How to get bottom of bridge radius correct?

[Froglips]

I was attempting to sand the bottom of my bridge so it would fit more uniformly around the arch of the top of my F-Style Mandolin ... And I totally screwed it up in the process.

How do I get the radius correct so I do not have to throw away this bridge?

Thank you for your time.

[Chris Keth]

If I were doing it, I would use my thingy like this to transfer the curve to the bridge. I would approach the drawn line and then fine tune it my trial and error for a perfect fit without going too far.

[Fretbear]

The first thing is to be extremely patient and to know that arch-top bridge fitting is an art and to gain the ability to fit one perfectly will not be gained overnight or without a learning curve. Frank Ford's method of scooping out a slight cup or hollow in the bridge feet and then sanding down the edges of that at first glance would seem like an advanced technique (which it is) but it actually does end up making the entire process easier and faster once you have grasped it. When properly fitted, you will not be able to insert the thinnest of papers anywhere along the edges of the base feet.

http://frets.com/FRETSPages/Luthier/...t/fitfeet.html

[Mike Bromley]

I have successfully planted many bridge feet by the tried and true wiggle-it-on-a-piece-of-sandpaper method...

Tape a piece of sandpaper gritty side up on the top and gently mirror the shape of the bridge feet.

[Lefty Luthier]

The best way to fit a bridge is the wiggle over a piece of 220 grit sandpaper process. On occasion, I have been asked to rescue an overeager amateur builder and have found that taking a bit of West Systems high strength glass powder mixed with their epoxy into a thick glue consistency, spread it over the bridge feet and then take small strips of aluminum foil to protect the finish. Place the bridge in position and tighten the strings to pitch. Next day loosen the strings, pull the foil off, clean off any glue bulges. You get a rock hard layer perfectly matched to the curvature of the soundboard.

[Mike Snyder]

It's very easy, with the sandpaper taped to the top, to sand a rocker bottom onto the bridge. IMHO. I've seen fixtures, probably home made, that hold the bridge true as the wiggle commences. Also read on the cafe of a technique with a fixture that holds the bridge in a static position as the sandpaper is manipulated. Personally, I no longer diddle with such tasks, greatly preferring to let a professional luthier do them right the first time in a fraction of the time it would take me to realize that I had ruined a perfectly good bridge. My guy likes to set the bridge with a lean back toward the tailpiece. It's so slight to be almost imperceptable, but seems to help prevent the foreward lean that I used to have on the junk I played years ago. He built the mandolin in my avatar.

[Froglips]

The best way to fit a bridge is the wiggle over a piece of 220 grit sandpaper process. On occasion, I have been asked to rescue an overeager amateur builder and have found that taking a bit of West Systems high strength glass powder mixed with their epoxy into a thick glue consistency, spread it over the bridge feet and then take small strips of aluminum foil to protect the finish. Place the bridge in position and tighten the strings to pitch. Next day loosen the strings, pull the foil off, clean off any glue bulges. You get a rock hard layer perfectly matched to the curvature of the soundboard.

WOW!!!! These are some freakin awesome ideas!

I feel really stupid not thinking of just putting some sand paper on the top of the Mandolin and doing it that way.

As far as the glue and glass powder method goes, does that not drastically change the tonal qualities from the bridge through the Mandolin top?

Thanks for the help everyone.

[Hans]

Stew-Mac sells a fixture that I have used for years to hold the bridge at the proper angle as you sand. It's pretty cheap, and worth it if you are doing it yourself.

[Scotti Adams]

You have to be careful when using the "wiggle" method not to rock the base back and forth as you are sanding..this will cause a ridge and the base will not seat correctly. The tool Hans speakes of is the tool to use. But..Ive also made a T handle out of 2 paint sticks..drilled holes to accept the bridge post..tighten down the knurls and sand away.

[Lefty Luthier]

WOW!!!! These are some freakin awesome ideas!

I feel really stupid not thinking of just putting some sand paper on the top of the Mandolin and doing it that way.

As far as the glue and glass powder method goes, does that not drastically change the tonal qualities from the bridge through the Mandolin top?

Thanks for the help everyone.

The rock hard epoxy/glass film does enhance high frequency transmission a tiny bit but mainly it assures perfect contact with the soundboard. Been doing this for years when necessary with kudos from customers.

[EdHanrahan]

... set the bridge with a lean back toward the tailpiece. ... seems to help prevent the foreward lean ...

... fixture that I have used for years to hold the bridge at the proper angle as you sand.

Not to change the subject but: What IS the proper angle for a bridge?

I ask that both relative to the top under the base, and relative to the string break angles going both toward the neck and toward the tailpiece. I recognize that different top contours and/or neck-sets could change some of those angles without changing others.

Having salvaged some "junk" myself, I've tried for a slight backward lean between the saddle and the vibrating end of the stings (maybe 87 degrees?), and let the tailpiece break angle and the base-to-soundboard angle take care of themselves.

I also note that some bridges, and not only the cheap ones, have a slight angle between the base and the saddle. Is that intentional, or just sloppy? Some bridges have had a penciled arrow on the bottom, presumably indicating orientation toward or away from the tailpiece, and that I've sometimes disageed with, mostly to eliminate forward lean.

Again, not to change the subject but I've never seen the topic of "bridge lean" addressed, or even mentioned. Thanks!

[Dale Ludewig]

I use the thing Hans mentioned. It works great in my experience. As to angle, I pretty much go straight up (tangent to the arch). I'd rather set it to lean back a 'little' rather than forward. YMMV.

[lenf12]

Here's the link for the Stew Mac bridge fitting tool. It highly recommended and not very expensive, especially if you plan on fitting more than one bridge.

http://www.stewmac.com/shop/Tools/Sp...tting_Jig.html

Here's a picture of it as well.

Len B.
Clearwater, FL

[Steevarino]

All good ideas so far. I use a fixture like StewMac sells, but I made my own. Theirs is well worth the price, even if you only ever plan on fitting a couple of bridges. It lets you set the angle of the bridge right where you want it (usually either straight-up, or just a hair of back-tilt), and it eliminates the "rocking" effect while grinding the bridge across the sandpaper.

The only thing I would add is that I start with 80 or 100 grit, then move on to 150, 220, 320, and then maybe 400 grit sandpaper. I like the coarser grits because they get the job done quite a bit faster, and the finer grits are a good idea because, as the grits get finer, the paper gets thinner. This allows the profile you get to be as close as possible to that of the mandolin top.

About the only other suggestion I would make is, if you have either a spindle sander, or even a drum sander you can chuck up in a drill press, you can save a lot of time by removing material this way. I use a pencil that I have belt-sanded in half (so it looks like a half-circle instead of a circle) and use it to trace the top of the mandolin shape onto the bottom edge of the base of the mandolin bridge. This can be a real time saver, especially if the curvature of the top of the mandolin is somewhat different from the contour that came on the bridge base.

In fact, I just used this method about a half-hour ago, while installing one of my mandolin bridges on a Davis F-5. I would guess it saved me about as much time as it took me to write this message~!~

[woodwizard]

Here's the link for the Stew Mac bridge fitting tool. It highly recommended and not very expensive, especially if you plan on fitting more than one bridge.

http://www.stewmac.com/shop/Tools/Sp...tting_Jig.html

Here's a picture of it as well.

Len B.
Clearwater, FL

That looks like a really cool tool for the job. Without the tool you really have to be careful as mentioned above to hold the bridge base pretty straight. It's easier said than done without a tool like that. Notice I didn't say impossible 'cause I have done it a few times successfully without one. It would make the job easier that's for sure and from the picture I think it would be easy to make one too. Or just buy it.

[Lefty Luthier]

That pencil idea is good. I have used a half pencil for years to set nut notch height above the frets but never on a bridge.

[amowry]

I use a modified version of Frank Ford's method, along with my own version of the Stew-Mac tool. Frank's idea of removing material from the middle of the bridge with a knife (I use a gouge) speeds up the process a lot. No need to use sandpaper for all of it.

I like to angle my bridges backward, so they roughly bisect the string angle. It does help prevent them from being pulled forward by the strings, and it makes intuitive sense to me.

If the holes in the saddle for the posts are too big, the saddle can tend to lean forward relative to the bridge base. I like a fairly snug fit there. A lot of bridges you see have holes that are quite a bit bigger than the posts and rely on the adjusting nut rather than the posts themselves to keep them in line with the base.

[Froglips]

What if you had to sand off so much that there is no longer a space between the two legs of the bridge? In other words, the entire underside of the bridge is now one long leg from one end of it all the way to the other side.

[Bill Snyder]

Some bridges are made that way from the start.

[300win]

Stew-Mac sells a fixture that I have used for years to hold the bridge at the proper angle as you sand. It's pretty cheap, and worth it if you are doing it yourself.

i'll second that> I have one and have used it to fit 3 bridge bases so far. They work great. On mine I adjust it to put a slight back angle, that way it does away with saddle tilt. They are very easy to adjust and use. Well worth the money, takes all of the trail and error of sanding one by just holding it with your hands.

[Steevarino]

What if you had to sand off so much that there is no longer a space between the two legs of the bridge? In other words, the entire underside of the bridge is now one long leg from one end of it all the way to the other side.

That works too! In my world that is sometimes called a "Full Contact" bridge, or maybe what one of my British customers calls a "Big Foot" bridge. It is standard equipment on Sam Bush F-5 mandolins, and some other builders use this style bridge exclusively, as well.

I have found that, while in the process of fitting the bridge to a given mandolin top, if you happen to lose the gap-in-the-middle, you have two choices. One choice is to leave it a "full contact" base. The other is to go ahead and sand or file another gap in the middle of the bottom edge of the bridge. I mentioned spindle and drum sanders earlier. Another way is to either use a file, or maybe a piece of wooden dowel with some sandpaper stuck to it.

After having fit quite a few mandolin bridges, I will say this... It is a bit more of a chore to fit a full contact bridge base than a gapped one. Not a lot harder, but you do have that much more surface area to get an intimate, gap-free contact with. There are possible tone issues too, but that is a whole different can of Mandolin Bridge Worms...

[Dale Ludewig]

What Steve said. I have pretty much moved to the full contact bridge base. A bit more time and difficulty but I have found no noticeable tone differences. On the other hand I've not done any scientific studies. Frankly, I've never understood the reasoning for the arch cutout in the middle of of the bridge base. Except that it saves time especially if the arching on the top is not a very smooth curve in that area. Tradition?

[Lefty Luthier]

Full contact bridges work fine in my experience. I especially like them on Mandolas where string loads are further from the rim. It is a bit harder to get them properly fit but certainly doable.

[Fretbear]

What Steve said. I have pretty much moved to the full contact bridge base. A bit more time and difficulty but I have found no noticeable tone differences. On the other hand I've not done any scientific studies. Frankly, I've never understood the reasoning for the arch cutout in the middle of of the bridge base. Except that it saves time especially if the arching on the top is not a very smooth curve in that area. Tradition?

I also like the "bigfoot"; I think it is as Dale says, another possible Golden Era nod to violinistic traditions like removable endpins.

[Gavin Baird]

I think if you look at the bridge as two seperate parts with the centre section connecting them; that as you put pressure on the two posts that with a thinner section between them, the two halves will probably assume a more uniform contact with the top...don't you think?...Gavin