Continued from this page:
Screwcutting and Power Feeds
Both Student and Master were available with either screwcutting by changewheels or (and by far the most common specification) a dual inch/metric screwcutting gearbox. However, detailed differences in the screwcutting were noticeable over the years, with the first Student models listed with either a gap or straight bed and with either changewheel or screwcutting gearboxes in inch or dual inch/metric versions. The range was arranged as follows: "Cub", "Penny", "Colt" and "Dime" The Cub was a gap-bed lathe with screwcutting by changewheels (and a 2-speed gear on the leadscrew); the Penny (later called the Dominion) was a straight-bed model also with changewheels; the Colt had a gap bed and a dual inch/metric quick-change screwcutting gearbox with 36 inch pitches from 4 to 60 t.p.i. and 11 metric from 0.5 to 6 mm. A special feedbox was available for the Colt (at extra cost) that had no metric capability and incorporated pitches of 5.75, 11.5, 23, and 46 threads per inch, but at the expense of omitting the 7.5 15, 30 and 60 pitches. The Dime was another straight-bed lathe with a screwcutting gearbox having 36 inch pitches - though at extra cost the dual metric/inch box from the Colt could also be fitted.
Later versions of the Student, and all models of the Triumph, had screwcutting gearboxes able to generate 45-inch pitches from 4 to 120 t.p.i. and sliding 45 feeds from 0.0025" to 0.068" with cross feeds set at exactly half those rates. 12 metric pitches could also be obtained (by just lifting a small lever on the face of the gearbox) of 0.25 to 6 mm pitch.
Just to confuse matters, the straight-bed, plain saddle model was also known as the "Dominion" and was listed in some catalogues as either a 6" x 24" or 6.5" x 36" model. Although at extra cost it could be fitted the dual inch/metric gearbox it was fitted as standard with either changewheels or an English-only screwcutting gearbox with two extra pitches of 11.5 and 23 t.p.i. By the mid 1960s a rare "Continental" all-metric model was also listed, a machine easily recognised by a distinctly different gearbox fitted with a Bantam-like joy-stick control lever on its front face. Both the Dominion and Continental boxes could be persuaded (at some expense) to generate metric and English threads respectively by means of translation changewheels.
Unlike their Harrison competitors, the changewheel models had no convenient and easily-operated 3-speed gearbox to allow a quick change in the rate of power feeds, instead a double gear on the feed shaft could be slid by hand into one of two positions to pick up a single gear on the leadscrew - and so select either a fast or a slow rate of feed. The changewheels were guarded by a cast-aluminium cover secured, not by a hinge, but on studs; although this arrangement was perfectly satisfactory for use on the gearbox-equipped models on the changewheel version, where access to the gears was required for every change of thread, the design was a considerable inconvenience.
All Mk. 1 and Mk. 2 Students and Masters had a 6 t.p.i. Leadscrew (with no metric alternative) that was used only for thread cutting and not power feeds - normally it was left disengaged by positioning the inch/metric thread-selector handle in its middle position. Power sliding and surfacing (along the bed and across) was provided by a separate power rod and fitted to all models with, on the Mk. 1, both selection and engagement by a single lever that was slid along the lower edge of the apron and lifted (and locked) into one of two slots. Releasing the feeds merely required the lever to be pressed downwards; unfortunately, the heavier the cutting load the greater the effort required to disengage it, but at least it was a positive in-out action and did not require (as on so many older lathes) a knob to be unscrewed to release a clutch.
The apron was double walled but open at the bottom, so precluding any chance of the gears being run in oil and requiring the operator to periodically supply lubricant. Unfortunately, in many cases, this simple task was ignored and allowed such severe wear to take place on the inside of the worm-wheel carrier that the whole assembly dropped so low that engagement became impossible.
A few Mk. 1 lathes had a ball-bearing "clutch" incorporated in the feed-shaft to protect the apron against overload damage - a mechanism that could be used to advantage in combination with bed stops to provide a reliable, automatic means of turning repeated shoulder lengths in production work. All models had a shear pin incorporated in the top gear of the screwcutting drive; when the leadscrew and power shaft refuse to rotate, this is the first place to look; if the pin has broken replace it with one made from aluminium, not steel. All versions were provided as standard with a thread-dial indicator built into a rather perfunctory cast-iron leadscrew guard.
Just before the introduction of the proper Mk. 2 the Mk. 1 was updated (as the jokingly-called "mark-one-and-a-half") with a completely redesigned and far safer apron. For the first time the bottom of the assembly was enclosed, allowing the gears to run in an oil bath, though oddly not all models had the sight-glass level indicator showing this feature was fitted. The function of selection and engagement of feeds was separated with a push-pull knob, protruding from the middle of the aprons face, selecting the feed and with engagement by flicking up and down a lever at the bottom of the apron - in the long-established Harrison manner. However, inside the apron the Colchester mechanism was quite different, and incorporated an important safety feature: an automatic feed knock-off for when cutting forces become too high. This was achieved by mounting the engagement mechanism on a separate carrier that transmitted drive to the carriage through a spring-loaded ball-bearing running up a ramp. To adjust the spring pressure (and so the release point) the engagement lever was given a threaded barrel (often found jammed tight) that the operator could screw in or out. By using a combination of a correctly-adjusted lever together with a bed stop a useful automatic-disengage could be set up by which means repeated runs could be made up to a shoulder.