During the 1950s and early 1960s the Clausing Division of the Atlas Press Company, Kalamazoo, Michigan, USA sold two very similar 12" lathes, the 6300 and 5400, each based on the same flame-hardened bed.
The 6300 was available with either a continuously variable speed drive system, which gave a range of 43 to 1300 rpm, or with a geared-head drive that, with a single-speed motor, provided ten spindle speeds from 30 to 1300 rpm. Fitted with the optional two-speed motor 18 speeds spanning 15 to 1300 rpm were available; the 6300 was originally the Model 200 and the 4800 the Model 100.
An optional extra for either type of drive was a combined multi-plate clutch and brake unit that could instantly start and stop the spindle whilst leaving the motor running; an important consideration if a single phase motor was fitted (the fewer times they are stopped and started, the better their long-term health). The operating handle for the clutch/brake unit was mounted on the front face of the stand, immediately below the screwcutting gearbox - a picture can be seen on the Model 5400 page.
On the 6300 the final drive to the headstock was by twin V belts (3 on the belt-drive 5400) that acted directly on a pulley fastened to the outboard end of the L00 taper, headstock spindle. This novel design meant that a separate input layshaft was not needed, although the final number of shafts and gears was the same as in a more conventional lathe of the same era such as a Colchester or Harrison.
Surprisingly for a lathe of this class no separate power shaft was fitted; instead, the leadscrew, as in many smaller lathes, had a slot running its full length that drove the various gears (via a key within a worm gear on the apron), that provided power sliding and surfacing feeds.
The screwcutting gearbox was completely enclosed and supported the leadscrew on two Timken roller bearings; like the substantial double walled apron, it ran in an oil bath.
Details of later Clausing lathes - the 5900 with redesigned variable-speed drive and others with the "squared off" appearance - will follow..