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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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Didn't read everyones replies, but I'd glue it on flat, this will counteract the stress and flatten the top some. Tops are flexible enough that it won't crack or loosen any braces. This works for any top that does not have an excessive radius built into it, new.
The only time I would form the bridge to the top of the guitar is on some Gibsons, like the Advanced Jumbo's and probably some other models, which have a significant radius built into the top during construction.

I know this works, because I have done it many times. Never had a bridge failure yet gluing them flat.

If you need, or want to flatten the top some before gluing on a flat bottom bridge this is what I do. Lay a 3/4" thick X 4" wide board on the top, clamp it at the side with cam clamps. This will push the top down flat. Now dampen the bridge plate inside the guitar. Next place a heating pad(the kind you use on your sore back) inside the guitar and turn on some heat, probably medium or high. Those pads don't normally get that hot. Let it set like that for a couple days, maybe longer, you can play it by ear.

Jim

Jim
Jim, Do you ever slightly dampen the wood on top or the bridgeplate below to help the process while glueing the new bridge? Would this cause other problems while glueing? Maybe keep the whole thing clamped for a few days before removing??

saw your addition...thanks
Brian,
No, I never dampen the area before gluing the bridge. I pretty much use thin hot hide glue for bridge gluing, and that alone will add plenty of moisture, to the top anyway. I don't worry about the bridge plate.
I see many posters are making this job more complicated than what it really is. It's pretty much straight forward, you just glue a flat bridge on the top, keep it clamped for 12 hours, and string it up after it sits for 24 hours.
Check out Frank Ford's site, frets.com., his ideas are about as logical as they come.

Jim
I also see some posters are making the job sound a lot simpler than it is - nature, ie: wood, has a way of sorting out what works over time and what doesn't.

Call us in a few years and tell us how it went - as I said - you are unlikely to see a customer again if your initial efforts have been ultimately unsuccessful, consequently you can then say "none of my customers have come back with a problem" - unfortunately that isn't always the case. Rusty.
Rusty,
I pretty much still have contact with every customer I have had over the years, and I can honestly say none have ever had the bridge fail because they were glued on flat.

I did my first bridge reglue in 1979, or so, and it's still on the guitar.

Most, if not all, bridges are glued on flat when originally constructed. There may be a few exceptions with guitars that have extensive radius built in during construction, and possibly some custom makers.
Jim
Jim, I don't think anyone would really try to argue too hard about glueing a flat bridge onto a new guitar with an contoured top if the top has been shaped/braced to a 25' radius (pretty typical). The rise over the bridge's span of 6" is only 0.38mm. Not a big enough mash to get too excited about.

However.I believe Rusty's original point regarding Brian's post is that it takes enormous strain over a long period of time to generate a curvature in the bridge area of the magnitude described by Brian, which was around 3mm or ~7X the original arch. Mashing that arch down abruptly without relieving it some is asking for trouble down the road.

In Brian's situation, the bridge pad is probably part of the problem and the best solution is probably to remove it first and replace it with a slightly oversized pad that would cover some of the extra bellying behind the bridge. And, yes, that pad should probably be clamped up flat during glue up. Then, reassess where you are with arch across the 6' span and either sand a little arch in the bridge or not.

As for sticking a heating pad set to medium or high inside a guitar for a couple of days, I simply downright cannot imagine that. It would never get hot enough for glue joints to slip a bit to accomodate the pressure on the outside, it's nowhere near hot enough to induce the wood to bend and stay bent, and in the end, it's just a recipe for completely desiccating the interior of a guitar. Who on earth would rationally want their guitar put through that?

Bob
Bob,
Not here to argue.
But everything I described has worked for me for over 20 years of doing repair work, including putting a heating pad inside a guitar. That's fine you have a totally different outlook.
Take Care
Jim
So, the slot should be routed after its been glued to the top, correct, to ensure a flat bottomed slot.?
Yep, the top will belly some under string tension and the bridge will go with it (to a lesser extent). This is usually manifest when the low and high E strings (mainly the top E) sound a little weaker - and this is more prevalent with bone saddles which have no "give" (non-conformal).

The trick is to place a small jack inside the box under string tension and screw it up so it pre-tensions the top so the belly remains when the strings are off (this is really difficult - Stewmac has a screw up scissor jack which stops the madness of trying to get a jack to turn with the strings on). But, if you are a believer in pragmatism and not cashed up enough for these little marvels, place a ruler/straight edge across the top of the guitar when strung up , note the belly amount by the space under the ruler at the edge of the guitar and then use a cedar (ie: soft wood) post shaved to length and inserted against the the back and top braces (if you can find an aligned set all the better) to gently pre-tension the top. Once the top is approximately where it was under string tension - ie; looks like the same gap under the ruler, use your router jig to machine the saddle bottom flat. Now, this works so don't rubbish it if you haven't tried it.

Alternatively, use a co-polymer transducer such as the Fishman Matrix/Martin Thinline Gold which is a bit conformal and then use a Graphtech Tusq saddle (or similar) which has a little give. If it still doesn't work and the outside strings are weak try slightly radiusing the bottom of the saddle which will help with the Fishman co-polymer but will not work very well with the old style piezo 'section' saddles.

My experience is that trying to balance a saddle in a distorted saddle bottom environment causes me much anxiety and wastes heaps of time and generally doesn't hold for long. Routing the bottom of the saddle slot flat takes time to set up and do but is a worthwhile step. R.
You can rout it before gluing to the top or after gluing to the top, doesn't matter. I've done them both way, with no problems.

Jim
Yes, Jim, I'm sure you have no problems with any of this. However, glueing saddles to new instruments is a great deal different than working with undersaddle transducer installation, repairs or aged instruments

Further, Fishman, the worlds premier undersaddle pickup supplier, Dan Erlwine (in his writings and advice) and myself all have had problems in this area and, as we deal with professional discerning musicians on a daily basis we have had to address the problems associated with rudimentary luthiery practices. I'm not here to argue, I'm here to explain what to do when one is not so lucky as to have 'no problems'. What doesn't matter to you does not necessarily translate into no problems for other newcomers or those with less professional standing. Rusty.
Rusty,
I don't work on new instruments, or build new instruments. I'm referring to putting a new bridge on an old guitar. I've found no problems routing the slot either before or after installing.
If I'm installing an undersaddle pickup, I may have to tweek the bottom of the saddle for perfect contact. I never mess with the bottom of the saddle slot.
I'm sure your way works, but what I do works for me, so I'm just passing on what I know.

Jim

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