The spindle, which runs in Gamet Super-precision opposed-thrust roller bearings, has a generous 38 mm (1.5") bore and is equipped with a hardened and ground No. 4 D-1 Camlock nose and a No. 5 Morse-taper socket. The headstock gears, all induction-hardened, run in a simple splash oil bath and are moved by two rather short levers on the front face of the headstock; as a safety measure the levers have to be pushed in before they can be turned and their finish, which is far too smooth, can make them difficult to operate with oily hands. Early series lathes were fitted with a headstock gear-selector drum in "Delrin" which distorted, preventing movement of the selector forks. The fault was quickly recognised by the makers and a replacement drum (part: 303-039) made available in metal. Screwcutting is provided through a well-supported, ball-thrust-equipped 28 mm (1.125") diameter leadscrew, of 6 mm pitch or 4 t.p.i., that is engaged only for threading (a thread-dial indicator is part of the standard equipment) with a separate power-shaft driving the sliding and surfacing feeds. Surprisingly, for a lathe often found in training workshops, the leadscrew is not protected by a cover as on the M250, but there are torque-limiting clutches on both the leadscrew and powershaft - though earlier models were protected by a limiter on the powershaft with just a mild-steel shear-pin through the splined shaft of the top changewheel stud to prevent over-load damage to the leadscrew. Both systems are adequate enough to militate the damage that might otherwise be caused by the over-ambitious cuts often set up by apprentices or - if one dare one say it - the over-enthusiastic amateur.
In comparison with earlier Harrison screwcutting gearboxes the "Universal" unit fitted to the M300 is a considerable improvement and capable of reliably transmitting much great torque. Formed from a one-piece casting it is completely enclosed and holds induction-hardened gears driven by changewheels that have included, over the years, one or more non-metallic intermediates to promote quite running. Controlled by three levers and an 8-position rotary dial the box will generate both English and metric threads: 35 Inch pitches from 2 to 56 t.p.i and 39 metric pitches from 0.2 to 14 mm. By the use of additional changewheels 18 Module pitches from 0.3 to 3.5 MOD and 18 Dimetral pitches from 8 to 56 DP can also be obtained. The range of power sliding feeds varies from 0.03 mm to 1 mm per revolution of the spindle in metric mode and from 0.001" to 0.040" in English; the power cross feed rate is arranged to be half the sliding rate. The box runs in an oil bath (a level window is fitted on the front face) with the lower gears distributing the oil by splash.
Fitted on either the left or right of the apron (to the customer's choice) the carriage traverse handwheel can be disengaged when using power feeds. The double-wall apron carries a supply of lubricant in its base (with a level window let into the headstock-end face) that can be distributed to the bed and cross slide by a simple hand-operated plunger pump. Feeds are engaged by a traditional push-pull plunger and engaged and disengaged a very light-action, flick-up-and-down lever the operation of which is not affected by the how hard the lathe is working.
Fitted with feed screws carrying twin ball-thrust races, that for the cross slide on early models had a nut closed by a wedge (in common with many conventional lathes made by the 600 Group) to allow the elimination of backlash - though when the assembly is badly worn this arrangement can disguise the imminent failure of the threads. Later machines were modified and the nut made solid--the change also affecting the design of the cross-feed screw, The satin-chrome finish micrometer dials are sensibly large with especially clear graduations with that on the cross slide arranged as a "direct-reading" type to shows the amount taken off the diameter rather than the radius of a workpiece. Whilst the top slide has a traditional screw-adjusted gib-strip, the full-length cross slide is fitted with a tapered type and the cross and top slides, as well as the saddle, are equipped with locking screws.
Carefully thought out, the specification of the heavily constructed, set-over tailstock was very user-friendly - with especially long and easy-to-use locking handles fitted with gently tapered and consequently comfortable handgrips. The 42-mm diameter quill, marked with inch, metric (and sometimes both graduations) is fitted with a ball thrust bearing, a micrometer graduated collar and accepts a No. 3 Morse taper of the tang type with, usefully, a drift slot provided to aid its removal. As a final touch, tailstocks on later versions of the M300 carry felt bed wipers to help reduce the inevitable wear to the front section of the sole plate.
Electrical stop, start and reverse of the spindle is by a "third-rod" system controlled by a lever attached to, and moving with, the apron. Both the electrical isolator and coolant pump switch are mounted on the left-hand face of the stand - the latter a unit that is well made and supplied with a chip tray, splash-back and locking tool cupboard as standard. Although safety equipment has varied during a long production run most machines have a useful "power-on" light, a rather small headstock-mounted, mushroom-headed emergency stop button, a long, foot-operated (and very powerful) spindle-brake and "motor-off" bar and electrical interlocking of the changewheel guard. The electrical system is built into the left-hand end of the stand and accessible thorough a door equipped with an interlocked isolating switch that can, for additional security, be padlocked in the off position.
Although of an excellent mechanical specification equipment provided as standard with the M300 has generally been sparse: a single toolpost, drive plate, a 5 to 3 Morse adaptor bush for the headstock spindle (often missing on used machines), 2 No. 3 Morse centres, spanners, keys, oil gun, final inspection chard and a handbook and parts list. An M300 weighs about 583 Kg (1288 lbs) in short-bed form and 685 Kg (1512 lbs) as a long-bed model..