Tuning a stringed instrument string requires a mechanical part to adjust the string tension. In this page, we’ll talk about the tuners typically used for guitars, mandolins, basses. Tuning pegs, such as used on violins, deserve a separate discussion.
History
18th Century
Preston tuners or machines (also known as peacock, fan, or 
tuners) is a type of machine head tuning system for string instruments, named for English cittern (English guitar) maker John Preston and developed in the 18th century. The tuning mechanism was also used on the German cittern known as the waldzither, and is associated with the early-20th-century instruments built by C. H. Böhm.
This type of tuner is almost obsolete, but is still used for the Portuguese guitar, itself historically closely related to the English guitar. The 18th-century incarnation of the design in England arranged the tuning bolts and hooks parallel with each other. 19th-century Portuguese luthiers developed the current fan arrangement to accommodate the extra 2 strings with the octave doubling of the lower courses and narrower fingerboard width; the English instrument had two single strings instead and a slightly wider fingerboard.
Johann Georg Stauffer (1778-1853) 
was an Austrian luthier generally credited with creating the worm-and-gear tuning machine. In 1825 Stauffer invented the machine heads named after him: a metal plate with an asymmetrical “scroll” headstock, machine heads with worm gears mounted on the plate, arranged in a single line on the upper side of the head stock (six-in-line). This “Stauffer” headstock and design was reproduced by his son Anton, and copied by many luthiers in the 19th century. The Stauffer-style scroll tuning machines have been in use since the 19th century. As of 2018, Stauffer style tuning machines are still made by some companies, and some luthiers continue to make “Viennese guitars”.
Stauffer and CF Martin
Christian Frederick Martin,
was born in 1796 in Markneukirchen, Germany, a centre for instrument making. Martin first studied with his father, Johann Georg Martin, a cabinet maker. At 15 years of age, he went to Vienna for an apprenticeship with Stauffer, and in 1825, Martin married Ottilie Kühle, the daughter the Viennese harp maker Karl Kühle.
Martin remained in Vienna until at least 1827,[10] after which he returned to his hometown and opened his own shop. After a long dispute with the guild of luthiers regarding the rights of cabinet makers to build guitars, Martin emigrated to the United States of America, where he introduced the mechanism developed by Stauffer and founded Martin Guitars. In 2008, the 175th anniversary of the Martin Company, the company released a tribute guitar: the “Martin 00 Stauffer 175th”.
20th Century
Kluson
John Kluson established a Chicago machine shop in 1925, specifically for making tuning machines, with the “Kluson-style” design having each mechanism enclosed in a stamped-sheetmetal shell.
The company was founded as a machine shop by John Kluson in Chicago in 1925. Kluson had previously run a machine shop for Harmony Company. Kluson Manufacturing soon found a niche making tuners for string instruments, most prominently for the Gibson Guitar Corporation, to whom they were a major supplier. They also supplied other major guitar makers such as Fender, Martin, and Rickenbacker, among others.
Kluson lost one of their most important customers in 1974 or 1975 when Gibson switched to Schaller tuning machines. The company continued until 1981, when it folded. The Kluson brand was acquired by Larry Davis of WD Music Products in 1994. Under his leadership the product was reconstructed with improvements and the brand was reestablished with some of its former customers, including Gibson Guitar Corporation and Fender. Kluson-branded tuning machines are now produced in Korea by WD Music Products based in North Fort Myers, Florida. The Kluson brand was briefly licensed to TonePro Sound Labs Int’l, who produced a diecast version of the traditional Kluson-brand tuning machines.
The rights to the name in Europe are held by Rockinger Guitars, who market Kluson-branded tuning machines manufactured in Japan by Gotoh.
Kluson Today
Kluson has re-engineered many of the original products lines, and now produces technologically improved versions of their original stamped steel tuning machines, #9 tailpiece, Waffleback tuning machines, and Gibson Firebird banjo tuning machines. In addition to re-engineering original catalog offerings, Kluson has also expanded to include Fender Telecaster, Fender Stratocaster, and Tune-o-matic bridges, tuning machine bushings, pickguard brackets, and other hardware made in the United States. The Kluson “Supreme” series was introduced as an 18:1 gear ratio upgrade to the original stamped steel tuning machines. A locking version of the vintage stamped steel tuning machines is also available. In 2015, the Kluson-brand expanded to produce a series of diecast tuning machines, the Revolution Tuning Machines, recognized as “Best In Show” by Guitar World after their debut at the NAMM Show.
Grover
A.D. Grover (1865-1927) held at least fifty patents for musical instrument parts and accessories. The company he founded (now Grover Musical Products) continued to refine the machine-head concept through the 20th century, particularly a design with the mechanism sealed in a cast-metal shell.
Grover Musical Products, Inc., is an American company that designs, imports, and distributes stringed instrument tuners (machine heads) for guitars, bass guitars, banjos, mandolins, dulcimers, ukuleles, and other instruments. Grover also imports and distributes tuning pegs for violins and bridges for five-string and tenor banjos. The company has four divisions—former companies that they acquired: Trophy Music Co., Duplex Percussion Accessories, Grossman Music Corporation, and The Clevelander Drum Company. Grover adopted its name in 1952 when it acquired A. D. Grover & Son.
Present-Day: Company Websites and References
Grover: https://www.grotro.com/Grover
Gotoh: https://g-gotoh.com/international/
GraphTech: http://www.graphtech.com/products/product-categories/machine-heads
Anatomy of Tuners
Tuners are an assembly of several parts; tuning knob, worm gear, pinion gear and capstan.
Traditionally, a single machine head consists of a cylinder or capstan, mounted at the center of a pinion gear, a knob or “button” and a worm gear that links them. The capstan has a hole through the far end from the gear, and the string is made to go through that hole, and is wrapped around the capstan. To complete the string installation, the string is tightened by turning the capstan using the tuning knob. The worm gear ensures that the capstan cannot turn without a movement on the knob; it also allows precise tuning.
Banjos usually employ a different mechanism using planetary gears – in this case the knob and the capstan both rotate on the same axis. A few guitars (e.g. the original Gibson Firebird, early Gibson basses and Mario Maccaferri’s plastic instruments) have used this design.
Tuning Knob :
The tuning knob is of course the part that you twist when you are tuning the guitar. Albeit not the “most important” part, it is the part which the user interacts with.
Tuning knobs come in all sorts of materials, colours, and shapes. I’m pretty sure that most tuners these days allow the user to remove and replace them, but there are still quite a few (one expects they are the least expensive options) that are fixed in place and require a bit of ingenuity to swap out.
In any case, we would like to determine what size of button to use. For the example of an Octave Mandolin, in particular, we have the choice of using ‘standard’ Mandolin 4-on-a-side pairs, or individual tuners. The Mandolin buttons are, as shown here, 17.4mm wide, with a spacing of 23mm. This gives a clearance of 2.6mm. Standard tuners have a button width of 23.85mm, with a usual spacing of 27mm, for a clearance of just over 3mm.
Worm Gear :
The worm gear extends through and is attached to the tuning knob
Pinion Gear :
which turns the pinion gear which winds the capstan.
Capstan :
The Capstan has a hole in it for the string to go through. The string is attached to the capstan and winds around it as the string is tightened.
Mounting Plate :
A bracket is used to hold all the pieces together, it often is used to mount the assembly to the instrument. In some arrangements, a common bracket is used for multiple tuners. such as for many Mandolins.
Full Assembly :
On steel string acoustic guitars the gears are usually enclosed so you cannot see them. On a classical guitar the gears are usually exposed.
Miscellaneous Stuff
CAD Drawings of Parts, including tuners: https://www.liutaiomottola.com/components.htm