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Rebuilt Cromwell S800   Mk. 2 Screwcutting Cromwell

Produced as both a plain-turning precision lathe and, as shown on this page, a backgeared and screwcutting model, the 3.5" x 20" Mk. 2 Cromwell is more often to be found mounted on a very heavy cast-iron stand with the words
Cromwell Coventry cast into its front face. While the headstock, compound slide rest and tailstock resembled those on the simple  Mk. 1 this was a very much more complex machine with the screwcutting and power sliding and surfacing feeds arranged in an unbelievably convoluted and expensive-to-produce way - and quite unlike that on any other lathe seen by the writer. As far as can be understood, the system used two separate back-to-back gear trains at the headstock end, a second gearbox by the tailstock, a permanently-engaged leadscrew for screwcutting and two powershafts - one for sliding (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 while the other was carried externally - and unprotected - and employed only for screwcutting. The changewheels were carried on square-ended studs that slotted into a beautifully-made quadrant arm with precisely machined T-slots. Only one train of gears 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 (they used 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 changewheel train. 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) that turned both the steel shaft  and 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. When moved to the right the lever set the rate of longitudinal feed shown on the tailstock-end gearbox cover, the central position providing a neutral and the left-hand position the screwcutting feed. 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 six gears arranged in pairs, gave six rates of feed (a plate on the box showed how to arrange the gears) and drove the leadscrew. The tailstock-end box used gears of the same pitch (16DP) as the later models - but with 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 lower power 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, when the clutch faces became, it became 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.
While 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: a 1
3/8" 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, draw-in bronze bearings (or white-metal bushes by Glacier) tapered on their outer surface.
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 the slide having covered ways and a single T-slot. While the twin-slot top slide was a copy, the single-slot was entirely original for, although 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 Cromwell with backgear, screwcutting and power sliding and surfacing feeds. Note the tailstock-end mounted feeds' gearbox with its swing-up cover, the overhung power cross-feed shaft and compound slide rest end plates in bronze.

The single lever immediately below the headstock operated individual dog clutches that engaged either screwcutting or the power sliding feed. Its central position provided a neutral with neither engaged

Reduction gearbox case opened showing the fixed-ratio drive and the large bronze gear with the drive shaft from the screwcutting changewheels passing through it.

The bronze gear pulled out from its housing. The hollow shaft had a gear formed at its other end (it can just be seen emerging from the casing) used to drive the power sliding and surfacing feeds

Reduction gearbox cover in place and the shaft to pick up the drive from the screwcutting changewheels protruding through the face

The beautifully-made quadrant arm used to carry the screwcutting changewheels. As no cover was provided, this was left exposed.

"Selector Gearbox". Here the drive arrived at the top - the bronze gear driven from the Reduction Gearbox and the shaft passing through it from the screwcutting changewheels. The drive passed down to the power-sliding shaft and then to the overhung power cross-feed shaft.

6 gears, arranged in pairs provided, via the leadscrew, 6 rates of sliding feed

Pick-off gears (there were six pairs to vary the ratio) transferred the drive from powershaft to leadscrew.

Backgear and 4-step V-belt headstock pulley

Early Cromwell Mk. 2 mounted on the maker's cast-iron cabinet stand

The large diameter, coarse-pitch leadscrew

Alternative top slides appear to have been offered-- some machines being found, as shown, 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. Other lathes had an entirely different arrangement with covered ways and a single T-slot. 

Changewheel and pick-off gear storage was on neat spiders

Cromwell Mk. 2 Screwcutting Charts

Engraved degree scale for rotating the top slide. On some lathes this casting part was in iron, on others bronze

Underside of the top slide. 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 this unit was highly unusual in employing the roof of the upper casting instead.

Countershaft motor bracket. The whole of the countershaft drive system was very heavily built.

A contrast in fortunes: while the facia used for the reversing switch, above, was over-engineered in cast bronze - an astonishing waste of money  - that for the on/off switch - right - appears to have been in ZAMAK. 

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