Introduced in the early 1960s, the TOS lathes Models SN40, SN45, SN50 and SN55 were named after their swings (double the centre height) these being, respectively, 400, 450, 500 and 550 mm. Although the SN40 could be had with between-centres capacities of 750 or 100 mm, and the SN55 1000 or 7000 mm, the SN45 was restricted to 1500 mm and the SN50 to 3000 mm. All versions had detachable gap beds these being able, in order of centre height, to take jobs up to 600, 650, 700 and 750 mm in diameter. The gaps did not increase in length with centre height with all, except the SN55, being restricted to mounting jobs of up to 230 mm in thickness on a faceplate; the SN55 was able to accommodate 300 mm. In all other respects, apart from capacity, the four models were of identical mechanical arrangement with the smaller three types even sharing the same 4 kW, 1410 r.p.m. 2-speed main motor. The drive from the motor passed to a speed-change gearbox that incorporated a multi-plate clutch, this also acting to provide reverse of the spindle; two series of 12-step spindle speeds were provided, these being from 45 to 2000 r.p.m. and 22 to 1000 r.p.m.
Suitable altered to benefit its extra size and mass - its bed being 450 mm wide instead of the 360 mm of the smaller versions - the SN55 had a 7.6 kW motor and 16 spindle speeds in two groups of 10 to 112 r.p.m. and 90 to 1000 r.p.m.
The rates of power feeds on all four models were identical and arranged to drive either direct from the spindle or via a "countershaft", these two settings giving ranges of, respectively, 0.05 to 0.8 and 0.64 to 6.4 mm per spindle revolution longitudinally - and at half those rates in traverse.
Screwcutting, using a 40 mm diameter by 6 mm pitch leadscrew on the SN40, SN45 and SN50 (48 mm by 6 mm pitch on the SN55) was arranged to give, on all models, 29 metric pitches from 0.05 to 40 mm; 35 English inch pitches from 1 to 80 t.p.i. and 26 module from 0.25 to 20.
Details of the lathes' construction and their full specifications - the SN55 also had a several other differences - can be found below and on this page..