Height Of Straightedge Above Bridge?
 

I see conflicting reports of the clearance a straightedge is to have above the bridge when setting a neck on a Dreadnaught. I have heard that Martin uses .094"

I find that number unlikely. I shoot for the straight edge on the top of the frets being as close to exactly in line with the top of the bridge as possible on the treble side, and a little below the top of the bridge on the bass side.

I know it seems high, but to get the 1/2" measurement from the soundboard to the top of the saddle, it seems that there needs to be some significant distance between the top of the bridge and the straightedge.

I am experimenting with making a measuring gauge so don't need a bridge to get an accurate neck angle. I have many guitars to compare it to. They all have varying gaps between bridge and straightedge but all have the critical 1/2" measurement. The amount of deflection in the top and different bridges is causing these differences.

I'm not sure what you are trying to do.

First, the "critical 1/2" measurement" isn't. It depends upon the manufacturer and design whether or not 1/2" is the target. For example, on guitars that I make, I don't use a 1/2", I use less. My guitars are designed for less - lighter bracing, thinner bridge. That a guitar is more or less than 1/2" doesn't mean much in and of itself. It is a common target for Martin - and Martin style - guitars.

There is no mystery here. It is simple-enough geometry. Start with a scale length. Add necessary compensation, fixing the position of nut and saddle. Choose a fingerboard thickness - uniform, or tapered - and fret height. Choose a target action at the 12th fret. Choose a target vertical height from outside of guitar top to bottom of strings at the saddle. Add a factor for deformation (rising) of the guitar top due to string tension - I use 1/16", which is pretty accurate for the instruments I make. The neck angle is whatever angle of the neck that simultaneously produces the target string height at the 12th fret and vertical height at the bridge, WHILE using the chosen fingerboard thickness, scale length, fret height and top deformation. Change any of those variables and the neck angle has to change to accommodate the change in variables.

The target vertical distance at the saddle is comprised of two components. One is the thickness of the bridge, the other is the height of the saddle projecting from the top of the bridge. The two equal the target vertical distance.

There is a minimum - and a maximum - that the saddle should project from the top of the bridge. At the minimum, there needs to be sufficient break angle of the strings over the saddle to provide a clear tone. A target of 1/8" is common. If the saddle projects too much from the top of the bridge, the projection places a larger torque on the front of the saddle slot in the bridge, which can cause the bridge to split at the sound hole edge of the slot. A common target is not more than about 3/16". One can vary the thickness of the bridge and the projection of the saddle within those targets while still obtaining the target vertical string height at the saddle.

In summary, these are the variables:

scale length
fingerboard thickness
fret height
string height at the 12th fret
target vertical string height at the saddle
deformation (rising) of the top under string load
(I didn't include string height at the nut, assuming it is the same as fret height. I also assume minimal neck relief: if relief is "excessive" it becomes a factor.)

bridge thickness + saddle projection = target vertical string height at the saddle

If you want it, I can post the equation that I developed and use to calculate neck angle. I have also laid it out geometrically in a parametric CAD system, allowing me to change any of the parameters to see the resulting geometry.

Thanks for all the info, it is a lot to contemplate. As far as the 1/2" critical measurement I was referring to, every acoustic I now have all have it, and they all have good volume and tone, except for one at one time, my 1998 HD-28. I had been shaving down the saddle years ago to get better action without realizing I was losing volume. At one point it had started to buzz at a party so I made a quick shim out of a popsicle stick. I was amazed at how loud it got. I attributed this to break angle only, I was unaware of the height of the saddle above the soundboard having a lot to do with it. So, for the 3 guitars I am building I made a block to show me exactly where to locate the bridge and determine the neck angle. right now the block is still high until I can get the exact dimension figured out. In the past I was shooting from the hip setting necks and they all came out good, they just varied a bit. I want dependable and repeatable results going forward. My Martin, now with a tall saddle, has the straightedge just hitting the top of the bridge with a little space due to the bridge rolling forward. I imagine 20 years ago there must have been quite a gap before the belly rose. That is why I am curious about this .094 spec I found in a video.

Well I have the clearance at .050", same as my Martin

https://i.imgur.com/xmhjEIM.jpg

https://i.imgur.com/36axRo4.jpg

That's what I've always heard, and what I look for in any acquisition.

Those two pictures show necks that have been way overset.

When you overset a neck you have to run a high saddle, a high saddle loads up the saddle slot and risks breaking the bridge in half

Steve

According to an authorized Martin Service Center they are not, they said Martin uses .094 as their spec. The Martin in the picture is my 1998 HD-28 that has great action right now with a measurement of .070 at 12th fret. The string height above the top is 1/2", with a good break angle at the saddle, right were it needs to be for optimum tone and volume. The bridge has never broken.

Mmm, let me think, yes I am a Martin warranty repair agent, that is however irrelevant to the conversation.

Your photos show what is known as an overset neck, overset necks are known to have issues, I described one on my earlier reply.

Steve

Apparently I have a factory installed "overset" neck that has perfect action, relief and playability. It seems to me a truly overset neck would have very little clearance at 12th fret, I have .070". I also have the ideal 1/2" clearance from soundboard to strings.

You clearly do not want to hear anything that is contradictory to your view point, 4 seperate people have replied dis-agreeing with the assumption you have re neck angle, I dont know why you even bothered starting the post if you dis-agree with those of us that do it for a living every day of the week.

Good luck with your builds.

I need many reports and evidence to base my conclusion. I have evidence right in front of me. I have 4 relatively new Gibson acoustics I will put on my measuring bench today.

A straight edge on the frets hovering over the bridge should barely touch the top of the bridge. .1 inch is way too high as has been mentioned. That would be considered over set. It might still work but it might cause problems too. In that second pic is looks to me like the bridge is not glued down. is the neck glued in yet? If so I would make another taller bridge for it. If not then reset the neck angle down a bit.