A heavily built, if conventionally designed industrial-quality lathe, the M400 had an exceptionally deep (380 mm - 15") and wide (360 mm - 14.125") bed. Although intended as a medium-duty production lathe it was also claimed by the makers to be capable of handling, through its wide speed range and great strength, both toolroom and general repair-shop work. It had a swing of 450 mm (17.75") and a capacity of either 1000 mm, 1500 mm or 2000 mm (40", 60" and 80") between centres; the induction hardened bed could be ordered either straight, or with a detachable gap able to accept material up to 642 mm (25.25") in diameter and 200 mm (8") thick. All models had 18 speeds progressing in a ratio of 1.46 from either 40 to 2000 rpm, or 31 to 1600 rpm, powered by a 7.5 kW (10 hp) 3-phase 1500 rpm mounted on an adjustable plate bolted to the back of the bed immediately below the headstock. Four V belts transmitted the power to the geared headstock - the input shaft of which carried individual clutches (made by Matrix) for forward and reverse drive; on the outboard end of the main spindle was a powerful "Fail-safe" self-compensating electromagnetic spindle brake manufactured by Warner. Spindle speeds were selected by a pair of neat, concentrically mounted dials that allowed both rapid, positive selection - yet the minimum of confusion for the new operator.
Fitted with a hardened and ground No. 8 D-1 Camlock nose the 65 mm (2.5") bore spindle carried a reduction sleeve to hold a 5 Morse-taper centre and ran in expensive Gamet pre-loaded Super-precision bearings. The headstock gears and bearings (for this was a machine of considerable strength and capacity), were pump lubricated from a separate oil tank held in the base of the machine.
Fitted with induction-hardened gears the dual English/metric screwcutting gearbox could, without changing or rearranging any of the changewheels, produce 51 inch pitches from 2 to 84 T.P.I. and 54 metric from 0.2 to 14 mm. By the use of additional changewheels 18 Module pitches from 0.2 to 3.5 MOD and 27 Diametral from 8 to 72 DP could also be generated. The 27 power sliding feeds varied from 0.04 mm to 2.84 mm per revolution of the spindle in metric mode and from 0.0016" to 0.112" in English with the power cross feed rate was arranged to be half the sliding rate. The gearbox was controlled by three smooth-finish levers (like those on other contemporary Harrison lathes these could be difficult to operate with oily hands), and a single rotary selector; an oil supply was held in the base of the headstock and circulated by the simple but effective means of splash.
Adjusted to the bed by a tapered gib strip the carriage was fitted with a conventional spoked wheel for movement by hand (the wheel could be disengaged when power feeds were in use) and the 32 mm (1.25") diameter leadscrew (of 6 mm pitch or 4 t.p.i.) carried a torque-limiter and was engaged only for screwcutting; the leadscrew clasp nuts were arranged to "float", a feature that Harrison claimed both reduce wear and improved threading accuracy. The powershaft was fitted with an adjustable over-load protection device that could also act as a carriage stop) provided the sliding and surfacing feeds used in normal work. The double-wall apron was of a completely new design for Harrison and, instead of the usual worm-and-wheel mechanism, was fitted with twin-bevel gear drive and dog clutches (a mechanism used as far back as the late 1800s) to the power sliding and surfacing feeds; this feature allowed the operator to instantly reverse either without having to stop the headstock spindle. A single lever was used to both select and engage the power feeds and returning it to a central position, to stop the cut, could be done without effort, no matter how hard the lathe was working. The apron carried a supply of oil in its base that was distributed to the apron's gears and spindles, the bed and cross slide by an automatic, "one-shot" pump.
Electrical control of the spindle was primarily from a lever attached to the right-hand face of the apron, where it fell conveniently to hand and, of course, moved with the carriage as a cut was taken. An emergency-stop button and "power-on" light on the headstock was provided and both the electrical isolator and coolant pump switch were mounted on the left-hand face of the stand.
Of the full-length type the cross slide and the top slide (which could be swivelled through 360 degrees), were both fitted with taper gib strips; the handwheels were of the typical Colchester/Harrison "semi-lever" type with the micrometer dials (dual-reading metric/English units were an optional extra) finished in non-glare satin chrome; the cross feed nut could be to eliminate backlash. When an M400 is encountered with dual English/metric dials it is worth knowing the cross feed screw and nuts can be either of English or metric pitch - to tell which is fitted look at the micrometer dials: that fitted to the outside position (nearer the operator) indicates the pitch of the screw e.g. if the outside dial is metric so are the feed screw and nut.
Robustly built the tailstock was fitted with a 73 mm diameter (2.875") spindle with 155 mm (6.125") of travel and a No. 5 Morse taper. The spindle was engraved with ruler marks in either inch or metric graduations (or sometimes both) with a rotary micrometer dial fitted as standard.
A variety of electrical-safety installations was offered, but usually consisted of a long, foot-operated bar fitted between the cabinet legs that controlled the action of a combined motor switch and spindle brake.
Unfortunately the stand, through heavily constructed and complete with a splash-back, was fitted with neither a slide-out chip tray nor any sort of storage for tools, accessories or work in progress.
An M400 weighed 1753 Kg (3857 lbs) in 1000 mm between-centres' form; 1956 Kg (4304 lbs) as a 1500 mm capacity machine - and 2159 Kg (4760 lbs) as a long-bed, 2000 mm model.
Overall lengths available were approximately: 2215 mm, 2690 and 3240 mm (88", 106" and 127") respectively..