just wasn’t a good fit.

If I take it off and re-glue should I expect the area will go back flat? Meaning, is it a waste of time to try to fix it? :)

Or could there be other causes? I should have used the vacuum with hide anyway. I’ve done it on ukes, but chickened out on the guitar. :D

Second pic just tried to show the direction of the deformed area.

Image

Image

Brad


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Author:  Colin North [ Sat Jun 06, 2020 11:36 am ]
Post subject:  Re: Bridge causing top deformation
Was the caul inside the full width of the bridge?
IE,did it fully support past the X brace?
Author:  bcombs510 [ Sat Jun 06, 2020 12:17 pm ]
Post subject:  Re: Bridge causing top deformation
Thanks, Colin. It did not, it was inside the X and almost the full width of the bridge patch but not quite.

Yank it off and make a caul that goes the whole way?


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Author:  Hesh [ Sat Jun 06, 2020 12:22 pm ]
Post subject:  Re: Bridge causing top deformation
The Stew Mac bridge caul has some idiosyncrasies that have to be considered or it may not clamp the wings well.

Because of it's design when we tighten our clamp in the middle the flex in the caul can cause it to flex and loosen the screw downs on the wings. Crank the wing clamps more and it flexes again and loosens in the middle....

Because of this I like to use two clamps with this caul both spread out to the areas of the high e and b pin holes and the low e and a pin holes.

Nevertheless even with my two clamps if I clamp down the wings too much the center loosens because of the flex but not as much as with one clamp in the center. You get used to it and it's worked for us for hundreds of bridge glues but you do have to be mindful of this.

Where I have seen this dimpling prior is on small builder guitars that the bridge was not radiused to match the top before it was clamped and glued.

In terms of long term issues if the bridge was not fitted to the top well and that's the cause or one of the causes the bridge wings are trying to shape the top to a radius that the internal bracing is resisting.

If it were mine I would remove it, fit the bridge and clamp carefully so that the clampage... (two clamps) is not excessive in some places and not enough in others. The SM caul can be problematic with this so be careful.

It's not unusual for us to use four clamps with this caul too if the wings don't fit well and there is a reason when we can't remove anymore material from say an original, valuable vintage instrument with super thin wings.

We never went the vacuum route and don't like it. It prevents us form a) cleaning up squeeze out while it's easy to clean up and b) seeing how well the bridge is all the down all around before calling it a day and c) HHG is all we use and our goal is to have everything down and clamped in 15 seconds or less and it really helps if you can see that it's in the right place too. :)

Hope you are doing well my friend.
Author:  Hesh [ Sat Jun 06, 2020 12:26 pm ]
Post subject:  Re: Bridge causing top deformation
bcombs510 wrote:
Thanks, Colin. It did not, it was inside the X and almost the full width of the bridge patch but not quite.

Yank it off and make a caul that goes the whole way?


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One of the reasons why we fit bridges well is so that the underside caul need not extend to the wings. If you fit the bridge so that it's down all the way around with only light finger pressure two clamps as described above with the same caul and Bob's your uncle. The wings are by design supposed to be coupled with the X legs making a full length caul very difficult and time consuming to make for every different instrument.
Author:  Colin North [ Sat Jun 06, 2020 12:40 pm ]
Post subject:  Re: Bridge causing top deformation
Hesh wrote:
bcombs510 wrote:
Thanks, Colin. It did not, it was inside the X and almost the full width of the bridge patch but not quite.

Yank it off and make a caul that goes the whole way?


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One of the reasons why we fit bridges well is so that the underside caul need not extend to the wings. If you fit the bridge so that it's down all the way around with only light finger pressure two clamps as described above with the same caul and Bob's your uncle. The wings are by design supposed to be coupled with the X legs making a full length caul very difficult and time consuming to make for every different instrument.

Much the same as I do, I don't myself, although some do.
Pic of my typical caul top and radiused bottom one + wing cauls, bottom caul runs to the X braces, good fit for the bridge, nor too much pressure on the wings, finger tight only.
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=44471&p=588150&hilit=+bridge+clamp#p588150
Author:  doncaparker [ Sat Jun 06, 2020 12:42 pm ]
Post subject:  Re: Bridge causing top deformation
I moved away from the whole “screw down wings” way of doing this and now just use three or four of the StewMac bridge clamps, the ones with the cushioned leveling screw. One or two in the middle of the bridge, one on each wing. This avoids the craziness Hesh was describing, which I had experienced, and which led me to ditch the fancier upper caul. Three or four independent, and independently adjustable, sources of clamping pressure are better for this job, I now opine.
Author:  bcombs510 [ Sat Jun 06, 2020 12:47 pm ]
Post subject:  Re: Bridge causing top deformation
Thanks, all.

Here was my setup:

Image

I did have two clamps but they need to be further apart per Hesh’s guidance, but.... listening to the description of the wing screws on the caul, I almost certainly over tightened them.

Aaaaaaannnnnndddd - glad it’s hide glue. ;)

Thanks!!


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Author:  Hesh [ Sat Jun 06, 2020 12:48 pm ]
Post subject:  Re: Bridge causing top deformation
doncaparker wrote:
I moved away from the whole “screw down wings” way of doing this and now just use three or four of the StewMac bridge clamps, the ones with the cushioned leveling screw. One or two in the middle of the bridge, one on each wing. This avoids the craziness Hesh was describing, which I had experienced, and which led me to ditch the fancier upper caul. Three or four independent, and independently adjustable, sources of clamping pressure are better for this job, I now opine.


Exactly and I agree. Some odd shaped bridges the SM caul does not fit well either such as my own Heshtone bridge so just use multiple clamps with protection from making marks. Good going Don. Hope you are doing well too.
Author:  doncaparker [ Sat Jun 06, 2020 1:55 pm ]
Post subject:  Re: Bridge causing top deformation
Brad—

I thought it would be useful to scratch a little deeper into why I prefer the multiple clamp approach, without the upper caul.

When you use the upper caul, it is a relatively unyielding, monolithic piece of aluminum, that you are using to try to glue two relatively flexible pieces of wood together (bridge and top/bridgeplate). You can use the screws over the wings to try to make the caul fit the bridge better over the wings, but it is still a very stiff caul. I don’t think stiff helps here. The bridge is flexible (particularly at the wings), the top/bridgeplate is flexible. I think multiple clamping points, all working independently, is what glues flexible things together. Using an inflexible caul to glue flexible things together runs a high risk of gaps at some points, where there is little or no clamping pressure. The problem Hesh described is basically this problem. If you leave the wing screws too loose, there is no clamping pressure being exerted by this inflexible caul out there. If you take the wing screws down too far, the clamping pressure gets applied out there, and none of it gets applied in the middle. You can correct for this by getting the caul adjusted exactly right, and applying additional clamps over the wings, but at that point, why bother with the caul at all? It’s just an impediment. I put the screwed down wings idea in the category of a cool idea that doesn’t really work very well. Not for me, anyway. Give me multiple clamps, all tightened until they feel right, any day.
Author:  bcombs510 [ Sat Jun 06, 2020 2:08 pm ]
Post subject:  Re: Bridge causing top deformation
Thanks, Don. It makes sense. I only have two of those clamps so I’ll need to order a couple more, probably the longer ones that have the stabilizing foot.

I should have said in the initial post, the bridge was fit very well. I spent more time than I ever have making sure it was tight all the way around with very little finger pressure. I was paying attention here because of using hide glue. Titebond is a bit more forgiving in this area from what I understand.

I’ll remove it and try again. The build is for me or at worst it’s going to a family member so I’m in no rush.

Brad


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Author:  bobgramann [ Sat Jun 06, 2020 4:45 pm ]
Post subject:  Re: Bridge causing top deformation
I would check for two things before I did major things: with the top under string tension, make sure that you don’t have a place where the top has detached from a brace, and check the top in that area to ensure that you didn’t inadvertently sand it too thin in the distorted area. Od distortions are often a telltale of a popped brace.
Author:  Terence Kennedy [ Sat Jun 06, 2020 7:57 pm ]
Post subject:  Re: Bridge causing top deformation
Count me in as another that became disillusioned with the StewMac camp after a couple of uses. I just could not be sure of even clamping pressures. I think if you use it you need an extremely rigid clamp on the center section.

I went back to individual clamps and cauls for the center portion of the bridge and wings and a radiused caul that spans the X and supports the wings.
Author:  Clay S. [ Sat Jun 06, 2020 9:39 pm ]
Post subject:  Re: Bridge causing top deformation
My thoughts are similar to Bob Gramann's - Possibly a thin spot in the top or a loose or weak brace. Also the shape of the bridge may be contributing to the problem. You might be able to add a finger brace to help spread the load.
Author:  bcombs510 [ Mon Jun 08, 2020 10:33 am ]
Post subject:  Re: Bridge causing top deformation
Heated the bridge up for about 15 minutes and it worked off pretty easily.

Image

Next question - now that the finish is cleared under the bridge footprint, do I need to do anything differently to fit the bridge? Should I clean up the bridge and fit it by sanding just below the bridge location where there is still finish? Or just stretch a piece of sandpaper over the footprint and fit it like normal?

I thought about maybe cutting a piece of tape that is the size of the area where the finish is removed to try to “level” the surface, then sandpaper on top and fit like normal.

I don’t want to over think it (too late probably :D).

Brad




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Author:  Barry Daniels [ Mon Jun 08, 2020 11:01 am ]
Post subject:  Re: Bridge causing top deformation
Any of those steps would be dangerous on a buffed finish. I would scrape the bridge with a razor blade and then check the fit carefully by eye, and maybe fine feeler gauges.
Author:  bcombs510 [ Mon Jun 08, 2020 5:06 pm ]
Post subject:  Re: Bridge causing top deformation
Barry Daniels wrote:
Any of those steps would be dangerous on a buffed finish. I would scrape the bridge with a razor blade and then check the fit carefully by eye, and maybe fine feeler gauges.


Good point. I could cover the entire area with the SM low tack tape and then a 1/4 sheet of paper on top. I don’t know if I have the talent to use a razor blade or scraper and do it by eye. :)

I will try though!

Also, I need to make the caul for the inside. I’m thinking making it of basswood since I have a hunk the right size and then maybe cork lined?

The caul will have to account for both the X and the finger brace and it’s all right there in the corner. You can see here the circled area of the bridge footprint, the caul will have to be notched for both braces.

Image

Brad


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Author:  jfmckenna [ Mon Jun 08, 2020 6:57 pm ]
Post subject:  Re: Bridge causing top deformation
I would second the suggestion to double check to see if the brace is loose. I posted something very similar to this last year or so and thought it was the clamp set up as well and it was driving me crazy. It wasn't till I looked under string tension using mirror that I saw it was indeed loose.

As for bridge cauls for every guitar I make, I make a caul out of mdf that fits the arch of the top if there is one and is tapered to fit the xbrace so that a clamp will have contact with the X.
Author:  bcombs510 [ Mon Jun 08, 2020 7:03 pm ]
Post subject:  Re: Bridge causing top deformation
Thanks, John.

It’s a good point. I inspected everything but did not do it under string tension. The bridge is off now, so I’ll have to deal with that first. :)

I took a look at a couple different ideas after work for the caul on the inside, but I’m not getting it. Should I just make three pieces, one for the bridge plate (which I already have) and two small blocks for outside the X and under the wings?

Anyone care to share a pic of their setup?

Brad


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Author:  doncaparker [ Mon Jun 08, 2020 7:48 pm ]
Post subject:  Re: Bridge causing top deformation
I might be labeled a heretic and burned at the stake, but if you use the StewMac clamps (3-4 of them), I think you can just double-stick tape a few scraps of wood in the right inside places to spread the pressure locally for each clamp, rather than trying to fashion a single inside caul. Like I said before, you are gluing two flexible things together. I think stiff cauls hinder the process, and I include the inside caul in that criticism.
Author:  bobgramann [ Mon Jun 08, 2020 8:00 pm ]
Post subject:  Re: Bridge causing top deformation
I just went back and read the first post in the series again. You don’t need clamps and cauls for hide glue. I went to using vacuum for bridge gluing years ago because, every now and then, my clamping setup would distort the bridge and leave an unexpected gap in a joint that fit perfectly before clamping. I have used hide glue the whole time—maybe 100 bridges so far. My process: I make sure the bridge fits the top perfectly. Since I don’t trust my eyes, I use a piece of 20 lb copy paper (3 mils thick) as a feeler gauge to make sure there are no gaps. I heat the bridge with a heat gun until it is quite warm (but not burning hot) when I pick it up and cradle it in my hand. I brush the hide glue on the bridge bottom, slap it on (using those tapered StewMac slippery guide pins cut flush with the top of the bridge), and apply the vacuum clamp. After 5 minutes, I remove the clamp and clean up the squeeze out with a warm wet paper towel, thoroughly dry it with a dry towel, and reapply the clamp for about two more hours. I use a roofing nail with the point ground flat to push out the index pins from inside. I don’t drill out the other bridge pin holes, ream it and slot them until the next day. The vacuum clamp does the job with no distortion. I’ve used no other clamps on a bridge since I got it. I hope this helps. And, do check for loose braces after you string it up.
Author:  bcombs510 [ Mon Jun 08, 2020 8:34 pm ]
Post subject:  Re: Bridge causing top deformation
Thanks, Bob.

I’ve only used vacuum on all the instruments I’ve built until now. I’ve used vacuum with titebond on guitars and vacuum with hide in ukes. I wanted to use hide on this guitar and should have used the vacuum but chickened out. Too many horror stories about bridges coming clean off I guess. ;)

I still can use the vac by putting a few layers of tape on the inside to cover the pin holes that have already been drilled. I may do just that.

Thanks for sharing your process. I do pretty much the same on ukes but I leave it for four hours. Thanks for building confidence that you have 100 instruments and no issues.

Brad


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Author:  bobgramann [ Mon Jun 08, 2020 8:55 pm ]
Post subject:  Re: Bridge causing top deformation
You only need a strip of masking tape over the holes on top the bridge. With that, the bridge will block off the holes in the top. 4 hours clamping doesn’t hurt anything. My vacuum pump is noisy, so I don’t mind turning it off.

I won’t promise that a bridge won’t come clean off some day. I’ve a a few customers bring one back for a reglue—always in the winter. I have to give them the humidity lecture. The top shrinks across the grain when it dries out. The bridge doesn’t shrink along the length of the grain. I would much rather have it pop off than have it crack the guitar.
Author:  Hesh [ Tue Jun 09, 2020 3:39 am ]
Post subject:  Re: Bridge causing top deformation
Brad our process to reglue a bridge is pretty simple.

1). Clean up the bridge patch of all old glue and clear the finish to very close to the foot print. We don't like going to the foot print because it can show and work that shows is lousy work when it does not have to be like that.

2). Like Barry we single edge razor blade as a scraper make quick work of fitting the bridge bottom so that the bridge and wings sit naturally down with no gaps all the way around with only minimal finger pressure. We see bridge clamps as holding things in place until the glue dries not as devices to force parts to fit that don't. Additionally it's prudent to scrape any bridge bottom within say 15 minutes of gluing in that scraping clears debris better than sanding which creates it... and there is the concept it makes of a better bond.

3). Then we glue and clamp and clean up squeeze out and hang for a day. Not telling you or suggesting what glue to use because that's not important here, Titebond original or HHG are both fine choices but have as you know very different working properties and requirements for use.

FYI we never sand a bridge to fit in on a guitar that has finish on it and that cavity where the finish stops and creates a well of sorts. We will take a bridge to the belt sander, Dan Earlywine's Dad's belt sander when there is a lot of material to remove or old glue and wood to get it to fit well but we always scrape too just before gluing. Holding a bridge on a belt sander can be a challendge but we have some tricks for this if you want to use a belt sander to help impart the radius on the bridge bottom. A single edge razor is very fast and easy though and it's trial fit, see a gap, scrape where there is no gap and repeat until there are no gaps. Not unlike fitting a dovetail in some ways.

I'm not confident that you will find any loose braces this does not look like that to me. Please do check though and let us all know what you found so that maybe I might learn something. There is no plausible reason for a new build that has not left the house, has not suffered a blow, has not experienced RH shock to have a loose brace other than a builder mistake and I know you wouldn't do that. :)
Author:  Burton LeGeyt [ Tue Jun 09, 2020 7:50 am ]
Post subject:  Re: Bridge causing top deformation
For an interior caul I made one that is oversize and a loose fit over braces/bridge plate etc... when it looks good I then kerf it pretty deeply with the kerfs situated along the grain lines of the top to make it flexible. It will conform to the interior without putting excess force anywhere that might be problematic and still allow for good clamping pressure on the top to be distributed well. I double stick tape it to the bridge plate so I don't have to fiddle with it and use 5 of the stewmac small cam clamps with small cauls I made to fit my bridge shape. I use hide glue.
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