Guitar Neck Relief
Lowest Possible Action
To Achieve the Lowest Action on a Guitar, the Neck
Relief must, essentially, be set to zero.
These diagrams shows why, if you want to achieve the lowest possible action on a guitar, you should have, essentially, zero relief, in the guitar neck. i.e. no bow, such that the neck is, essentially, perfectly straight. This is in contradiction to main stream views.
Pretty much all guitar luthiers, guitar vendors, guitar players, as exampled by the numerous YouTube “instructional” videos claiming that some relief is required. They do this, despite, having something like 1.6 mm high E action at the 12th fret, and 2 mm low E
I am sitting here with 4 guitars that I have set up with around 0.65 mm to 0.75 mm action
at the 12th fret for both high and low E strings, with no fret buzz, achieved by having all the frets uniformly breathed on by the strings when fingered at the 1st and last fret of the fingerboard. The method works. Period. However, it is crucial that the frets are leveled exactly. Pretty much all vendor guitars as new, are not, so, I suggest a pro does that first.
The diagram shows the string clearance from its next fret that would cause a buzz. Whilst it can be seen that the clearance is greater for nut end frets in the “with relief” case, this does not matter if the frets have been leveled correctly. If the clearance at the nut end for the “zero relief” case is set the same as the clearance at the worst case “with relief” case (bow center), then the overall bridge action can be set lower. It is somewhat stunning that the vast majority of those involved in guitar subject matters have failed to understand the basic geometry and physics of the situation. My guess is that there has been a failure to understand that, although it notionally looks as if there is greater fret clearance closer to the nut because the action is higher there, the fret clearance after the maximum bow point is decreased to a greater extent, that requires the bridge to be set higher than it would need to be if no relief is used. This is true, irrespective of the string gauge or how hard the sting is played. Zero relief will always enable the lowest buzz free action to be achieved.
I found these links, after the fact.