Why do instruments require neck resets?
All stringed instruments are designed for a neck angle specific to that model instrument. Therefore, when the neck angle is improper, the instrument will not play or sound as it was designed. As a stringed instrument ages the string tension (almost 200 pounds on a flat top acoustic steel string guitar) begins to actually change the shape of the instrument. More specifically, the top of the instrument begins to sink in under and around the fingerboard tongue (the part of the finger/fretboard that extends past the neck and attaches to the top of the body). Also, the sides around the neck block (where the neck and body join) pull inward, more so near the top/side glue joint.
What is the importance of the neck angle?
As the neck angle worsens the saddle must be adjusted lower and lower to make the instrument playable. Of course, if the neck angle becomes too poor, a saddle adjustment will not allow for proper playability. Many musicians and builders alike attribute a loss of volume and tone to a low saddle.
How is a neck reset?
In order to reset the neck angle, the neck is removed from the instrument. Many traditionally built stringed instruments have a dove tail joint securing the neck to the body (although, a number of contemporary makers use neck joints that are secured with a series of bolts). The heel of the neck is modified to shift the neck angle back to its original projection. The joint must be refitted using shims. The neck is then re-glued using the type of glue originally used by the manufacturer. A properly fitted dovetail joint is so strong that aliphatic resin glue works just fine (we normally advocate the use of hide glue for wood to wood glue joints). Typically a new bone saddle must be fabricated as well. A set up with fret leveling completes a neck reset. This ensures optimal playability.