email: tony@lathes.co.uk
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Holbrook Lathes - Model "Minor" (1960s)

- Holbrook Minor Continued on Page 2 -

An Instruction & Maintenance Manuals is available for the Minor

Holbrook Home Page

By the 1960s, when the new Holbrook "M" range of lathes was introduced, the company was in the hands of Herbert, at the time the United Kingdom's largest machine-tool maker and distributor. The "M" range consisted of the 5-inch centre height  "Minor" and three other (very rare) types, a 6.125" "Major", the almost identical 7.25-inch "Magna" and the 8.25" "Marquis". The 'Minor', with a capacity between centres of just 20 inches, was Herbert's smallest centre-lathe and neatly fitted under the market position occupied by makers of such high-quality machines as CVA, 'Dean, Smith & Grace, 'Schaublin', Weiler (etc.) and Holbrook's own, larger lathes. The "Minor" - not, perhaps, the most marketable of names - was designed for the "efficient production of small precision components particularly for the electrical and instrument manufacturing industries." Both "early" and "late" models were made, obviously as a result of feedback from customers, but these were only relatively minor changes that reflected a tidying up of some mechanical details, an improvement in potential reliability together with some easier and more certain operating procedures for the user. That the lathe was superbly built cannot be in doubt - one wag claiming that when the expensive-to-produce design was laid down the accountant must have been distracted by being locked in his office with a particularly-attractive secretary. Every part of this carefully contrived machine was built up to a standard, not down to a price, even prosaic covers being in abnormally thick material - that over the motor in half-inch thick cast aluminium and the headstock front-cover extension in 6 mm steel with a ground finish.
Of enormous depth, the V and flat-way bed was double-diagonal braced, hardened and ground and supported on a full-length cast-iron stand; being a toolroom-class machine no gap was offered.
Fitted with not only an infinitely-variable spindle-speed drive, the lathe also had independent variable-speed control to the power feeds - along similar lines to the arrangement found on the long-established and popular Hardinge HLV. This arrangement ensured that, whilst the initial settings of spindle speed and feed rate could be at the 'text-book' approved  levels, they could (if necessary) be changed by empirical experimentation until the perfect settings were arrived at for any particular combination of workpiece diameter, material and tool setting. (Another important advantage of this type of drive is that it reduces the number of gears involved in the transmission of power - gear drives are bad news for surface finish, they induce vibrations that, whilst usually hidden in the "roughness" of ordinary turning, become much more evident on the finer finishes achievable on high-quality lathes.) Unlike the Hardinge HLV, where controls were scattered inconveniently around the lathe, on the Holbrook everything was grouped closely together at the headstock end with all but the screwcutting gearbox levers mounted on a heavy, cast-aluminium panel that was, unfortunately, given such a beautifully-finished, stove-enamelled coating that it resembled shiny plastic. From this one panel the electric and mechanical controls were cleverly integrated so the machine could be started in either low or high range, with the brake stopping the spindle and gearbox but leaving the motor and variator running ready for an instant restart. As an alternative, the operator could choose to use a short joystick for spindle direction control - a forward or reverse movement of the lever producing a corresponding direction of spindle and feed-shaft rotation with, in addition, a braking effect produced when moved to the centre and a "spindle-free" position to the right. If the operator wished to bring everything to a halt, use of the joystick, rather than the other individual controls, caused the spindle, motor and variator to all stop at the same time. So convenient was this joystick arrangement, and so well did it work, that the makers had to caution against over-enthusiastic use, pointing out the damage that could be caused by overloading the electrical system with too frequent stopping, starting and reversing. Later models had the variator control wheel moved to the front of the stand where its isolation made for slightly safer and surer operation. Because the bed was so large, the motor to drive the power sliding and surfacing feeds was able to be mounted neatly out of sight inside it (at the headstock end) with its spindle connected directly to the screwcutting gearbox.
Isolated in a ventilated compartment inside the stand, the spindle-drive system consisted of a large-frame 3 hp, 900 rpm 3-phase motor flange-mounted to an Allspeeds  KOPP swash-plate variable-speed unit with drive by a flexible coupling to a 2-speed gearbox that incorporated separate electro-magnetic "Matrix" clutches (for engaging the high and low-speed ranges) together with a third Matrix unit adapted as a brake. All three units sat together on a massive cast-iron plate that could be adjusted to set the final drive belt tension. The speed range was either 80 to 3000 r.p.m. or, optionally, 80 to 4000 r.p.m. (though some lathes, in defiance of the catalogue specification, are actually marked 100 to 4000 r.p.m.). Mounted on the front face of the headstock was a beautifully clear tachometer with revolutions for forward and reverse shown in separate quadrants. Final drive to the forged-steel 1.125" bore,  Dl-4" nose headstock spindle (running in precision taper-roller bearings) was by, at first, multiple V-belts and later a 50-mm wide (10-v) heavy-duty  Poly-V type. If problems are encountered with the electrical controls on a Minor suspect ageing of the original selenium rectifiers: however, these are easily replaced by new solid-state types.
On early examples of the Minor oil for the screwcutting gearbox and headstock was lifted by a gear pump on the end of the main motor shaft from the base-mounted gearbox. Later machines had the oil supply rearranged with the base gearbox retaining its own lubricant (and presumably metal particles with which it would have become contaminated) and the pump that circulated oil between headstock and screwcutting gearbox driven from the drive to the large mechanical tachometer.
Continued below:

Mechanical art: the Holbrook 5" x 20" "Minor"

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Totally-enclosed and positively-lubricated, the gearbox was used only for threading and engaged by a push-in knob on the outer face the changewheel cover. By combining the rotation of two dials thirty different pitches of thread from 8 to 64 t.p.i. could be generated and, in addition, by operating an (optionally-fitted) "doubler" sixty became available. Little known is that inside the box was space to mount another seven gears - and this set, if fitted, would have given fifty-one pitches, or with the "doubler" in action, an astounding total of 102. As other threading ranges were also listed as being available to
special order and could be built into the machine from new, one can only surmise that the "missing" gears would have been fitted as required in case a customer required the box to generate a particular pitch. In addition, shown in the metric section of the Operator's manual, is an extra set of gears that do not appear to mesh with anything and whose function is, at the moment, a mystery. If required, additional special threads, or metric pitches, could be obtained (in the usual way) by mounting changewheels on a slotted and adjustable quadrant arm. Although not clear as to the exact positioning or method of operation (though it did involve micro-switches and electrical reverse) the makers claimed that "Micrometer-adjustable stops facilitate length control and allow the production of threads at high speed with accuracy".
Of double-wall construction, the oil-bath apron incorporated independent (safety) clutches to engage the sliding (along the bed) and surfacing (across the bed) power feeds; these could be quickly adjusted if the aluminium plate on the face of apron was removed. As a further refinement, a push-in knob on the apron controlled the selection of gears that allowed feeds 6-times slower than normal to be obtained. The carriage traverse handwheel, fitted with a micrometer dial, could be disengaged from its shaft by a control knob in its centre--a feature designed to eliminate the flywheel effect of the heavy handle and so improve the accuracy of delicate screwcutting operations. On early lathes, the rim of the handle was properly knurled for oil fingers to grip but later machines, with the escape of the accountant, looked to have had cheap, bought-in, components that lacked that genuine "engineering" look and feel.
The immensely rigid and wide "wingless" saddle slid along the bed on V and flat ways whilst the cross slide, unusually on this class of machine, carried two transverse T-slots that allowed the fitting of rear toolposts, milling slides, etc. When fitted with hydraulic copying, a plain cross slide was fitted with fittings at the back to carry the necessary unit. The zeroing micrometer dials were of a reasonable, if not large size, where one division represented 0.005" of tool travel; each was locked by a simple thumb screw passing through its rim. As an option, a dual inch/metric dial could be ordered. The top slide could be swivelled through 360 degrees but was fitted, as standard, with only a cheap "American-pattern" toolholder. As an option a quick-action, top-slide withdrawal mechanism could be ordered for fitting (as original-equipment) to help with high-speed screwcutting.
Attention to detail did not stop with the lathe's main elements, even the tailstock receiving some of Holbrook's legendary pedantic concerns with the 1.25-inch diameter, No. 2 Morse taper, 4-inch travel barrel hardened, ground, lapped, fitted with a micrometer collar and lubricated from an oil-bath complete with a dip stick.
An unusual accessory on a lathe already fitted with a screwcutting gearbox was a "thread chasing" attachment. This classic method of thread production relied upon a "Master Thread" or "Former" carried behind the headstock and a long "transmission bar" on which the cutting-tool slide pivoted and slid (the Master Thread was also variously known as a
hob, leader or former and available in various pitches). A "half-nut", held in the end of an arm connected to the "transmission rod", pressed on the thread and conveyed its pitch, via an adjustable toolholder, to the workpiece. The interconnection of the cutter holder and the half-nut allowed the nut to be lifted out of engagement and the cutting tool returned by hand to the start without stopping or reversing the lathe. A little additional depth of cut could then be applied by the tool slide or "stop screw", the half-nut rested back on the Master Thread - and the cut restarted. While this system is able to produce very fine and absolutely accurate threads, and is especially suited to delicate operations on thin-wall tubes used to construct such items as microscopes, the length of thread that can be cut, and its pitch, depends upon the availability of the appropriate thread master - although in the case of the Holbrook additional gearing was provided to extend the threading range of each Master Thread by several multiples.
Available in both English and "all-metric" versions, in 1964 a Government Missile Development Group was quoted just over £1300 for a machine with basic equipment and 3 and 4-jaw chucks; Today (2019) that would equate to nearly £34,000.
Although mounted on a wide and stable stand the "Minor", due to its very heavy construction, is a top-heavy machine and great care needs to be exercised when moving it. If you assume it will, at the slightest provocation, fall over - and then take the necessary precautions - all should be well.
Approximately 5-feet 6-inches (1676-mm) long, 2-feet 5-inches (737-mm) deep and 3-feet 11-inches (1194-mm) high, the lathe weighed, with standard equipment, around 1680 lbs (760 kg.)..  Photographs continued on Page 2

Headstock Controls.
Almost certainly another pre-production picture showing a rev counter with only one scale - productions versions showed revolutions in both forward and reverse in separate quadrants. 

Apron with clutched selection of power sliding and surfacing and, at the bottom in the middle, the push-pull knob by which the rate of feed could be varied by a 6 : 1 ratio. Notice the immensely rigid, wingless saddle with the casting extended to its full width both to the left and right of the cross slide.

Chase Screwcutting Attachment - an unusual fitting to find on a lathe already equipped with a full screwcutting gearbox.
In the "Chase" of screwcutting a rear mounted holder carried a "Master Thread" - tucked up underneath the headstock - and a long "transmission bar" on which the cutting-tool slide pivoted and slid. The Master Thread, also known as a
hob, leader or former, was available in various pitches.
A "half-nut", held in the end of an arm connected to the "transmission rod", pressed on the thread and conveyed its pitch, via an adjustable toolholder, to the workpiece. The interconnection of the cutter holder and the half nut allowed the nut to be lifted out of engagement and the cutting tool returned by hand to the start without stopping or reversing the lathe. A little additional depth of cut could then be applied by the tool slide or "stop screw", the half-nut rested back on the Master Thread - and the cut restarted. While this system produced was able to produce very accurate and fine-pitch threads - and was especially suited to delicate operations on thin-wall tubes used to construct items as such microscopes - the length that could be screwcut, and the number of threads per inch or mm, depended upon the availability of the appropriate thread master - although in the case of the Holbrook additional gearing was provided to extend the threading range of each
Master Thread by several multiples. 
A very simple form of this screwcutting mechanism, which shows very clearly the layout and method of operation, can be seen on the Goodell-Pratt Pages.

- Holbrook Minor Continued on Page 2 -

An Instruction & Maintenance Manuals is available for the Minor

Holbrook Home Page

Holbrook Lathes - Model "Minor" (1960s)
email: tony@lathes.co.uk
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