question about single-action truss rod slots

Discussion in 'Luthier's Guitar & Bass Technical Discussion' started by Luthierwnc, Jan 9, 2018.

  1. Luthierwnc

    Luthierwnc Member

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    Hi All. I'm looking at building a single cutaway solid body using a 627mm Gibson scale. The instrument will have a body join at the 16th fret bass side and 19th fret on the treble. I haven't done a lot of electric building in a while and usually then with double action rods. I'd like to use a piece of 3/16 cold or stainless rod threaded for 10-32 in the traditional Gibson style for this guitar -- weight is a factor.

    In the past I generally cut the slot at full depth for the entire rod run -- usually on the tablesaw. Then I would make equal length wedges that were grooved on the upside with a 3/16" bullnose bit to cradle the rod inside the slot. Those were glued in to put the ends of the rod higher than in the middle. I suppose I could make a router sled to do the same thing. Crushing in a grooved cap strip and planing it flat finishes that part of the joinery.

    My question is both theoretical and practical. Is there an advantage to making the effective truss rod curve symmetrical? Sometimes with a single rod you get that hump near the 12th and/or have to crank the rod almost to a backbow to get the last frets near the peghead to behave. I'm playing with the idea of having about two thirds of the run from the body relatively flatter with a more pronounced upbow near the 6th.

    I've looked around and haven't seen much on truss rod arching. Most diagrams -- oldies -- show the low point at the center of the sweep. Although I thought I remembered a Fender schematic using a rod anchored at the nut that was closer to what I described. Can't find it now.

    I'd love to hear your theoretical ideas and even more from people who have tried different rod curves.

    FWIW, I plan to straddle the rod with two 1/8" x 3/8 carbon fiber bars. Mahogany neck, cocobolo fingerboard. 16 degree angled peghead made with a scarf-joint and a 1/16" Indian rosewood peghead cap. Three per side tuners and the truss nut will be under a traditional cover at the peghead. The neck heel will be supported with 1.25" of wood by the 16th fret so I don't have a very long unsupported run of neck.

    Thanks, Skip

    PS apologies to metric builders. I do American guitars in English and everything else metric -- so I'm multi-measurement. 25.4! Cheers, sh
     
    Cal Webway likes this.
  2. Jack Daniels

    Jack Daniels Supporting Member

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    Lots of opinions on this. Lots of info on this board as well. Just a few post below this were several other questions on single rod construction. To me, it sounds like your overbuilding with the extra carbon rods. I’ve never needed them.

    You may not get detailed answers as some of this is proprietary to some builders. You are on the right track with thinking about where necks bend and where they don’t. Under the 6-7th frets seem to be where most builders place the deepest sections. The neck does not move much where it leaves the body, but does move some. Keep that in mind.
     
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  3. Luthierwnc

    Luthierwnc Member

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    An 18" rod on a Gibson scale guitar puts the center point between the 8th and 9th fret. So your thoughts of 6/7th fret as the low point gets me closer to the mark.

    I got into the habit of using carbon rods when I was building guitars in New Mexico. Almost anywhere you shipped them would be more humid so it took some of the sproing out of the adjustment. There is also a lot to be said for using them with angled pegheads. Not so much with a scarf of "V" joint but for necks cut from a single blank, it really stiffens the place where Gibsons so famously snap. I just run them long and sand them flush with the belt sander before putting on the headplate.

    Thanks and cheers, sh
     
  4. Jack Daniels

    Jack Daniels Supporting Member

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    Here is food for thought. This is not my picture, and may not be 100% accurate to how a neck moves, but studying it will provide some good insight to where to make your rod react. Also, you may want to think about your carbon rods and where they may make the most sense. You want some relief, so making the neck perfectly flat and immovable might not be the right answer.

     
  5. Terry McInturff

    Terry McInturff 40th Anniversary of guitar building! Gold Supporting Member

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    I'd like to be able to comment regarding the exact curve that I use, but it works as an integral part of my entire neck recipe...and I am paid to teach my neck build, and so it wouldn't be fair to my clients.
    I CAN comment regarding the generation of the slot however. Your method of generating a flat-bottomed slot via tablesaw, and thereafter gluing-in curved "ramps" isn't a terrible idea, and in fact this is the method that Roger Simonov used in his mandolin book as I recall. You'll get better results that way as compared to a curved router sled; the router method requires that more than one pass is required, and you end up with a slot that's too wide due to the deflection of the cutter/slop in the plunge.
    I recommend using a 3-wing bullnose cutter on a shaper. One pass.
    Using your current method will work. Be sure to wax the rod well (I use Gulf Wax paraffin) and don't clamp down on the filler piece very much at all....seat it, and no more. Be crafty regarding the fit of the filler strip (the "spline"). This should be sized to seat down with thumb pressure. Not a good idea to impose tension on the neck via this step, and go EZ on the clamps for an uber-smooth rod action.
     
    Last edited: Jan 10, 2018
  6. Luthierwnc

    Luthierwnc Member

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    Thanks gents. I was thinking of a fishing rod. Thicker at the grip since you don't want flex there but lots of whip on the unsupported end. That's why I like the idea of putting the bottom of the curve around the 7th. It also makes the first 60% of the run (from the body) relatively flatter than the last 40%. Presumably that helps with that mid-hump. I'm sure you guys have done refrets with a bulge around the 14th (flattops). Cranking the rod only makes it worse but it doesn't level frets one through 5. You can sand through the dots trying to get the board flat.

    For through slots I use a table saw but for finite lengths I use a router table. Usually 7/32" or 1/4" flat bottomed to depth and then 3/16" round-nosed bit to groove the spline and wedge supports. I could use 3/16" for all but I wrap tape or shrink tubing around the rod to keep it quiet and need a little slop. To your point Terry, the spline fits but isn't tight so I can tap it in place with a mallet. It's also only as wide as needed so it can flex to contour to the intended rod shape.

    Something I haven't always remembered to do is groove the splines first. Then I can leave the fence in place for both the truss slot and the truss nut cavity with a different bit after I glue the peghead veneer on.

    Terry; I don't have a shaper but I do have a three-wing 3/16" slotting router bit with a ball bearing. Years ago I machined a piece of 1/2" rod as an extender for it and other bits for better height control. I suppose it would be fairly simple to make a thin template to match the rod contour and screw it to the neck blank. Don't know where I'd find a round-nosed version of that though. For a few bucks my local sharpening shop can probably reshape the carbide tips. For a one-off guitar, that's probably a lot more work than use.

    Cheers, don't know when I'll be in Pittsboro next but lunch is on me when I am. Chicago -- might be a while but thanks again just the same. sh
     
    Terry McInturff likes this.
  7. Jack Daniels

    Jack Daniels Supporting Member

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    I do not use tape or heatshrink on mine. I cut the slot 3/16 where the rod just fits in but 0 slop. Encapsulate the rod with wood and not air.
     
    Terry McInturff likes this.
  8. Terry McInturff

    Terry McInturff 40th Anniversary of guitar building! Gold Supporting Member

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