Official Luthiers Forum!

I'm surprised you guys don't have an acoustic guitar section. Anyways, I'm no luthier, just a player interested in his instrument. One thing I can get my head around is how you're supposed to obtain a straight edge (on all planes) on a neck with a continuous radius (as apposed to a compound radius). Wouldn't you need to have a compound radius or cone shape to accomplish that? This is of course assuming that the neck gets wider from the nut down. I also have a picture in my head that using a set radius, all points down the middle of the neck would get higher in relationship to the neck edge the closer you get to the body (away from the nut).

That might have been a little confusing so I'll to clear anything up that didn't come through. I would really appreciate some help on this, it's one of the little obstacles standing in the way of putting together the pieces of the guitar design puzzle in my head. It seems like compound vs. set radius would have an effect on intonation and that's sort of the reason I'm interested in the topic. I don't think I've ever played a compound radius so I don't know if the change in geometry is great enough to have a noticeable effect, but anyways... thanks in advance for the help.

A constant radius fretboard is, if you can imagine, a section cut out of the walls of a cylinder (say 32" diameter, 16" radius) with one end narrower than the other.
A compound radius is a similar section cut out of a cone.
On a compound radius fretboard the string action can be taken a little lower as the fretboard is parallel to the string path, this is not the case on a constant radius board due to the widening of the string spacings.

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The very first forum is an acoustic forum. We had to beg for the electric section!

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Mike

Ah, I missed the acoustic forum because of the big fancy banner in front of it

Parallel to string path is a good way to put it, I imagine that's what I'm picturing in my head when comparing constant vs. compound radius. It seems like there would be a more even increase in string action on a compound radius. If that is the case, is choosing between a constant or compound radius important to correct intonation? Or is it merely a playability thing?

I didn't follow your whole train of thought, but you might be overlooking the bridge radius.

Yeah this topic might be a little complex to do over the internet, either that or I just can't explain myself clearly. Hopefully there's someone who has a general idea of what I'm trying to say, if not I guess my best bet would be to look a up a good luthier's book. Something that goes in depth on guitar scale geometry and intonation.

There will be no difference in intonation, as long as the intonation is set correctly. Intonation corrects for the extra tension put on the string when fretting, which is why you should not use a harmonic to set the intonation. So, the lower you have the action, the less the string is stretched when you fret, and the less adjustments need to be made for intonation. However, you should always take your playing style into account with intonation as well. If you press really hard on jumbo frets you will pull the string more sharp etc. As far as the radius (compound or constant) think of it as the radius is falling away from the center of the fret board. Before the radius is applied, the center of the board should be perfectly flat. The radius is then sanded on until it is down the length of the fretboard with the center still being perfectly flat and theoretically untouched. So, the edges of a constant radius fretboard will actually get thinner as you progress from the nut towards the bridge. In theory, a compound radius fretboard should have the same thickness at the edges even though the fretboard is getting wider because the decrease in radius as you move toward the bridge. This is not an exact science though and depends on scale length and how much of a radius change you have down the fretboard. You can get the same action on both of them, but on a compound radius fretboard with the same action as a constant radius, the compound radius will allow bigger string bends before fretting out on the higher frets. So, if you like really low action and big bends, then maybe compound is the way to go. However, the same bends can be achieved on a constant radius, you just have to accept a higher action so it comes down to playing style.

So to try and answer your initial question, achieving a straight edge down the string is done essentially the same way, but on a compound radius neck, you need your bridge set up at a bigger radius than the 22nd fret. For example, if your compound radius goes from 9" at the nut to 14" at the 22nd fret, your bridge should be about 16" radius or so. On a constant radius, your bridge should be the same as your hole fretboard. Either way, if you think about it as a conic section, or cylindrical section as mentioned above, it will still create a straight line as long as the center of the fret board is true. Hope that helps.

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John Thiessen
http://www.iszacguitars.com

The effect of the cylindrical radius on a tapered fretboard is such that, when the center of the fretboard is straight, the edges of the fretboard are effectively backbowed in relation to the string, as the cylindrical shape causes the edges to roll away. It's more noticeable on bass guitar fretboards with 1.5 inch nuts and 2.5 inch fretboard ends due to the more extreme taper of the fretboards. The effect can be measured with a long 24 inch straightedge. When the neck is set straight according to the CENTER of the fretboard, the straightedge held parallel to the EDGES will gently rock on each edge. This means the action has to be set higher in order to get the correct amount of relief for the outer strings, while the middle strings will have more relief in comparison, unless measures are taken to correct the fret plane during dressing.

A properly compound radiused fretboard will have each string effectively in it's own straight fret plane, and is easier to obtain a low action setup accordingly, as each string will have the same amount of relief with evenly dressed frets.

I compound radius my bass guitars for the better action, but use a 10" or 12" cylindrical radius on my electrics as I don't like the feel of the compund radius on them, and those radii allow a low action with easy string bending despite the cylindrical radius geometry.

As far as intonation goes, the scale is derived by taking the scale length and dividing it by a specific number, to derive the first fret. Then, the distance from the first fret to the theoretical saddle is divided by the SAME number to derive the second fret, and so on and so forth on up the neck for each successive fret. This means that fret scales are actually PROPORTIONS, based on the scale length of the instrument. On a tapered fretboard, the distance from the nut to the end of the fretboard is longer parallel to the edges than in the center, as the fretboard taper makes the fretboard a trapezoidal shape. In the center of the fretboard the distance from the nut to the first fret is slightly shorter than the distance from the nut to the first fret on the edges. Effectively, the strings in the center are a slightly different scale than the strings on the edges, but the relationship of the edges to the center is proportional, and so the fret layout self corrects as the scale length increases towards the edges of the fretboard, since the scale length is also proprtional, and the distance between each fret increases proportionaly to the increase in distance from the nut the the fretboard end as you move from the center of the fretboard to the edge of the fretboard, and so intonation is not affected by fretboard taper.

As far as compensation goes, the strings are not exactly the same distance away from the fretboard along their entire length, and are not all the same tension. As the string is deflected towards the fretboard, they increase in tension and rise in pitch slightly, and are therefore sharper than their theoretical pitch should be. The saddles are moved backwards, lengthening the string and flattening the note, to compesate, which is why intonation adjustments are known as compensation. Wound strings reuire less compensation than plain strings, and thicker strings require more compesation than thinner strings, within each string type wound or plain.

Does this help?

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Oh man, I totally forgot about this thread, bad habit of mine. Anyways, Cocobolo, I was able to skim the first part of your last post and YES!!! the straight center line of the fretboard and the 'backbowed' edges is EXACTLY what I was picturing in my head, i.e. when I said that it was impossible to have straight edges. I knew someone would have a better way of putting it than I did. I'm going back to finish the rest of your post to see what else I can learn but man, it feels good to know that what I was picturing in my head was correct because the neck geometry was bugging me for awhile. Thanks!

Okay finished up the rest of your post but I'll definitely have to go back to properly digest it all. This is really great, you've answered a lot of the questions I've had about intonation and neck geometry, one of course being the actual shape of the neck in regards to compound vs cylindrical, but also about something I didn't even mention: the difference in scale length between the center of the fretboard and the edges. I was always wondering about that because I knew the frets at the outside strings had more space between them but I was always confused because I guess I wasn't looking at the scale length as a whole. If I interpreted what you were saying correctly then the theoretical scale length (NOT nut-to-bridge) is longer at the outside strings than the inside and the scale auto-corrects since the frets are still proportional to the overall scale length. That's actually kind of neat, a very elegant design.

I think I saw you mention a couple of other points I had some interest in so I'm gonna go back and reread your post but thanks for all the help. I really feel like I have a much clearer understanding of my guitar, just what I was looking for.


*edit - I'm an idiot, thank you theguitarwhisperer NOT cocobolo. haha