Bridge Radius For "Compound" Neck

Discussion in 'Luthier's Guitar & Bass Technical Discussion' started by Bobisadrummer, Jul 10, 2018.

  1. Bobisadrummer

    Bobisadrummer Supporting Member

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    I know there's a few formulas and plenty of discussions about what to set the bridge radius for fixed and "true" compound radius necks, but I have a USACG 9.5"-12" compound radius neck and per the folks at USACG, the neck starts at 9.5" before the first fret and becomes 12" by the 12th fret, whereupon it stays 12" for the rest of the neck to the heel. Obviously there'll be some folks who come in here and say the obvious "just do it at a string by string basis." but I'm curious... because the neck isn't following a true cone shape (if it was, the 12" radius would be at the last fret and I'd set my bridge at around 14.75"-15") but it seems like if I set my bridge radius to match the 9.5" the upper neck will be off, and if I match it to the 12" the first few frets will fill funky... Anyone with any experience with this?
     
  2. walterw

    walterw Gold Supporting Member

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    that’s why the right answer is "just do it at a string by string basis." :)

    measure all strings to be the same distance off a given reference fret (i use 17) and you have your matching curve, compound or not, perfect cone or not.
     
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  3. Bobisadrummer

    Bobisadrummer Supporting Member

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    But that's where I'm seeing the problem. Frets 12-21 have 12" radius, which means if I set my bridge to match the 12" how does that effect frets 1-11 that go from 9.5" to 12"? Granted the height is tiny, but even the smallest of measures have oddly impactful changes when playing. I've even read that despite a fret board having a fixed radius, you'd still want to have a flatter bridge radius due to the strings being narrower at the nut than the bridge.
     
  4. walterw

    walterw Gold Supporting Member

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    Didn’t catch this before.

    Thoroughly weird, not sure why anybody would want that in the first place. It means that by definition there’s a kink in the profile at the 12th fret!
     
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  5. Tony Done

    Tony Done Member

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    I set mine at about 15", but try and it see is the best approach if you are sensitive to differences in string height. - You don't need a degree in applied maths for that. :)
     
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  6. Boris Bubbanov

    Boris Bubbanov Member

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    Sounds like things have changed at USACG, even more than I thought.

    Walter is right, the conical radius must continue on past the 12th fret or the quality of the neck hangs in the balance. The way I understood what I bought was, the fan radius continues to evolve all the way to the end of the heel EXCEPT there's some "fall away" incorporated into the last few frets.

    Walter's description of the approach is right on, except I would say you can actually do hard measurements with tools or you can use your eyes and other senses. But to get each string "right" and let the net radius at the saddles sort of take care of itself and you're better off not trying to impose a sum certain radius at the bridge - if you have to screw up the setup of some strings to achieve that radius.
     
  7. Gclef

    Gclef Member

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    I remember reading something a while back regarding the warmoth 10-16 radius.

    I think the bridge radius works out to 18.5 inches or so.

    Easy way to figure it out? Don't. Just check the string height at the 21st, 22nd or 24th fret (whichever is highest)

    The actual radius doesn't matter when a neck has a full compound radius like warmoth.


    Good info on usacg necks. A straight radius on the higher frets is no bueno
     
  8. Bobisadrummer

    Bobisadrummer Supporting Member

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    Yeah I have a set of radius gauges, I'll measure the radius up and down the neck to double check if it stays 12" past the 12th fret or not, hopefully it's just some mistaken info. On fixed radius guitars I normally just set the saddle height for the high and low e to how I like it (just above fret buzz) and then set the rest of the strings following my radius gauge and it's worked well for me.
     
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  9. Dave Weir

    Dave Weir Gold Supporting Member

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    I can’t even see how you could make such a thing. 12” radius is easy. Conical slightly more complicated, but leveling is a little easier. You’d have to jig it up twice and wind up with fall away too far up the neck and only on the outer strings. Hard to imagine doing this intentionally.
     
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  10. Bobisadrummer

    Bobisadrummer Supporting Member

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    I'm not very familiar with CNC machines, but I know USACG uses them, so that might be how they do it.
     
  11. Dave Weir

    Dave Weir Gold Supporting Member

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    Maybe. I guess you could use a cnc machine to make one leg of a table shorter if that’s what you want.
    I looked at their website and don’t see any evidence they make it like you say. I think someone has mis spoke or you misunderstood. It just doesn’t seem like there would be any manufacturing or performance advantage.
     
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  12. walterw

    walterw Gold Supporting Member

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    ha!
     
  13. Bobisadrummer

    Bobisadrummer Supporting Member

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    "Hi Bob, your neck starts at 9.5 and transitions to a 12 but doesn't go beyond the 12 radius SO we start your radius at 9.5 inside the first fret and by the 12th it is at it's final radius and stays that way all the way to the end. Hope that helps!"
     
  14. Boris Bubbanov

    Boris Bubbanov Member

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    There's a significant probability that the CNC machines were set up in the Rosamond days, and the people there now are trying to simulate what Tommy & Co. used to do, and they don't actually know what the science or numbers really are. And the strange information they're conveying to the customers now is just a misunderstanding.

    Really hard to get my brain around necks being shipped with a kink like that - my guess is they're not actually like that.
     
  15. Dave Weir

    Dave Weir Gold Supporting Member

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    So pretty clearly not you misunderstanding. Maybe they really do this and they've figured out a new secret formula. But more likely it is like Mr Bubbanov suspects.
    Even if it was like this, it would be pretty hard to measure, and you could clean it up with a good fret level so it plays fine. Set you bridge radius around 15" salt to taste.
     
  16. GuitarInnovations

    GuitarInnovations Member

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    Someone else around here explained correctly that a compound radius is in fact conical. So, if I am understanding correctly, a 12-16 compound radius would benefit from a 20 bridge.
     
  17. KGWagner

    KGWagner Member

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    Somebody else did the math one time, and as it works out, the 10" to 16" progressive radius ends up wanting something like an 18.3" radius at the bridge.

    As a practical matter, you don't need to be that close. Fret height anomalies and playing style will force compromises that often require the compensation to be less than ideal.
     
  18. Ricardo Montiblan

    Ricardo Montiblan Member

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    The 9.5” radius is at the nut and it gradually tapers to a 12” radius by the 21st fret so the radius keeps getting flatter all the way up the neck. In the old days they used to do the radius on the CNC and clean them up by hand. But for about 15 years have used a multi radii swing sander to radius which has the fall away built in. I would probably set up the bridge at about a 14 then tailor it in. Hope this helps!
     
  19. TJT79

    TJT79 Member

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    A radius which is conical for part of tje neck then straight for the rest makes no sense. There would be a bump at the 12th fret! When levelling the frets there would be no way to avoid changing the radius.

    I'm not saying they haven't made it, just that it seems that someone doesn't understand the geometry involved!
     
  20. walterw

    walterw Gold Supporting Member

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    that explanation sounded authoritative :)
    do you have an inside scoop here?
     

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