At one time the planer was considered an indispensable part of any machine shop; using very inexpensive tooling they were able to machine large components with relative ease. Some examples were huge, with beds fifty or sixty feet long and able to accommodate work sixteen feet high, whilst other were fitted with either fixed or pivoting grinding or milling heads in place of the ordinary "clapper-box" tool holder - and so became known as "planer-millers" or "planer-grinders" or, in some regions, "plano-miller"and "plano-grinder". On larger examples it was common to find more than one tool box fitted; the cross-rail would often carry two, side by side, with several extra ones sometimes mounted at the bottom and sides of the cross-rail support columns.
Some planers were made open sided, and so able to accommodate even larger jobs, and as the species developed it became increasingly difficult to differentiate between planers and true millers - examples made by the huge American Niles-Bement-Pond Company included: Multiple Spindle Horizontal Milling Machines, Horizontal Slab Milling Machines, Horizontal Slab milling machine, Rod Milling and Fluting Machine, Duplex Milling Machines, Forge Milling Machines, Plate Planers, Rotary Planers and End Milling Machines. Although the name changed according to the specific use, the principal of operation remained essentially the same, a long sliding table passing beneath (or between) single or multiple cuttings heads.
Variations on the theme include Pit and Breast planers where the workpiece rested in a pit (or on a table) and the columns carrying the cross-rail and toolheads travelled over it. These massive machines were generally reserved for the heavier kinds of armour-plate work.
Planers have been made in almost every size increment imaginable, the smallest being tiny hand-operated units designed for bench mounting.  Although rare, examples of planers suitable for the smaller workshop do occasionally turn up and are very sought after, not only for their novelty and historical value, but because they are still capable of their original task - the economical machining of large components in a limited space.

The Milnes' drive system (though not unique) was ingeniously arranged.  A countershaft unit with a 3-section "fast-and-loose" pulley set was driven by a single belt which passed between two striker "hooks"; the hooks were controlled by a mechanism linked into the machine's motion in such a way that they could throw the belt from one outside pulley to the other. If the belt-shifting mechanism was disengaged, the belt came to rest on the centre pulley which, being free to spin on the shaft, stopped the feed.
Reference to one of the photograph below will show what appears to be a single shaft emerging from the drive pulleys and carrying two pinion gears, each engaged with its own crown wheel; in fact, there are two shafts, running concentrically with each other - and each independently driven by one or other of the outer pulleys.
With the belt positioned in the middle, or idle, position, the drive was started by lift a lever which engaged the belt-shifting mechanism. The belt was moved first to the right-hand pulley which drove the outer of the two pinion gears; this advanced the table slowly under the tool box and metal was cut. As the table came to the end of its travel a trip bar caused the belt shifter to fling the belt across to the left-hand pulley, and so engage the drive to the inner of the two pinions. Being "higher-geared" this returned the table at high speed to its starting point with the tool-carrying clapper box being knocked backwards on its hinge as the workpiece passed beneath it; from the returned position the cycle automatically began again. The cutter head was also power fed across the job by the use of a simple ratchet feed, operated by a long rod which reached down and engaged with the table drive mechanism.