Of massive construction and great rigidity, the 'plano milling machine' evolved from the early, simple - but oh-so-useful - small hand-operated planers such as these Victorian examples from the 1800s by Brittain,  Fomm, and Plant. Simultaneously, larger machines with power feeds were also in production, a good example of the variety available being those made by the English Company Milnes.
It did not take long before it was realised the overhead rail could hold not just one,, but two or even three toolholders - and the vertical supports used to house them as well. Hence, a job -  or multiple sets of identical jobs - could be set up on the table and several cuts taken simultaneously. From there, it was only a small step to replace the cutting-tool holders with powered spindles that held rotating cutters or grinding wheels - hence the terms "plano-miller" and "plano-grinder".
The Köllmann plano millers shown below are typical of the type sold during the 1950s; they had multiple sets of powerful milling heads that could be individually controlled as to their r.p.m. and also angled. Hence, it was possible to set up massive castings and machine several faces in one pass - a system that significantly improved efficiency and precision.
The Köllmann range consisted of around 40 models and sub-series with tables ranging in size from the 55" x 16" of the M30 to the 167" x 79" of the MV64 and available with a variety of fixed and travelling columns, fixed and swivelling spindles, combinations of one horizontal and one, two or three vertical spindles, and fixed and swivelling spindles. In other words, anything that the customer desired for his particular needs. The full range of Köllmann plano-millers, including the Types Models M30, M31, M32, MV32, M33, M34, M40, M41, M42, MV42 M50, M52, MV52, DFF350, EMA, EMAV, MAV, ME and the MA, Series, MB Series, MBV Series, MC Series, MCV Series, MF Series, MD Series, MG Series, MGV Series, MFV Series, MBV Series, MCV Series, MV Series, VF5, VF5s, VF6, VF6S, VF5B, VF6B, SF and HZF, etc. Some possible combinations of features and specifications can be seen towards the bottom of page 2.