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      email: tony@lathes.co.uk
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      SM Lathe


      Believed to be French in origin this (approximately) 150 mm x 1010 mm (6" x 40") gap-bed, backgeared and screwcutting lathe was typical of the lighter general-purpose workshop type manufactured from 1880 to 1900. With its slender, flat-topped, V-edged "English" style bed, deep gap, simple headstock components and using (easily-blunted) carbon-steel tools it would still have been equal to most simple tasks - though the long-suffering apprentice, working hard at the treadle, would have quickly tired of his master's demands to "Keep the crank turning." An unusual feature on a lathe of this class was the use of (7 : 1 ratio) backgears of helical form  - normally the preserve of more expensive, sophisticated machines. This type of gearing was usually chosen for its strength and quiet running - though the additional end thrust generated appears not to have been taken into account in the design of the headstock bearings or their support posts - these being as slender as any contemporary machine fitted with ordinary spur gears. The headstock spindle, which ran in simple. split rectangular bronze "boxes" capped by compression plates, was solid - the end thrust being absorbed against a remote plate held on a pair of outrigger bars. Of 30 mm diameter - and 6 mm pitch - the headstock spindle thread was, like most lathes of the time, without a location register between spindle thread and abutment face to add support to chucks and faceplate Although the expected treadle and flywheel system was employed to drive the spindle the pulley arrangement was unusual - with a 2-step "cone" drive by flat belt (of 90 and 125 mm diameters) for heavy-duty turning combined with three narrow V-grooves (90 mm, 110 mm and 165 mm diameters) to take a round leather "gut" drive for lighter and high-speed work. The "combination" headstock pulley was beautifully machined from one piece of cast iron with the flat sections to the left and the V--with one extra-large in diameter for a low speed--to the right. This possibly unique set-up was, in all probability, a highly effective way of getting the very best out of a difficult-to-operate system. In the Victorian picture (below) the rope can be seen rising from the outside rim of the flywheel - on one of its two low-speed settings - with the single, narrow, high-speed pulley visible much closer to the wheel's centre. The flat belt would have passed round the two outer flats by the rim, running over the V-grooves when moved to the inner position. The system required the use of three belts - two different lengths of round and a single flat--all easily dismounted and reassembled by clips for the flat and hooks or neat, screw-on eyes for the round. The fact that the round belts may have run considerably of line would have been of little consequence, the drive still working efficiently as it did on many other lathes with a similar degree of off-set.
      Screwcutting was by changewheels through a robust, spring-loaded  tumble-reverse mechanism - the relatively fine pitch of both the gears and (6 mm) square-thread leadscrew suggesting either advanced thinking on the part of the designer - or a manufacturing date rather late in the Victorian period.
      Like many lathes made in the same era the entire saddle top was arranged as a boring table - in this case with three transverse T slots - and the compound slide rest as a separate, detachable unit. Of course, the feedscrew were bereft of micrometer dials and would originally have been crank driven, the "balanced" handles in the example illustrated being later replacements. However, as yet more evidence of careful design,  the cross-feed screw was allowed to protrude through the back of the casting and squared off to accept the drive crank. If - for some special job - the slide was removed and bolted to a milling machine or drill being able to turn the screw from the back might have been a useful facility.
      One particularly awkward aspect of many early lathes was the crude arrangement provided for moving the saddle by hand. This often consisted of a bed mounted rack, engaged by a gear connected directly to a large crank handle, the practical effect of which was that a small turn of the crank towards the headstock moved the cutting tool a disproportionately long way towards the tailstock. To remedy the situation a previous owner of this SM engineered a particularly neat and effective modification - a full-circle handwheel, complete with a micrometer dial, mounted on a plate replacing that originally used to blank off the leadscrew clasp-nut housing. A reduction gear incorporated in the drive both slowed the rate of feed and ensured that, as the handle was turned, the carriage moved in the correct directional sense.
      Even a cursory glance at the "SM" lettering on the bed shows it to have been fastened separately, and rather crudely, to the casting pattern - thus raises suspicions that the manufacturer may have been building to order for machine-tool merchants or other distributors. It would therefore be unsurprising if another lathe, identical to the SM, was found to be badged differently. However, as no further information is available about SM, should any reader have a similar lathe, or details about the maker, the writer would be interested to hear from you.

      A fine contemporary picture of a treadle-driven SM with both round and flat-belt drive. The original caption placed the photograph in France. The round rope drive can be seen rising from the outside rim of the flywheel (on a low-speed setting) with the single, narrow, high-speed pulley visible much closer to the centre. The flat belt would have run around the two outer flats by the rim, covering the V-grooves when moved to the inner position. The system required the use of three belts - two lengths of round and a single one of flat--all easily dismounted and reassembled by clips for the flat belt and hooks or neat screw-on hook eyes (with tapered internal threads) for the round.

      The one-piece construction of the unusual dual flat-belt and round-rope drive headstock pulley is clearly visible in this picture

      The headstock spindle ran in simple split rectangular bronze "boxes" capped by compression plates and was solid - the end thrust being absorbed against a plate held on outrigger bars.

      Lightness and simplicity - like most other contemporary lathes there was no location register for the chuck or faceplate between spindle thread and abutment face.

      How the unusual "spring-loaded" oilers functioned has yet to be determined

      Modifications: in its ex-factory state the bed-mounted rack would have been engaged by a gear connected directly to a large crank handle, the practical effect of which was that a small movement of the crank towards the headstock moved the cutting tool a disproportionately long way towards the tailstock. To remedy the situation a previous owner of this SM engineered a particularly neat and effective alteration - a full-circle handwheel, with micrometer dial, mounted on the plate replacing that originally used to blank off the leadscrew clasp nut housing. A reduction gear incorporated in the drive both slowed slow the rate of feed and ensured that, when the handle was turned, the carriage also moved in the same direction.

      A clear picture of the modified carriage drive. The crank handle engages the leadscrew clasp nut

      Apron - rack-pinion gear and leadscrew clasp-nut housing

      Like many lathes made in the same era the entire saddle top was arranged as a boring table - in this case with three transverse T slots and the compound slide rest being a separate, detachable unit. The "balanced" ball handles on the compound slide rest feed screws are a later edition - the square ends of the screws indicating that, originally, harder-to-use crank handles would have been fitted. Note the bronze gib block fitted to the cross slide.

      but as more evidence of careful design  the cross-feed screw was allowed to protrude through the back of the casting and squared off to take the crank handle. If the slide was removed and bolted to a milling machine or drill - for some special job - being able to turn the screw from the back might have been a useful feature.

      A spring-loaded tumbler assembly was far from common - it being more common for a bolt to be undone before the lever could be moved and the direction of carriage travel reversed.

      Evidence of skilled ownership - a beautifully executed dovetail repair to the carriage rack

      Tailstock with open casting and a simple (but properly square-threaded) barrel screw

      The tailstock barrel thrust was taken against two semi-circles of steel retained by radially-disposed screws


      email: tony@lathes.co.uk
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      SM Lathe
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