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Has anyone tried this method?

Discussion in 'Luthier's Corner' started by rubo, Nov 25, 2004.

  1. rubo


    Aug 25, 2003
    I've see lots of pictures where they show step by step how a bass is build. And every single one shoes the truss rod is at the top of the neck then it's covered by the fingerboard. What if we took the neck and drilled a hole in the middle ( not at the top) of the neck from the heel to the headstock, but making it a little smaller for the truss rod to fit in. Then heating up the neck so the hole expands then inserting the truss rod, after it cools down it will sit very tight. Will that truss rod work in the middle of the neck, or is there to much tension. Also if it does work, will that make for a better and stronger neck then the regular method.

  2. LajoieT

    LajoieT I won't let your shadow be my shade...

    Oct 7, 2003
    Western Massachusetts
    Well I'm sure people with a lot more knowlege on the topic will weigh in I'll kick it off with a few things that I can surmise.

    One thing is the basic theory that heating the neck will make the hole you've drilled through the middle larger is in fact the opposite of what will happen. The wood will expand, not the hole, so as the wood expands (in all directions) the neck will get larger in proportion (however in much smaller proportions than it sounds like you are expecing) the hole will actually get smaller (as the wood around the hole expands into it.

    Secondly, truss rods aren't inserted from one end of the neck into a hole like that. They are typically laid into a grove cut into the neck wood along it's face (as in strait down the fret markers) and the grove is hidden behind the fretboard. The main function of the truss rod is a squeezing tension which requires both ends of the rod to be exerting force toward the middle of the neck. They do install truss rods from either end of the neck which is why some have the adjustment at the headstock and some have the adjustment where the neck meets the body, but either method requires that the rod be secured at each end. The method you mention could work, but you would also need to cut out enough access at each end of the neck to make the anchor attachements, and you do not want the truss rod to be that tight in the chanel or it will not be able to move as the wood expands/contracts or as adjustments are made to the truss rod. In the end it sounds like it would be too much work for something that is easily hidden behind the fretboard.
  3. rubo


    Aug 25, 2003
    I know about the groove, but with my method and using one piece neck , the only way in would be by drilling a hole thru the neck correct? So if the heat won't work, then we can drill a bigger hole.
  4. LT had the word -

    Did you know that some trussrods, especially single action trussrods, work better if they are in a curved channel? It's curved so that the ends are higher than the center. That way, the compression action tries to pull the trussrod straight and this pushes up on the center top of the TR channel and really straightens the neck. Boring a curved TR hole won't be easy with your method.

    You might also have some extra thinking to do on the fixed end of the TR. To make any kind of a solid attachment to the neck will require passing the TR out the headstock end to, at least, affix some sort of hardware to keep it solid in the hole.

    So, as far as I know, there's the Fender "skunk stripe" method of a slot cut from the back and there's the top rout under the fretboard. I can't think of another way I've seen to do a trussrod. But that's not to say there isn't and maybe you've found the next thing to keep us up late at night :D
  5. rubo


    Aug 25, 2003
    I guess I was way off on this one. Oh well, there are no stupid questions, only stupid answers. :rollno:

    Now different question. If you take a wooden piece which is cut out for the neck before you shape it and add the truss rod & fingerboard, you apply heat to it, then cold, and repeat the procces few times. Then if it's bend you cut out only the streight piece from that block. Will that make for a better neck - meaning it will be suited more for climate change. Or will that whole procces ruin the wooden block?
  6. schuyler


    Aug 5, 2003
    Atlanta, GA
    if the wood is properly dried and seasoned to begin with, what you're suggesting won't make a bit of difference. traditionally, luthiers (and other woodworkers) air-dryed lumber for long periods of time (sometimes 50 years or more), which let the wood go through temperature and humidity cycles like you're talking about. modern kiln drying methods attempt to replicate this in a shorter period of time, and are generally successful enough, though it seems many luthiers still like to let the wood age for awhile before working it. the neck laminates for my next project were glued up over a year ago, and the rest of the wood has been sitting around for at least six months. so far this seems to work for me... at least i haven't had any problems with my necks warping! (knocks on wood)
  7. pilotjones

    pilotjones Supporting Member

    Nov 8, 2001
    Sorry, this is not really true. It would happen this way if only the outside were heated, or only the inside; but if a piece is heated evenly, the hole expands by the same percentage as the outside. This is one of the non-intuitive things you learn in first year engineering, which becomes obvious once the full explanation is given. (Full explanation available for anyone really interested.) The real question is, would you be able to heat it enough to get a decent press fit once it cooled, but not so hot as to damage any glue joints? I doubt it.
    This is true of traditional single-acting truss rods. A double-acting truss rod, however, could easily be inserted into the end of a bore, provided there weren't any obstructions beyond the bore. They need no anchor on the far end.
  8. pilotjones

    pilotjones Supporting Member

    Nov 8, 2001
    Interesting thing- there are a few patents for making a curved channel in an assembled neck. The neck is bent the opposite way of the intended curve of the channel; a straight hole is gun-drilled from one end; and the pressure is released, resulting in a curved channel.
  9. Well, if you are using a single action TR, the curved hole/slot would be helpful but if you can't anchor the other end, it won't work anyway.

    Then, of course, the curved slot wouldn't be necessary for a Double action TR.

    I've actually used this method to make curved channels. I didn't think far enough to realize that this would work for a gun drilled hole. :)
  10. LajoieT

    LajoieT I won't let your shadow be my shade...

    Oct 7, 2003
    Western Massachusetts
    Well I guess I can see how that would be true about the whole thing expanding, and yes it is both non-intuitive and obvious at the same time (I'm SO confused). I'd love to hear all the techie explanation if you have the time.

    And I can barely understand the single action truss rod, I have yet to get into understanding the Double-Action ones...

    Another question that crossed my mind goes back to the old six string Parker bass that was for sale at the low end. I was said to have a truss chain (I think Ken Smith mentioned it and that it was the only he knew of). But I was wondering if you could drill the hole/channel and get a threaded anchor into the wood at the far end with the chain. It still sounds like a lot of work for minimal if any advantage, but I'm just trying to make the original concept work.
  11. Since when wood expands under heat? Metal does, but wood? :confused:

    Also, the drill bit length needed to make a hole in the neck would be around two feet, right? The deflection of the drill bit would make it virtually impossible do drill a straight hole, IMO. Do two-feet long drill bits exist anyway?

    The few double-action truss rods I've seen required a rectangular or U-shaped channel, not a round one.

    Finally, I don't see any advantage in using such method.
  12. pilotjones

    pilotjones Supporting Member

    Nov 8, 2001
    Neglecting a few freakish materials that actually contract, all solids expand when heated, to varying degrees.

    Expansion of oak, for instance, is roughly 40% that of steel.
    They're called gun drills (amongst other names also, probably). And gun drilling is certainly difficult, but doable.
    That's been my experience also. But that's not to say that you couldn't make one for a round bore if you wanted to.
    One possible benefit would be to be able to offer a customer a true one-piece neck, with no glue joints.
  13. pilotjones

    pilotjones Supporting Member

    Nov 8, 2001
    Quite true.

    Here's an idea though for the far end anchorage: do a flat-bottom counterbore at the proper position at the "anchor" end of the rod channel. This could be either at the underside of the heel for a headstock-adjusting rod, or at the headstock for a heel-adjusting rod. Then insert a metal-- or even wood-- device resembling the little "turn disk doohickeys" that are used in all that cheap particleboard furniture. The anchor end of the truss would have a little boat-shaped bit welded on the end to engage the disk, while preventing twisting (instead of the usual ball end on the furniture connector shafts). Think teenut, but perhaps in the shape of a woodruff key.

    Voila: a removable single-action rod.

    The "turn disk" should do a decent job of spreading pressure to prevent splitting.

    Now who's got $1000 and a patent application?
  14. Oh, I always thought only metals expanded when heated.

    Called like this because they're used to bore gun barrels, I guess?

    Is it really worth all the pain? I mean, I don't think having a glued-on fingerboard is a disadvantage, since most builders are using that method without any problems (well, there's Wish but that's another story).
  15. pilotjones

    pilotjones Supporting Member

    Nov 8, 2001
    I'll bet if someone started offering true one-piece necks, and claiming some advantages to them, he could find some customers.
  16. Probably. Thanks Peter.
  17. pilotjones

    pilotjones Supporting Member

    Nov 8, 2001
    Here's a long-winded technical thing, so feel free to pagedown, everybody.
    Well, rather than the technical explanation, here's a shot at a demonstrative thought experiment.

    Imagine you have a 1" diameter disk of some material that expands a lot with heat. Specifically, this material expands linearly by 50% with a rise of 50°.

    You heat it up by 50°, and it expands to 1.5" diameter. Cool it down again, and it returns to 1" diameter.

    Now imagine you have the same 1" disk at room temperature. You draw a centered 0.5" inch circle on it. (It really doesn't have to be centered, but it's easier to think about that way.) You heat it up. After heating, you again have a 1.5" disk; the circle that was drawn on it has now grown to .75" . You cool it down again.

    Now, you get out your x-acto knife and cut the disk on the 0.5" diameter line that you drew before. You now have a 0.5" small disk, surrounded by a 1" OD (outside diameter)/ 0.5" ID (inside diameter) "washer".

    You heat this. The 1" washer OD expands to 1.5", as it did before; the 0.5" washer ID, which is also the small disk outside edge, expands to 0.75", as it did before.

    Finally, you cool it down; throw away the small disk; heat it up; you have a washer with 1.5" OD and 0.75" ID. The hole has grown by 50%, as did the outside.

    What has happened is that in an isotropic material, when it expands, the relationship between any to points grows by the same ratio - regardless of where on the object they are. This includes points on opposite sides of a hole.

    It is true, as mentioned earlier, that if for example the inside only of the washer were heated, the inside material would be "forced inwards" by the cool, unexpanded outer material, shrinking the hole. (There would be internal stresses developed, causing this to happen.) But if it's uniformly heated, it expands uniformly. (Assuming you start with no internal stresses, the uniformly heated object will also have no internal stresses.)

    Anyone who might have experience with a heated press fit or "shrink fit" of a bearing on a shaft will testify that the bearing must be completely "heat soaked" before attempting the shaft engagement.

    And we now return you to the previously scheduled episode of, "The Death of Mary, Queen of Scots." :)
  18. Tip yer hats boys.
  19. LajoieT

    LajoieT I won't let your shadow be my shade...

    Oct 7, 2003
    Western Massachusetts
    Thanks for the Explanation Peter, Makes perfect sense when you think about it that way!!!
  20. Suburban


    Jan 15, 2001
    lower mid Sweden
    Are you teaching, or ever thought about teaching tech?
    If not, you might as well do that. You've got what it takes...