The Harrison M Series lathes have been in production for many years and considerable numbers of the smaller and very popular models, the M250 and M300, are now in the hands of enthusiastic amateurs and second-user professionals. The digits following the M indicate, in all cases, the swing in millimetres, half the swing being the "throw", more usefully described as the centre height. The range has been the subject of continuous development and, by the mid 1990s, consisted of the established and largely unchanged M300 model together with the M350, M450 and M500. By the late 1990s the last three machines became, with alterations to centre height, bed length and specification the M390, M460 and M550 - and two newer, larger machines were introduced (though with a short production life), the M600 and M750. Today the range continues in production but fitted with variable-speed drive (though the M300 continues to be available as a geared-head model) with models numbers of: V350, V390, V460 and V550. Whilst these articles cover the older, conventionally driven M Series lathes they do not concern the up-to-date models; an article about those will have to wait until they become, in some years' time, cheap enough to be accessible to the keener model and experimental engineer. M250 Lathe
An Operating, Service & Parts Manual is available for the M250
Now out of production, the M250 of 145 mm (5.7") centre height was designed to appeal to schools, training establishments, repair shops and those needing a strong, properly-built but compact lathe for light production work. Never as popular in the educational market as the more robust M300, as a second-hand machine it has enormous appeal to the more enthusiastic home user and hence demand usually outstrips supply with consequently high prices - but of course an excellent investment potential and an easy resale. It was available with a choice of either 500 mm or 750 mm (20" or 30") between-centres and had an induction-hardened bed as standard - but lacked the option of a detachable gap; interestingly, the width of the bed, at 190 mm over the ways, was greater than the centre height, a design feature that many years ago would only have been considered necessary for a lathe built to "toolroom" standards.
Early models had 9 speeds from 40 to 1500 rpm (or, optionally, 80 to 3000) whilst the later (1998 on, approximately) M250 ran from 52 to 2000 rpm - the electric motor used on the slower model was 0.9 kW, with a 1.3 kW employed to drive the two faster machines.
Fitted with a hardened and ground No. 3 D1 Camlock nose, the 35 mm (1.3") bore spindle ran in Gamet Super-precision bearings with the hardened headstock gears lubricated by oil splash. On post 1989 machines the headstock had the same spindle nose as used in the Colchester Bantam Mk. 2, a No. 4.5 Morse taper into which fitted a hardened adapter sleeve to take it down to 3 Morse. The handbook and catalogue both contain incorrect information about this fitting stating, variously, 4 Morse and 5 Morse for the spindle bore.
Available to cut either metric or English pitches, the screwcutting gearbox was available (at some considerable cost) with sets of conversion changewheels to change either to the other. The two standard gearboxes - that give either 33 inch pitches from 3 to 72 T.P.I. or 33 metric pitches from 0.25 to 8 mm - were fitted with an oil supply circulated by splash from the lower gears dipping into the lubricant and flinging it around.
Fitted with European Safety-Neurosis, full-circle smooth-faced handwheels, the carriage could be driven by either a fully guarded, 25 mm diameter leadscrew (in 6 mm or 4 t.p.i. pitch) or a power-shaft; however, the leadscrew was intended to be used exclusively for screwcutting whilst the torque-limiter protected power-shaft provided the normal sliding and surfacing feeds. The torque limiter is normally a reliable unit but, if dismantled and the stack of Bellville spring washers assembled incorrectly, it can give trouble. If yours is playing up or not releasing, or releasing too soon, the answer can be found here. Although the top slide had a traditional, parallel-type of screw-adjusted gib strip, the cross slide was fitted with what many regard as the superior tapered type, able to be set more accurately and provide much better support; both the cross and top slides, as well as the saddle, were fitted with locking screws.
Situated in a convenient (and hence safe position) the electrical controls were on a sloping panel immediately below the headstock; although the angle of the panel makes the switches much easier to use than if they were mounted on a vertical surface, the panel has no hood and, even though sealed with a wide gasket, it would probably be unwise to allow coolant to flood over it.
Well made, with a deep chip tray and a splash-back as standard, the stand could be supplied with a full-length kick-stop safety switch - though unfortunately the spindle was not braked (an interesting article about part of the electrical system and transformers can be found here). However, despite the cabinet's box-like structure, no shelves or cupboards were provided as standard in which store tools, accessories or work in progress. For an extra charge a cabinet door (part number 911001 for the short bed) could be fitted to take advantage of the space beneath the chip tray.
The short-bed M250 weighed approximately 365 kg (785 lbs) and the long-bed 457 kg (1008 lbs)..