Portass "Big Dreadnaught" Lathe
- a photographic essay -
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Virtually unknown, this larger Portass lathe (we'll call it the "Big Dreadnought") was listed by the makers during 1951 as a 4" x 24" - but was actually slightly larger with a real centre height of 45/8". Priced at a considerable £144 : 7s : 6d (the 6d was important…) it was supplied on either a sheet-metal or massive cast-iron underdrive plinth (with a cast leg under the tailstock end of the bed and a chip tray between the two supports) and fitted with screwcutting by changewheels using a 3/4" z 10 t.p.i. leadscrew. Mounted inside the stand, the motor was fitted with a long flat pulley and drove up to a simple countershaft fitted with a type of clutch known as "fast-and-loose" - which is, unfortunately, nothing to do with a women but an arrangement where two pulley are mounted side by side, on a common shaft, with one free to rotate and the other fixed. When the belt is on the loose pulley the drive remains unconnected but, when shifted across to the fixed (in this case by a lever protruding horizontally through the front face of the stand) the drive is engaged. The wide motor pulley was, of course, necessary to span the width of the two upper pulleys. Final drive to the headstock was also by a flat belt, this running over 3-step cone pulleys and passing over the front and back faces of the headstock - a bolt-on cover at the front giving access to the belt run. Bored through 5/8", the spindle ran in bronze bearings, had a No. 2 Morse taper nose and carried a 3-step flat-belt pulley. To accommodate the belt run, the backgear assembly was mounted above the spindle line - hence the unusual height of the headstock - with a meaningless advertising puff in the sales literature describing it as being of the "top-geared" type. If the lathe was not fitted to the maker's stand a cover plate at the back of the headstock could be removed to allow drive by a rear-mounted remote countershaft. The unusual backgear assembly was not the only eccentricity - because the gap in the bed was too long (it allowed work 13-inches in diameter to be swung) the front spindle bearing and spindle nose were extended outwards to give the cutting tool a better chance of reaching up to the chuck jaws. With a faceplate fitted the problem was even more serious - and only fresh air supported the front end of the saddle at the forward limit of its travel.
Robustly constructed, the carriage was fitted with a heavy apron holding bronze leadscrew clasp nuts and step-down gearing for the hand traverse. Of a decent width, the cross slide carried a top slide able to be rotated through 360° and equipped with a 4-way toolpost. Most unusually - for this was seldom seen on any Portass lathe - both slides had micrometer dials, neatly engraved on the flat rim of the handwheels. It is believed that a batch of these lathes, with their centre height increased to 51/8", was exported to Indonesia during 1963..
There was also a very seldom-found short-bed version of this lathe with the flat-belt drive coming from behind and so obviously intended for bench mounting.