Fitting the bridge on a arch top mandolin.

[kkmm]

I had a mandolin that I repaired, and the bridge does not fit the top at all.
I have done bridge fitting on many mandolins as long as it is not way too far off using the proper method (shown in FRETS.COM).

The last one is really hard to fit, after sanding for 20 mins, I gave up, there are still gaps between the bridge and the top.
I came up with an idea: use epoxy to fill in the gap. Here is what I did:
1) lay piece of plastic sheet between the bridge and the top
2) put on blue tapes on both sides of the bridge
3) spread thick layer of epoxy under the bridge, then lay the bridge on plastic sheet (i.e. on the top sound board where it's supposed to be)
4) tune the strings
Leave it overnight, the next day, I take out bridge again, remove the plastic sheet, the blue tapes and sand off some expoxy residue on both sides of the bridge (then put it back on , etc...).

It was worth a try for me as the worst case, I only need to buy a new bridge and redo, but it works well so far (for the last two months).

I am wondering if this is a good method or a crazy method to avoid.

[billhay4]

I think this is a good method that has been successfully tried before. It certainly gets the contour right.
Bill

[multidon]

Your method would fill the gaps but I think your sound isn't as good as it could be. Those places where the epoxy contacts the top will not transmit vibrations to the top wood as effectively as wood to wood contact. Epoxy is plastic, it isn't resonant, it actually dampens vibrations. You have to ask yourself, if this is a good thing to do, why isn't this method in repair books? On the FRETS.COM Web site? Why aren't professional luthiers and repair people doing this? Or Gibson, or Weber? If it was a good thing wouldn't other people be using it? I think you should avoid this in the future. There is no substitute for wood to wood contact. I believe you gave up on your sanding too soon. I have spent over an hour sanding a bridge to get it right. If you are impatient you can use a faster-cut sandpaper but then you have to be more careful not to go too far.

[bmac]

From your description it is hard to tell what the problem was.... Unless the top of your instrument is really peculiar in shape the bridge sanding will work. But from what you described you simply stopped too early, or were not holding the bridge feet in proper alignment with the top. If you ran out of steam then perhaps your sandpaper was not a coarse enough grit to start with. You mentioned 20 minutes and then gave up... It has always taken me roughly an hour start to finish.

Not that your method with epoxy wouldn't work. It most likely will. but it creates a cosmetic issue which I suspect negates the time saving advantage. I tend to agree with Don.

[kkmm]

I did not know that it could take more than one hour to fit the bridge to the top.
I usually spent about 15 to 20 mins on previous mandolins. OK, I learn something new today.
The epoxy method does work (I play this mandolin for more than 2 months now after this "fitting"), may not give the best result as you guys pointed out.
I will take time to redo this, this is not something irreversible.

[HoGo]

Wood to wood contact is overrated. If you have 90% contact doesn't imply you're going to get 90% of sound. And, BTW, I recall Charlie Derrington mentioned he used some kind of epoxy to restore Bill Monroe's Loar. Sometimes, when top is very unevenly deformed, proper fitting of bridge may be too time consuming and not worth the effort. This is not something one want to see on expensive instrument but I think it's OK for lesser instruments.

[Stephen Perry]

1. You're not going to get wood to wood contact without removing finish.

2. A key worry is internal reflection at the interface of dissimilar materials. I would worry about the epoxy.

3. Chalk fit like a violin bridge, no worries then. Will take a bit of skill and time.

[8ch(pl)]

I have done this type ofbridge fitting several times and it is a standard for me. I posted about doing this here some years ago. I use waxed paper for a release device, but the plastic will work as well.

Epoxy is very hard and I believe it will transmit sound well. It is important to get the bridge bottom surface to as close as possible to a match with the top of the instrument. This will allow only a very thin layer of epoxy to fill gaps. If the strings are brought to tension like kkmm states in his post, it will clamp down the bridge to reproduce the actual contours present when the instrument is in use.

One difference that I have done is to clean off most of the excess epoxy when it is gelled, but not fully hardened. It will come off by scraping the sides with the edge of a sharp knife. I try to leave a very thin amount to sand out after hardening. Epoxy is not that easy to sand by hand.

The Jigs that some folks use to sand bridges , in my opinion, are not all that accurate, the thickness of the sandpaper will prevent a true contour match. They will get close, but the thinner the sandpaper, the closer the match. This is just my thoughts, i am not trying to disparage the Jig or discourage it's use.

The use of Black Epoxy on an ebony bridge will improve the look.

[HoGo]

I didn'twant to get any deeper into this... bu actually there is NO direct wood to wood contact in ANY standard mandolin. There is always a layer of plastic in the way. People call it finish. Sometimes it is considerably thick.
I'm not sure what you mean by "internal reflection" I think that may be something happening with light or at completely different frequencies. We are in hundreds of Hz here. Most of what is actually happening is that the bridge moves (rocks or whatever) and it takes soundboard with it. Mostly simple physics at those low frequencies. Since the top is loaded by all the forces (string pull and bridge pressure) it tends to follow the movement of bridge. If there are larger gaps they may cause buzz or loss of energy transfer as the gaps close and open periodically. IMO, any internal loss of energy in thin puddles of epoxy (or within ebony) really doesn't matter here.
PS: I've never used epoxy fitting, I just don't see reason not to do it in some situiations.

[Stephen Perry]

The physics isn't all that simple. Check out marimba bars, for example. In a standard bridge, the saddle is especially important as an independently resonant piece. The base as well. Lots going on, but not necessarily all of it at the contact surface.

I'm not concerned about internal reflection from the contact between dissimilar materials myself, but haven't dismissed it as an influence. I like to minimize the influences.

[Philphool]

kkmm

I used exactly your epoxy method to fit a bridge to a Michael Kelly mando some years ago for the same reasons you gave.
Worked okay. I wouldn't use it for a really nice instrument, but there was certainly no worsening of sound from the mando that I was fixing.

But I still wouldn't use the method on a mando that I really cared much about, although I'm not sure that it's a particularly bad thing to do. (I sure sound wishy-washy)

[multidon]

This has morphed into a very interesting thread. I really have never heard of anyone using this technique before, and now I am hearing that some do. But I am hearing it's ok "in some situations" and " I wouldn't do it on an instrument i care about". It it's such a great technique, why isn't it's use more widespread? Why doesn't everyone fit their bridges this way, in all situations and on instruments they care about? Maybe just maybe it's because... it just isn't as good as old fashioned sanding? Those who are advocating this technique are at the same time dissing it with qualifiers. I say, just take the time and do it right, no shortcuts. And as far as those who say there is no wood to wood contact, i think you are being disingenuous and you all know exactly what i was talking about. The finish is very thin (at least, it should be), and is not enough to impede vibration, especially in the case of nitro, which is akin to wood on a molecular level. The vibrations of the bridge eventually take off at least some finish under the feet (that mark it leaves when you change its position proves that). Finish isn't the tone killer that epoxy would be, IMHO. Of course i could be wrong about all of this. It wouldn't be the first or last time.

[Marc Berman]

If epoxy is such a "tone killer" then I think Peter Mix's carbon fiber mandolins have a problem

[Mandobart]

I'm no luthier, but I've had good results using the StewMac jig.

[belbein]

I'm agnostic on whether using epoxy deadens sound or not. However, a few notes on using epoxy. First, if you're going to use epoxy just as a filler, not as a structural agent, get yourself something similar to West System's 407 or 410 filler; mix it with 2 part epoxy until it's peanut butter thickness. I use waxpaper as a releasing paper--the stuff might stick to plastic and make a mess. And make sure that within about 5 minutes after you put it on, that you scrape all of the excess off your bridge. It might be more than 5 minutes--it might be 20--but when it starts to harden, it seems to be a geometric process and goes very, very fast. This stuff is easier to sand than plain epoxy and some other fillers but "easier" is like, "It's easier to build the Great Pyramid of Giza with hand tools than the Golden Gate Bridge."

As far as the "if this is such a good idea why doesn't everybody do it" argument ... I think that it is certainly true that the age-old tried and true methods have much to be said for them and probably deserve what we lawyers call a rebuttable presumption. BUT ... if the first guy who made a mandolin had had expoxy, I'm not sure that he would have used hide glue. It's true that hide glue has its advantages, as it turns out, but that's not why they chose hide glue. It's simply all they had. Similarly, in boat building: the reason they used pegs and spikes and long, straight trees was that they didn't have fiberglass, expoxy and aluminum for masts. It doesn't mean that you can't experiment. It just means you don't want to experiment on someone's Lloyd Loar #00001 or whatever.

If you have questions about expoxy and fiberglass, feel free to contact me. I have no connection with West, though I have spent so much money on their stuff that I should own the company. I'm no expert but can tell you what's worked (and not) for me.

[kkmm]

I knew about the Stewmac jig for fitting bridge. I don't have one but use something similar (home made) to fit the bridge. This works for me on many mandolins I have gone thru. Only this last one is quite challenging, with thin gap here and there and, being exhausted, I use the epoxy method to get over with. I have no lack for sanding papers (used quite a lot when I built an ukulele almost from scratch). This mandolin is now my favorite, i.e. I play it a lot more than the others.
Yes the sound may not be as loud as it could be (I have no idea how loud it was as it come to be in a damage state, I had to repair it). However, loudness is not really the key to good music (for me), it's more of the dynamic, interpretation of the song, etc.. that makes good music.

[Stephen Perry]

Chalk fitting should be superior to sanding.

How about bondo?

[8ch(pl)]

I think we are missing the point in just how much this process is supposed to do. The bottom of the bridge needs to still be brought to as close as possible a fit to the top. Epoxy will penetrate the wood about 1/100th of an inch, according to what I have read in an article in Wooden Boat Magazine. Then the gap in the fit should be very small, probably about .050 or .060 inch. The mandolin sitting level while the epoxy is setting should allow the adhesive to bridge a gap of this size.

No filler material is needed. It would be ideal to fit the bridge dead on to the top by scraping and /or sanding, but this is not always possible. I would use this method on any mandolin that I own. My only Carved top being a Samick A style is beside the point. My Mid Missouris have not required it.

[Stephen Perry]

Why wouldn't chalk fitting always be possible?

[StevenS]

The choice of materials used in the tone generating mechanisms and how they are fit together can have huge to subtle impacts upon the sound of an instrument.

This (shortcut) approach is adding a layer of material that is very different from wood, much harder and stiffer, adding mass rather than removing it, and avoiding the reductive process of shaping the base of the bridge for optimal response (which removes mass, changes the shape and maximizes the accurate fit between surfaces.

As with many "shortcuts" the end result may be a detour away from the real goal rather than toward it -- if your desired goal is maximizing the tone and responsiveness of your instrument.

Which means that once you better understand what you are trying to achieve with your instrument, you are farther from that goal rather than closer . . . and, in this case faced with the additional task of sanding back hardened epoxy or correctly fitting a new bridge.

Steve

[Stephen Perry]

Sanded fit is a shortcut, too.

[HoGo]

Sanded fit is a shortcut, too.

But don't tell it to Frank Ford, he's using sandpaper as well.
IMO, it really doesn't matter HOW you remove the material. Once the bridge bottom is polished and waste is in the bin no one knows whether it was removed in dust form or shavings.
Chalk fitting is always possible but on $100 mandolin. How much would "proper" fitting cost. I can imagine epoxy fitting would take me 15 minutes or so (not counting the setting time of epoxy).

[Stephen Perry]

I don't have any issues with people using sandpaper. I'm just a violin guy. Having fitted a goodly number of violin and mandolin bridges, always looks that sanding is a shortcut. Chalk fitting gets whatever fit one is willing to aim for. Doesn't really matter what the fit is for. I've used the general approach on boats, engines, guns, etc. One could chalk fit and sand, but what I generally see people doing with sanding is moving the bridge around on a piece of sandpaper. Usually with a jig. I do that as a second step to get the angle of lean set and remove material for a rough fit. Then I work with scrapers. One could almost always do a little more. One can also set up a fit to be closer to right considering the distortion under load. I do that for cello bridges. The tops distort under string pressure.

The economics of the situation certainly figure in. One of the reasons I don't carry $100 mandolins. I prefer to like the things leaving my shop!

A real test of the effect of filler would be nice. Would indeed figure into the economics of the situation. A test of 50% contact, 70% contact, 90% contact would also be helpful.

[8ch(pl)]

I agree with everything you say on that Stephen, it is the way it has been done since Amati. using scrapers etc to remove the proud areas across the bottom of the bridge. The jigs are only accurate to a point. If the skill is lacking in an owner (like me)who wants a good fit, the epoxy is a viable option, Bondo, thick Epoxy etc. is not needed.

I worked for many years as an Aircraft Refinisher, doing fiberglass repairs painting and markings on several different Aircraft, but mostly on Sea King Helicopters. The Sikorsky Sea King has a lot of Fiberglass panels and skin areas. It was just a step for me to adapt the experience to my mandolin bridges. It would appear that others in the Cafe have found the method independently.

I make no claims to being a luthier.

[NAS]

If epoxy is such a "tone killer" then I think Peter Mix's carbon fiber mandolins have a problem

Because there is epoxy then there is Epoxy. Just the same as with paints and lacquers etc.

Structural pre-pregs that need to be cured in an autoclave are very stiff and also somewhat resonant, the sort of muck that is sold as 5 minute epoxy at DIY stores has all the structural and resonant properties of rubber. The original 24 hour Araldite is better but still not hard, that is why it is difficult to sand, it deforms and does not sheer under the action of the abrasive grains.

In between there are thousands of products all with their uses. I have used two part epoxy based adhesives to build both aeroplanes and racing cars (serious sports prototypes, LMP etc) and used appropriately they are wonderful, however for fitting bridges, I don't think so!