Steel string and classical guitar tuning machines vary widely in quality. Following are a few basic principles that can be used to evaluate quality:

    All guitar tuning machines have a worm-and-cog gear mechanism that translates the rotation of the button (the piece you turn) into rotation of a post or roller (the part around which the string is wound). The spiral-shaped gear on the shaft attached to the button is called the worm, and the circular-shaped gear attached to the post or roller is called the cog, or sun gear. The key to a tuner’s smooth operation and long life is exact mating between the teeth of the cog and worm gears, as to both shape and alignment. These qualities are dependent upon how precisely the gears are made. Generally speaking, higher precision will tend to be reflected in a higher price for the tuners.

     Note that regardless of the quality or price of the tuners you buy, their longevity can be shortened considerably if the holes drilled in the headpiece for the posts or rollers are misaligned.  Poor alignment will induce stresses that will wear out the gears prematurely.

     If you have a chance to try out a set of tuners before you buy them, make two tests. First, note whether there is slop in the gears when you turn the button; that is, whether the button turns a bit before the roller starts to turn. Second, check for looseness by wiggling the shaft to which the button is attached and then wiggling the posts or rollers. The best machines will be tight in all directions, yet will turn smoothly. High-quality tuners always have cogs with teeth that are machined to a concave shape which mates more closely with the curved teeth on the worm gear. Also, the cogs will usually be made of a different, softer metal than the mating worm gear so the cog teeth will conform to the worm teeth as they wear in. Cheaper tuners use cog gears which are cast, rather than machined, with teeth that run straight across. These can clash with the curved worm gears and wear out more quickly. Also, on better tuners the posts will go all the way through the buttons and lock on the ends. Cheaper tuners have buttons that are simply molded onto the posts. Another quality indicator is how the worm gear is held onto the base plate. The cheaper alternative is for the worm gear to be suspended on two hook-shaped posts that are just bent away from the base plate. The more secure and precise (and more expensive) solution is to mount the worm gears in two support blocks that are soldered or riveted and peened upright onto the tuner plate.