Smallpeice Cromwell S.800 Precision lathe with a rev counter built into the lamp-support tube.

Continued:
The post-war "Cromwell S.S. & S.C. 3.5" x 19" Super-Precision Model S.800", was a very expensive, fine-quality machine designed for use in toolrooms and by instrument, gauge and optical-equipment makers. In 1946 the first machines were advertised at £448 (though few can have been available for delivery) when a Myford ML7 of the same capacity was just £46. As might be expected, the attention to detail and use of fine-quality materials was exemplary - with even the 6 felt wiper housings on the saddle, the vernier scale on the leadscrew handwheel, the end brackets on the cross and top slides the hinges on the changewheel cover being in bronze. Although the lathe and its stand weighed nearly half a ton, almost four times as much as the Myford, it was not designed for heavy-duty roughing work, that would have been done on another lathe and the work transferred to the Cromwell for accurate finishing; even so, in amateur hands, when pressed into general service duty, it will easily outperform any equivalent-sized machine in this respect. The lathe began life in the 1930s as the now very rare Mk.1 flat-belt drive model, developed through the screwcutting (and plain-turning) Mk. 2 to become (with considerable modifications) as the well known and highly-respected Mk. 3 (S.800) with electronic variable-speed drive. The lathe was further developed as a 4-inch centre height "Series 2" with a mechanical variable-speed drive system by the continental manufacturer Prym; in contrast to the often-encountered Mk. 3 this is a very rare model - and few can have been produced.
The spindle of the Mk. 3 was driven by a "Ward-Leonard" electrical system that comprised a 3-phase motor attached to a DC generator that in turn supplied current for a DC motor controlled by an early form of electronic variable-speed drive. The result was a step-less speed range, with dynamic braking, from 20 to 2000 r.p.m. as well as a geared range from 3 to 290 r.p.m. The operator controlled the spindle speed with a single black plastic handwheel positioned conveniently to hand on the front face of the stand's headstock-end plinth. Today, unfortunately, very few Cromwells are found with the original electrical system intact for, over the years, as the original components failed, many were fitted with conventional V-belt drive countershaft units. However, now that miniature computer-controlled phase inverters and similar devices are available, it is possible, by using just a single 3-phase motor in place of the DC motor, to completely restore (and possibly improve) all the functionality and wonderful simplicity of the original speed-control system. The final drive to the spindle was by a flat belt, the tension of which could be adjusted quickly and easily by screwing in a large, knurled-edge knob that rose through the chip tray immediately behind the headstock.
The headstock spindle was made from nitride-hardened steel, ground and lapped to provide a perfect surface finish; it ran in continuously adjustable, precision plain-bearings of the taper-split type, with internal cones at either end that engaged with cones on the adjusting nuts; this method of construction ensuring that the bearings were held firmly outwards against the taper bore of their housings. The flat-belt spindle pulley, sealed in its own compartment against the ingress of oil and dirt from the backgear assembly, ran on its own, independent ball-bearing sleeve and so removed from the main spindle all effects of belt pull and belt-transmitted vibration. Whilst other manufacturers of precision lathes boasted of finishing the spindle bearings when the headstock had been bolted to the lathe bed, Evans reversed the process and finished the tapered bearing seats first and used those as a datum to mount the headstock on a jig (which replicated the bed mounting) and then machined the headstock casting base relative to the spindle line -  the internal taper for collets being ground into the spindle after the lathe was assembled. To enhance rigidity, chucks and other spindle fittings were mounted on a combination of a screw, taper and large-diameter spigot face. The headstock was backgeared with operation by a wheel on the front of the headstock (that disengaged a small gear on the spindle from a surrounding internally geared sleeve) and a second rotary control on top of the headstock by which means the bronze drive gears were slid into mesh with the steel spindle gears.
Precision lathes as made by (amongst others):  Boley, Lorch (also badged Lorch Schmidt) in Germany, Schaublin in Switzerland and a variety of American manufacturers, foremost of the latter in post war years being Hardinge with their HLV and later HLV-H models, but also including :- B.C.Ames, Derbyshire, Hjorth, Potter  Pratt & Whitney, Rivett, Stark, Wade, Waltham Machine Works,  Rivett, Cataract, Elgin, Remington, and Sloan & Chace.
Continued below:

Mk. 3 Cromwell (S.800) Lathe with rear cover removed showing, on the bottom shelf, the 3-phase motor and DC generator and, at the top, the DC motor that drove the spindle by a flat belt.

Continued:
Each lathe bed was the subject of considerable attention by skilled craftsmen; the casting was rough machined, seasoned for several months to relieve stresses, machined accurately, hardened and finally ground and hand-finished. Its inspection was rigorous; the maximum permissible error, in convex only, was 0.0002" over the entire length of the ways; the makers claimed to be able to take two beds at random from stock, mount them face to face, and find a variation in flatness of only 0.0001" over their entire length. The bed had, in effect, three V ways: one at the front and one at the back to locate the saddle and another of "truncated" form in the centre with bevelled edges and a flat top (just like that used for the main bed on many precision bench lathes) was used to hold the headstock and tailstock; this arrangement can be seen in the photographs here.
Surprising, although the compound slide rest worked very smoothly (especially the 5 inches of top slide movement) its feed screws and micrometer dials were only of modest diameter - although the latter was given a vernier scale engraved into the fixed inner section of the dial assembly. The screw was supported in a bolt-on bracket and, whist cast iron would have sufficed perfectly well for this unit, Cromwell chose instead to use bronze. Power cross feed was fitted as standard, operated by a separate shaft beneath the leadscrew that drove a bronze worm and wheel - the latter with a cone clutch formed in its centre into which was pushed, by a bronze wing nut 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 on the small wing nut to keep the drive engaged. 
Carefully thought out, the design of "magazine" screwcutting gearbox offered the same wide range of threads and quick operation of a conventional unit yet managed to eliminate the usual open slots in the front (through which dirt could enter) and allowed the its tumbler gear to run in an oil bath. A rotary dial selected four sliding and surfacing speeds that could be engaged instantly for every position of the main tumbler - whilst  a lever (just inboard of the point where the leadscrew entered the box) could be used to switch immediately from fine to coarse feed. Complete with English, BA and metric sections the screwcutting chart was engraved into a thick brass plate mounted on the inside face of the changewheel compartment's hinge-open cover. Instead of the usual split-nut to engage the leadscrew, the apron carried two 'solid' nuts, spaced well apart and adjustable relative to one another, as in a micrometer, to compensate for wear and to eliminate backlash. Two handles were fitted to the saddle traverse, one at each end of the leadscrew (working through worm-and-wheel gearing) with a large 0.0001" vernier scale dial at the right hand end to assist in the accurate turning of shoulders and other lengths and a smaller dial on the headstock-end handwheel.  The leadscrew (1-inch diameter by 4 t.p.i. or 6 mm pitch Acme-form) was known to hold true to within 0.0005" over twelve inches - and, if necessary, a lathe could be provided with a N.P.L. certificate guaranteeing the accuracy of the screw to within 0.004" over its full length of thirty inches - however, the normal accuracy over this length, as shown on factory test certificates (where each lathe was individually checked against a N.P.L. master leadscrew) was actually better, at just 0.002". Unlike a conventional lathe there was no quick-action rack feed fitted to the carriage and, when moved under hand control, the leadscrew had to do the work; a separate power shaft was fitted to drive the standard-fit power cross feed.
Using cast-iron for its chip tray and floor base the cabinet stand was fitted with heavy-gauge, plate-steel ends and sheet-steel side panels; as the slatted tool cabinet cover on the right hand side was lifted, a light came on to illuminate the inside; even the tool tray itself was not allowed to escape the designer's attention to detail: each spanner and key slotted into its own milled and named recess and, as the tray was withdrawn, it angled downwards to present its contents to the operator.
Supplied with each lathe was a dividing plate and indexing plunger; a headstock collet draw-tube with a centre formed on the end of a solid collet; drive plate, tailstock centre, adjustable saddle stop, a light unit, tool tray and tools, a single-tool tool post and "left and right-hand tool rests".
Rolls Royce bought numbers of these machines for their tools rooms, and it is not unusual to find their label, or that of NCR (National Cash Registers), Metal Box or various aircraft companies adorning Cromwell lathes.
A now-rare Mk. 1 version of the S.800 was also built (and illustrated at the bottom of the page) as well as a simple 3.5" centre height plain-turning lathe with an open 3-speed flat-belt drive. One of the latter is known to have survived in a dismantled condition, and photographs may become available. If you have, or know of, either of these types of early Cromwell lathes, or have access to any Cromwell or Smallpiece literature, the author would be pleased to hear from you..

Cut-away section of screwcutting gearbox.

The drive pulley was contained within its own compartment sealed against oil and dirt. In this picture the headstock spindle is locked for direct drive with the internally-geared sleeve (moved by the bronze lever) in position over the smaller spindle gear.