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.
This is either due to stringing too thick steel strings on a guitar that was built for softer sinew/nylon strings, but it can also be completely natural as wood is plastic and "creeps" when subjected to forces over a long period of time. Even such hard woods as maple and red beech bend over time. Mahogany is a unique wood that holds up very well when subjected to long-term bending, it is also a very popular wood choice for necks.
A bent neck can be bent back by heating the neck and bending it over in the other direction with forceps and counters. If you bend the neck back like that, you can "start over" with a neck, it will be straight for a few more decades before it slowly bends again. Have noticed that it becomes more difficult every time you heat to reshape the neck, if you want to heat and bend it is best to do it in one or two passes. Mahogany necks are the hardest to heat straight. To prevent the fingerboard from shrinking from drying out, you should oil it with fatty teak oil before heating.
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.
A method that was common in the 1800th century and that actually works is so-called "bar frets". They are strong rectangular bands that are pressed down into suitably narrow grooves in the fingerboard and which pre-tension the neck into a "back bow" right at the surface of the fingerboard. With properly mounted "bar frets" you can get a neck that stays straight without either a drawbar or a carbon fiber rod with tensioned strings. The problem is that sooner or later the frets wear out and the wood in the fingerboard must be hard, e.g. ebony. When the bands need to be changed, it takes both time and skill to reposition them correctly. T bands with a "tang" are very easy to mount and are the ones used nowadays, "bar frets" are only used in exceptional cases and never by factories.
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.
Lightweight epoxy resin.
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.