Tom Senior's first milling machines were a pair of small, very-well made but light-duty flat-belt drive horizontal models with traditional round overarms and intended for mounting over the front edge of a bench. Although the "S" Type was designed for model and other types of light engineering, even so, the table was enormously deep and the end of the main spindle incorporated both a No. 2 Morse taper and a drive slot, so easing the direct load on the taper under heavier cuts. The 1.25" diameter spindle bearings, protected by dust covers and felt seals, were in bronze and two Hoffman ball thrust races absorbed spindle axial loads in both directions. However, one weakness of the design was the use of a hardened point on the end of the drop bracket (instead of a proper bearing) to support the arbor. With not only a significantly better cosmetic appearance than other small millers on the market, all the sliding surfaces were also ground and then hand frosted - and with the exterior surfaces of the 14.5" x 3.625" table also finish ground. The square-thread 10 t.p.i feed screws were not fitted with micrometer dials - though they were available at extra cost, as was (in the absence of a backgear for slow speeds) a large drive pulley to fit on the end of the spindle. Early tables had just one T-lot - later ones had three - with movements of four inches across, twelve inches laterally and ten inches vertically. The flat-belt drive cone pulley had diameters of 6 inches, 4.5 inches and 3 inches and accepted a belt 1.25 inches wide. The machine weighed approximately 100 lbs.
Senior's other early miller, manufactured from the late 1930s onwards, was a very much more robust and better-specified machine, sometimes listed as the "Heavy Bench" and available in two sizes, the No. 0 and the No. 1. The latter was an industrial-class model with a self-contained drive system, a heavy cast-iron main column and an unusually large base that held the motor and coolant equipment. Like most of the heavier models produced by Senior in the years that followed, the No.1 was fitted with internal backgearing that allowed very much heavier cuts to be taken. Power feed to the table was provided by a carden shaft chain-driven from an extension to the main spindle drive. One rather unusual fitting - which might have been an optional extra - was an epicyclic gearbox fitted to the end of the table's longitudinal feed screw. Unlike the similar (though none-epicyclic) arrangement available as an option on the Clausing Type 8520 and 8525, this unit appears to have had just one reduced drive ratio - and not the ability to select a fast or slow rate of hand-driven feed. The table was 6.5" x 24" with a longitudinal travel of 14", in traverse of 4" and vertical 10.5". Relatively small at just 2.25" in diameter, the overarm supported a spindle running in a plain bronze bearing 1.590"
In diameter. All dovetails were machined at an angle of 55 degrees. .