The H series was the last in a line of exceptionally high quality toolroom lathes made by Holbrook. Introduced in the 1960s, it was visually like a giant Holbrook Minor, but entirely mechanical in conception.
The H Series range was initially introduced as the H13, H15 and H17, with 15 ½ ,17 ¼ and 20 inch swings respectively. The H13 and H15 were essentially the same lathe, with a raised swing for the larger machine. They were available with either 36 or 60 inches between centres. The H17 was larger in every dimension and available in 42 or 66 inches between centres or, to special oder, longer at intervals of 24 inches.
When Holbrook realised they were under-selling the capacity of their machines, they renamed them the H15, H17 and H20. These names will be used for reference in the rest of the article, as they are more commonly found.
All the H series lathes were very heavily built. The smallest machine, the short bed H15, had a D1-6 camlock spindle nose with a 2 1/8 spindle bore. It was 8ft long and weighed 2947kg. The largest, the long bed H20, had a D1-8 camlock spindle and 2 5/8 bore. It was 12 ft long and weighed 3982kg. Before transporting one of these lathes, it is worth noting that Holbrook had a habit of underestimating the mass of their machines: A long bed H20 could easily exceed 5 tons in reality.
The H series bed was exceptionally deep and wide, although narrower than earlier Holbrooks. The design of the bed was entirely new, with substantial diagonal cross bracing, carefully thought through to allow easy swarf dispersal into the cast base. The bed was hand scraped to match the base and the two could be considered one unit, being incredibly heavy and rigid. This is one lathe that is not top heavy, despite the size of the headstock.
The headstock of all the H series lathes was all geared with 16 speeds in geometrical progression and a helical final drive. All gears were made from alloy steel, hardened and profile ground for quiet and smooth running. As a result, a good H series is near silent when running. All headstocks had gears of 8DP. Three options of speed ranges were available, with the middle range the standard: 10-666rpm; 15-1000rpm and 22-1500rpm. Early models had a reversing clutch and brake, operated from the feed box or apron. Later models had a fwd clutch and brake, with the motor reversing. All models pulling up started the spindle. Gears were quickly and easily selected using a single lever at the top centre of the headstock front. The spindle, forged to massive proportions and incredibly thick walled, was supported by precision taper rollers at the front, with a floating rear bearing. It only carried 3 gears: the single helical drive gear and the two feed gears for forward and reverse. All gears were cascade lubricated using a pump that ran as soon as the motor was started. Oil was picked up from the bottom of the substantial sump and passed through a magnetic filter before being delivered to an oil tray at the top of the headstock. The tray had various holes and pipes to ensure delivery of oil to all the required places: nothing was forgotten. Excess oil overflowed the sides of the tray and cascade lubricated everything: a true belt and braces solution. An oil sight glass showed everything was working correctly. Due to the nature of this system any owner should be very careful to level the lathe before use, otherwise some oil passageways may not be covered in oil.
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Power was supplied to the headstock via a 7.5 HP motor on the H15 and 17 or 10HP on the H20. The optional 1500rpm top speed machines were fitted with 10 and 15HP motors respectively. All machines were fitted with 4 V belts driving the clutch pulley at 1330rpm. A micrometer switch prevents the motor from running with the change gear cover open and the integral electric cabinet at the tailstock end of the machine contains relays and overloads for the motor. All pushbuttons were mounted on the headstock in a convenient position near the spindle and facing the operator.
The feedbox of the lathe again consisted of hardened and profile ground alloy steel gears of 12DP on the H15 and 17 and 10DP on the H20. Change gears on the banjo were 14 and 12DP respectively. The feedbox was of an ingenious design, offering 60 imperial pitches from 1.25 to 76tpi and a range of standard metric leads from 0.25 to 9.5mm. It was totally enclosed and lubricated by a lever operated pump on the right of the box.
The interior of the box was largely conventional, except for the provision for metric threading. On all machines, one lever selected metric or imperial leads. It did so by reversing the direction of the feed through the entire gearbox. Standard lathes then needed a 127 gear fitted on the banjo, but many machines were optionally fitted with a special banjo incorporating a 127 that was automatically slid into mesh by the same lever. This gave instant, accurate metric threading for the majority of metric pitches. Lathes with this feature were recognisable by the threading chart on the front of the feedbox which contained metric pitches and was much wider as a result.
All H series lathes had single tooth dogclutch leadscrew reverse as standard, but operation from the apron, with automatic micrometer adjustable knock outs, was optional. It is easy to identify these lathes as they have a lever at the top right corner of the apron.
The apron and saddle of the H series were again of huge proportions, each weighing about 70kg on the smallest H15 model. The saddle on the smallest machine was about 2ft long and rode on a front V way and rear flat. The gearing in the apron was massive: 12DP on the H15 and 17 and 10DP on the H20. The apron and saddle, including V ways, halfnuts, crosslide and nut, were all lubricated using a hand operated pump on the lower corner of the apron. Early models had a lever on the side of the apron, later models a push button on the front. The oil was directed everywhere by carefully designed drillings. Feeds were operated using light action levers that were raised to engage with fingertip pressure. They incorporated automatic overload clutches and the longitudinal (sliding) feed was interlocked with the massive, bronze half nuts. On all models a micrometer adjustable threading stop was fitted as standard. When engaged it allowed precisely 2 turns of the crosslide screw and could be used for internal or external threading. The micrometer adjustment to the right side of the of the crosslide handle allowed advance of the tool in increments of 1 thou. Taper turning was optional and had to be fitted by the factory.
The tailstock was of equally huge proportions for the size of the machine. The H20 was fitted with a winding handle as standard. An engine crane is required to remove any tailstock and hence many handwheels get broken when someone foolishly tries to lift one by hand. All machines had 6 inches of spindle travel, graduated in 16ths of an inch with a micrometer dial on the handwheel. The H15 and 17 spindle was 2.5 inch diameter and the H20 3inches. All were fitted with MT4 tapers, which looked very small indeed! The upper half of the tailstock was dovetailed and taper gibbed to the lower half with Allen screws to provide set over for taper turning. All tailstocks clamped to the bed with a lever operated cam and the spindle was locked with a lever operating a split clamp.
The H series Holbrook lathes are arguably one of the finest and most useable lathes ever built with the power and rigidity to tackle almost any job. Finding the space for one and moving it to that space is not for the faint hearted, but once installed the effort is quickly forgotten. Very few H series lathes were ever made as their cost was astronomical, so finding one may be even harder than moving one. They are incredibly well made and hard wearing, using first class design and materials, quality controlled in house to the highest standards. The writer knows a few other H series owners, most of whom are astounded by the size, rigidity and quality..