Rockwell Horizontal miller with the optional table power-feed attachment Model 21-820 - an infinitely-variable speed device with both jog and rapid approach controls. The unit was also marketed by Rockwell for fitting to Bridgeport, U.S. Burke, Millrite and Clausing millers.
With the exception of a spindle and its drive system, the Rockwell Horizontal Miller was identical in every respect to its vertical counterpart. To change the spindle speed, two handles were loosened on the rear-mounted motor plate so that it could be swung upwards and the belt tension released. A spring, anchored below the plate, automatically gave the right amount of belt tension as the plate was lowered. Besides the 5-stage Gates Polyflex belt countershaft, the miller's spindle was fitted with a lathe-like 6 : 1 ratio backgear assembly which gave a range of slow speeds suited to slab milling and other heavy-duty uses. With the standard 1725 rpm motor the horizontal spindle speeds were, in gear drive: 60, 135, 240, 300 and 385 rpm and in direct (belt) drive: 375, 845, 1500, 1875 and 2400 rpm. Whilst the spindle ran in two oil-bath lubricated Timken taper roller bearings unfortunately the nose taper chosen by the company was the NMTB No. 30, a type that, whilst popular in the USA at the time, has now fallen in disuse with a corresponding increase in the difficulty of finding suitable fittings.
An optional, infinitely-variable speed table power-feed attachment was available (model 21-820) complete with both jog and rapid approach controls; it was also marketed by Rockwell for fitting to Bridgeport, U.S. Burke, Millrite and Clausing millers.
Because the horizontal miller retained the same arrangement to support the horizontal overarm as was employed in the vertical miller to hold the ram, it was possible to combine both functions in one machine, as illustrated at the bottom of this page, when the machine became known as the Model 21-122..

The simplicity of the conversion from vertical to Horizontal is illustrated by this picture of the pivoting motor-mount plate and the compact drive which used a pair of narrow 8M Gates Polyflex belts running on 5-step pulleys. Sometimes disparagingly referred to as a "boot-lace drive", this system was originally developed for automobile use and is able to transmit a surprising amount of power, even when forced to run around small-diameter pulleys.

To change the spindle speed, two handles were loosened on the motor plate so that it could be swung upwards and the belt tension released. A spring, anchored below the plate, automatically gave the right amount of belt tension as the plate was lowered.

Besides the 5-stage Gates Polyflex belt countershaft, the miller's spindle was fitted with a lathe-like 6 : 1 ratio backgear assembly.

Because the horizontal miller retained the same arrangement to support the horizontal overarm as was employed on the vertical miller to hold the ram, it was possible to combine both functions in one machine. In this form the miller was known as the Model 21-122

The table was 24" x 6.5" with 3 T slots and longitudinal, traverse and vertical travels of 16.5", 6.75" and 16.5" respectively. Handwheels (in detachable support housings which doubled as coolant and chip collectors) were fitted to both ends of the table; this was a feature which must have added considerably to the cost of manufacture, but its convenience would have been appreciated by every owner working against the clock, or faced with a job where, for example, a slot was being cut which required several changes in longitudinal  direction.

The enclosed, box-form cast-iron knee had internal ribbing and cross bracing. Its dovetail slides stayed in contact with the supporting ways at both extremes of its 16.5-inch travel.

Each axis of moment of the table (the one shown is the table longitudinal feed) was fitted with tapered gib strips  - a far superior system to the usual strip of metal  and adjuster-screws.

The cabinet base was 17.5 inches wide and 26.5 inches deep front to back. It was fitted with a thick, plywood floor.