Compound radius transition points

Discussion in 'Luthier's Guitar & Bass Technical Discussion' started by BluePat, Sep 17, 2019.

  1. BluePat

    BluePat Member

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    Hi all,

    Done a few of these now, preferring a 10 to 14" end to end.

    My question is, to those who have done compound radii to their necks, where are your specific transition points located?

    Mine have been 10" from nut to between frets 5 and 6, then 12" from that point until the space between frets 12 and 13, then 14" to the end of the neck.

    How would that differ if one were to do a 10-16"?

    All thoughts and opinions welcome
     
  2. burningyen

    burningyen Member

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    Isn't a compound radius supposed to be a cone, not stacked cylinders?
     
  3. Slash

    Slash Supporting Member

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    This is what I always believed... It transitions gradually across the length of the fretboard not in steps.
     
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  4. BluePat

    BluePat Member

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    True that, the graduation would be smooth and less "stark" than an abrupt 10 to 12, or 12 to 14...

    My question is more of a "at what point on the fretboard does a 10" graduate to a 12?"
     
  5. burningyen

    burningyen Member

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    Assuming a cone-shaped profile it's a straight-line relationship, so on a 10-16" compound fretboard, it should be 12" exactly 1/3 of the way from the nut to the last fret.
     
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  6. walterw

    walterw Gold Supporting Member

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    what? no!

    no "transition points", that just means your fretboard isn't flat under the strings. any transition means a hump in the profile of the fretboard

    maybe "compound radius" is a bad term here, what we're after is really a progressive radius, where it continuously flattens in an even manner from the first fret to the last fret.

    for me it's done by sanding my starting and ending radii at each end of the neck and then "connecting" those curves with a full length flat sanding beam.
     
  7. B. Howard

    B. Howard Silver Supporting Member

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    My standard compound radius on all new builds (unless specified different by the client) is 12" at the nut, 20" at the saddle. The only accurate reference for shaping the board is at the 12th fret and in my case is a 16" radius at that point.
     
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  8. Tone_Terrific

    Tone_Terrific Supporting Member

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    Where are the standard points of reference?
    The nut, fine, but is a 10 - 16, say, 16 at the saddles? end of fretboard regardless of fret number, a certain fret position? or?
     
  9. walterw

    walterw Gold Supporting Member

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    i've only ever seen it specified at nut and end of the neck.

    i've certainly never paid attention to trying to math out the saddle radius, that's adjustable. it's first fret to last fret that has to be "pre-made" when fretting the neck
     
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  10. Tone_Terrific

    Tone_Terrific Supporting Member

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    I would expect that the setup jig is set to provide a 10- 16 at the end of the fretboard but the length of the fretboard can vary and I doubt they rejig when going to a 24 fret neck, not to mention other scale lengths...not that it much matters. Terry probably knows:)
     
  11. dotmkr

    dotmkr Member

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    Pat,
    You don't need transition points. That is because there is a continual transition from nut to the FB end.
    If you do it with hand tools then you can trace a curve on the each end of the fretboard before fretting. Then remove the wood above the traced line. That will give the shape you want. Keep in mind that you will want it to be level so to achieve that you will draw your curves with the peak at the same distance from the fretboard bottom.

    I think I understand your confusion leading to your question. The compound radius you are wanting is usually measured at the first and last fret. But in your head you are imagining that the radius is smaller if measured at the nut than first fret position. And larger at the end than at the last fret. True, but really not significantly different except in theory. It is so small that you can't even measure it.

    So there is no need to measure a radius on the surface of the board in between the 10 and 16. The board still needs to be flat from end to end. And that will result in a constantly changing radius. There will be a point along the length that the radius will be 12 or 14 or 12.22445558888. But you don't need to measure it.

    So to summarize do this to build a compound curve board.


    Make two templates, one for each end. The templates will be the desired curve at the nut end and the desired curve at the body end. For you 10 inch and 16 inch radius. ( these don't even need to be a radius. They could be parabolic or the Nike swoop)

    Select a fretboard blank and plane the bottom flat. Cut the taper shape close to your plan or leave it a rectangle. Either way will work.

    Determine the thickness of your fretboard. Usually around 0.25 inch and mark a line parallel to the bottom.

    Trace your templates on each end of the board with the highest point of the radius on the thickness line.

    Use a block plane, sander or other tool to remove all the wood above the template lines being careful to look at the board end to see of you are nearing the drawn curve. And eventually the surface of the board will transition from the 16 to the 10 inch radii.
     
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  12. Dave Weir

    Dave Weir Gold Supporting Member

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    The ratio of the radii at any two frets should be the same as the ratio of the string spacing at the same two frets. 10-14 seems like it would be more accurate than 10-16, unless your strings really spread out.
     
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  13. dotmkr

    dotmkr Member

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    Dave I don’t quite get what you mean? I don’t even use a variable curved board except on violin family instruments. Does your method help in fingering fast passages or something or is it an esthetic thing like the golden ratio?
     
  14. Dave Weir

    Dave Weir Gold Supporting Member

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    I was more replying to the OP.
    First, the notion of "Transition points" seems really strange. Like one would sand part of the neck with one radius block and then switch to another. That seems both complicated and in-effective. I think most people understand a compound radius to be a segment of a cone shape, with a smoothly changing radius. When I read your post, it seems like we are on the same page.

    I was looking at the choice of the two radii, 10 and 16. It seems too extreme of a cone. A cylindrical board has a certain amount of error that a proper compound board will fix. but if you go to far you get an opposite error.

    It doesn't have anything to do with fast fingering (trust me on this) or golden ratio. But there is a particular cone that is defined by the two outer strings. And your fret board should be a section of that cone. You can choose different starting radius, 7.5", 10" 12", whatever you want, but the end radius should be calculated, not just chosen.

    As an example, if the string spacing at the nut is 1.5, and at bridge is 2.125, the ratio is 1.416667. If you start with a 10" radius, the radius at the bridge would be 14.167". At the 12th fret 12.083". At the 24th fret, end of the board, 13.125".

    I could be totally wrong about the math, so maybe think it through for yourself and see if it makes sense. Mr. Howard seems like a pretty smart guy whose put a lot of thought into this stuff and he seems to have arrived at a little different formula. Or maybe his string spacing is different. Maybe there is something he is accounting for that I'm missing.

    And also, the error by the strings spreading out over a standard straight radius board is pretty small. Smaller than the relief some people intentionally put in their necks. The error on a straight 7.5" board is more than the error on a straight 12" board.
     
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  15. Tone_Terrific

    Tone_Terrific Supporting Member

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    One end of the cone terminates in a point.
    The other end is restricted by the spread of the stings at the saddles and the range of adjustment of those saddles wrt radius.
    Then you cut the cone at the point where you would like to start your fretboard i.e. the nut and use a section of the cone.
    I'll bet few, if any, meet the ideal calculable spec..which I cannot calculate anyway.
     
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  16. Dave Weir

    Dave Weir Gold Supporting Member

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    It’s probably not too important to get exactly mathematicaly correct. I’ve never had any actual problems with the straight 12” radius.
    But the math isn’t too complicated.
    Radius at the end of the board equals radius at the nut times width at the end of the board divided by width at the nut.
     
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  17. Tone_Terrific

    Tone_Terrific Supporting Member

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    12 does seem to be a very effective compromise all around.
    My point was that if dude A wants a 1 5/8 nut and dude B wants a 1 3/4 nut, the fretboard would have to come from a different section of the surface of the cone and it cannot happen without changing other dimensions like width and saddle spread....not that it really matters.:)
     
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  18. KGWagner

    KGWagner Member

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    It might help to see a tool that does that sort of thing...

    [​IMG]
    The pivot points can vary to create whatever radius (in practical terms) you want at either end.

    That happens to be from Grizzly, and you can get one of your very own for ~$1,200. Free shipping, if that helps...
     
    Last edited: Sep 20, 2019
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  19. Tone_Terrific

    Tone_Terrific Supporting Member

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    Keeping the board ends fixed in terms of radius and overall board width and then adjusting the nut width will drive the string spacing apart or closer together at the saddles, I would think.
     
  20. walterw

    walterw Gold Supporting Member

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    agreed. i think anything flatter than say fender's modern 9 1/2" is thoroughly "flat enough", you'll be able to get low action without choking on wide bends even if it's a cylinder and not a mathematical cone.

    the compound i like to do on vintage radius boards is from the stock 7 1/4" at the nut to say 10" at the other end; that's close to a mathematical cone, entails minimal sanding removal of wood and allows for wide ends without choking. i feel like it's the roundest i can leave a fretboard without having to compromise on the action
     

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