Superbly constructed, of a most ingenious design and packed with interesting and complex features to assist the operator in every way possible, the Boley 5ZL was an outstanding example of a small toolroom-class lathe. With a 130 mm (5") centre height and admitting 500 mm (20") between centres, the machine weighed 900 kg and was built to be as rigid as possible.
Carried on a cast-iron stand (with integral chip and coolant tray and recessed in the centre to allow an operator to sit down) the bed of the 5ZL was arranged on lines identical to those on the much earlier Schaublin 102VM - this comprising an uncovered central section with a flat top and bevelled sides to carry the tailstock with, on V-ways set on wings protruding from the front and back faces of the casting, to guide the carriage. Long protective covers in sheet steel were fitted to deflect the worse of swarf and coolant.
Running in massive, plain, hardened bronze bearings, the 23 mm bore headstock spindle was protected from the effects of belt pull by a 3-step flat-belt pulley running in its own bearings, the drive being transmitted by a peg (experienced owners report that the spindle bearing oilers could be difficult to adjust accurately and require regular attention). The threaded spindle nose was "almost" a DIN800 pitch - with the risk of an accidental undoing of a chuck or other fitting in reverse being mitigated by a threaded ring and wedge arrangement that clamped the chuck to the spindle register with three brass pins. Collets were secured in the spindle nose with a simple and accurate draw-back, lever-operated collet closer.
Of a most interesting and involved design, the drive system provided a very wide and useful range of spindle speeds that spanned 28 to 3550 r.p.m. - with power coming from a 2-speed 3-phase 1.3/2.2 kW (alternatively in some years a 1.5/2 kW) motor mounted in the headstock end of the cabinet base. The motor was connected, via a 2-step pulley (that gave the initial high and low ranges), to a 2-speed gearbox with a built-in, electrically operated, 1:4 drive that gave increased speeds in reverse. The box was controlled by dual electromagnetic clutches combined with a brake - the engagement of which was controlled by the flick of a single switch and allowed most speeds to be changed without stopping the spindle. The final drive to the spindle was by a wide flat belt with the oil-bath lubricated helical backgears clustered just behind the spindle nose in their own sealed compartment. It was not necessary to open the headstock cover to move the belt from pulley to pulley, instead a lever-operated mechanism was employed that operated on the belt by the lower pulley - another older idea pinched from Schaublin idea and one that enjoyed the same rather uncertain, clunky action. The clutch and brake lever also doubled to control the forward and reverse return of the carriage - and for setting semi or fully-automatic screwcutting cycles (though the latter required extra equipment in the form of the d32* threading accessory). When used for the batch production of threaded components, the four-times accelerated reverse was especially useful and, of course, eliminated the need to disengage the clasp nuts. Unfortunately, the main electrical controls were clustered in a vertical line low down on the front face of the tailstock end plinth -leaving the red stop button conveniently out of reach as the unfortunate operator was wound into the mechanism. An on/off rotary switch was fitted to the top edge of the splash-back, easily within reach of the operator's right hand and topped by a short tool tray in wood.
Screwcutting was by changewheels which, being fitted to large diameter studs and retained by quickly detachable locking discs, were easily dismounted and changed. There was no option of a screwcutting gearbox, the makers deciding instead to offer the convenience of a well-thought-out power-feed system that enabled the operator to set the most appropriate cutting speed with the minimum of effort (a change in the power sliding or surfacing feed rate did not disturb the screwcutting setting). To run the feeds, a separate V-belt-driven, 9-speed quick-change gearbox was used in conjunction with a ratio-lowering worm-drive to turn a power shaft - with nine sliding rates available from 0.63 to 0.4 mm and nine facing from 0.032 to 0.2 mm per revolution of the spindle. The massive 36 mm diameter, 6 mm pitch leadscrew was fitted beneath the bed, on its centre line, and with the enormous bronze-lined clasp nuts directly below the toolpost to give the shortest possible thrust path. Fully protected against the ingress of swarf and coolant, the leadscrew was lubricated from a perforated copper tube that ran for its full length directly above it. Further protection included a 2.5 mm shear pin and - most usefully - an adjustable trip for automatic disengagement.
A hardened stop - with a fine-pitch screw and able to accept a slip gauge - was fitted for the automatic disengagement of the separate power-shaft-driven sliding feed (and also available as an option for the surfacing) with an adjustable-tension drop-out worm providing an instant break in the feed no matter how heavy the cutting load. As an option, adjustable six-position stops were available - that for the longitudinal travel being mounted on top of the feed gearbox and the one for the cross motion having the facility to mount a clock gauge.
Ingeniously, below feed rates of 0.05 mm per rev. the leadscrew could be "back driven" from the powershaft, via the changewheels, to give a range of micro-fine feeds down to as low as 0.01 mm on standard machines - the makers offering to engineer in even finer rates if desired. A set of 25 changewheels was provided of 25, 26, 27, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 87, 90, 95, 100, 105, 110, 120, 125 and 127t. Neatly boxed and held in a compartment built into the right-hand face of the stand, they allowed metric from 0.2 to 24 mm pitch, English from 4 to 80 t.p.i. and module from 0.3 to 8 to be cut.
Of massive proportions, the carriage had an apron carrying a comprehensive set of all-lever controls including a single one to divert drive to the lead or feedscrew; forward and reverse selectors were provided for both longitudinal and cross traverses and a separate lever for an instant trip to disengage either feed. The carriage handwheel was fitted with a useful and very large zeroing micrometer dial, the crisply engraved graduations able to indicate a travel of as little as 0.1 mm.
Cross and top slides were fitted with tapered gib strips and beautifully engraved micrometer dials, that on the cross slide being notable large at 80 mm and with one division indicating 0.05 mm off the diameter of the workpiece. An alternative screw was offered, with a 2 mm pitch, with one division on the dial giving 0.02 mm off diameter. A face lever, the action of which prevented any change in the dial's reading as it operated, was used to lock the setting - Boley even going to the trouble of fitting it with red and green indicator marks visible through a small hole in the dial's periphery. The cross slide was machined to take rear toolposts for parting-off, forming and chip-breaking work, etc. Able to be rotated through 90° each side of zero, the top slide had its degree graduation lines engraved on a sloping surface at the front and hence clearly visible to the operator from his normal working position.
Although the standard toolpost was a simple "bridge" clamp, with tools sitting directly on top of the top slide, a rather ungainly-looking quick-change toolpost was listed as an option. However, as the distance between the top face of the top slide and the spindle axis was only 22 mm, this accessory did have the disadvantage of requiring, when using larger tools, of needing to set the lower face of a tool holder below the slide level, thus restricting the angle to which the unit could be turned.
Able to be set over on its base by 10 mm in each direction for the turning of slight tapers, the tailstock had its 40 mm diameter, 80 mm travel, No. 3 Morse taper spindle locked by a collet arrangement on the front end - a design more commonly found on watchmakers' lathes and intended to completely eliminate the misalignment that occurs with an ordinary wedge-type or "split-casting" lock.
*d32 Threading Accessory. This was a unit used in place of the ordinary toolpost and in conjunction with the automatic saddle-reverse mechanism. It incorporated a semi-automatic feed where the operator only had to move the operating lever against a stop at each pass. To obtain an extra-fine finish, the overall rate of feed could be set by a rotary button, with the in-feed cam so arranged that the finishing cuts were carried out with a rate gradually reducing to zero..