The 3.5" x 20" Mk. 2 Cromwell was produced as both a plain-turning precision lathe and, as shown on this page, as a backgeared and screwcutting model. It is usually found mounted on a very heavy cast-iron stand with the words Cromwell Coventry on the front face. Whilst the headstock, compound slide rest and tailstock resembled those on the simple  Mk. 1 this was a very much more complex machine and with arrangement of screwcutting and power sliding and surfacing feeds of an unbelievably convoluted and expensive nature - and quite unlike that on any other lathe. The system used two separate back-to-back gear trains at the headstock end, another gearbox by the tailstock, a permanently-engaged leadscrew for screwcutting and two powershafts - one for sliding speeds (with final drive via the leadscrew) and the other for power cross feed. At the headstock end of the lathe one fixed-ratio gear train was enclosed in a housing and used for power feeds whilst the other was carried externally - and unprotected - and employed only for screwcutting. The changewheels were carried on a beautifully-made quadrant arm with precisely machined T-slots to accept the  square-ended studs used to carry the gears. Only one train could be driven at once and relied upon the operator taking off and reversing a gear fitted to the end of the headstock spindle. In order to achieve a drive from two gearboxes using a common output shaft the lowest (bronze) gear within the reduction box was pinned to a large diameter, hollow bronze shaft with a gear cut at its furthest end. Passing through the bronze shaft was a steel bar keyed, in the usual way, to the lowest gear of the screwcutting changewheels.  The concentric bronze and steel shafts emerged into a second "Selector Gearbox" where the bronze gear drove a train of gears - used to power the sliding and surfacing feeds -  and the steel shaft  the 4 t.p.i. leadscrew. A lever on the front face of the Selector Box was arranged to operate separate dog clutches for the sliding and screwcutting feeds - the surfacing feed being in permanent engagement. Because the drive from the reduction box was at a fixed ratio, in order to provide a variety of sliding speeds the powershaft passed though the apron and drove a reduction "pick-off" gearbox at the tailstock end of the bed. This box, supplied with a selection of 6 gears arranged in pairs, gave six feed rates - a plate on the box showed how to arrange the gears - and drove the leadscrew. The tailstock box used gears of the same pitch (16DP) as the later models - but had a smaller centre hole. An additional fixed-ratio sliding feed was also achieved by arranging for the slotted powershaft to drive a gear in the apron that meshed with the carriage-traverse handwheel shaft. The lowest shaft was overhung at its right-hand end and, as it passed through the apron, drove a bronze worm and wheel - the latter with a cone clutch formed in its centre into which was pushed, by a knurled-edge knob on the face of the apron, a steel gear that meshed with a gear on the cross-feed screw Unfortunately, if the faces of the clutch became polished, it could be almost impossible to apply enough pressure to keep the drive engaged.  Just one rate of power cross feed was provided - and in an outwards direction only.
Whilst the main elements of the Mk. 2 - bed, carriage and tailstock  - were almost identical to those used on the Mk. 3 (the S800) the headstock was completely different and carried either a 3-speed flat belt or 4-speed V-belt pulley. The spindle end, though it resembled that on the S800 in having twin registers to support backplates and faceplates, had different dimensions with a 13/8" outer parallel register, a 15/8" by 12 t.p.i thread and a 13/4" parallel register. A conventional eccentric-shaft backgear assembly was provided with a nut and sliding dog to uncouple the bull gear from the spindle pulley. Like both earlier and later versions the headstock spindle ran in a pair of adjustable bronze bushes tapered on their outer surfaces..
Alternative top slides appear to have been offered-- some machines being found with one identical to the type, with exposed slideways and two T-slots, used by Boley on their pre-WW2 Model 3L and 4L lathes; this unit needed a cross slide formed with a semi-circular support flange on its right hand side. Other lathes had an entirely different arrangement with covered ways and a single T-slot. Whilst the twin-slot top slide was a copy, the single-slot was entirely original for, whilst a normal V-edge was used, with a gib strip, instead of the base casting being flanged for the upper casting to run on, the roof of the upper casting was employed instead - a most unusual arrangement. The tailstock was given a separate sole plate that allowed the top to be set-over for taper turning.
If you have a Mk. 2 Cromwell lathe of any type, the writer would be pleased to hear from you.
Mk. 2 Continued here.