Action Height
"Action" is the term used to describe how far the strings are from the frets. This measurement is usually taken at the last fret by measuring from the top of the fret to the bottom of the string. You can use a feeler gauge but using a simple mm rule is just as effective, in fact I don't use feeler gauges for anything Many times you will hear action measurements including a measurement at the 12th [or some other fret]. Measuring at the 12th fret assumes every necks bow profile is the same and they never are. The bow profile of your neck will ultimately dictate the kind of setup that works best for it. [bow is the amount of relief or "frontbow" the neck is dialed into, bow profile describes where the relief is] Some necks will show bow only from the 7th to the nut, some will show a very long smooth even bow throughout the whole neck, and many other profiles. I measure action at the 24th, but only to get a number to advise others, I would never setup a guitar with a mm rule, each guitar is unique and has it's own peculiarities.
Action is also dependent on fretboard radius, some will be 430mm, some 250mm, some in between. A 250mm radius fretboard requires higher action so the strings don't choke while making large bends. The flatter the board, the lower you can set the action without choking.
Playing style also can dictate how low, if you don't bend strings you can set it lower than someone who plays with lots of big bends. If you have a really heaving picking hand [or bad picking technique] your wound strings will give more buzz than somebody with a much lighter hand [or perfect technique]. If you have a heavy pickhand you might need to raise the low side as high as 4mm to completely clean it up, again assuming clean fretwork and correct [for that neck] relief.
While on the subject of buzz, lighter gauge strings will have more buzz than heavier, and different brands will buzz more or less than others. I use D'Addario's which are a very pliable string but also buzzier because of this. A set of DR Tite Fit is much less pliable and will not buzz as much given the same action and setup.
As a average and a starting point, on a 430mm 24 fret board, with around .4 - .5mm of relief measured at the 9th with the low E string fretted at the 1st and 24th, and set for light to medium pick hand with minimal low E buzz, measured at the 24th fret - 2mm on the wound E and 1.7mm on the high E. I will set it as low as buzz will allow on the wound side, and as low as bending will allow on the high side and still get full tone on big bends, assuming no problems with the fretwork or the bow profile. You can go lower on the high side but I also like to feel the string under my fingers and around 1.7 it's also keeping the action fairly even across the fret [high to low E]. Action much higher on the wounds than on the unwounds is awkward and I prefer the balance.
On a 250mm radius board with .4 - .5mm of relief it's more like 2mm on the wound and 1.8 on the the high side. As low as you can get the high E with clean clear big bends. To get lower on the high side without choking under bends in the upper frets you will need less relief in the neck.
There are many different ways to setup a guitar,
Silly Low - Some like the neck perfectly straight or with the slightest amount of relief, and the action right on the frets. Perfectly straight neck and extremely low action will give you playability that's unparalleled for some people, a very low resistance to fretting, but there are sacrifices. The lower the action the more the strings are being choked by the frets. It has a way of disguising fret buzz by smoothing it out. On a medium action you'll hear the buzz quite well as the string is usually just pinging off of one or possibly two frets. On silly low the string is pinging off of so many frets it's not as apparent as "buzz", but what all that contact with the frets is doing is robbing your notes of the full tone that string could offer if it was allowed to breath. Processed signals can virtually hide the choking, but if you ever plug into a clean channel you'll notice right away. You can get away with silly low if you've got an extremely light pick attack. Typically in a setup this would be a perfectly straight neck to .2-.3mm of neck relief, and action height at the last fret of 1.5mm or lower. Not good for playing big bends without impeccable fretwork and a flatter radius fretboard.
Low - Adding just a little more neck relief and raising the action will give a cleaner tone, longer sustain, and tolerable buzz with a medium/light pick attack. This is usually the typical setup for guitars I ship. I like just a hair more neck relief than some as it gives cleaner wounds though most of the neck, but will get some light choking on the wounds through the upper frets. Good clear tone on the majority of the neck with and a light fingering feel and enough resistance that you can feel the strings under your fingers. Typical is .4-.5mm of neck relief and 2mm string height on the low E, and 1.8mm on the high E at the last fret.
Low revised - Since I give every guitar a fret level now I typically keep the relief down in the .3mm range to reduce the amount of "boing" from the slightly higher action in the 4-12th fret range. The overall action is still the same at 2mm bass and 1.8mm treble. As I mature I have grown to like this setup better in the middle range. Tastes change, and you should set your guitar up to your taste.
Medium - You can still add a little neck relief but after a certain point more relief just isn't an advantage. Raising the action height is and will continue to clean and fatten up your tone and increase the feel of the strings under your fingers. Medium relief is .5-.6mm of neck relief action of 2.5mm on the low E and 2-2.5mm on the high E.
High - Lots of frontbow and high action will give you a very clean playing guitar with full tone, and lots of extra calluses. However much neck relief you want and action on the low E at 3mm or more, action on the high E over 2.2mm to as high as you want.
There is another school of thought that the higher the action the less relief the neck needs since the height of the action itself produces little buzz. Relief with high action is more about keeping the action more even the length of the neck and not as much about buzz. Like anything in a setup it's all about the compromise and the preference of the individual player.
The wound E is going to vibrate in an ellipse that will vary by how hard it's struck. Strike it light and it will have a small ellipse, strike it hard and it will have a much larger ellipse. A higher gauge string will vibrate in a smaller ellipse because it is strung "tighter" than a smaller gauge string to get the same pitch. Your action should be compromise of how much buzz you can take, the feel you want from the strings, and the tone you want to produce.
Setting the Action - Action is my last adjustment in a setup, after neck relief and the trem angle are set I will dial in the string height. With it approximate, I will then break and adjust the nut height since this is dependent on everything being in correct setup to get as low as possible without open string buzz, then do a fine tune on the action after the nut is set for the final adjustment before playing the guitar to determine if it needs any further tweaking.
Action Adjustment
Adjusting the action varies by the type of guitar but since this is an Ibanez site you'll find Ibanez directions. Final action adjustment is the last tweak to make and should be done when the neck relief is correct [for you] and the trem angle is correct. After it's set make a final check of the setup to be positive nothing else has changed. Once action height is set it should never need adjusting, the only things that will change it are the natural changes your neck will make as it warms/cools/humidifies/dries, and the trem angle which will change for the same reasons, as your neck moves, and your body swells and shrinks. Adjust what is changing, don't just tweak the string height as a quick fix instead of tweaking what has caused the action to change.
Floating Bridge - Edge, Lo Pro, Pro Edge, and all of the double locking Floyd variants are adjusted by raising and lowering the trem studs that the knife edges pivot on. There is a small 1.5mm Allen adjustable set screw inside the studs on all Edge and Lo Pro equipped guitars that must be kept tight for the best tuning stability. Never try and loosen it with the allen wrench but use the 4mm Allen to back out [Lefty Loosey] the studs a hair to free the set screws. If raising the action raise it until it's correct and retighten the set screws. If lowering the action use the 1.5mm Allen to back out the set screw enough to allow the bridge to be lowered. When correct retighten the set screws. Important! After you've tightened the sets using the 1.5mm Allen use your 4mm Allen to tighten the studs down on the set screw to really lock the threads together and give a very solid fulcrum for the bridge. It should only be about 1/16th of a turn so don't try grinding everything together or you'll either break the head off the stud or spin the stud anchors in the body, use good sense when applying torque, but you do want the threads locked together. Adjusting the string height to a large degree will alter the tuning slightly, which can alter everything. If you are making extreme action adjustments be sure to retune, readjust the trem angle, and then recheck the action. Yes, you can adjust the studs with the strings at full pitch without fear of damaging the knife edges or the studs.
Vintage Trem and Fixed Bridge - Adjusting the action of these types of bridges entails adjusting the height of the 2 adjustment screws on each saddle. Each string is adjusted independently so it is important to keep the bridge radius in sync with the fretboard radius. Do this by measuring each string at the last fret. With this type bridge I always like to keep the same radius to the saddles themselves so they're more of a nice arc than a stepped feel across the bridge.
Gibson Type Stoptails - These are all generally adjusted by raising or lowering the mounting studs for the bridge using wheels built into the studs. On a Gilbraltar type you have to loosen the screws on top of the bridge to be able to raise or lower the bridge using the wheel adjusters. It is advisable to slack the strings to decrease the tension on the bridge to make these adjustments. Tune and check the action, repeating as many times as necessary to get correct.
