Colchester Magnum LB800
Totally enclosed, the screwcutting and feeds gearbox held hardened and ground gears with all shafts running in anti-friction bearings. 70 metric pitches could be generated from 0.2 to 112 mm; 88 imperial from 1/4 to 112 t.p.i.; 37 module from 0.3 to 24 MOD and 41 diametral from 1 to 60 DP. Direct changes of pitch were selected by three levers and a single knob on the face of the gearbox but with the range of pitches - fine, medium and coarse - by a lever on the face of the headstock marked, respectively, L, M and H. The H position was for generating very long leads suitable for oil grooves and similar jobs and was intended, of course, for use only at slow spindle speeds, these being marked in red on the spindle-speed selector dial. The maker's handbook has essential data about the limitations of the spindle-speed to feed-rate settings.
Feed to the sealed, splash-lubricated apron for screwcutting was by a leadscrew 45 mm (1.75") in diameter, with a pitch of either 12 mm or 2 t.p.i. together with the usual separate power shaft - on long-bed versions both being supported by adjustable, intermediate hangers. Power sliding and surfacing feeds were driven through a constant-mesh wormwheel and spur gear assembly and Vee-tooth clutches, these being adjustable and fitted with an overload trip that could be set to work against adjustable stops in either the sliding or surfacing direction. Built into the apron was a rapid-traverse motor that drove the sliding feed at 4000 mm (60 inches) per minute and the cross feed at half that rate - engagement being by pressing a button built into the top of the joystick lever.
Of minimalist appearance, the apron's mechanical controls consisted of just a (safety disengage-able) handwheel for manual movement of the carriage, a lever to engage the screwcutting clasp nuts and a directional joy-stick lever that acted to both engage and set the direction of feeds. In addition, on the face at bottom left, was a manually operated plunger pump that directed lubricant to the bed and cross-slide and, to its right, a fluted knob to set the loading at which the power feeds tripped out. For main motor start, stop and reverse, a line of push buttons and warning lights was provided on the apron's upper right-hand face.
Using tapered gib strips, the cross and top slides were fitted with micrometer dials reading either to 0.001" or 0.02 mm. The dial on the cross feed was set to indicated the reduction in workpiece diameter while that on the top slide showed the tool travel. As fitted to all later Colchester lathes, the cross-feed nut was split and, by means of a central wedge, could be adjusted to remove backlash. For safety, because a rapid power cross-feed was fitted, the cross-feed handwheel was spring loaded outwards into a disengaged position and had to be pushed inwards to engage a dog clutch; however, a latch was provided to lock it in position - though if the operator forgot to disengage this before using rapid feed the danger remained of a spinning wheel catching loose clothing.
Able to be set over for the turning of slight tapers, the tailstock had a 6 Morse taper barrel with a travel of 190 mm (7.5 inches). For heavy drilling operations an eye bolt was provided at the front, this being coupled by a clevis pin to the carriage (the pin was stored in a parking position). The maximum feed force was limited, of course, to that available from the saddle feed mechanism i.e. 1000 Kg f (2200 lbs f).
Supplied as standard with each new Magnum was a 380 mm 3-jaw chuck, a simple slotted toolpost, a driving plate, headstock spindle reducing bush to 6 Morse taper, a full-length rear splash guard, chuck guard, thread-dial indicator and a set of spanners, a handbook and an accuracy chart.