At what height does a straight edge pass over the bridge?

[ken cierp]

I read here on our forum and endlessly at some other forums about how high from the bridge should a straight edge pass over to achieve the proper neck set. The point I must make is that premise is a bunch of baloney! It’s the tail wagging the dog. True if all the parts are matching and the process used compliments those angles (which sadly is seldom the case) a reasonable close measurement/distance will result. This will occur with a neck set that is a straight line along the neck and finger-board extension area on the top. Here the thinking goes haywire when the maker gets obsessed with this straight edge measurement over the bridge and then starts dinking with the neck angle. By ruining the straight line set many of aspects of build are being jeopardized. These include playability and most importantly the built in hump or shi slope at the FB extension….

What is not widely published is that major makers use different thickness of bridges and/or different heights of saddles ---- they do not compromise the straight line of the finger-board plane. Of course you could shave the bottom of the FB extension or add shims neither of which look quite right. Here’s excerpts from something Frank Ford posted elsewhere – talking about a 1968 guitar but nothing has changed,maybe more bridge selections on the shop floor.


Reply by Frank Ford on November 11, 2009 at 9:42pm

All D-18s in that period had rosewood bridges, NOT ebony. It was a transitional time, and the switch was from Brazilian rosewood to Indian rosewood. I'm unaware of any particular change in saddle width, and the depth was variable. At that time, bridges were made in three thicknesses, 1/4. 5/16, 3/8, and the appropriate bridge was selected to accommodate the neck angle. Each bridge would have a different depth of saddle slot, of course.

Compensation was determined by the position of the bridge - saddles were not carved to achieve any degree of compensation. Any error in intonation is typically an error in placement of the bridge. That era was also a transitional time for nut and saddle material. Ivory, Micarta, ivory, and white plastic were used in the 1960s, although not in that order. The saddle was short, set in a blind-end routed slot as today's are. The "through-cut" saddle ended earlier in the 1960s.

As the folks at Martin suggested, you make the bridge the appropriate thickness to suit the neck angle and achieve a reasonable amount of saddle sticking above the bridge. So, I can't recommend a "starting point" for the bridge height for just that reason - it is determined by neck angle.

[Dave Bagwill]

Great info. Thanks.

[John Link]

I believe string height is quite important. It has a large say in how string energy is transmitted to the top because it determines the length of the lever - an 8/16" height exerts significantly more leverage than a 5/16" height. No need to go into other specifics, such as angle of attack, etc. Let's just assume for this discussion that height alone makes a difference, and is worth bringing under maker control.

When I built David Russell Young style necks and tops, I found that: a), it was easy to adjust neck angles; b), the "correct" angle always coincided with parallel alignment of the fingerboard with the top of the upper bout; and c), the string height always came out to 7/16" in the finished instrument. I theorize 1/2" might be more in keeping with the drive and volume I would like to have, but 7/16" is what I always got. But Young's method, while reliable and consistent, was not particularly intelligible - it simply worked and I did not mess with it.

The DRY system leveraged the ease of adjusting the simple butt joint for getting the neck angle to suit his other key measurements. I would not want to think of what it would take to get a dovetail to change angles and still retain a perfect match on all 8 faces. A MT joint is much easier to wrap my mind around.

The method Ken describes on his KMG site is the best I've encountered for gaining control over this little beastie. Martin Co. uses it, as Ken documents, and that is not a small recommendation. But the real reason I will adopt it is that it is adjustable and intelligible. With a little high school trig and/or experimental trial, the maker can determine ultimate string height to suit the sound sought. Want a lot of leverage and you can have it without messing up the straightness of your fingerboard. Likewise if you want less leverage. (I am assuming fingerboard thickness remains constant.)

If you retop old guitars, Ken's method (and Mega Mold) would be useful as well, though you would be sanding down the lower bout instead of the upper to increase leverage. While the need to decrease leverage on an old instrument is rare, it too could be accomplished by adding to the linings, then replacing bindings with taller ones.

[John Parchem]

The height of the string over the top when it breaks over the saddle is an important metric in terms of transferring energy to the top. So I would argue that there is an acceptable range for that height. That is one of the reasons SS have a back neck angle and classical guitars often have a slight forward neck angle. The optimal range of string height off of the top at the saddle is different.
I think when people talk about a desired height over the bridge for a straight edge they are talking about making that particular bridge work. While building a kit only a single bridge is shipped. If the straight edge is too low to that bridge you will have to have a high action or very low saddle to the bridge height to make that bridge work, If you end up too high over the bridge you will end up with too much saddle over the bridge. So one can define a height (dependent on the desired action) where the result is the knowledge that the top angle, the neck angle and bridge combination you have is working. This is straight forward geometry.

How one deals with this information is a different story. I agree that within a range changing the height of a bridge or making a bridge the height that works with the top and neck angle is a workable solution. There are others. This issue really comes up when the top is glued on before the rims are properly profiles and corrections need to be made.

[Tim Benware]

I have gone to making my bridges AFTER the neck is glued on so I can keep the neck angle as machined in as close to ideal as possible i.e. little or no adjustments. But there always seems to be some tweaking necessary. Not sure how much it plays in to the equation but the top radius chosen (25, 28, 52 etc) will raise the top different heights and the neck angle should match that. Supposing you machine in a top radius of 28' but actually get 25' how much tweaking will that result in to get the proper bridge/saddle height off the top? I have come to feel it is MUCH easier to control the thickness of the bridge (assuming you have machined in the correct neck angle and with the 5 or so I have done so far with the MM I've been almost dead nuts on) to get to that ideal. And as Link said I really don't think the difference between 7/16" and 1/2" is going to be telling except to the most discerning of guitars players and ears.

Having said all that, the bridges I've made have come in between .298" and .365" in thickness. Good enough for me.

[ken cierp]

I know I can't hear the variances from optimal break angles, bridge thickness, saddle height, orverall string height etc.-- what I can hear is the how much better a well set-up instrument versus one poorly set-up sounds in the hands of players at any skill level. Good intonation, make it easy to play and it will sound fine.

[MarkAndrew1]

I think I have a headache :( See this is the part
of building that scares me the most. I'm getting
ready to glue my top, followed by the back and then
the NECK......I've followed Ken's instructions to a tee
but the neck angle and bridge location steps seem
like the most difficult and critical.

Proper neck angle and bridge placement can mean
the difference between a very good or very bad playing
guitar. Getting this portion correct is something I really
want nail....