Manufactured by the Ruttner Company in Zürich, Switzerland, the GRAL lathe was, for its size, of a most usual and patented design. Of the "sliding" type, its bed gave both an increase the capacity between centres while also providing a deep and variable-width gap. The lathe was similar in execution to other sliding-bed lathes including the much larger American Harrington and Smith-Drum and the British Stanley in its interesting early and late forms.
The patent description - they are never the clearest and repeat facts endlessly - translated from the German and paraphrased with additional details reads:
Lead Screw Lathe
"The present invention relates to a screwcutting lathe with a leadscrew. Lathes are already known in which a gap is provided in the bed that makes it possible to machine a diameter that exceeds the centre height of the lathe. Although a lift-out bed section is often used to bridge the gap and allow the carriage to pass over it, the design can impair the precision and rigidity of the lathe and lead to inaccurate machining.
This invention is characterized by a bed designed in two parts, consisting of an upper and a lower section. The upper bed, that carries the carriage and a No. 1 Morse taper tailstock, can be displaced relative to the lower bed to form a variable-width gap
The lathe is provided with a lower bed (1) which has a gap (2). The headstock (which accepts W20 Schaublin collets) is (3) and the 3-jaw chuck (4). The headstock is carried on the lower bed (1). The usual type of V-belt driven countershaft drive system (5) is arranged at the back of the headstock.
The lower bed (1) carries an upper bed (6) on which is mounted the carriage (7) and the No.1 Morse taper tailstock (9). This upper bed can be slid left to right to open a gap and extend the between-centres capacity and then locked in the required position by clamps. A screwcutting leadscrew (8), is mounted on the upper bed (6) and drives the carriage (7). The leadscrew (8) is driven via a train of gears under a cover (10) using an extendable shaft (11) that passes through the lower bed (1) to connect to the screwcutting changewheels, these being fitted, in the normal position, on the outside face of the headstock.
By shifting the upper bed to the position shown in the patent drawing, the gap (2) is exposed and the centre height of the lathe increased."
The bed carrying the carriage and tailstock was arranged with a separate V and flat way for each, these both appearing to have been hand-scraped in the usual Swiss manner of seeking a perfect fit. Although screwcutting, the lathe lacked a gear-driven slow-speed backgear, the V-belt-drive countershaft compensating, to some extent, by having a 2-step drive by a Z-section V-belt from the motor pulley. Final drive was by an A-section V-belt to a pulley overhung on the left-hand end of the headstock spindle, a design economical in execution and allowing the headstock spindle to be supported by two relatively closely-spaced bearings. The drive arrangement provided four low and four high speeds and, if fitted with the usual 1400 r.p.m. motor, the writer would guess that the speed range ran from around 95 r.p.m. through 150, 205, 275, 380, 590, 820 to a top speed of 1100 rpm - a range that might be criticised for being rather too fast at the bottom end, yet not quite fast enough at the top.
Although no backgearing was fitted, the lathe did have a large gear just inboard of the spindle drive pulley; this drove a smaller gear carried on a shaft enclosed in a housing on top of the headstock. The shaft emerged to the left-hand side and, on all the examples of the lathe so far discovered, not guarded in any way. The purpose of this fitting is not known.
The drive system posed three problems for the designer: being able to slacken belts to change speed, allowing for the fact that the two- step drive from the motor would not have the same belt tension in each speed - and how to adjust, separately, the tension of the motor-to-countershaft and the final drive belts. It was solved the countershaft being hinged from the back of the headstock and pulled towards it by a pair of strong springs. The motor was bolted to a pair of sliding rails, these being pushed backwards by a screw thread (connected to a small handle on the face of the headstock) against the spring pressure - the pictures below should make the design clear. This simple but effective system is believed to have been unique to this lathe
Covered by a swing-open guard in cast aluminium, instead of the expected 12 or so changewheels, a generous set of 18 was provided - and tumble-reverse included in the drive. Connection to the leadscrew was by a shaft - arranged to extend as the bed was slid open - that passed beneath the lower bed to a train of gears arranged under a cover on the tailstock end of the bed.
In the manner of some Emco lathes, the cross slide was machined with two transverse T-slots, these allowing the swivelling top slide to be positioned at will. Both top and cross slides were fitted with unusually large micrometer dials- clearly, the designer was somebody who used a small lathe and realised the importance of this feature.
If you have a GRAL lathe or any literature about them, the writer would be pleased to hear from you.
Wenn Sie eine GRAL-Drehmaschine oder Literatur darüber haben, würde sich der Autor freuen, von Ihnen zu hören
Si vous avez un tour GRAL, ou toute documentation à leur sujet, l'auteur serait heureux d'avoir de vos nouvelles..