|
Although the centre lathe (or in American terminology the "engine" lathe) is capable of a great variety of work (and can be adapted by the use of capstan heads and cut-off slides to become a repetition machine capable of turning out hundreds of identical parts every hours), for true mass production it was quickly realised that specially designed and built lathes would be far more efficient and cost effective. Of the many types developed and used during the 20th century was the "multi-tool" machine; unlike the capstan lathe this was a relatively simple affair, strongly built to absorb years of continuous use and fitted with several toolholders at both the front and rear of the spindle line. The front toolholders were generally mounted on a sliding carriage and cut along the work, reducing it to different parallel diameters, while those at the back, which were individually controlled as a group, were fed towards the work. The sliding feed of the carriage and the cross feed of the rear toolpost were controlled by simple, adjustable screwed stops and under human (and some times hydraulic) control it was a relatively easy matter to set up a job so that accurate work was produced with a minimum of effort or expenditure. With the advent of better grades of high-speed steel, and later tungsten tooling, tool-wear rates were reduced to the extent that once set a machine could run for several shifts before having to be reset. The only disadvantage of the lathe was its limitation to turning, facing, profiling and boring operations, but its relatively low cost when compared to a capstan lathe was a considerable compensation; another advantage was that it freed up the more complex machine tools in a factory to do jobs for which they were specially equipped.
The "Multi-cut" was of American origin, from the Reid Brother's Company of Massachusetts, USA. Originally built by Smallpeice in Coventry the design appears to have been passed on the Herberts after the destruction of the Smallpeice factory - a number were also constructed in 1949 by Evans in Portsmouth, a company also responsible for manufacturing the Cromwell precision lathe. The Multi-cut was of simple but rugged construction with a very deep and heavy bed having a pair of widely-spaced V ways machined in its front surface to carry the sliding carriage. To maintain accuracy over as long a productive life as possible the saddle was made enormously long (32 inches) so the usual wear was evened out over as much of the bed as possible. The top of the saddle was provided with a longitudinal T slot into which screw-feed slides, toolposts and other attachments could be quickly mounted, dismounted and adjusted. Power was provided to the carriage drive by a 3-step V belt drive from the headstock spindle to a lower matching pulley that was connected, via pick-off gear to a shaft; sliding on the keyed shaft worm gear that engaged with a gear on the shaft carrying the large hand-traverse wheel; the drive was then taken onto the rack normally used for hand feed. The feed could be instantly engaged by a lever on the left-hand side of the "apron" and disengaged by a separate lever to the right, or by a pre-set trip dog. Faster rates of feed were available by changing the standard single-start worm on the worm shaft for a twin or 3-start and slower rates by fitting a special "reduction" gear.
The rear tool slide was operated by a cam-feed mechanism consisting of a plate cam mounted on the rear of the hand-wheel shaft. The cam actuated a bell-crank lever that was linked to a threaded rod that passed along the back of the machine; the final part of the motion was conveyed by a diagonal-pitch rack, just visible in the lower picture on this page.
The headstock spindle was driven through worm gearing and could be supplied in either low, medium or high-speed versions; the latter used taper roller bearings while both the former were fitted with bronze bushes. The speeds were changed by reversible pick-off (removable) gears; one pair was supplied with the lathe with four additional pairs available that, if fully employed, allowed a range of 10 speeds to be obtained in ranges of 52 to 362 rpm, 127 to 875 rpm and 190 to 1320 rpm. according to whether the low, medium or high-speed headstock had been specified.
An interesting British addition to this class of lathe was the Britan, a machine first made in the late 1940s but - because of its low operating costs, versatility and rugged construction - still widely used into the 21st century..
|
|
