Originally built to handle work connected with the timing fuses of large shells, this miniature precision milling machine has always excited the imagination of machine-tool enthusiasts. Although difficult to judge from the pictures, the Derbyshire Micromill was a tiny machine, about the size of a portable typewriter, with equally miniscule movements of its slides: the head could be moved vertically through just 1.73" (44 mm); the cross travel was 1.26" ( 32 mm) and the longitudinal feed 3.15" (80 mm). Because the table travel was so short,  only 3.15" (85mm), a lever-operated cam was chosen as the easiest and cheapest way of providing a smooth, sensitive yet positive movement. The cam, in the shape of a sector and turning on the end of a block of metal screwed to left-hand face of the cross slide, carried a slot near its periphery formed in the shape of an arch; fitted closely into this was a lug that extended from the end of the table.
The ball-bearing headstock spindle, which could be run at up to 20,000 rpm, carried a draw-in 3/8"-diameter cutter arbor and was mounted on a two-part slide that allowed the operator the choice of either a quick-action cam or (very fine) screw-driven feeds; the screw-feed travel was 1.73", the cam feed 0.984".
Various models of Micromill were offered over the years but, whilst each differed in some small detail from the others, or used a different drive system, all shared, with the exception of the table, the same major components. Early variations manufactured included the Model 651 with a table work surface of 67/8" x 23/8" and a single, central longitudinal T slot; the Model 750 with a more heavily built and larger table of 77/8" x 23/8" with a single T slot along the length and two traverse slots and the 750-M with a 12"-long table machined to replicate the Derbyshire lathe bed. Changes could also be specified as to how the head and table were moved: the standard set up provided a cam-operated head (with a screw for fine feeds) a cam-driven longitudinal feed and a screw for the traverse motion. Alternatively, at no extra cost, both head and table could be moved by rack and pinion, or the table by a screw in both directions. When a cam-operated table was specified the model was given a suffix C (as in 651-C), with a  rack-and-pinion drive the suffix R and with a screw feed the suffix S. Later models included  the 750-12R (really the same as the older 750-M); the vertical-head equipped 750-SV (screw-feed table) or RV (rack-feed table) and the 750-CV with a cam-operated table and vertical head.
The machine could be used as a miller or grinder and required a 1/4 hp motor of 1725 rpm for the former and 3450 rpm for the latter..