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New bridge going on a top with a pretty good belly in it. I'm afraid that clamping a bridge with a truly flat bottom will cause too much stress on top/bracing and cause a crack. Is it wise to sand the bridge bottom on the actual top of the guitar to get some of the contour before clamping?

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Hi, Brian. Could you give us a little more information? Is the guitar a classical or and X-braced steel string, for example? Can you give us an estimate of the amount of curvature of the top over the length of the bridge? Is this a guitar that you are making or is this a bridge replacement repair job?

Cheers,
Bob
Hi Bob...Its an x braced steel string...bridge replacement...belly is just shy of 1/8". Maybe I should do my best to flatten the glueing area on the top instead? I just cant get a dry fir that looks clean and tight.
Hi,
I wouldn't flatten the bridge area of the top. In my limited experience, a bridge gets a countour sanded on the bottom to match the guitar top. i have a piece of 3/4" plywood that i sanded to have a belly of an average guitar top. i use that as a sanding block to shape the bottom of the bridge, Then i fine tune the bottom of the bridge by laying a piece of sandpaper facing up, right on the guitar top and scoot the bridge on top of that until I have a nice clean joint. Sometimes it goes quick.. sometimes it takes a while, but it comes out nice in the end.
good luck!
That it's x-braced makes it a lot tougher. By design, that area is just not very flexible at all, so there's no way that it's even going to compromise with a half squeeze and half shape-the-bridge approach and stay put very long. Something will give, either the top or the glue joint. The added tension on the top in that area wouldn't be all that great for the tone, either.

With that much belly in the top, there might also be a bit of a challenge in getting the right action. Can you estimate how high the saddle has to be over the top to get you the right action? It might be that the situation calls for a bridge that's not as tall as standard.

In that case, the prudent course might be to make a new bridge from scratch rather than adapting an off-the-shelf replacement. The reason relates to the depth of the saddle slot. If you take 2.5mm or so of thickness off the bottom of a replacement bridge, you're running the risk of making at least the center portion of the saddle slot way too thin and the bridge could fail. You might want to draw out a schematic of the relationship in heights for the fingerboard, the top and the action/saddle top to figure out just how thick the bridge is going to have to be, the depth of the saddle slot, etc. Then add the 2.5mm or so extra for the belly and get out the chain saw.

This, of course, is the long route, but I think it's the surest path to getting the bridge replaced in a stable configuration. As for taping sandpaper on the top to aid in contouring the bridge, I wouldn't do it. Too much of a chance of errant grit bouncing around elsewhere on the top and scratching it. You're better off hogging out the contour using scrapers ;and sanding sticks or curved forms and then refining it away from the guitar. You could even make a replica drawing of the arch and make an ad hoc sanding form from it.

Bob
Oh, yeah,(as if I didn't go on long enough) what was the nature of the bridge failure in the first place that lead to the need for a replacement bridge? Was it the original bridge or a replacement that failed? There might be some clues there. It's just that 2.5-3mm is alot of belly for a top over just the span of the bridge and it makes me suspicious that something else might be going on with the top.

Bob

P.S How about someone else weighing in on this. I think the water is about to go over the top of my waders.
I wonder what was wrong with the old bridge?

I have a fixture that I have made to hold the bridge for my drum sander that I put a small couverture in the bottom of the bridge. Two reasons I do this , I want a very flat bottom with the curve that will fit the top. I want the curve a little more than the top so the ends of the bridge will press down tight as that is the hardest place to clamp and hold until dry. I will try tomorrow to get some pictures of the fixture for the ones that are into this thing. It took a long time to make the fixture but the time saved is well worth it.

Use the old bridge if possible as it will save a lot of time.

Ron
Man!!! I think you guys are over thinking this situation. Usually the area of the bridge should be flattened a bit to remove at least some of the hump. It helps to replace the bridge plate at the same time, or at least cap the bridge plate with hardwood with the grain parallel to the top wood. The parts all succumb to the string tension over time, and that usually needs to be remedied so the parts all go back to where they started, or nearly so. This is assuming all the parts are otherwise healthy and not damaged.

I usually replace an old bridge (Martin, Gibson, Guild, etc.) with a new one with a flat bottom, just like they would do at the factory. This helps keep the top flatter like the guitar was designed. The stresses will cause the bridge to flex and eventually curve a bit, up on the back side and down on the front.

Unless I'm missing some salient points to this discussion I think that fitting a bridge to a bulged guitar top is enabling the demise of the top.
Some good points here, Michael. So. I think that some guidance for Brian on how to remove the hump without damaging it would be in order. I was pondering ($3 word for over thinking) how you could do that when the water came over the top. I will now be a listener-in.

Cheers,
Bob
Michael is on the mark here. Arching a bridge to fit a bellied top is bass-ackwards. Are the X-braces loose? Is the bridgeplate cracked, or plywood? Is the top plywood? Glue the braces or replace the bridgeplate to flatten the top and glue on a standard stick-in-the-throat flat bridge. If the top is laminated, a new flat ebony bridge will go a long way to flattening it alone.
Assuming there's nothing else wrong (cracked braces, split bridgeplate, etc) I've had good luck using this little set of wonders from StewMac. You heat 'em to 150°, dampen the wood underneath the bridge area and clamp as needed. They won't completely remove a bad bellying, but they go a long way in bringing it into something reasonable.

http://www.stewmac.com/shop/Tools/Clamps,_support_tools/Thompson_Be...
Seems to me that conventional woodworking practice has gone out the window here. If I were to say to one of my cabinetmaker colleagues that they should glue a flat piece of rosewood onto a curved and braced piece of spruce which has taken a set over time and is under tension their response would be along the lines of "this will end badly". You will be able to do it initially under clamping pressure but our business give 24 month guarantees on our structural work and I'm not about to risk our reputation by dodgy practice.

Best try to flatten out the top (the Stewmac belly reducer may help as indicated), and regarding conformal bridge bottoms: if you have no alternative go ahead and make a new bridge with a curved bottom to match the face of the soundboard - use Stikit sandpaper over safety tape (black plastic low tack tape used in the aluminum siding industry to protect new surfaces) stuck to the old bridge area to accurately/conformally shape the bottom of the bridge (this is also how to shape the feet of an archtop bridge - as per Dan Erlwine). A poorly matched bridge bottom will not glue properly and is a recipe for failure in this situation.

The other thing this allows is that undersaddle transducers have half a chance of working if the saddle slot bottom is not distorted by being forced into an arch - but , the precision approach here is for the top to be preloaded and a jack adjusted in the box to maintain the top loading for saddle slot bottom machining with the bridge glued in situ.

However, all this is, at best, the best I can offer - most bellied tops/bridge replacements I see have the problem that the front of the bridge nearest the soundhole stays relatively flat because of the concentration of bracing in that area and the back of the bridge, over time, assumes a curved shape where the belly starts - this gives a complex topography to the area to be glued and is just, simply, a problem which has a very time consuming fix.

Bottom line with this kind of work is that it all looks OK when we initially do it but if it fails over time the customer may not wish to give me a second chance and I will not know whether my repairs have integrity/longevity. I worry about this kind of work. Rusty.
So, tried contouring the bottom of the bridge to match the top, but I did all of the sanding behind the actual bridge location. The problem, as noted by Rusty, is that the belly starts at the pin holes, so the front of the bridge area is relatively flat, and the back half is arched. So the bridge began to fit the back half better and the front half worse. FUN!
THe StewMac belly reducer concept looks good, but $100 is out of the budget right now. I've already done some VERY light sanding to the bridge area of the top, so I will probably try to get the top as flat as I can using both methods.

Its a 40 year old x-braced steel string. Bridge plate looks solid and strong. The bridge looks to have been replaced at one point, then shaved down considerabley. (Total bridge height at the front edge was only 3/16).
It didnt really ever fail. When I first got the guitar, I strung it up and the tone on the bass end was fantastic, while the tone on the treble side was weak. In addition to a poor break angle, I think that the saddle was biased toward the bass end of the slot (due to more force from bigger strings). A flat bottomed saddle wont sit flat in an arched slot.

the saga (and the learning) continues.

Thanks for all the replies on this!!! More to follow I'm sure.

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