Would this work?

[G7MOF]

I'm fitting a new bridge to a carved top mandolin. To get the finishing curve could I put some sand paper on the top body curve and gently sand the base of the bridge on top of the body to get the shape required or is there a better way?

[Oliver A.]

You've hit on the standard method. You can buy a roller jig from StewMac that helps hold the bridge in position ie: upright while you roll the jig with bridge back and forth over the area you wish the bridge to fit. Personally, I do it by hand. Just be careful to keep the bridge perpendicular to the top (or a very slight back angle) while you move it back and forth. If I'm fitting to a finished mandolin, I will apply some 1/4" pinstripe tape to the edges of the sandpaper so that the tape wraps around the edges before taping the entire piece of sandpaper to the top. This will protect the top from the scratchy edges of the paper. I use a piece of 120X paper about 1 1/2" wide. Take short back and forth strokes with the bridge so that the fit is very localized. After the bridge feet fit the top perfectly, I hollow out the bottoms of the feet just a little with a highly curved scraper. I feel this gives the bridge a more solid footing.

[Rob Roy]

I'm fitting a new bridge to a carved top mandolin. To get the finishing curve could I put some sand paper on the top body curve and gently sand the base of the bridge on top of the body to get the shape required or is there a better way?

yes, that's the way luthiers do it. I scribe a line on the base of the bridge and then get very close using a spindle sander. I put stickyback paper, 220 then 320 on the top of the mando and carefully use short strokes back and forth front-to-back. the tricky part is keeping the bridge perpendicular to the top and not wobbling as you move it on the top. I found what works best for me is to pin one end of the bridge in place with finger pressure then swing the opposite end in a short 1/2" arc. less wobble that way. You just have to go slow and be patient.

[MikeEdgerton]

You might want to put a layer of something under the sandpaper to protect the finish. The edges of the sandpaper can move a little and cause scratches. I usually use blue painters tape.

[Marty Jacobson]

Also, watch out-- sandpaper can often have little pieces of grit embedded in the paper backing, and it can scratch the top of the instrument pretty badly.
Another way to do this is to use carbon transfer paper, get a mark on the bridge base indicating the high spots, and then scrape away the high spots.
None of these methods are perfect or quick. I have never gotten a perfect bridge fit, and I don't know that it's possible to get a perfect bridge fit. Just like polishing isn't really polishing, it's just making finer scratches, and at some point we call it good enough. Same deal with bridge fitting. It'll never be a molecule-to-molecule fit.

And then the top deforms a little bit when you string it up. So that's always fun.

[pops1]

I have the stu mac jig, but it doesn't work that well. I like the side to side method, but I usually clamp something to the sides to keep the bridge on a straight keel.

[Marty Jacobson]

Oh yeah, forgot to mention that one reason why the sandpaper method is tricky is that the bridge needs to fit one spot, but you're moving it over a stripe of the top. The side to side method seems to be easier to do, but can result in outermost edges that get sanded too much, leaving a gap at the edges. The neck-to-tail motion has less of this, but you still have to position the bridge pretty accurately and you can't make the sanding strokes too long or you will get inaccuracies. The carbon paper method takes this problem out of the picture. I use a combination of both methods.

[fscotte]

Lets think about this so called perfect fit. As Marty said I don't think its possible to get a perfect fit. Since the bridge is wood and the top is wood, the two surfaces with per-fect themselves with time and pressure.

Secondly, if you're worried about little gaps where there isn't a perfect fit, think about the fact that you have a large gap right in the middle between the two feet.

So I think that as long as you have some of the feet making fairly decent contact, your tone will be fine. The danger of less contact in the surface area is increased "ground pressure" on the parts that do make contact. Thus possibly causing a crack in the top or deforming the top.

[Ivan Kelsall]

I've posted this before but it won't harm to do it again. Notice that Wayne Henderson sands 'across' the top,not back & forth = no rocking & rolling & rounding off the bridge feet. i did that when i fitted a CA bridge to my first Lebeda mandolin. It's much easier to do than the 'dragging' style you get when sanding back & forth. Stew Mac brought out a holding device to keep the bridge base stable, because it can cause the bridge feet to become rounded off doing it the 'trad.' way.
Place a piece of writing paper underneath the sand paper to prevent grit from getting to the mandolin top,or you're in for a heck of a lot of re-polishing !,
Ivan

[gtani7]

prevent grit from getting to the mandolin top,or you're in for a heck of a lot of re-polishing !,
Ivan

Thanks for this. Hendersen's top seems ot have a little bit of dust or, worse, marred spots in the finish from sandpaper at 5:12.

[fscotte]

To assist in keeping the bridge level while sanding, You can make a piece of wood much wider than the bridge that will fit in place of the saddle, and then use the nuts to hold it in place. That way you have something more to hold onto instead of the bridge. Much easier to keep it level.

[bernabe]

Hendersen's top seems ot have a little bit of dust or, worse, marred spots in the finish from sandpaper at 5:12.

I've played this mandolin and fit and finish was not that great. It was explained to me by the 2nd owner that it was a "rush" and built in "8 days". Whether true or even possible, I wouldn't take all the methods and procedures in the video as his standard way of doing things or as thorough as normal for him.

As far as sticky back sandpaper mentioned by RobRoy, Id be hesitant as not all papers are the same. I use the sticky Klingspor paper for different applications and its extremely difficult to remove from anything it adheres to without a little heat. I could see the finish possibly coming off. Again, may depend on the specific brand of paper, but...

Like pops1, I also have the Stew Mac bridge fitter and works great. Keeps the "wobble" out and as in the side to side method, you gotta use short strokes. Its not needed but cant see how it "doesn't work well" for someone. It does what a human hand does while keeping it in perfect position at which you can adjust the perpendicular or slight backward lean you desire.

[Larry Simonson]

To be picky, the radius of curvature, f hole to f hole, will be smaller than the radius of curvature of the bridge feet by an amount equal to the thickness of the sandpaper assemblage.

[fox]

The method I use is ... sanding the base to a near fit... sticking down a piece of 2" wide parcel tape over the instrument in the bridge position.... mixing epoxy with a little dust from the sanding.... placing the complete bridge in the correct place.... bed down the bridge on a little of the epoxy mix.... fitting the strings & tune up to tension.... leave overnight & it works!
The parcel tape I use may have a different term in the US but it is the micron thick, shiny finish tape that is used as cardboard box packing tape.
I found out it is best to apply a thin bead of the epoxy in the middle of the bridge position rather than apply the epoxy to the bridge foot. A thin line in the exact central bridge position will allow the epoxy to spread out and avoid air pockets.

[Marty Jacobson]

The method I use is ... sanding the base to a near fit... sticking down a piece of 2" wide parcel tape over the instrument in the bridge position.... mixing epoxy with a little dust from the sanding.... placing the complete bridge in the correct place.... bed down the bridge on a little of the epoxy mix.... fitting the strings & tune up to tension.... leave overnight & it works!
The parcel tape I use may have a different term in the US but it is the micron thick, shiny finish tape that is used as cardboard box packing tape.
I found out it is best to apply a thin bead of the epoxy in the middle of the bridge position rather than apply the epoxy to the bridge foot. A thin line in the exact central bridge position will allow the epoxy to spread out and avoid air pockets.

I know some top-shelf builders who use this approach. It does work, but the hassle of dealing with the epoxy or CA glue is similar for me to the work it takes to finish the job normally.

[Folkmusician.com]

You can get a great fit with sand paper and a knife or scraper. It is very simple in concept, but difficult to get precision in practice. This is because the top is not perfectly uniform. Id do this entirely by hand. Nothing taped down, no jigs. I have tried both and they don't seem to work as well as free hand. I hold the piece of sandpaper with my thumb and index/middle finger and stretch it tight to the top(carefully as not to crack the top in this weak area), Right hand moves the foot across the paper. The very simple tricks to this are short strokes (I use my finger and thumb to control the motion, keeping it centered and downward force. This last one is where you run into problems if you haven't done a lot of bridges. The top changes under string tension, so the bridge needs to be fit under tension. I am creating this with downward force while sanding.

So, sanding is to mark the spots that I am going to scrape with the blade. This is why the tape is not held down. I continually sand, scrape, then check the fit on the bare top. Once the bridge is reasonable close to fitting, I do this over and over, each time get a little close to seating the bridge. This can take 30+ minutes of non stop, sand/scape/check, over and over. During all of this, I am also adjusting for the lean angle.

All of this can be done without scratching the top. I do use a very thin film under the sand paper. I have not been able to use paper, tape or anything that doesn't 100% conform to the top without adding thickness. The thickness will slightly change the radius of the top and you won't be fitting directly against any deformities in the top.

I do it on all of our new mandolins.

[Ivan Kelsall]

When i fitted a new CA bridge to my first Lebeda mandolin,i used low-tack masking tape to attach a piece of writing paper in the area where the sanding was to be done.I attached the sandpaper to the writing paper to prevent it moving. I had already marked the position of the existing bridge on the top of the mandolin using a 'Chinagraph' pencil,rather like a very soft wax crayon.

I removed the bridge saddle & sanded the base from side to side as per the Wayne Henderson method. I'd never seen a mandolin bridge fitted or done it before,but it just made sense to do it that way to prevent the base rocking on it's edges.

I got the job done,which included radiussing the top of the saddle - it was straight when i got it - & slotting it. It took around 4 hours,but i did a good job. The base fit perfectly & there was no slot buzzing etc.,in actual fact,i cut hardly any slots at all & let the strings bed themselves in to their own diameter - it worked.

I posted my method on here,& Gail Hester posted that she sands her bridges the same way - great minds etc...... !,
Ivan

[theCOOP]

Great thread. Just about completely answered in the first two responses.

[sunburst]

yes, that's the way luthiers do it...

Luthiers are much like everybody else in the world, and that means they don't all do things the same way.
Here's a quick explanation of my method.
First, I make a mark on the bridge base corresponding to the top contour and cut and spindle sand to the mark, then:
For new mandolins, with the finish not completed, I prepare a strip of sandpaper about 1" wide and several inches long. I place it on the mandolin top in the bridge position starting at the center of the top and extending over the f-hole (or similar position in the absence of f-holes), then place the bridge in position with one foot on the sandpaper. While holding the bridge in position, I pull the strip of sandpaper so that it moves under the bridge foot. That marks the underside of the bridge foot, and then I scrape away the part that was marked with a small curved scraper. Repeat that until the bridge foot makes full contact. I alternate bridge feet so that the fit doesn't change on one side as a result of removing material from the other.
For a finished madolin, the procedure is the same, but the top is protected with thin masking tape. Usually three strips side by side covering the entire center area of the top.

[Larry Simonson] To be picky, the radius of curvature, f hole to f hole, will be smaller than the radius of curvature of the bridge feet by an amount equal to the thickness of the sandpaper assemblage.

Because only one bridge foot at a time is sitting on sandpaper with this method, that situation is somewhat alleviated.

[mtucker]

Beside's getting the side-to-side radius of the bridge and top to match, on my mandolin there seems to be a slight bit more material removed from the base of the bridge toward the tail of the mandolin, a very slight kant (back) to the bridge to prevent it from being pulled (tilting) forward over time by the strings. Am I imagining this?

[Mandodork]

Many mandolins need the bridge fit that way. It depends on where the bridge placement sits on the lengthwise, (neck to tailpiece), curve of the top.

I've only used it a couple of times, but the Stew-Mac jig worked great for me. It's easy to fit the bridge squarely, or with a little back tilt to prevent forward lean, as needed to compensate for the lengthwise curve of the top.

Keep in mind that you need to keep checking the vertical adjustment of the roller, as the bridge will tilt back more as material is removed. (I learned that the hard way.)

[mtucker]

Thanks. Makes sense now that i think about it. The arch top is a compound radius.

[bernabe]

Keep in mind that you need to keep checking the vertical adjustment of the roller, as the bridge will tilt back more as material is removed. (I learned that the hard way.)

I think you mean forward, not back.

[Marty Jacobson]

I think you mean forward, not back.

It gets shorter, so it tips back more. Assuming "back" means towards the tailpiece.

[pops1]

The bridge foot on my Woodley had a trough in the middle of the foot where it contacted the top. I am guessing for easier fitting as you are fitting only the outsides of the foot and the center is hollowed out. It sounded great so it wasn't a detriment to sound. Has anyone else used this method?