Freeman Keller
Doctor of Teleocity
Geometry 101
I’ve been thinking about doing this thread for quite a long time but several recent threads tell me this might be the time.
If any of you have followed my ranting, er posting, you know that I am quite focused on the geometry of the guitar. I frankly don’t care what your guitar looks like or sounds like or what fancy pickup you put in it or how many coats of whatever you put on it – if it doesn’t play easily it is not a good guitar. Geometry is simply all those parameters that define whether or not you can make the guitar play easily. Basically if you have good geometry you can adjust the action to play well and easily, without buzzes, and have enough adjustment to change the action as needed.
Geometry is not the same as action – action is the height of each string above each fret, geometry is the thing that allows you to adjust the action. Another way to say that is geometry is the relationship between the ends of the strings (nut and saddle) and the fretboard. I’m going to define some terms so I can use them later – these are my definitions and there is nothing sacred about them.
- Fret plane. The tops of all of the frets on the center line of the neck. This assumes that the frets have been leveled and at least for now, there is no relief in the neck. If you lay a straightedge on the frets between the 3rd and 4th strings it follows the fret plane. The fret plane can be extended to the bridge
- Neck angle. The angle between the fret plane and the body of the guitar, usually measured relative to the side next to the neck heel. Neck angle can be positive or negative, zero is an acceptable angle. I will show measuring the neck angle with a protractor that will read somewhere around 90 degrees, the actual angle is 90 minus that reading.
- Overstand. This is a term from the bowed instrument world (violins) that indicates how much the fretboard is raise above the body of the instrument. It normally does not include the thickness of the fretboard itself. It may be angled or wedge shaped, in that case it is usually taken at the neck/body joint. It will make more sense when you see some pictures.
- Arching. Some guitars are flat on top, some are not. The amount that the top is arched or domed is important when we talk about geometry. I will assume that the outside rim (top of binding if the guitar is bound) is in a level plane, by laying a straightedge across the top behind the bridge we can measure the difference at the rim, assuming the gap is the same on both sides.
- Action. We have already mentioned action but my formal definition is the height of the strings above the first and twelfth frets. Action is a very personal thing – different players like totally different feeling guitars. Action often varies with different styles of guitar and sometimes style of playing (a Flamenco player or bluegrass flat picker might like a completely different action than a heavy metal shredder or an acoustic finger picker). For the sake of his discussion I am going to use a very average “medium low” action that I happen to prefer (and many people who play my guitars seem to agree). However this is one of the things that YOU should change in the following discussion to fit your preferences
- Action values for this discussion are first fret clearance between 15 and 20 thousands of an inch and 12th fret clearance between 60 and 100 thousands. Note that I am going to use decimal inches in all of this – sorry, that’s the way I think the best. Most players prefer action rising slightly across the neck – I might set the high E at 0.014 (1st fret) and 0.060 (12th) and the low E (0.018 and 0.090). I’ve done a whole ‘nother thread on setups and this is discussed in lots of detail. For most of this discussion we will assume that the relief is so small as to be negligible. Relief however is a factor in action and needs to be considered when you actually set it.
- Electric vs acoustic vs other instrument. The following discussion works equally for both acoustic and electric guitars, and interestingly, I find I like the same range of action values for both. My electric and acoustic guitars have a completely different feel when playing, but that comes from my choice of string tension and not action.
- Fretboard radius. I’m going to make all of my measurements on the center line of the fretboard, with the assumption that the radius is consistent (no compound radius here) and that the saddles are the same radius (or can be adjusted to be the same).
- Future fudge factors. I think it is very important to have some fudge room in any adjustment. I don’t like things at their maximum or minimum setting. Guitars change over time (particularly acoustics), playing style change, I want to be able to change my action accordingly.
So my refined definition of geometry is those parameters (neck angle, overstand, thickness) that allows me to have playable action (0.015 at the first fret, 0.075 at 12) and some adjustment in case something changes. There are an infinite ways to get there.
The last part of this little preamble is my observations that if I make the fret plane just hit the tops of the saddles at their lowest possible adjustment for most guitars (and bridges). It works for most guitars, most bridges and most players but it is up to you that it is going to work for yours.
OK, what I propose to do is design the geometry for two guitars, working backwards. I’ll start with a few assumptions about what I want to build and playable action. From that I’ll try to derive the geometry, with luck that will be recognizable and then I’ll show the actual guitar. Being that I am a total retro grouch I’m going to do this with pencil and paper (and some colored markers to make it easier for all of you to see). Obviously a nice computer aided design software would be faster and more accurate (particularly the “offset” command) but pencil will be accurate enough. Besides, I can’t cut wood to three decimal places…
I’ve been thinking about doing this thread for quite a long time but several recent threads tell me this might be the time.
If any of you have followed my ranting, er posting, you know that I am quite focused on the geometry of the guitar. I frankly don’t care what your guitar looks like or sounds like or what fancy pickup you put in it or how many coats of whatever you put on it – if it doesn’t play easily it is not a good guitar. Geometry is simply all those parameters that define whether or not you can make the guitar play easily. Basically if you have good geometry you can adjust the action to play well and easily, without buzzes, and have enough adjustment to change the action as needed.
Geometry is not the same as action – action is the height of each string above each fret, geometry is the thing that allows you to adjust the action. Another way to say that is geometry is the relationship between the ends of the strings (nut and saddle) and the fretboard. I’m going to define some terms so I can use them later – these are my definitions and there is nothing sacred about them.
- Fret plane. The tops of all of the frets on the center line of the neck. This assumes that the frets have been leveled and at least for now, there is no relief in the neck. If you lay a straightedge on the frets between the 3rd and 4th strings it follows the fret plane. The fret plane can be extended to the bridge
- Neck angle. The angle between the fret plane and the body of the guitar, usually measured relative to the side next to the neck heel. Neck angle can be positive or negative, zero is an acceptable angle. I will show measuring the neck angle with a protractor that will read somewhere around 90 degrees, the actual angle is 90 minus that reading.
- Overstand. This is a term from the bowed instrument world (violins) that indicates how much the fretboard is raise above the body of the instrument. It normally does not include the thickness of the fretboard itself. It may be angled or wedge shaped, in that case it is usually taken at the neck/body joint. It will make more sense when you see some pictures.
- Arching. Some guitars are flat on top, some are not. The amount that the top is arched or domed is important when we talk about geometry. I will assume that the outside rim (top of binding if the guitar is bound) is in a level plane, by laying a straightedge across the top behind the bridge we can measure the difference at the rim, assuming the gap is the same on both sides.
- Action. We have already mentioned action but my formal definition is the height of the strings above the first and twelfth frets. Action is a very personal thing – different players like totally different feeling guitars. Action often varies with different styles of guitar and sometimes style of playing (a Flamenco player or bluegrass flat picker might like a completely different action than a heavy metal shredder or an acoustic finger picker). For the sake of his discussion I am going to use a very average “medium low” action that I happen to prefer (and many people who play my guitars seem to agree). However this is one of the things that YOU should change in the following discussion to fit your preferences
- Action values for this discussion are first fret clearance between 15 and 20 thousands of an inch and 12th fret clearance between 60 and 100 thousands. Note that I am going to use decimal inches in all of this – sorry, that’s the way I think the best. Most players prefer action rising slightly across the neck – I might set the high E at 0.014 (1st fret) and 0.060 (12th) and the low E (0.018 and 0.090). I’ve done a whole ‘nother thread on setups and this is discussed in lots of detail. For most of this discussion we will assume that the relief is so small as to be negligible. Relief however is a factor in action and needs to be considered when you actually set it.
- Electric vs acoustic vs other instrument. The following discussion works equally for both acoustic and electric guitars, and interestingly, I find I like the same range of action values for both. My electric and acoustic guitars have a completely different feel when playing, but that comes from my choice of string tension and not action.
- Fretboard radius. I’m going to make all of my measurements on the center line of the fretboard, with the assumption that the radius is consistent (no compound radius here) and that the saddles are the same radius (or can be adjusted to be the same).
- Future fudge factors. I think it is very important to have some fudge room in any adjustment. I don’t like things at their maximum or minimum setting. Guitars change over time (particularly acoustics), playing style change, I want to be able to change my action accordingly.
So my refined definition of geometry is those parameters (neck angle, overstand, thickness) that allows me to have playable action (0.015 at the first fret, 0.075 at 12) and some adjustment in case something changes. There are an infinite ways to get there.
The last part of this little preamble is my observations that if I make the fret plane just hit the tops of the saddles at their lowest possible adjustment for most guitars (and bridges). It works for most guitars, most bridges and most players but it is up to you that it is going to work for yours.
OK, what I propose to do is design the geometry for two guitars, working backwards. I’ll start with a few assumptions about what I want to build and playable action. From that I’ll try to derive the geometry, with luck that will be recognizable and then I’ll show the actual guitar. Being that I am a total retro grouch I’m going to do this with pencil and paper (and some colored markers to make it easier for all of you to see). Obviously a nice computer aided design software would be faster and more accurate (particularly the “offset” command) but pencil will be accurate enough. Besides, I can’t cut wood to three decimal places…
