A considerable step up in weight and size from the 8-Series machines, the U.20 (Universal with swing table) and O.20 (Plain Horizontal) weighed some 3300 kg with the ram-head O.20-BA considerably heaver at 4000 kg. With the main column and foot cast as-one, all versions were fitted with a 1500 x 350 mm table with three 18 mm T-slots on 80 mm centres. However, although the table was significantly larger than the 8-series machines, the longitudinal travel, at 1000 mm, was the same, and the vertical increased by only 10 mm to 480 - the main improvement coming in the traverse that was increased by a full 70 mm to 400 mm. 18 rates of power feed available from 12 to 640 mm/min longitudinally, 3 to 160 mm/min in traverse and the same vertically. Rapids were provided on each axis at a rate longitudinally and across of 2000 mm/min and vertically at 500 mm/min. All feeds passed through a positively lubricated gearbox, that included a multi-plate clutch, and were driven by a separate 2.5 h.p. motor. 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 operate. Low-voltage electrical controls were part of the standard equipment and, being pendant-mounted, could be swung by the operator to any convenient position.
Models U.20 and O.20
Running in specially selected, high-precision taper roller bearing the spindle incorporated a dynamically 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. Drive came from an enclosed, based-mounted 10 h.p. motor driving upwards using a 4-step V-belt pulley (with an attached electromagnetic clutch) to a 5-shaft, pressure-lubricated gearbox holding hardened and ground chrome-nickel gears. 12 speeds were provided that spanned 25 to 1200 r.p.m. Supported by a very heavy overarm, the far end of the cutter arbor ran in a pair of needle roller bearings.
The first U.20 milers (like all the maker's machines) were fitted with an I.S.A taper, later ones with the more common and useful I.S.0. 50 - as did the oil-bath lubricated vertical heads, the T.V.20 and double-swivel Universal T.U.20 both being equipped with an I.S.O 50 spindles. The miller had two mechanical, plunger-type oil pumps, one for the spindle gearbox and another for the feds with a third hand-operated one, located in the saddle, to distribute oil to the table and knee ways.
Although the ordinary vertical heads were correctly sized for the machine, the others (the Boring Head with Auto Down-feed and the High-speed Head) were not, being the same units as offered for the smaller - and larger - models. However, the M.20 Slotting Attachment was unique to the U.20 and had a stroke adjustable between 0 and 120 mm. Unlike the smaller Riva millers, the 20-Series could not be fitted with a special head intended for worm milling. The Universal Dividing and Spiral-milling Head was identical to that used on the smaller and larger millers and had a130 mm centre-height unit with its spindle revolving in roller bearings driven from gearing at the right-hand end of the table. 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.
Identical to the U.20 and O.20 in respect of column, knee, table, feed rates and horizontal drive motor, the O.20-BA was fitted with a special self-contained, oil-bath geared overarm that converted the machine into a form of ram miller. The arm could be moved so that the spindle line was as close as 70 mm to the inside face of the column - or as far away as 850 mm. Fitted with an ISO-50 nose, an externally-mounted flywheel and 100 mm of fine down-feed by handwheel, the spindle was held in a housing that could be swivelled in two planes. Driven by a 6.5 h.p. motor 12 speeds were available from a low of 40 to a high of 1300 r.p.m. selected by the juxtaposition of three levers on the left-hand face of the arm..