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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 and 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) used to hold the headstock and tailstock.
Surprisingly, 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. Typical of Cromwell ingenuity, the top slide was of unusual design 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 bolt-on end flange of each slide as also in bronze, an expensive way of providing a bearing for the feed screw to run through. 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.
With the advent of the S800 Cromwell designers dropped the convoluted and expensive-to-produce screwcutting and feeds arrangement of the Mk. 2 and chose instead a straightforward, quick-change gearbox drive. Careful thought was given to the design of the box: it was laid out as a magazine type, to offer the same wide range of threads and quick operation of a conventional unit, yet eliminate the usual open slots in the front (through which dirt could enter) and also allow 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. Despite the simplification of the screwcutting and feeds arrangements Cromwell just could not help adding an interesting and well-engineered complication: because the coarse-pitch leadscrew was in permanent engagement with the carriage - and would have been difficult to turn with the usual handle at the tailstock end - two handles were provided,, one at each end of the leadscrew with each working through easily-turned worm-and-wheel gearing. A large 0.0001" vernier scale dial was fitted at the tailstock end of the leadscrew, 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". 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 dropped down at an angle 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.
If you have, or know of, any type of early Cromwell lathes, or have access to Cromwell or Smallpiece literature, the author would be pleased to hear from you.
*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.
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