In the 1930s Pratt and Whitney offered three very high-quality centre (or "engine" lathes as they are known in the USA) of 13", 16" and 20" swing. While all looked very similar they were, of course, as they rose in size, of proportionately more massive construction and with speeds, screwcutting ranges and controls altered to suit.
The lathes were designed to appeal both to production and toolroom engineers and much was made of their ability to not only "hog" large amounts of material off a bar in one pass but also to cut chips which were described as, "as fine as lace, 0.00014" thick, 2" wide and 200 feet in length without a break".
Mounted underneath the spindle line the headstock "backgears" were engaged and disengaged by a lever immediately below the spindle nose. All the headstock gears were made from a hardened and ground chrome vanadium steel.
Being set at an angle of 20 degrees to the line of its operating screw, the top-slide handwheel had room to be fitted with a good-sized micrometer dial
The cross feed screw was fitted with a quick-withdrawal mechanism for use when screwcutting and an automatic, pre-set disengage and reverse bar was fitted above the leadscrew with stops having precision-adjustable micrometer sleeves. The stop and reverse mechanism control was duplicated by a lever on the right hand-side of the apron..

Spindle Speed Reducer
This device, which contained a 6:1 ratio planetary gearing system, was designed for use in relieving and precision thread chasing.
By selecting the lowest spindle speed and employing the Reducer, speeds as low as two revolutions per minute could be obtained.
The device was also adaptable as an indexing unit, with a 60 hole ring and pawl engagement.
It was possible to cut threads with 2, 3, and 6 starts by using the index numbers on the face and engaging the leadscrew when they came into alignment with the appropriate mark. Because the unit was revolving so slowly, judging the exact point of engagement was claimed to be a simple task.