By the late 1960s Clausing lathes had become rather squarer in appearance, a styling and design change clearly seen in the 1968 Clausing 5900 Series machine illustrated and described below. The lathe was based on a robust, flame-hardened, ground-finished bed that was well braced between elliptically shaped, rearward-draining, swarf-clearance holes.
Made in a forged, chrome-steel the headstock spindle had a hardened nose, was bored 13/8", and ran on precision Timken taper-roller bearings. Two types of collet fitting were available, a simple spring-collet assembly (which used either a hand-operated or lever-type draw tube) or the beautifully-well engineered English Burnerd Multisize collet set that ranged in capacity from 1/16" to 1". Each Multsize collet spanned a 1/8" gripping range and so could be used for both English and metric-sized material.
A conventional tumble-reverse mechanism was fitted to the changewheel drive that connected to a screwcutting gearbox of the traditional Norton Quick-change pattern with a single tumbler and open base - a design as fitted, for example, to thousands of early South Bend lathes. A knob protruding from the headstock-end guard was used to select either fine feeds or screwcutting ratios.
The drive system on the 5900 was of the variable-speed type that gave from 52 to 2000 r.p.m. with 26 to 280 r.p.m. in backgear and 180 to 2000 r.p.m. in direct belt drive. Speed changes were made hydraulically by a conveniently-positioned dial of the top of the headstock - a lovely idea for operator convenience and the speeding up of jobs, but just one more thing to go wrong in the future. A less expensive version was also offered that employed the 4900 drive system with a motor mounted in the base of the cabinet stand, driving upwards using a V-belt to a rigidly-mounted, Multi-V Step countershaft unit. Reflecting contemporary changes to drive technology, from there the motion was taken by a slip-free "toothed" belt to an overhung pulley on the end of the headstock spindle. To change speeds the stand door had to be opened, an over-centre lever operated and the motor to countershaft V belt repositioned on its pulleys. Twelve spindle speeds were provided: in backgear: 52, 62, 75, 125,174,237 r.p.m, and in open gear: 370, 450, 540, 900, 1250, and 1700 r.p.m..