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how to rabbit bridge
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Author:  mikemcnerney [ Mon May 15, 2017 12:34 pm ]
Post subject:  how to rabbit bridge
I did a search first 7 found a lot on gluing bridges but have yet to find a method to cut the rabbit/rebate on the bridge perimeter. I've hard anywhere from .001 to. 005 deep
and .008 to .062 wide.
My bridge has a curve bottom so routing seems awfully tricky; maybe some kind of scraper
(razor blade) with a guide?
Please help
Author:  Colin North [ Mon May 15, 2017 12:54 pm ]
Post subject:  Re: how to rabbit bridge
This is what I use, upside down proxxon router(plunge spring out) with a 3mm bit, with a micro-adjustment screw (locking, underneath, bearing on a steel plate)
The "table" is radiused to my 25' tops. The bearing at the back is a spare laminate trimmer guide I had, and regulates the width of the rebate.
Rebate depth is set by trial and error, using small pieces of binding cut-offs or similar, adjusting depth of cut until it just slips over the finish.
Got the idea from previous OLF threads, esp. re Dave Collin's use of a similar type using a dremel. just I wasn't using this router.
In use it is real easy.
Author:  J De Rocher [ Mon May 15, 2017 12:59 pm ]
Post subject:  Re: how to rabbit bridge
A search of the forum for "rabbet bridge" pulls up a number of threads on this topic.
Author:  Hesh [ Mon May 15, 2017 1:36 pm ]
Post subject:  Re: how to rabbit bridge
Mike for simplicities sake rabbiting bridges is optional. Of course some of us prefer to do this method because we believe that it makes for more wood-to-wood contact resulting in possibly.... a superior bond for a guitar bridge. Collings and Taylor also use this method. Some of us also guarantee our work. For us it makes sense but likely doesn't for everyone.

The vast majority of guitar bridges have not been rabbeted and the music goes on anyway.

If this bridge reglue is a one-off and not something that you want to tool-up to do repair work every day for I wouldn't worry about it. Instead I would simply expand the bare wood area under the bridge to very close to the bridge perimeter, work on a decent dry fit, do a final scrape, understand and abide by the requirements of the glue of choice and be done with it. Next....

A decent job of gluing on a bridge without a rabbit can also last a lifetime.
Author:  mikemcnerney [ Mon May 15, 2017 2:56 pm ]
Post subject:  Re: how to rabbit bridge
it was the idea that i had to make a base to the given radius ie25ft. ish that was nagging at me.
This is the washburn bridge I posted about last week so i think I will just go for the exact fit thingy.
I am not becoming a repair guy.
thanks
Author:  James Orr [ Mon May 15, 2017 3:40 pm ]
Post subject:  Re: how to rabbit bridge
Consider using a scalpel to trace the bride along the top and to cut through the finish, but be careful not to cut down into the wood.
Author:  kencierp [ Mon May 15, 2017 3:54 pm ]
Post subject:  Re: how to rabbit bridge
I shave a tiny bevel all along the perimiter, which effectively reduces the bridge foot print -- been doing that for many years
Author:  david farmer [ Mon May 15, 2017 5:39 pm ]
Post subject:  Re: how to rabbit bridge
Don't be a victim of GPS!(glue patty syndrome)
Get wood to wood contact!



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Author:  Colin North [ Mon May 15, 2017 6:44 pm ]
Post subject:  Re: how to rabbit bridge
Hmm, someone forget to do the rabbit/rebate on the bridge?
Author:  Woodie G [ Mon May 15, 2017 8:12 pm ]
Post subject:  Re: how to rabbit bridge
We just finished a reglue on a Collings, so this is fresh in my mind:

- We use a 6" x 8" x 1/2" piece of MDF that is radiused to the top radius (28' for new production, but may be flat on some other brands, depending on factory practice) with 5/16" clearance hole and a Ridgid trim router with fine adjustment mounted underneath

- We mount a 1/4" spiral down-cut bit and use spruce bracing scrap to set depth using a micrometer to measure the depth of cut. We set the depth by measuring the depth of the pocket with feeler gauges, and mill to the deepest measurement.

- Once set, we slide a freshly made guide fence (more like a finger) into the bit as it is operating to mill a zero clearance relief cut, stop the router, then set the width of the rabbet and clamp the fence

- The fence is just a 4" long x 5/8" wide x 3/8" thick scrap of spruce with one end rounded in plan-view to 5/16" radius

- We make the long grain cuts first, followed by the cross-grain, and then inspect and repeat if necessary

- We use a scraper to adjust the depth of the rabbet, but always keeping depth as close to the pocket depth in that area as possible

- We do a dry clamp up to check the radius of the corners on the rabbet (this gets done with a sharp chisel)...we selectively scrape the bottom of the bridge or the rabbet if we see a gap to fit things, but the most important element of the fit is great wood-to-wood contact.

One thing about the Collings ...the bridge had already been reglued by a local builder, but the rabbet was not deepened to account for the scraping done to clear the patch, so there was about 0.010"-.015" of dead white glue across the entire rear edge once the bridge was removed. The lesson here is that pocketed bridge prep and reglue, as well as removal is different than a net-sized glue-up. Poor knowledge of the mechanics of the Colling bridge joint and the necessary repair considerations on that first repair cost the owner a couple extra hours of labor to make things right.

Another point on pocketed bridges is that if gluing with Titebond or similar glue subject to cold and hot creep under load, leave just a bit of extra width (.020"-.030") in the rabbit on the front edge to accommodate that movement without shearing the finish - the reduction in gluing area is in not significant in that area, and add a beveled edge there on the lacquer as well. That way, if the bridge moves, it will be less likely to separate the finish. We see a few guitars a month with serious separation of lacquer in front of the bridge, and it's usually a reliable marker for a Titebond or AR/PVA-glued guitar that got too hot or has a thick, creep-prone glue line under the bridge.

As others do here, we have gone to making the rabbet and pocket as close to a 1/32" undersize as possible...while the boss remains convinced that bridge issues are seldom related to gluing area, there's no reason to give up any without good reason.
Author:  david farmer [ Mon May 15, 2017 10:53 pm ]
Post subject:  Re: how to rabbit bridge
Glue patty syndrome on a collings: (happens to the best of them.) unlike the instrument in the first set of photo's, Collings covered the cost of repair [:Y:] )
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Woody G, your post illustrates the intricacies and difficulties making a good rabbeted bridge joint. I believe it is more, not less difficult to do well than cutting the finish back to the very perimeter of the bridge. The rabbet tends to hide trouble.

I have a rabbeting rig in the drawer when I need it but 90% of the time I scrape around the very outside of the bridge with the back of a #11 blade and glue the whole thing on. Even on a Collings. It looks fine and it feels good!
I think there is an undue fear of doing it this way. Especially on a new instrument when the finish is likely softer and more forgiving.

I've already decided on the first ad slogan for my giant guitar company. "Farmer Guitars, We glue the whole friggin bridge on!"
What do you think? :)
Author:  Woodie G [ Tue May 16, 2017 6:40 am ]
Post subject:  Re: how to rabbit bridge
Those photographs are marvelous, Mr. Farmer! Very close to what we saw on the first Collings with lifting bridge in the door during my tenure here - a pocket that was cut perhaps 3-4 thousandths too deep, and a rabbet not adjusted for that difference. Interesting to see that the 'glue patty' pictured (so wonderfully evocative a term!) appears to be of that thickness or greater at the rear edge of the bridge, but did not form at the front edge due to lack of contact between the fi,s on the patch and bridge.

I would agree with you on relative level of complexity and just sheer finickiness of a well executed pocketed bridge installation - although I've only been in this shop for a bit over two years, and part-time at that, it would appear to me that the process is more time-consuming and has more potential points of failure than a traditional job. On the plus side, the appearance is cleaner, there is a potential for a better joint with a very thin glue line (although that potential appears to be unrealized in the case of your second set of photos), and a reduced risk of expensive finish repair and touch-up work if the bridge joint does fail due to shock or creep.

I like your slogan - but Legal would insist on adding disclaimers:

"In no way does the limited liability corporation known as Farmer Guitars suggest, imply, or otherwise state that a partially glued bridge is any more or less legitimate a bridge attachment choice than any other method, including - but not limited to - bolts, pins, locking wedges, force of habit, aerospace epoxies, or acts of one or more captive demonic entities. Farmer Guitars does not condone the practice of employing physically or emotionally enslaved, bound, or captive laborers - demonic or otherwise - and complies with all applicable Federal, State, and local fair wage and hour laws and regulations."

Perhaps Mr. Orr can lend a hand to clean up the language a bit - just a stab in the dark here!
Author:  david farmer [ Tue May 16, 2017 10:32 am ]
Post subject:  Re: how to rabbit bridge
I think glue patty syndrome can also caused by not getting the parts clamped together in time with AR glue. It's forgiving with open time but there is still a limit. I think That may be what happened on the Collings above. It had a rabbet at least close to sufficient depth.

On reflection, My giant Guitar company probably would need a legal dept. One day an employee sucked into the buffer is bound to get upset. Reason enough to stick with a one man repair business.
Author:  James Orr [ Wed May 17, 2017 11:13 am ]
Post subject:  Re: how to rabbit bridge
Woodie G wrote:
Perhaps Mr. Orr can lend a hand to clean up the language a bit - just a stab in the dark here!

:D
Author:  Mike Franks [ Wed May 17, 2017 6:25 pm ]
Post subject:  Re: how to rabbit bridge
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Here is my simple fixture which has worked for a few hundred bridges. I use an air pencil die grinder with a 1/8" diameter up-cut router bit. The maple plate was radiused using my top radius dish. The finger fence is adjustable and made from a micarta nut blank (what else are they good for?). The die grinder is adjustable for depth of cut.

Mike Franks
http://www.mjfranksguitars.com
Author:  Mike Mahar [ Thu Jun 01, 2017 9:14 am ]
Post subject:  Re: how to rabbit bridge
Cutting the rabbet seems to be pretty straight forward using the jigs described above. How do you mark the pocket outline in the finish? Do you have a template for your bridge that is 2mm smaller on all sides? Is there some other way that I can't seem to think of?
Author:  david farmer [ Thu Jun 01, 2017 10:01 am ]
Post subject:  Re: how to rabbit bridge
One method is to mark final position of the bridge with some tape, jog the bridge inboard by eye and use it as the template to score. Justs remember to stop short at the ends.

Photo of rabbet reduction using the Above method.
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Author:  mikemcnerney [ Thu Jun 01, 2017 10:31 am ]
Post subject:  Re: how to rabbit bridge
that all looks self explanatory. I shall proceed thanks
MM
Author:  kencierp [ Thu Jun 01, 2017 12:58 pm ]
Post subject:  Re: how to rabbit bridge
I am not a scraper -- I prefer to mask, but here's a way to make a mask or under-size template.

http://www.kennethmichaelguitars.com/bridgemask.html
Author:  Tai Fu [ Fri Jun 02, 2017 7:37 pm ]
Post subject:  Re: how to rabbit bridge
mikemcnerney wrote:
I did a search first 7 found a lot on gluing bridges but have yet to find a method to cut the rabbit/rebate on the bridge perimeter. I've hard anywhere from .001 to. 005 deep
and .008 to .062 wide.
My bridge has a curve bottom so routing seems awfully tricky; maybe some kind of scraper
(razor blade) with a guide?
Please help


You use a hare brush...

Seriously though since the tops of a guitar has a curve, and the bottom of the bridge must have one to match, it's complicated (in my experience) to rabbet them and get them to not look like the bridge is coming off. A router or a laminate trimmer is not a precision tool and so it's not actually very easy to cut such a precise rabbet consistently (as in you will have a variation of more than .005") You will need a really good jig, and a very steady hand (the jig must be very stiff or else your hand will push things just enough to make the rabbet too deep). I believe factories who do this have fancy CNC stuff to make this a breeze.

I just score around the bridge, route off most of the finish, come in with a chisel to finish, and glue.
Author:  Woodie G [ Sat Jun 03, 2017 2:46 am ]
Post subject:  Re: how to rabbit bridge
Tai Fu wrote:
Seriously though since the tops of a guitar has a curve, and the bottom of the bridge must have one to match, it's complicated (in my experience) to rabbet them and get them to not look like the bridge is coming off. A router or a laminate trimmer is not a precision tool and so it's not actually very easy to cut such a precise rabbet consistently (as in you will have a variation of more than .005") You will need a really good jig, and a very steady hand (the jig must be very stiff or else your hand will push things just enough to make the rabbet too deep). I believe factories who do this have fancy CNC stuff to make this a breeze.


You might consider adopting a jig design and process for use closer to what has been described in the thread, and consider using another laminate trimmer or router for the job - perhaps the same one that you use for removing finish on the bridge patch, which one would hope is free of the truly horrific depth errors you described re: your failed bridge rabbeting attempts.
Author:  Hesh [ Sat Jun 03, 2017 5:34 am ]
Post subject:  Re: how to rabbit bridge
Hesh wrote:
Mike for simplicities sake rabbiting bridges is optional. Of course some of us prefer to do this method because we believe that it makes for more wood-to-wood contact resulting in possibly.... a superior bond for a guitar bridge. Collings and Taylor also use this method. Some of us also guarantee our work. For us it makes sense but likely doesn't for everyone.

The vast majority of guitar bridges have not been rabbeted and the music goes on anyway.

If this bridge reglue is a one-off and not something that you want to tool-up to do repair work every day for I wouldn't worry about it. Instead I would simply expand the bare wood area under the bridge to very close to the bridge perimeter, work on a decent dry fit, do a final scrape, understand and abide by the requirements of the glue of choice and be done with it. Next....

A decent job of gluing on a bridge without a rabbit can also last a lifetime.


Don't think that I've ever quoted myself before....;)

Rabbiting is optional AND again billions.... of bridges have been glued on without the rabbit and most of them stay put. For the ones that don't we will continue to endlessly toil by candle light with sharp chisel in hand to rake in significant economic gain invoicing clients to glue their bridges back on......

It's a tough job but someone has to do it....... :? :D

Mike specs don't matter just keep this in mind:

1) When scoring be mindful to not score top fibers.
2) Depth of the rabbit is dictated by the thickness of the finish ledge AND if some Yahoo who got there first did not dig a hole in the top to inlay the bridge......... Don't laugh... we've seen it before, subterranean bridge installation.... :?
3) Inset is personal preference. I like a bit more because I'm freaking old and can't see, Dave Collins likes a bit less because he can. What's important is that you expand the wood-to-wood gluing area or why bother? I know some of you like any excuse to make and use a new jig but every optional operation does represent some risk.
4) Regarding domes and curvature on the bridge bottom the jig or at least ours does and any jig with a radiused table will permit consistent depth routes on curved bottom bridges. We do it very nearly ever day.

PS: Big thanks to Woodie and David F. for hanging in with this thread. [:Y:] [uncle] [headinwall] :o [clap] :D
Author:  Colin North [ Sat Jun 03, 2017 5:46 am ]
Post subject:  Re: how to rabbit bridge
Quote:
it's complicated (in my experience) to rabbet them and get them to not look like the bridge is coming off. A router or a laminate trimmer is not a precision tool and so it's not actually very easy to cut such a precise rabbet consistently (as in you will have a variation of more than .005") You will need a really good jig, and a very steady hand (the jig must be very stiff or else your hand will push things just enough to make the rabbet too deep

It's not complicated at all as can be seen from the posts.
Author:  mikemcnerney [ Sat Jun 03, 2017 6:34 am ]
Post subject:  Re: how to rabbit bridge
I have wondered about those soft spruce fibers splitting off.
Is that an argument to pre strengthen those fibers, sizing so to speak, presumably with a thinned down version of the same glue you are using?
I guess i'll call titebond on Monday & see if yellow glue can stick to a 'very smooth yellow glue' surface.
Author:  kencierp [ Sat Jun 03, 2017 8:10 am ]
Post subject:  Re: how to rabbit bridge
Titebond glue and other PVA's do not adhere to previous dry glue layers -- the bond will fail.

As Hesh indicated plenty of guitars have survived without having perimeter rabbits.

As I mentioned earlier, my compromise is to simply sand a tiny chamfer all the way around, which like the rabbit privides clearance from the coating ledge.
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