All Sunderland gear-planing machines work on a principle established in the early years of the 20th century by the inventor and patentee Sam Sunderland of Keighley in Yorkshire, England. Sunderland realised that if the principle of the involute curve could be applied to a gear-cutting machine, the gears produced would (to simplify matters) always mesh perfectly and so produce a smooth and consistent delivery of power from one to the other. Even better, when the gears (either spur or helical) were allowed the necessary clearances to accommodate expansion due to heat and the avoidance of excessive friction, the involute rule still held and the meshing remained unchanged. The practical solution lay in his realising that a rack (in effect a gear laid out in a straight line) would always mesh perfectly with a gear of the correct specification, if the rack were to be made be made of a cutting steel it could generate a gear with the correct involute form - but not only that, in normal use only a small stock of cutters would be needed as each could cut any number of teeth providing they were of the same pitch and pressure angle (in comparison, ordinary "form" cutters are limited to a small range of teeth and dozens are required to cover the needs of a commercial workshop).
Possibly the easiest way to understand the Sunderland method is to imagine a gear blank in a soft material being rolled backwards and forwards, under pressure, along a rack until the tooth form of the rack is reproduced in the blank - the analogy being that as the rack (cutter) is planing the blank to produce teeth and spaces, the blank is being rolled along the rack and the tooth form generated directly. In addition, even when blanks are over or under-size they can still be cut so as to run together correctly - an example being when stronger teeth are required by either increasing the outside diameter of a gear or the "addendum". As Sunderland wrote, "….uniformity in the shape of the teeth is obtained as perfectly as possible by a commercial method and all gears that are cut by the same cutter will gear correctly with one another."  The easiest way to understand the method is to watch it in action: here are two rather shaky videos showing a Sunderland No. 19 and 27SDH at work.
Once established, the Sunderland Company (soon to became part of J.Parkinson & Son, the Shipley-based milling machine makers) called their system "rack generation of involute gears" and employed a hardened and ground high-speed steel cutter (or cutters) having straight-sided, rack-shaped teeth that, in operation, are made to reciprocate parallel with the axis of the gear being cut while simultaneously travelling at a tangent to its pitch circle - the gear rotating slowly at the same linear speed on the pitch line. To keep the length of the rack cutters down, after the traverse of one or more pitches this movement is stopped and the cutter slide automatically withdrawn from the gear teeth at the end of the pitch movement - this action permitting the cutter to step back one or more teeth on the partially-formed blank. The cutter is then automatically re-engaged and the generating motion resumed. Thus, on its cutting stroke, the cutter is in contact with several teeth at the same time - but with a different part of each individual tooth - and so planes over a comparatively narrow band at every stroke. With a different part of each tooth submitted to the action of the cutter at every stroke, every tooth is acted upon by the same teeth in the cutter in exactly the same way, the result being the production of a uniformly shaped gear. In addition, by using multiple or gang loading of blanks loaded to the full capacity of the machine, a very high rate of production can be achieved.Based on simplicity and what the makers called (as a reference to the involute principle) "mathematical accuracy", the design of a Sunderland gear machine calls for a gear blank (or blanks) to be mounted on a horizontal arbor held in the work spindle at one end and supported by a sliding pedestal at the other. Two setting adjustments are available: the cutter headstock can be slid along the bed to suit various gear diameters and the cutter box moved along the cutter slide to suit the axial position of the blank. For spur and single helical gears an infinetly variable adjustment of stroke length, to suit the width of the blank,  is made on the crank-driven cutter slide while the hand-wheel, used to move the headstock along the bed, is also used to set the feed depth with the setting, once made, locked.
The rate of feed - stated as the number of strokes of the cutter slide per pitch movement - is selected by a dial on the feed gear box with a lever to stop and start the feed as required. Changing the setting of cutting strokes per minute is  varied by easily-changed  pick-off gears mounted on fixed centres.
A train of "pitch changewheels" are arranged to suit the pitch of the rack cutter and the particular gear to be cut and "division changewheels" selected to give the number of teeth required. Charts, matched to the capacity of the machine, are supplied to show the setting of changewheels for division and for all standard pitches D.P., C.P. and Module and provision made to rotate the work spindle to check the truth of mounting. A counter is usually fitted, complete with an automatic knock-off device, to stop the mechanism on completion of the cut.
Lubrication provision varied throughout the decades with, on early models usually a combination of automatic and one-shot hand-operated plunger pumps. Later models, including the G Series built in the 1970s and 1980s, used two automatic centralised ststems were employed, one to feed oil continiously to the cutter head and feed gearboxes, the other an intermittant arrangement that lubricated the workspindle and various slideways. On the later machines the machine was interlocked with an oil pressure switch that would not allow the main motor to either start, or continue running, if the pressure was too low.
As all Sunderland gear planing machines work on the same principle, the maker's original very detailed instruction manual can be used as guide for the setting up and use of all models, though individual publications were also prepared for some types..