Adjusting the trussrod before dressing the frets?

Discussion in 'Luthier's Corner' started by reverendrally, Jan 31, 2013.


  1. reverendrally

    reverendrally

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    Got my neck all ready to dress the frets. Having put a straight edge a cross it, I've noticed it has a very slight back bend in it. Like 1mm from end of the board to the other. I'm guessing string tension will more than fix that.

    I'm wondering whether to adjust the TR up a little (double acting) to bring it up close to straight before flattening the frets down, or leaving it back bent and going from there.

    What would y'all suggest?
  2. Geoff St. Germaine

    Geoff St. Germaine

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    So the fretboard itself shows the backbow? If so, then yes, adjust the truss rod so that the fretboard is perfectly level.
  3. Bruce Johnson

    Bruce Johnson Supporting Member

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    The answer to that depends on how tight the truss rod is now, in that slightly backbowed condition.

    A double-acting truss rod has a dead zone in the center, where it's loose. That is, tightening it in the one direction (clockwise in most cases) causes the neck to bend back. Turn it the other way and it will loosen, go through the dead zone, then start tightening again in the reverse direction, causing the neck to bend forward.

    What you want is for the frets to be flat while the truss rod is partially tightened in the normal clockwise back-bend direction. You don't want the frets/neck to be flat with the truss rod real tight, nor do you want it flat with the truss rod loose in the dead zone. If the rod is partially tight, then it will have a good range of action to counteract the tension of the strings.

    When you string it up, the neck will pull forward some amount, depending on how flexible or stiff it is. Ideally, at full tune, it will have a small amount of relief with the truss rod partially tight. From there, you can tighten the rod a little more to bring the neck down to flat, or back off the rod a bit to add some relief. Again, the point is that, throughout the range of adjustment from flat to mild relief, the truss rod should always remain tightened some amount in the normal direction.

    Side note: This is a common reason why some bass necks are unstable and move around a lot with weather changes. For whatever reason, when the bass is up to tension and in normal setup, the truss rod is loose and applying no counteracting force. That makes the neck much more sensitive. A little change in humidity, the wood expands, and the neck bends.

    Anyway, back to your original question: The first thing that you should do, before you start any leveling, is to adjust the truss rod. It's probably quite tight right now, which is why the neck is backbent. Loosen the truss rod all the way down through the dead zone, and push on it some to flatten it out. Then, put a straight edge on it and start tightening the truss rod until you see the neck beginning to move. At this point, don't worry about how flat it is. You are watching to see when it begins to bend backwards. If all is well, with the rod at maybe 25% of full tightness, you'll see the neck bend maybe 1/16". Stop right there. That's the loading condition that you want the neck to be in, with the frets flat and no strings. Look at it now with the straightedge and see how close to flat it actually is in that condition. If it's fairly close, go to it and level the frets.

    If, with the truss rod partially tight, the neck is seriously bowed forwards or backwards, beyond what you can take out with fret leveling, well, then you have to make a value judgement. To make it right, you'll need to fix it by pulling the frets, level the fingerboard, and refret it. If you cheat by cranking the truss rod real tight or real loose, just to get the neck close enough to flat that you can level it, then you'll end up with a neck that will tough to set up under tension, and probably unstable.
  4. reverendrally

    reverendrally

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    Just did what you suggested. Frets seemed to have leveled ok. Now it's time to crown them... ah, the bit I enjoy. :(
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  6. Bruce Johnson

    Bruce Johnson Supporting Member

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    A fret crowning tip:

    Do you have a Japanese-style waterstone, of the type used for sharpening planes and chisels? I find that they work great for crowning frets, even stainless frets. I use the Fine stone (4000 grit, or something like that) and turn it up on edge, to preserve the main flat surface. Rub it on the fret dry, no water, and it will quickly wear a little rounded groove in the stone. Use that groove for your crowning. It will cut fairly quickly and leave a smoother surface than the commercial crowning files.

    Before crowning, I level the tops of the frets with an Arkansas Oilstone, with a little WD-40 wiped on it. That works great for leveling.

    It's interesting that the oilstone cuts the fretwire without grooving the stone. The waterstone, dry, wears quickly to the contour of the fret. So I use them both!
  7. reverendrally

    reverendrally

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    Thanks for the tip. I've got a stewmac fret crowning file. For leveling, I made a straight sanding block a while back that covers the whole fingerboard. I use that for initial flattening and then the radiused stewmac block after that to get the radius on the frets right. Seem to work really well.
  8. narud

    narud Supporting Member

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    So if you have a neck that is severely forward bowed with no tension and you pull the frets to level the board, do you level it with some truss rod tightened or with the rod slack?
  9. Bruce Johnson

    Bruce Johnson Supporting Member

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    You level it with the truss rod partially tightened, as I described above. You want the neck to be flat in that loaded condition. So, put it in that loaded condition, then trim it to flat.
  10. reverendrally

    reverendrally

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    *like* :)

    Thanks heaps for your insights and advice Bruce. I've discovered recently that it's tricks like this that sort the men from the boys. ;)

    Pete
  11. narud

    narud Supporting Member

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    thanks bruce! just wanted to make sure what you wrote about leveling the frets was the same procedure for the board.
  12. narud

    narud Supporting Member

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    just thought about one more question if you dont mind bruce. once you level the board with the rod engaged, do you refret with the neck straight and the rod still engaged, or loose rod with forward bow and let the frets pull the neck straight?
  13. Bruce Johnson

    Bruce Johnson Supporting Member

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    Think of it like this: Leveling the fingerboard and leveling the frets are doing the same thing, but the fingerboard is the coarse leveling, and the frets are the fine leveling.

    With the frets pulled, partially tighten the truss rod to load up the neck, then trim the fingerboard as flat as you can. Get rid of any lumps, correct the radius as needed, and check the depth of the slots.

    Next, put in the frets. The process of hammering or pressing them in may move the neck a little. Back off the truss rod, push the neck around a bit, then bring the truss rod back up to partially tight again. Now, level the frets, then crown them and polish them.
  14. FunkyMan

    FunkyMan

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    Nov 27, 2007
    Hello Bruce, one question: this means that the frets located in the middel zone of the fretboard, (lets say from fret 5 to 18) are going to be lower in relation to the rest of the frets when sanding? Due to the back bow?
  15. lettsbasses

    lettsbasses

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    This. Of course the neck needs to be flat before you level frets. The act of sanding/polishing the levelled frets is enough to re-crown the frets at the same time unless they are in poor shape ith really bad flat spots. I work my way down the sides of each fret starting at 240 or 320 grit and work my way up to 800/1000 grit wet/dry.
  16. Bruce Johnson

    Bruce Johnson Supporting Member

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    Not necessarily. It could also be the other way, with the middle frets higher. It depends on how straight the neck and fingerboard are when the truss rod is partially loaded.

    I'll say it again: The point is to first put the neck under partial load with the truss rod, and THEN trim it to level as needed. That trimming may be a slight leveling of the tops of the frets, or in more drastic cases, it may mean pulling the frets and recutting the fingerboard.

    This is all about making the truss rod do its job correctly, so it will brace the neck to help keep it stable, and allow the neck to be easily adjusted through a range from flat to mild relief.

    If you take the other approach, which to first adjust the truss rod to make the neck as flat as possible, and then level it, you make the job easier, sure. But you run the risk of defeating the function of the truss rod, and causing big problems later.

    Suppose the neck has a significant forward bow, and you have to crank the truss rod to 80% to make it flat (unstrung). If you level the frets with it cranked up like that, it now has hardly any reserve left. You string up the bass, and the tension pulls the neck into a small amount of relief. But the customer wants the fingerboard flat! So you put on a long arm wrench and crank the truss rod up from 80% to 110%. That's how truss rods get broken. Or stretched so far that they run out of threads.

    On the other hand, suppose the neck has a small backbow (unstrung). You loosen the truss rod to get it to flatten out. If it wasn't very tight when you started, then loosening it down to "loose" isn't going to do much. So, if it's double-acting, you keep turning it backwards, through the dead zone, until the truss rod tightens in reverse and pushes the neck flat. Suppose that you level the frets with the neck in that condition; being bent forward by the truss rod. Now, when you string up the neck, the string tension pulls the neck off the truss rod, leaving the truss rod slack and loose. This makes the neck really sensitive to movement, from weather changes or anything. The truss rod is stuck in the dead zone, where it can only apply a little load in either direction. It effectively has slop. The relief goes a little too high, so you tighten the rod a little bit in the positive direction, and the neck slams to flat. Then it starts to buzz from a bit of backbow, so you turn the rod backwards as gently as you can, and it slams into too much relief again. This is what's referred to as an unstable neck. You are constantly chasing it, readjusting the truss rod to increase and reduce the relief. This loose truss rod condition is also the usual cause of truss rods that buzz or rattle.

    This is why most of us "hand made" Luthiers do this pre-tensioning of the necks before leveling. It sounds like a small detail, but it's important.

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