To cut a carbon fiber rod in the neck of old parlor guitars I have done for a long time. Have you seen as many old guitars as I know you know that almost all the throats have been bent over the years of the string.
Det beror antingen på att man strängat på för tjocka stålsträngar på en gitarr som var byggd för mjukare sensträngar/nylonsträngar, men det kan också vara helt naturligt eftersom trä är plastiskt och ”kryper” när den utsätts för krafter under en lång tid. Även så hårda träslag som lönn och rödbok böjer sig med tiden. Mahogny är ett unikt träslag som håller väldigt bra när den utsätts för böjning under lång tid, det är också ett mycket populärt träval för halsar.
En böjd hals kan man böja tillbaka genom att värma halsen och överböja den åt det andra hållet med tvingar och mothåll. Böjer man tillbaka halsen på det viset kan man ”börja om” med en hals, den kommer att vara rak några decennier till innan den sakta böjer sig igen. Har märkt att det blir svårare varje gång man värmer att forma om halsen, ska man värma och böja är det bäst att göra det i ett eller två pass. Mahogny halsar är svårast att värma rak. För att inte greppbrädan ska krympa av uttorkningen bör man olja in den med fet teakolja innan man värmer.
Historically, many ways have been tried to make the neck stiffer. Tables have been folded into ebony or other hard and rigid types of wood, T angles and square tubes in aluminum and steel have been tested. No solution was really good.
En metod som var vanlig på 1800-talet och som faktiskt fungerar är så kallade ”bar frets”. Det är kraftiga rektangulära band som pressas ned i lagom trånga spår i greppbrädan och som förspänner halsen till en ”back bow” precis vid ytan av greppbrädan. Med rätt monterade ”bar frets” kan man få en hals som håller sig rak utan varken dragstång eller kolfiberstav med uppspända strängar. Problemet är att banden förr eller senare nöts och att träet i greppbrädan måste vara hårt, t.ex. ebenholts. Då banden behöver bytas krävs det både tid och kunnande att sätta om dom på rätt sätt. T band med en ”tang” är väldigt enkla att montera och är de som används numera, ”bar frets”används bara i undantagsfall och aldrig av fabriker.
The drawbar was invented by Gibson in the late 1920s and is an adjustable stiffener that can also reshape the curvature of the neck. The problem with a lot of metal in the neck (the drawbar must be strong to hold) is that the weight increases and that the metal is not good for the sound. One thing not everyone knows about the drawbar is that you do not get the smooth fine bend on the neck you think when you tighten the screw. The bar compresses the wood in the neck which is not homogeneous all the way, some parts are pressed together more than others. The result is that the bend of the throat becomes a little curvy and you can get out of the band rattle as even a few hundredths of a mm can be the difference between band rattle and not. The drawbar is as much a problem as a solution in my opinion.
With carbon fiber you got a material that was light, extremely stiff, not crawling and also has a good sound. It didn't take long before you started to fold carbon fiber rods into guitar necks, though the drawbar is still standard in mass-produced guitars. A guitar neck with or without a carbon fiber bar gives no noticeable change in tone or weight. With the carbon fiber rod you have no adjustment possibility, but if the neck is straight when manufactured and then never bends, you do not need to adjust! 🙂 In mine Gammel Gura I use a 1 x 1 cm thick carbon fiber rod with an 8 mm hole in the middle. In the hole I paste a round bar in birch, sometimes I put in a massive round bar in carbon fiber instead if it is a larger guitar that should have more powerful strings.
When it comes to stiffness, it is the shape of the rod that is the most important parameter, the material in the middle does not contribute as much to the stiffness as the material along the edges. I found this online: ”In a round rod with a 1 unit thick wall and a 3 unit diameter hole: The hollow shape maintains 87% of its ”strength”, with only 63% of its mass (or to put in another way, you only lose 13% of the strength, but lose 37% of the weight).” So you get almost the same stiffness from a rod with holes in the middle and lower total weight. I make sure to put birch wood in the middle of the carbon fiber rod which contributes to the stiffness and tone (the latter I think anyway!).
I always fold a carbon fiber rod whether the neck is stiff or soft for two reasons. Partly to ensure that the neck does not bend directly when tensioning the strings, but also to keep it straight for a long time.
The neck with a milled groove, wooden cover plate on top of the carbon fiber rod for the skin glue to attach to, two round bars to reinforce the carbon fiber rod and neck foot, the hollow carbon fiber rod.
This is how the parts are assembled. Also the neck foot receives a reinforcement in the form of a round bar in birch. Cheap old guitars with tapered neck foot almost always have a cracked and glued / screwed neck foot when two separate pieces are not matched with the conductor in different directions to hold better.
Adhesive with a rough epoxy resin used to bond the carbon fibers when the rod is manufactured instead of the tougher and more expensive epoxy adhesive. It is messy and the whole neck is wrapped in glad pack and tape. Always use plastic gloves when epoxy is allergenic.
The resin cures for 12 hours and glue the parts together well. The epoxy glue doesn't even attach to a happy pack!
Equals to the wooden stick in height with the neck surface with a few small planes and flat sanders with my abrasive board.
Clear! With a carbon fiber rod in my neck, I know that the neck will only give in a little grand when the strings are tightened even though the neck is made of soft poplar wood. The next step is to grind the grip board to a very small relief in the position it ends with the strings tensioned before the straps are finally assembled.