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Looking like a scaled-up 2-Series machine - for that's what it was (though at 1550 kg it weighed twice as much) - the Riva 6 was built with its substantial main column and foot cast as-one. It was fitted at first with a 1000 by 250 mm table - a swing type for the Model U.6 and plain for the ordinary Horizontal U.O - driven by a take-off from the main motor. Later, from the early 1970s, after the machine had been given more angular styling, it was fitted with a 1100 x 250 mm table driven from an independent 2 h.p. motor. All tables had three T-slots 16 mm wide on 60 mm centres. Longitudinal, traverse and vertical movements were, respectively: 730, 250 and 450 mm with 12 rates of feed, from 10 to 500 mm/min in both directions horizontally (with rapids at 2000 mm/min) and from 5 to 250 mm/min vertically (rapids 1000 mm/min). The revised machines had horizontal feed rates set at 12 to 650 mm/min and 4 to 250 mm/min vertically with rapids of 2500 mm/min and 833 mm/min. Both types had the drive arranged to pass through a gearbox fitted with, on the first models, an ordinary multi-plate clutch and on the later (at least on some versions) by a self-adjusting, maintenance free electromagnetic version. The whole of the feed gearbox and clutch assembly was built into one casing and lubricated by an oil bath and a pumped supply through a distribution tube (the miller had two mechanical, plunger-type oil pumps, one for the spindle gearbox and another for the feeds with a third, hand-operated one, located in the saddle, to distribute oil to the table and knee ways). Once running, the operation of the table feeds and rapids was by a number of long handles all of which reached towards the operator and required the minimum of effort to move. Although earlier models are thought to have had plain steel table feed-screws, later versions had ones that were hardened, with ground threads; though the micrometer dials on all types were on the small side, they were clearly engraved and given a non-reflective, matt chrome finish.
Running in specially selected, high-precision taper roller bearings, the spindle incorporated an externally mounted dynamically balanced flywheel assembly that was claimed to smooth out the drive, provide a reserve of torque for larger jobs and give a better surface finish when using large-diameter cutters. The first models were equipped with an I.S.A taper, later ones with the more common and useful I.S.0. 40 - a configuration that also applied to all the vertical and slotting heads. 12 speeds were provided, from 25 to 1200 r.p.m., driven by a base-mounted 3 h.p. motor on the first models and by a 4 h.p. on later.
A very wide range of accessories was offered, all sized appropriately for the machine, including five vertical heads the Types T.V.6, T.U.6, T.V.U., M.6 and C.R.6
Lacking a quill feed, but containing gears finished on Gleason machinery and running in an oil bath, the T.V.6 and T.U.6 heads were driven from the horizontal spindle but arranged with step-up gearing to provide a slight increase in speed - the range running from 30 to 15000 r.p.m. While the Type T.V.6 had a single swivelling end, the T.U.6 had a double.
In addition to the same Universal High-speed head fitted to the U.2 and O (the T.V.U., with a lever-action quill and a spindle driven by its own 2-speed, 0.6/0.4 h.p. motor that gave, via V-belts, speeds of 500, 750, 1000, 1500, 2000 and 4000 r.p.m.) Riva offered a much more sophisticated unit the Automatic-feed Boring Head with an I.S.O. 40 taper, 12 spindle speeds from 30 to 1500 r.p.m. and both a lever-action quick feed and a fine feed through worm-and wheel gearing and a handwheel fitted with a very large diameter micrometer dial. The spindle had a travel of 150 mm and the head could be tilted over 45° in each direction from upright. 2 rates of power down-feed were available: 0.05 and 0.1 mm per revolution An inclinable slotting head, the M.6 had a stroke travel adjustable between 0 and 80 mm.
An effective and well-designed 130 mm centre height Universal Dividing and Spiral-milling Head was offered; complete with a tailstock, this was driven from gearing at the right-hand end of the table with its spindle revolving in roller bearings. It was sold complete with 2 indexing plates, an adjustable intermediate rest, two centres, a catchplate, an extension arm to carry gears for differential indexing (13 gears were supplied) and a set of ready-reckoner tables for performing divisions - with 2200 already calculated for spiral pitches.
A special Fixed Vertical Head, the C.R.6 was offered and designed exclusively for rack and worm milling. This could handle Modular gears from 0.5 to 8 and DP equivalents and was available with a special holding attachment to fit on the Universal Dividing Head.
For hobbing (cutting straight and helical gears, worms and splines, etc.) Riva supplied an automatic 175 mm centre height, self-contained attachment the Type D.175 that was powered from gearing on the right-hand end of the table.
Just one rotary table, the 350 mm diameter Type T.G. was listed for the 6-series machines. This had a No. 4 Morse taper hole in the middle, an indexing plate and a helical gear ratio of 1 : 120. As an option the T.G. could be supplied fitted for a self-acting drive. Two inclinable Universal Table were available, the Pu.U.300 and P.U.600, that differed only in their table size: one being 300 x 160 mm and the other 600 x 300 mm; each was mounted on a circular base, graduated to 360°. By the early 1970s the 6-Series millers were being offered with either Optical Length Readers (reading down to 0.01 mm and read using a 10 to 1 lens) or an Olivetti Digital Readout - with those distinctive, flickering, neon-like red digits..
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