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Hi, I've have an Acoustic guitar that I'm putting back together, I need info where to place the bridge.( It's been in pieces for a long time), To get the scale length do I measure from the nut to the 12th fret which is 13" or from the 1st fret that is about 1/4" below the nut which is 12 3/4 to the 12th fret, I guess its a early acoustic having a fret so close to the nut. Thanks.
 

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This should give you the details of where to place it. As stated it is imperative to get it right for the guitar to play in tune.

Locating and Gluing the Bridge of the Steel String Guitar

Locating the Bridge

The location of the bridge is absolutely critical. If the distance from the nut, down the centerline of the fingerboard, to the center of the bridge saddle slot is not within about 1⁄32'' of where it should be the guitar will not play in tune, and it will take some extreme measures to rework it so it does. So bridge placement is an operation which should be taken very seriously. You'll want to get this right the first time. A variation of the old carpenter's rule of thumb about measuring twice and cutting once is appropriate here. As you will see, I am going to recommend measuring three times before actually committing the bridge to glue.

Anyone who is making more than one instrument on the same plan will want to consider making a bridge location jig to make this process a little more mindless. A simple piece of plastic or MDF in the shape of the fingerboard but extending in length to past the bridge, with a couple of pins that fit into the saddle slot or a few small pieces of wood that engage the front and sides of the bridge is worth making if you plan to make more than one instance of an instrument. Folks that make instruments on a production basis will also usually have other jigs and tools that aid in the bridge location and gluing process.

The first thing to do in the process of locating the bridge on the top is to mark the centerline of the bridge on its underside. This is easy to do by measurement if the bridge is basically symmetrical about its centerline. Since all the surfaces of the bridge which will show in the completed instrument are at this point sanded and polished, I don't like to mar those surfaces with marks. So to make the centerline apparent on the front (the surface toward the end of the fingerboard) and the back (the surface toward the tail end of the guitar) I'll cut two little arrow heads of masking tape that point to the centerline and stick them on. The front of the bridge looks like this:


and the back of the bridge looks like this:


With good construction technique and some luck, the centerline of the fingerboard should be aligned with the centerline of the top of the body. This is easily checked with a long straight edge. In this case the bridge can be laterally oriented by placing it on the centerline of the top so that the two tape arrow heads point at the centerline. But in those cases where the center of the fingerboard is not aligned with the top centerline, you'll need to extend and transfer the centerline of the fingerboard onto the top, and use that line as the center of the bridge. I like to make that mark when necessary using masking tape as well, since the top is already completely finished at this point in assembly.

Once the bridge is located on the fingerboard centerline it needs to be slid into position at the appropriate distance from the nut. For this and some subsequent measurements the roughed out nut should be inserted in the nut slot.


If you haven't made the nut yet, just wedge a small scrap of wood into the nut slot to serve as a stop for the end of the ruler. The long ruler is butt up against the nut and aligned with the centerline of the fingerboard. Slide the bridge into position so that the front-to-back center of the bridge saddle slot is located at the distance from the nut that is the sum of the nominal scale length of your instrument plus the nominal compensation value.


If you are working from a full sized plan, the compensated saddle position should be indicated on the plan, and you can just take the distance from the nut to the compensated saddle position directly from the plan. On my plans there is a red reference line perpendicular to the instrument centerline that is the nominal bridge position. The angled red line beneath that is the compensated saddle position.

Other plans will have something similar. If you are working instead from dimensions, look for something like "distance from nut to bridge saddle at centerline" or some such. The instrument in the pictures has a 25.5'' scale length and an additional 0.125'' of compensation at the centerline. So the front-to-back center of the saddle slot is located 25.625'' from the nut.

In theory, and assuming the saddle slot was accurately cut when the bridge was made, the bridge is now located at the proper position. But I like to double check the locations of the ends of the saddle slot to be sure that there is the correct compensation there. Here, two more measurements are made. From the full size plan the distance from the point of intersection between the nut and the left side edge of the fingerboard, and the intersection of the extension of that edge of the fingerboard and the front-to-back center of the saddle slot is taken. I know, that description is difficult to parse, so let me describe how to take the measurement on the instrument. Lay the long ruler flat on the fingerboard so that it is butt up against the nut and one edge is aligned with the left side edge of the fingerboard. Take the measurement to the front-to-back center of the saddle slot, as shown. When you do this, lay the ruler down so you do not disturb the location of the bridge.


That measurement should be the same as appears on the full size plan. With the ruler still in place, take a short ruler and measure from the edge of the long ruler to the end of the saddle slot, like this:

Make a note of that distance. Now, do the same measurements along the right side edge of the fingerboard, again being careful not to disturb the bridge. The long measurement should be the same as that on the full sized plan, and the distance to the end of the saddle slot on this side should be the same (or at least close) to that on the other side. If everything checks out OK, great. But if not, take a look at the differences between the measured values and what they are supposed to be and consider how the bridge must be moved in order to get things to where they are supposed to be. Sometimes a small rotation of the bridge is all that's needed.

Moving the bridge around at this point is tricky, and it is real easy to move it too much. About the best technique I've found for moving it in small increments is to lay a finger on the top, next to the bridge and then roll that finger into the bridge, nudging it a bit. Needless to say (but I'll say it anyway) since bridge location is so critical, if you do need to fine tune the location of the bridge on the top you should then go back and take all of the location measurements, starting with the alignment along the fingerboard centerline. By repeated small movement and repeated checking you should be able to get things in the right place. If you can't it is a good idea to step back and assess the problem. As mentioned, if the measured distances from the nut are off by much more than 1⁄32'' it will be difficult or impossible to get the instrument to play in tune.

If you just don't have the data to check the nut to saddle lengths along both edges of the fingerboard, and if you are sure the saddle slot was angled and cut correctly, and if the bridge itself is symmetrical about the centerline, there is another way you can check lateral placement of the bridge. Lay the long ruler on the fingerboard so it butts up to the nut at one edge of the fingerboard. Measure from here to the tip of the bridge wing on that side, like this:

Now take the same measurement on the other side of the fingerboard to the other tip of the bridge wing. It should be the same. If not, reposition the bridge until these measurements are the same. Once again, each time you move the bridge, go back and do all the positioning checks.

When the bridge is finally correctly positioned it is a good idea to tape it down with two pieces of tape, one at the end of each of the wings. Lay the heel of your hand on the top of the guitar near the bridge, then gently drop your fingers down onto the top of the bridge to hold it in position. Then apply the tape with the other hand. Be aware that, being carved thin, the bridge wings tend to bend up a little. If you push down hard on the end of one of the wings while taping it may pop the bridge up, messing up its positioning. If this happens, repeat the bridge location process.

At this point the bridge should be correctly positioned and lightly taped in place. Since the following steps will scar the top, now is the time to re-check all the bridge positioning measurements.
 

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Hi, I've have an Acoustic guitar that I'm putting back together, I need info where to place the bridge.( It's been in pieces for a long time), To get the scale length do I measure from the nut to the 12th fret which is 13" or from the 1st fret that is about 1/4" below the nut which is 12 3/4 to the 12th fret, I guess its a early acoustic having a fret so close to the nut. Thanks.
Yes, you described a zero fret which would result in 25.5 " scale. Measure from the center of the zero fret, from the "peak" to the 12th fret , then x2 is your scale length.
I measured my zero fret Klira acoustic, it appears to have a little less of the full .125" compensation recommended above which I know probably accounts for its "not perfect" intonation.

That compensation mentioned in the article is important. Your instrument will play sharp if you leave out the compensation.
 
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