Abstract:
A versatile four-way adjustable truss system in a stringed musical instrument is disposed uniformly within the instrument neck that provides or supports a fingerboard or fret-board on its top side. The bottom surface of the truss may be exposed along its full length and made flush with the bottom neck surface and smooth to the touch. The truss is made adjustable at both ends in either tension or compression and is securely fastened to the neck at an intermediate fastening point so as to form two substantially co-linear neck sections either of which can be adjusted independent of the other, via an associated adjustment nut constrained in a corresponding thrust cavity configured in the neck, to satisfy a desired section profile requirement in a range that includes both concave and convex curvature.

Description:
FIELD OF THE INVENTION 
   The present invention relates to stringed musical instruments generally of the guitar family, and more particularly to an improved adjustable truss implementation, in a stringed instrument neck supporting a fretboard or forming a fretless fingerboard, that enables adjustment of curvature independently in two sections of the neck in a range that includes both concave and convex to satisfy profile requirements unique to each section. 
   BACKGROUND OF THE INVENTION 
   In stringed musical instruments such as guitars and bass guitars, a main component is the neck that provides or supports a fretboard or fretless fingerboard. The neck is typically made from wood and is ordinarily supplied initially as being nominally flat along its length, free of neck curvature. For purposes of the present disclosure, “neck length” refers to only that portion of the neck that is associated with the fretboard/fingerboard, and is not intended to include any substantial portion of the neck located within the body of the instrument, e.g. as in thru-neck type instrument construction. Even a newly fabricated neck may already have some amount of inherent curvature, concave or convex, and such bowing or arching is not always symmetrical over the neck length. 
   When the instrument is strung and tuned, the high tension in the strings, in the order of one or two hundred pounds, sets up a strong continuous stress load that tends to bow the neck concavely. Depending on the strength of a particular neck, string tension often introduces some amount of concave curvature that results in a non-uniform increase in the string-to-fret separation, known as “high action”, making the instrument difficult to play by placing excessive demands on fingering technique. 
   In initial setup or refurbishing of a stringed musical instrument, a luthier or technician typically uses all available adjustment capability to achieve the desired string-to-fret spacing. In the absence of truss tension adjustment, the two main adjustments available are the bridge and nut heights. Many instruments are provided with truss rods with tension adjustment that can reduce neck curvature only if the curvature is concave, and their effect is spread over the full length of the neck. 
   In the art of the luthier, even a substantially flat neck with uniform low string spacing throughout the neck length would be considered no more than a marginally acceptable compromise. Ideally the luthier would prefer some slight concave curvature, known as “relief”, over a lower pitched section of the neck and a straighter profile over a higher pitched section. This represents a fortuitous circumstance that rarely occurs in manufacture and setup, and cannot normally be accomplished by adjustment in trussed necks of known art, thus requiring laborious “relief” dressing of particular regions of the fretboard by a skilled luthier. Conventional tensioned truss rods or even the relatively rare truss types that can be alternatively placed in compression for adjustment regarding curvature, are all subject to the inherent limitation of known art: their effect is spread over the full length of the neck. 
   Stringed musical instrument owners, manufacturers, luthiers and repair technicians all face a random variety of variables in instrument neck curvature. Some new necks may already exhibit some amount of curvature: concave or convex. Especially in absence of truss reinforcement, and even with a tension truss, a selection process may be required in quality control wherein necks found to have excessive concave or convex curvature may have to be discarded and replaced. Hopefully an initially straight neck might be strong enough to resist bending and thus remain playable, while a neck with slight convex curvature could even tend to straighten and thus could improve in uniformity and playability after one or more service refurbishments including bridge height adjustments, over a period of time. More typically, undesired neck curvature is likely to worsen with time, and could end up becoming even less correctable with existing truss construction of known art. Asymmetrical curvature affecting only a section of the neck length cannot be adjustably corrected by any truss system in the field, and instead may require extensive dressing of the crowns of the frets by a skilled luthier. 
   DISCUSSION OF KNOWN ART 
   U.S. Pat. No. 5,233,122 issued in 1993 to Kim for GUITAR WITH NECK TRUSS ROD SUPPORTING CONSTRUCTION discloses an extension member at one end of a rectangular metal truss rod anchored to a front board of the guitar body in a dovetail manner, exemplifying a type of truss that attempts to prevent neck “cracking and bowing” by functioning strictly as a “brute-force” non-adjustable neck-stiffening beam element with no longitudinal stress applied. The truss is entirely enclosed in the neck and located immediately beneath the fretboard. 
   U.S. Pat. No. 5,864,073 for LAMINATED NECK FOR GUITARS AND COMBINATION THEREOF WITH ADJUSTMENT SYSTEM issued in 1999 to Carlson, assigned to Fender Musical Instrument Corporation, typifies traditional trussed stringed instrument neck construction of a type that has found wide usage in known art. A metal truss rod is fully enclosed in the neck beneath the fretboard in an arched shape along a non-uniform groove of varying depth routed in the main neck section and enclosed by a separate filler part beneath the truss rod. This approach imposes a burden of structural complexity and addresses only concave neck curvature through longitudinal tension applied to the truss, failing to provide reverse compensation capability for correcting convex neck curvature. 
   In addition to the aesthetic compromises and adjustment difficulties arising from front-access adjustment, the practice of fully enclosing the truss imposes a serious serviceability shortcoming: if the threaded adjustment means on the truss becomes stripped or the adjustment tool interface such as a screwdriver slot in the end of the truss rod becomes deformed to a point of malfunction, removal of the truss for repair or replacement is extremely difficult, e.g. requiring removal of the fretboard from the neck, and in some instances truss repair/adjustment may be practically impossible, rendering the instrument unrepairable. 
   U.S. Pat. No. 4,557,174 issued in 1985 to Gressett, Jr., assigned to Fender, discloses a GUITAR NECK INCORPORATING DOUBLE-ACTION TRUSS ROD APPARATUS, described as providing “compressive or tensile loading of the truss rod for flexing of the neck in either direction”. A sleeve  33 , located in a central region of the truss for purposes of transmitting lateral thrust from the truss to the neck, is in “sliding metal-to-metal relationship between the truss rod and the sleeve  33 ” and thus fails to provide a longitudinally anchored point in the mid-region of the truss and thus fails to enable separate independent curvature correction adjustment of each half. Clearly this &#39;174 patent addresses only simple full length neck bowing. 
   U.S. Pat. No. 4,953,435 issued in 1990 to the present inventor, Emmett H. Chapman, for REAR-ACCESS TRUSSED NECK CONSTRUCTION FOR STRINGED MUSICAL INSTRUMENTS, discloses improved trussed neck structure that was incorporated as a refinement in the Chapman Stick (registered trademark) where uniform low action is desired to facilitate a special string-tapping playing technique (see U.S. Pat. Nos. 3,833,751, 3,868,880 and 4,953,435 by the present inventor). A substantially straight truss member is disposed uniformly in a groove along the rear side of the neck such that a surface of the truss is exposed along its full length, flush with the rear neck surface. A readily accessible rear-access threaded fabrication/service adjustment member provides convenient capability of applying an adjustable amount of either tension or compression as required to offset an unwanted neck-bending tendency in either direction, concave or convex, thus correcting and securing the neck in a straightened, stabilized condition. 
   Years of experience have revealed many instances where further degree of neck adjustment capability is required. In some necks, particularly long necks, there may be unwanted curvature that, in the absence of compensation, is not uniformly distributed along the total length; instead it may be asymmetric, e.g. predominant in one or other half of the total neck length, so that it cannot be fully compensated by adjustment of a full length truss rod whether in compression or in tension. Thus a need has been identified for further refinement in which each half section of the total neck length is made independently adjustable for correcting either convex or concave curvature. 
   U.S. Pat. No. 6,051,765, issued in 2000 to Regenberg et al for a GUITAR WITH CONTROLLED NECK FLEX, encloses the truss in an inverted U-shaped channel member that fits into a U-shaped channel machined into the back of the fingerboard. First and second spacers are welded or otherwise fastened onto the truss rod, separated from each other at predetermined locations along the truss so as to divide the total truss rod length into three regions with the two spacers, each acting in compression against the fingerboard to act on curvature. Since the truss rod can be deployed only in tension, i.e. for reducing concave neck curvature only, it fails to provide correction for convex neck curvature. These “spacers” exert vertical pressure on the neck and fingerboard to create upward force at selected points along the board when the truss is tightened in tension. Since the apparatus is buried within the neck under the laminated fingerboard, each spacer&#39;s location must be preset and the spacer welded or fastened to the truss in an arbitrary location in fabrication, after which the spacers cannot be adjusted or relocated to customize the truss action along the neck length. The three “regions” thus operate in a manner that differs from the two separately adjustable sections of the single truss or two in-line half-length trusses of the present invention. 
   In the above-described and all other known prior art in neck trusses for stringed musical instruments, even when both regular and reverse compensation capability are provided, they act over the full length of the neck, and as such, in a neck with compound or asymmetric curvature where the two sections of the neck require corrective compensation in different amounts and/or opposite directions, trusses of known prior art are inherently difficult or impossible to adjust in a manner to attain an the ideal neck profile commonly sought by luthiers, i.e. that of a concave curve along the lower pitched section, known as “relief”, and straight profile along the higher pitched section. 
   OBJECTS OF THE INVENTION 
   A primary object of the present invention is to provide an improved trussed neck for stringed instruments, including adjustment means for controlling neck profile relative to the strings including desired curvature in either convex or concave direction in either or both sections of the full length of the neck in order to accomplish a desired combination of straightness and concave curvature known as “relief” by independent adjustment of each half section of the truss to place it in compression or tension as required in order to control the contour of each of the two sections of the neck independently so as to enable flexibility of adjustment regarding fingerboard curvature and accomplish a desired final contour that requires minimal dressing of frets. 
   It is a further object to provide an embodiment wherein the truss system allows the neck to be fabricated as a single piece of material, the front side serving directly as a playing surface thus eliminating any need for a separate fingerboard part, and the rear side containing the truss exposed in a channel, thus facilitating truss/neck assembly and eliminating any need for additional neck parts such as enclosed strips. 
   It is a still further object that the truss be readily removable for service and/or replacement without removing the fingerboard from the neck. 
   SUMMARY OF THE INVENTION 
   These and other objects and advantages have been accomplished in the improved four-way adjustable trussed neck structure of the present invention in which a substantially straight truss member, which may be implemented as one piece or as two substantially co-linear sections, is disposed uniformly within the neck, optionally in a channel configured along the rear side of the neck such that a flat surface of the truss is exposed along its full length, optionally flush with the rear neck surface and extending fully to the channel edges so as to feel smooth to the touch. The truss is made adjustable at both ends in either tension or compression and is securely fastened to the neck at an intermediate fastening point so as to form two substantially co-linear neck sections either of which can be adjusted independent of the other, via an associated adjustment nut constrained in a thrust cavity configured in the neck, to satisfy a desired section profile requirement in a range that includes both concave and convex curvature. 
   A full understanding of this invention will be gained through a study of the accompanying drawings along with the following descriptive text. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side view of a stringed musical instrument neck of a known non-truss type illustrating a flat fingerboard and a closely spaced parallel location of the strings. 
       FIG. 2  depicts an instrument neck as in  FIG. 1  showing (in exaggeration) concave curvature induced by string tension. 
       FIG. 3  depicts an instrument neck similar to that of  FIGS. 1 and 2  but equipped with a known tension type of truss rod that can be adjusted from one end to act in a manner and direction to offset and minimize concave neck curvature of the type shown in  FIG. 2 . 
       FIG. 4  depicts an instrument neck with convex curvature that cannot be corrected by a neck truss of the tension type shown in  FIG. 3 . 
       FIG. 5  depicts an instrument neck fitted with the inventor&#39;s proprietary dual mode type of neck truss that can be deployed in tension to offset concave curvature, as shown in  FIG. 2 , or alternatively, in compression to offset convex curvature as shown in  FIG. 4 . 
       FIG. 6  illustrates concave curvature in the left hand half section of the neck. 
       FIG. 7  illustrates concave curvature in the right hand half section of the neck. 
       FIG. 8  illustrates convex curvature in the left hand half section of the neck. 
       FIG. 9  illustrates convex curvature in the right hand half section of the neck. 
       FIG. 10  illustrates concave curvature in the left hand half section and convex curvature in the right hand half section. 
       FIG. 11  is a side view of an instrument neck equipped with a truss system of the present invention showing enlarged cross sections at midpoint and end points. 
       FIG. 12  is a bottom view of the instrument neck and truss shown in  FIG. 11  including enlarged views at the midpoint and end points. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a side view of a stringed musical instrument neck  10  and tuning headstock of known art without a neck truss showing string location  12  spaced uniformly above the fingerboard  14  by a string-support nut  16  at the left hand end and a bridge (not shown) on the instrument body at the right hand end. For simplicity fingerboard  14  is shown as fretless, however the present description applies equally to a fretted fingerboard, i.e. a fret-board. In either case it is generally desired for ease of playing that the fingerboard  14  be kept substantially straight, i.e. longitudinally flat so that the nut and bridge can hold the taut strings at a close spacing above the fingerboard or frets, known as “low action”, that is substantially uniform throughout the neck length to generally facilitate finger-stopping and string-tapping while preventing vibrating strings from buzzing against frets or fingerboard  14 . 
   The fingerboard  14  can be made as an integral part of the neck, but more typically it is made as a separate thin layer of different material that is attached to and thus shaped by the neck  10 . To the extent that an instrument falls short of providing ideal low action everywhere along the fingerboard/fretboard, it is more difficult to play: the player is forced to compensate and develop corrective playing techniques. 
     FIG. 2  depicts an instrument neck  10  as in  FIG. 1  but with concave curvature of the neck  10  and fingerboard  14 . The amount of curvature shown in FIGS.  2  and  6 - 10  is exaggerated for clarity of illustration; in actuality players generally desire the “action” (string-to-fret spacing) to be low while avoiding string buzz on the frets. 
   When the strings  12  are tensioned as required for tuning and playing purposes, stress in the order of hundreds of pounds acts on the neck in a manner that inherently tends to bend the neck and introduce concave curvature. Even if this curvature is compensated by adjustment of the bridge support (located beyond the right hand end in  FIG. 1 ), the result is a compromise in which excessive concave curvature remains as a undesired problem with increased sting spacing, i.e. high “action”, in the mid-region of the fingerboard that cannot be corrected by setting of the bridge. 
   Since the neck  10  is typically made from wood, it is subject to both initial warping and variations in strength against warping under the continuous stress in the instrument. Unfortunately neck curvature may develop and increase further with the passage of time to an extent that makes the instrument virtually unplayable. This disadvantage of non-trussed instrument necks led to the improvement of incorporating some form of truss for neck reinforcement, preferably adjustable. 
     FIG. 3  depicts an instrument neck  10 A equipped with a known truss rod  18  of a type described above in connection with U.S. Pat. No. 4,557,174, intended to provide adjustment with regard to neck curvature, e.g. of the type shown in  FIG. 2 . However, this structure, being deployable in tension only, fails to provide capability of correcting curvature in both directions, i.e. concave/convex, and, as described above, is subject to other disadvantages and limitations including inability to adjust two neck sections independently. 
     FIG. 4  depicts an instrument neck  10 A with a tensioned truss  18 , as in  FIG. 3  but shown here deformed by convex curvature that cannot be corrected by the truss  18  since it can be deployed in tension only; tensioning the truss  18  would only increase the amount of convex curvature. 
     FIG. 5  depicts an instrument neck  10 B fitted with a special known type of dual mode neck truss rod  20  that in addition to being deployable in tension to correct concave curvature as in  FIG. 2 , can alternatively be deployed in compression to correct convex curvature (as shown in  FIG. 4 ). Adjustment is implemented by a machine nut engaging a threaded end of truss rod  20 , the opposite end being anchored in the neck  10 B. Truss rod  20  is square in cross-sectional shape and is located in a channel machined in neck  10 B, optionally with one flat edge exposed flush with the bottom side of neck  10 A. This type of truss system is disclosed in U.S. Pat. No. 4,953,435 by the present inventor, described above. 
     FIGS. 6-9  show string locations  12  over a fingerboard  14  illustrating four types of asymmetric neck deformation that affect predominantly only half of the neck length and thus cannot be satisfactorily straightened by any known instrument neck truss systems including those of  FIGS. 3 and 5 . 
     FIG. 6  shows string location  12  over a fingerboard  14  with concave curvature in the left hand half neck section only, shown somewhat exaggerated for clarity. If the amount of concave curvature shown in the left hand section could be adjusted to an ideal amount, and the right hand section held straight, this pattern would satisfy the standards of a luthier seeking optimal “low action”, i.e. close string-to-fret spacing. 
     FIG. 7  shows string location  12  over fingerboard  14  with concave curvature in the right hand half section of the neck. 
     FIG. 8  shows string location  12  over a fingerboard  14  with convex curvature in the left hand half section of the neck. 
     FIG. 9  shows string location  12  over a fingerboard  14  with convex curvature in the right hand half section. 
     FIG. 10  shows string location  12  over a fingerboard  14  with an S shaped profile, i.e. concave curvature in the left hand section and convex curvature in the right hand half section, a condition that cannot be corrected by the single full length trusses of known art. If the concave curvature in the left section could be reduced and the convex curvature in the right section eliminated to a substantially straight condition, the overall profile would conform to the desired viable pattern as described in connection with  FIG. 6 . 
   Similarly, each of the conditions shown in  FIGS. 7-9  could be transformed into a desired viable pattern if each half section could be adjusted for curvature independently in either direction. 
     FIG. 11  is a side view of a stringed musical instrument neck  10 B equipped with the four-way adjustable truss  26  of the present invention which is capable of correcting asymmetric concave and/or convex curvature as in  FIGS. 6-10  by independent adjustment in each of two sections of the neck. 
   The enlarged cross-section  22  at the mid-region of neck  10 B shows truss  26  anchored to neck  10 B by a pair of screw fasteners  32  with flat countersunk heads in a secure manner that prevents any relative displacement laterally or longitudinally. At each of the end regions  20  and  24 , an adjustment nut  30  is threadedly engaged with a threaded end section of truss  26  which is fitted snugly in a channel configured along the bottom side of neck  10 B. 
   Adjustment nuts  28  are located in thrust cavities  30  configured in the neck  10 B. At each of the end regions  20  and  24 , the nut  28  can be rotated in one direction to bear against the side of thrust cavity  30  to deploy truss  26  in tension or rotated in the opposite direction to bear against the opposite side of thrust cavity  30  to deploy truss  26  in compression. Optionally flat washers may be deployed as shown at the pressured side of each nut  30  for spreading the distribution of the force applied to the neck  10 B. 
     FIG. 12  is a bottom view of the instrument neck  10 B and truss  26  shown in  FIG. 11  including enlarged bottom views of above-described items in the midpoint fastened region  22  and the end point adjustment regions  20  and  24 . Thrust cavities  30  are dimensioned in width to fit closely on each side of the nut  28  and in length to allow entry of an open-end wrench for adjustment. 
   Truss  26  is typically made square in cross-section, but could be made in another shape such as round, elliptical or rectangular. If the truss has an exposed surface, it should be made flush with adjacent neck surfaces and extend fully thereto with no substantial gaps, thus providing a smooth feel of the thumb on the neck. As an alternative to exposing one surface as shown, the truss could be entirely enclosed within the neck but preferably close to the bottom surface for effective adjustment action regarding curvature. Alternatively the truss could be located at or near the top surface of the neck: in that instance the effect of truss tension/compression on direction of neck curvature would be reversed. 
   Fastening of the central region of the truss  26  to neck  10 B could be implemented with alternative fasteners such as pins instead of screws, an additional number of fasteners, or providing an additional metal member welded or otherwise fastened onto the central region of truss  26  and securely captivated in a thrust cavity similar to thrust cavities  30  configured in neck  26 . Such pins or other alternative fastener members may be oriented in any direction, for example transverse instead of the perpendicular orientation of screws  32  shown in  FIG. 11 . 
   As an alternative to locating the ends of said truss in close proximity to the ends of the neck as shown, the invention could be practiced with the truss made shorter or longer relative to the neck at one or both ends. 
   As an alternative to the one-piece dual truss described above, the truss can be made from two separate truss sections interfacing at the designated truss-to-neck fastening region that includes the boundary between first and second adjacent sections of said neck. In this embodiment the truss ends in the mid region of the neck are attached by truss-to-neck attachment means located in the designated truss-to-neck fastening region, so as form a two-piece dual truss that is functionally equivalent to a one-piece dual truss. 
   This invention may be embodied and practiced in other specific forms without departing from the spirit and essential characteristics thereof. The present embodiments therefore are considered in all respects as illustrative and not restrictive. The scope of the invention is indicated by the appended claims rather than by the foregoing description. All variations, substitutions, and changes that come within the meaning and range of equivalency of the claims therefore are intended to be embraced therein.