Checking the neck angle on a archtop
 

I know for a flat top you put a straight edge on the fret board and see where it lands in relation to the top of the bridge to check neck angle. How do you do it for an arch top guitar?

Actually, neck angle is responsible for achieving a target vertical distance from the surface of the top to the bottom of the strings measured at the bridge. That distance is related to the torque (moment) the strings exert on the top/bridge. For Martin style guitars, that target is 1/2”. The neck angle is correct when the string action is at the desired height above the frets and the 1/2” target is achieved. The bridge thickness is then what is left after the desired saddle projection - about 1/8” - is subtracted from the 1/2” target. That leaves a nominal bridge thickness of 3/8”. Martin uses several bridge thicknesses to maintain the saddle projection with varying neck angles (ie tolerance on 1/2” target).

For classical guitars the target vertical height is 10 or 11 mm, which necessitates a different neck angle than steel string guitars - the nut is higher than the plane of the top, rather than the nut lower than the plane of the top.

For arch top guitars, the target vertical height is often a little more than 1”. (The 1” value is a target for laying a straight edge on the frets and measuring vertical height at the bridge.)

So you mean on Martin type guitars the distance from the guitar's top to the bottom of the strings should measure a half inch? On an archtop the distance between the bottom of the straight edge to the top at the bridge should be a little more than an inch?

Thank you.

Those are common design targets, though not universal. For example, on the steel string guitars that I make I use less than 1/2” as the design target: I do use 1” for my arch tops.

Any individual instrument, particularly as it ages, might or might not be at the designed target. That an instrument is not precisely at its design target does not necessarily mean that a neck reset is needed. The need for a neck reset can be determined on a case by case basis. For steel string guitars, a typical indicator that an instrument might be in need of a neck reset is that the string height (action) on a property set up guitar is too high while there is too little saddle projection from the top of the bridge. One factor in the determination is bridge thickness.

What are the symptoms you are experiencing that lead you to question the neck angle of an arch top?

Charles, I don't have a problem. The reason for my question is that I bought a 1935 Kalamazoo KG-21 and I will take delivery of it tomorrow. The seller (GC in Fort Wayne) says the action and intonation are fine with no issues. I have never owned an archtop and wondered how I would determine the neck angle on this guitar. These guitars reportedly don't have a metal rod in the neck (not that this would pertain to the neck angle???). Thank you.
Michael

Those Kalamazoos don't have an elevated fingerboard, so the neck angle measurement may not be quite as high. I see those with bridges in the 7/8" range, and they seem to work just fine.

For those following the discussion: The Kalamazoo fingerboard is glued directly to the top, similar to "flat top" guitars, rather than have the fingerboard elevated above it, as is common for arch tops.

http://imperialvintageguitars.com/wp...93632226-3.jpg

Elavated fingerboard:

https://2.bp.blogspot.com/-Hfr1Buma8...1a09cba18e.jpg

I seem to end up with neck angles of around 4.5 degrees, using the Benedetto method of floating fretboard, etc. Sometimes a little less. You can easily measure neck angle by putting a protractor on the side of the body, and measuring the angle at the bottom of the fretboard. I target a 1" height above the body for a measuring stick placed on the fretboard (with frets installed), and try to finesse the neck set to accomplish that. That gives me 1 1/8" bridge height with a 1/16" action at the 12th fret. I have no idea how this would relate to a Kalamazoo, though.

As you state, the amount of bow in the neck is irrelevant to the neck angle. It influences playability, but is a separate variable.

One thing you can look for is whether or not the top is deformed - pressed inwards - in the area around the fingerboard. That can indicate the neck angle changing. You can also look to see that there is no gap at the heel, indicating movement of the neck in the joint - not likely, but possible. Otherwise, if the guitar plays and sounds fine, and there is sufficient bridge height/break angle, I wouldn't worry about the neck angle.

Thank you Charles! You've been a big help and I learned a lot! Take care.