Enclosed in a particularly stiff, very modern-looking angular-style headstock, the dynamically balanced spindle (made from a heat-treated alloy-steel) ran in high-precision ball races, a design change (from the usual plain bearings) that was also adopted during the 1930s on some contemporary models from other American makers including Ames, Wade and Hardinge. The front bearing assembly was constructed from a matched pair of races, set back-to-back and pre-loaded - while the single bearing at the rear was free to move and allow for expansion. On early versions of the lathe the grease-packed bearings could, after several months' of use, be recharged with lubricant through removable plus into which a grease gun fitting could be screwed - later models were given maintenance-free, sealed-for-life bearings. The all-important spindle nose was formed as a miniature version of the then popular and effective American long-taper key-drive type, a fitting that not only provided a robust and secure mounting but also allowed high speeds to be run in reverse with complete safety. In order to stop the spindle rotating as fittings were changed (an important point often neglected by many makers) a locking pin was arranged to engage with a notched wheel at the left-hand end of the spindle. In order to ensure absolute concentricity between the spindle nose and its various fittings and collets, the headstock was first assembled and then locked to the bed (twin eccentric screws engaging in the bed's central T-lot). Once in place the usual trick of grinding the spindle mouth, chuck seat and tapered nose was adopted, the bed acting as reference plane and mounting for the grinding head. Collets were retained by either a simple screw-type draw-tube or by a lever-action assembly, in each case end thrust being taken against a separate ball-thrust bearing. With a spindle able to pass a 55/64" bar, the maximum round collet capacity was ¾", square 17/32" and across the flats of hexagon stock 21/32".
Interestingly, the drive pulley was mounted outboard of the left-hand end bearing, a rather daring design for the time - though concealed inside the enclosed headstock - and adopted post-WW2 by a number of precision lathe makers including Pultra, IME and on some Lorch models. The arrangement brought some advantages, for example, the headstock (being a closed box) remained immensely stiff and, by incorporating a circular opening around the end of the spindle, it was possible (having first removed the hand turning ring and collet-closing assembly) to change the drive belts without disturbing the bearings.
Of conventional design (though the top slide T-slot was in steel and bolted on) the compound slide rest registered against the front bed bevel and featured the usual long-travel (5") exposed-ways top slide. Both feed screws were of Acme form, 7/16" x 10 t.p.i. (or, optionally, 2 mm pitch) with that on the cross slide (with 4.25" of travel) completely enclosed, so protecting it from the wearing effects of swarf and dirt. Balanced handwheels were fitted, the retaining screws, as on most other Rivett lathes, passing down the stem of the ball-ended section. Micrometer dials (13/16" in diameter) also followed Rivett tradition in being clearly graduated in 0.001" intervals (or 0.02 mm) and fitted with very effective and sensitive knurled-edge locking screws (protruding through the ends of the balanced handwheels) that did not cause the setting to alter when tightened. Long bronze feed nuts were fitted, though (surprisingly) neither could be adjusted to remove backlash. The cross slide was equipped with a screw-adjustable rear stop (that could be transferred to the top slide) to aid threading and repetitive production work while the top slide, fitted with a full-circle graduated degree disc, could be rotated through 360°. As a less-expensive lathe, the 715 had, instead of the expected eccentric, quick-set toolholder, an ordinary rocker type was fitted (known in Europe as an "American Type") held in a T-slot, and able to take an Armstrong or similar brand of toolholder up to 5/16" wide and ½" deep.
Able to be set over for the turning of slight tapers, the No. 1 Morse taper tailstock mirrored the headstock in its use of clean, sharp-edged, very modern styling. With its outer end covered (an adaptation of the usual precision lathe design where this was left exposed) the hardened, ground and lapped 7/8-inch diameter spindle remained fully supported within the casting, no matter what its position. 3-inches of travel was provided, driven by Acme screw running through a bronze nut, with the casting cut away across its central section, to expose a finely engraved ruler scale graduated at intervals of 1/16". In addition - and fitted as standard - was a micrometer dial marked in divisions of 0.001". A special tailstock was also offered that mounted a high-speed grind spindle - an arrangement designed for the drilling and honing of very small holes.
Stands and Drive System
A choice of two stands was offered: a compact 45" x 24" all-welded heavy steel-plate cabinet with a collet and tool drawer and locking storage cupboard, or a traditional open-type with cast-iron legs braced by steel tie rods and a large, 60" x 26" by 2.25-inch thick 5-ply laminated wooden top with side and backboards. The open stand was intended for laboratory and toolroom use - where it was necessary to mount the auxiliary motor unit and "overhead" to power high-speed grinding and milling spindles, or if space was needed for tools, small parts and sub-assembles to be laid out and a trail fit of components made.
Both stands used an identical drive system with a 2-speed 900/1800 r.p.m. motor (though some single-speed fittings have also been found) fitted with a 2-step V-pulley that drove to a matching pulley mounted on the rear of two shafts that extended from a simple clutch-gearbox unit bolted to the underside of the bench. Fitted to the output side of the rear shaft were high and low-speed pulleys that drove pulleys on the front shaft that were free to turn until locked by a sliding dog clutch. Final drive to the spindle was by twin V-belts, assembled as a specially-matched pair. In order to allow a quick release of belt tension for speed changes the motor was mounted on a hinged plate controlled by a lever hinged on a bracket bolted to the front face of the stand's lower shelf. By this (relatively complex) means the operator could juggle belt and clutch lever positions to give eight spindle speeds that spanned a very useful 150 to 3500 r.p.m. To promote smooth, vibration-free running, all rotating parts were dynamically balanced and ran in self-aligning ball races. Usefully, in addition to individual screw adjustment for each belt's tension, all could be changed without any dismantling being necessary.
Electrical control was simplified to a single switch and pilot light (to indicate current on or off) mounted low down on the front face of the headstock.The lever was turned to the right for high and low forward speeds and to the left for low and high in reverse - with, in each case, intermediate stop positions between every setting. Sensibly, the makers constructed the switch handle if the form of a large T, so enabling an oily hand to obtain a secure grip.
In addition to the usual wide range of collets, collet closers and centres, the expected T and triangular rests in various sizes (hinged and plain) for hand work, drive dogs, fixed and travelling steadies, a faceplate with radial T-slots and a saw table, Rivett also offered a number of more advanced extras. Amongst these more expensive items was a complete outfit to transform the lathe into a hand-operated capstan unit, a lever-operated tailstock for sensitive drilling; a revolving-spindle tailstock for the drilling of especially fine holes and lapping work (this required an overhead drive to give speeds of 6000, 8000 and 10,000 r.p.m. in reverse direction to the lathe spindle). An unusual design of milling attachment was also listed, almost certainly unique to Rivett, that bolted to the bed in place of the compound slide rest. The unit consisted of consisted of a baseplate on top of which was a screw-controlled longitudinal slide able to be moved through 1.125" and swivelled through 360°. Mounted on the base and arranged vertically was a casting shaped to accept the ordinary compound slide rest (and indeed, other bed-mount fittings). Whilst the assembly had a wonderfully varied number of movements its usefulness was somewhat diminished by the lack of a full-sized T-slotted table (as available for the 918 version) - its work-holding being restricted to the twin T-slots of the top slide. However, the makers did offer a special angle-plate (termed an Angle Iron by Rivett) and a small machine vice.
One important accessory - and a unit common to this class of lathe - was the Universal Grinding Attachment, a device that allowed the lathe to be pressed into use as small cylindrical grinding or honing machine for internal, external, parallel and taper work. Supplied complete, ready for mounting on the lathe bed, the assembly comprised a base plate, fitted with a 5-inch travel slide that could be swivelled through 360°. On top of this was a bearing housing carrying a hardened, ground and lapped spindle (with 2.5-inch of hand-driven travel) that could be hinged clear of the work (for measurement and inspection) and then dropped back into position without loosing the previous setting. The spindle end was formed with a Pratt & Whitney No. 4 taper to mount grinding wheels and, using a 3-step pulley with drive from a separate motor mounted behind the lathe bed, could be made to run at 6000, 8000 and 10,000 r.p.m. in reverse direction to the lathe spindle. With lathe and grinding spindle both on top speed, an effective maximum r.p.m. of 35,000 r.p.m. could be obtained, a figure high enough for all but the very smallest of work.
Not listed, but almost certainly available if requested, was an attachments for the relief of cutters complete with its own drive pulley, gear set and a universally joined and splined shaft drive to the top slide.
*Other American makers from the 19th and 20th centuries included: Levin, Bottum, American Watch Tool Company, B.C.Ames, Bottum, Hjorth, Potter, Pratt & Whitney, Rivett, Wade, Waltham Machine Works, Wade, Pratt & Whitney, Rivett, Cataract, Hardinge, Elgin, Remington, Sloan & Chace, and (though now very rare) Frederick Pearce, Ballou & Whitcombe, , Sawyer Watch Tool Co., Engineering Appliances and Fenn-Sadler the "Cosa Corporation of New York" and UND..
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