Largest of the Celtic range, the 10" centre height by 40, 60, 80, or 100 inches between-centres Mondiale Celtic "20" was designed along very similar lines to the Company's smaller lathes, even to a V-belt drive headstock. The lathe was mounted on two massive cast-iron plinths spanned by a deep slide-out chip tray; unfortunately there was no form of storage, not even an open shelf, for either tools or accessories. Like the Model 17 the headstock casing was double-walled, internally braced box with a centre section that sealed the 5-step V-pulley against the ingress of oil from the speed-reduction and leadscrew-drive gearing; the drive pulley ran on its own large roller bearings to isolate the effects of belt pull. As with the smaller Celtic models the bore of the (heat treated) No. 6 Morse taper Timken taper roller bearing headstock spindle was undersized for the capacity of the lathe and the nose fitting a similarly too-small American long taper L1 - in comparison the equivalent Colchester Mastiff had a 3.5-inch spindle bore and, with an 11-inch D.1 Camlock fitting, a nose able to carry much more massive and useful chucks.
Drive came from a 3-speed gearbox mounted inside the left-hand cabinet plinth with its operating lever, a large chrome-plated rod, on the font of the stand; the box was oil-splash lubricated and held induction-hardened gears running on ball-bearing supported, hardened and splined shafts. Bolted behind the gearbox (and protruding through the back of the bed-support plinth) was a push-button controlled 2-speed 6.5-4 HP electric motor that gave, in combination with the four-speed headstock reduction gearing a total of 18 speeds that ran from 12 through 16, 20, 25, 31, 40, 50, 62, 80, 100, 125, 158, 200, 250, 316, 400, 500, 632 to 1000 rpm.
Although many contemporary competitor machines offered either a spindle brake or clutch (and sometimes both) no such option was available for the Celtic 20, a situation that must have frustrated many operators as they watched their bonus-payment seconds slip away because a heavy job would not stop rotating quickly enough.
Available with or without a detachable gap piece that allowed material up to 30" inches in diameter and approximately 10" thick to be swung, the bed had V-ways of a type similar to that first used on the beautiful American Wade No. 7 precision bench lathe where the (asymmetric) front V was both laid back at a shallower angle than normal and made especially wide; the rear surface was narrower and, in order to better absorb tool thrust, set at a steeper angle. Although not hardened (a process restricted to the extensive options' list) the bed was of robust construction and very deep in relation to the machine's centre height.
With every thread or feed selection made by rotary levers (and a consequent absence of sliding controls) the oil-bath lubricated screwcutting and feeds box, with its hardened gears, could be completely sealed against the ingress of dirt and swarf. By using just the ordinary set of changewheels the box was able to generate 63 English pitches (from 96 to 13/16 t.p.i.) and 35 metric (from 0.45 to 40 mm pitch) - a range greatly superior to the 38 English and 27 metric of the Colchester Mastiff. Sliding and surfacing feeds totalled 54 from 0.003" to 0.252" per revolution of the spindle on longitudinal travel and from 0.0008" to 0.064" on the cross feed. With a different changewheel set on the quadrant arm a complete range of Diametral and module pitches could be produced and the 5-segment screwcutting chart included these thread sets in its layout. The 1.375-inch diameter leadscrew (1.969" on the longest bed model) could be had with an inch or mm pitch ground thread and its bronze engagement clasp nuts were adjustable for backlash; the leadscrew could be disengaged when not in use.
With its double-wall construction, oil-bath lubrication and ingenious mechanical design the all-helical-geared apron was well engineered for its task - and used throughout the Celtic range. The drive shaft from the gearbox transmitted its power through a worm and wheel to a shaft that passed vertically though the centre line of the casting; at the top of the shaft a double-sided dog clutch (operated by a very large combined selection and engagement quadrant lever on the face of the apron) moved up or down to select power surfacing and sliding feeds respectively. Although the apron was a robust and reliable piece of engineering its feeds engagement mechanism suffered from the usual drawback of all quadrant-lever operated systems: the awkward double action of pulling out the location plunger and swinging the operating arm sideways meant that an instant (and certain) disengagement of the feeds was impossible. A simple flick-in-and-out lever, preferable working in a vertical plane between fixed positions (and through some sort of overload protection) would have been much preferred in this situation.
On the right-hand face of the apron was a pivoting point for a lever that operated, via a long control rod, an electrical switch to reverse the spindle - a useful safety feature, especially on longer-bed versions where the operator could find himself some distance from the headstock-mounted push buttons.
Entirely conventional - but without T-slots in its top surface - the saddle carried a compound slide rest with proper taper gibs that were adjustable, with the usual sensitivity, by a push screw at the front and a stop screw at the rear. Two cross slides were available, both with a travel of 12.5": a plain-top standard version version - unfortunately shorter than the slide that it ran on and with a cast extension cover at the rear to protect the feed screw - and one able to accommodate a rear-mounted toolpost, its support block and other accessories. The top slide could be rotated only 45 degrees in either direction but came complete with an indexing (though 8-positions) 4-way toolpost and was held down by four bolts..
As with every other Celtic lathe the weakest point in the specification was probably the tailstock; the (unhardened) 2.65"-diameter No. 4 Morse taper barrel was "microfinished" its travel was short at 6" (Colchester Mastiff 9") and the two bed clamp nuts required a loose spanner to operate - the latter gambit may have saved the makers money but was guaranteed to waste time in the customer's workshop when the spanner was inevitably "borrowed" for some other task. The top section could be offset in the usual way for taper turning and had a maximum movement of 5/8". The weight of the lathe varied with the between-centres capacity as follows: 40" 3500 lbs; 60" 3900 lbs; 80" 4200 lbs; 100" 5300 lbs..