How far to reasonably lower the bridge?

[Houngan]

On my build I'm about ready to glue the neck. Fitted dry I measured the 12th fret string height (straight edge across the bridge to first fret, metal ruler providing the first fret height, probably a bit high) at 4mm or .16. Optimal seems to be .09 or .08, so I need to find another .07 of lowering.

I only know three ways to do that, lower the bridge, change the neck angle, or futz with the truss rod. Of those lowering the bridge .14 should give me .07 at the 12th. Is that reasonable, crazy, or what? How do you do this final check before gluing? Thanks!

[rcc56]

1. How high is the bridge? Is the bridge adjustable? Optimum height of the bridge should be somewhere around 5/8" to 3/4" above the center seam. If it is very much higher, the right hand will be forced into a clumsy playing position. Very much lower, and tone and response of the instrument will be negatively affected. This is assuming that your instrument has a carved top. Flat or canted top mandolins traditionally use a lower bridge. .07" between the bottom of the string and the top of the 12th fret is close to the highest action that is acceptable. Most of us measure in 64ths of an inch, and 5/64" [.078"] is the maximum. 4/64" [.062"] is more comfortable.

2. A change in neck angle is appropriate if the bridge height and string height are not within the above specifications.

3. Truss rods are for straightening the neck, not for adjusting string height. If you over-adjust a truss rod to change the string height, rattles and buzzes will appear. I recommend that a neck be set dead straight with no string tension on it. It will pull forward when it is brought up to tension.

I am a repair person, not a builder. Others may have better tricks for measuring geometry on an unfinished instrument. But you can tape a block of wood in the bridge position to simulate bridge height, and temporarily insert cardboard into the dovetail joint to secure the neck while you are making a measurement. If your action measurement is high, you will have to tip the neck back by cutting away material from the inside of the heel. When you think you've got it right, cut a wood shim to fit tightly in the space between the male and female surfaces of the dovetail. Then measure again. Remember that the action will come up slightly when you tune the mandolin up to pitch. When you have it right, you will glue the neck on with the shim in place. Go to frets.com and read the articles on setting guitar necks. The basic procedure is the same.

[fscotte]

What kind of neck joint are you using?

[Houngan]

What kind of neck joint are you using?

Erm, I guess it's a set neck? No bolts, vertical dovetail. To adjust I'll have to shim and sand.

[fscotte]

If its a DT joint, is it possible to raise the neck a bit?

I'm interested in how you're dry fitting the neck. When the strings are brought to pitch, the top will sink a bit, essentially requiring the bridge to be "higher". Also, are you factoring in fret height? You may also use a tad taller fret to compensate as well.

I usually dry fit with an unfretted fretboard. Laying the ruler flat on the board, I try to aim for 3/4" bridge height. If I need higher, then I lift the neck in the joint. If I need lower, then I set the neck lower in the joint. Once cut, the neck angle is difficult to change, so leave it.

[Houngan]

I'm within a mm of having the neck seated, like so:



And I'm measuring by running a straight ruler between bridge and 1st fret (held slightly over the 1st with another metal ruler's width) then using a digital caliper to measure the gap (measure depth from top of ruler to 12th fret, subtract width of ruler.)

The neck joint looks like this:



With a dovetail expanding from there to the top.

[fscotte]

That's why pics are a huge advantage when asking questions like this. So it appears there is no room left to raise the neck since the heel is already short.

What is your current bridge height? Your best bet would be to lower the bridge, providing it's got enough wood on the feet or the saddle. You can sand off 1/16" from the feet and then another 1/16" from the saddle. Then cut the grooves for the strings.

Do remember that the top will sink a tad with the strings up to pitch. Perhaps 1/32" or less.

[rcc56]

You can rough out the neck angle now, but it would be better to glue the back on before you do the final fitting and gluing of the neck.

I don't quite understand your measuring technique. There should be a simpler and more accurate way to get a measurement. Set a block or your bridge at your desired height with the instrument strung up, then lower the block by 3/32" to 1/8" to account for the desired string height above the 12th fret. In other words, if you want your final bridge height to be 3/4", adjust the bridge to height to 5/8" or 21/32". Then lay your straightedge flat on the frets. Adjust your neck angle or height in the joint until the straightedge is flush with the bridge top. You can do this by re-angleing the tenon and/or adding material to the bottom of the heel, the cheeks of the mortise, and the underside of the fingerboard extension.

Did you read the frets.com info on setting guitar necks? The principles are the same.

[rcc56]

By the way, since that is a tapered mortise and tenon rather than a true dovetail joint, it is very important to get the gluing surfaces fitted as accurately and tightly as possible to avoid joint failure later.

With the mortise and tenon you are depending completely on the quality of the glue joint to hold everything together.

With a true dovetail joint, the shape of the joint plays a large part in holding things together. If correctly fitted, the glue mostly is there to prevent movement rather than to provide structural strength.

Despite old sayings and manufacturer's claims, glue is not necessarily really "stronger than the wood itself."