Sliding-head lathes with 32+ mm capacity

7 min read

Steed Webzell charts the evolution of sliding-head lathes offering greater than 32 mm bar capacity, finding a potentially bright future for this relatively 'young' class of machine tool

Those with an eye to this technology area will have noticed that a number of sliding-head lathe manufacturers have released models in excess of 32 mm bar capacity over the course of the past year or so. The upshot is that any potential user of this technology today has a fair selection of machines over which to cast his or her discerning eye. Among those hoping to attract many admiring gazes is the ST-38 from Star Micronics GB (01332 864455), a 38 mm capacity machine with 12 CNC axes shared between two C-axis spindles and three live turrets – X and Z-axis rapids are 30 m/min. Here, a 3-axis and a 2-axis turret can perform combinations of turning and prismatic machining concurrently on round or hexagonal bar, rotating at up to 7,000 rpm in the main 11 kW spindle. Another 2-axis turret is devoted to simultaneous back-working on a parted-off component in the 7.5 kW/7,000 rpm counter spindle. Each turret has 10 tool stations driven by a 4 kW/5,700 rpm motor for carrying out prismatic cross-working and end-working operations. As each station can accept two tools, a total of 60 cutters can be resident in the machine. It has 350 mm of headstock stroke in Z, allowing parts up to this length to be produced in one chucking. Citizen Machinery UK (01923 691500) also has an offer in this area – the latest (fourth) generation of its top-of-the-range M32-VIII CNC sliding head can provide increased bar capacity to 35 mm. With this machine launch, a third Y-axis cross feed, with a nine-tool multi-head, is added to the back tool post, an additional B-axis and adjustable angle rotary tool device is fitted to the gang tool slide. Both spindles have been improved to 8,000 rpm and the speeds available for each driven tool increased with a much greater torque of 9.5 Nm available at the cutting tip, giving a higher rate of metal removal. The 10 station all-driven turret, with each position able to half-index and carry multiple tooling arrangements, has a 1.5 kW, 6,000 rpm motor and an extended Z-axis stroke of 212 mm. As the turret can be indexed at any position, without retraction to its return position, the index time is now within 0.5 seconds. It is the two-axis gang tool post that now carries an additional B-rotational slant axis able to provide simultaneous 4-axis continuous contouring in conjunction with the main spindle Z1-axis feed. It is programmable between -10° and +95° for producing angled features, using up to three driven tools. Image: Citizen's machine not only features 35 mm bar capacity, but also has B-axis New from the Tornos stable is the Delta 38/5, a 38 mm capacity machine with an 11 kW spindle and Tornos' innovative guide bush design to minimise bar remnants. The Delta 38 is available in two versions, both offering driven tooling possibilities and equipped with five linear axes and two C axes. It can be fitted with up to 31 tools, including a maximum of nine rotating tools. The spindle/guide bush, with integrated motor system, ensures a rigid assembly and reduced scrap length. Also featured are: increased collet gripping force to avoid problems with push-back when machining; large working and swarf area; and the ability to house existing guide bushes and collets used on current 32 mm models when machining below 32 mm. THE RATIONALE So what's the rationale for sliding-head lathes offering capacity over 32 mm? Well, according to Steve Totty, operations manager at Star GB, a big driver of growth has been the realisation among users that sliding-heads are not just for traditional long, slender part production, but also for machining short components of larger diameter, normally associated with fixed head mill-turning. "With the arrival of machines such as the Star ST-38, the range of sliding-head applications is set to increase," he says. "More companies, especially in the automotive, pneumatic and hydraulic sectors, will be able to take advantage of the fast cycle times, close tolerances and high productivity achievable on sliding-heads." Image: Star's 38 mm capacity ST-38 Geoff Bryant, managing director at Citizen Machinery UK, agrees, stating that Citizen's recent move to offer a 35 mm capacity option on its top-of-the-range M32 sliding-head has been largely user driven. "Traditionally, due to the machine configuration using a guide bush, the sliding-head concept was widely used for long slender parts using a single cut-to-depth with the interface between the cutting tool and the material being very close to the machine headstock," he says. "However, the versatility under NC then CNC technology has seen it also adopted for shorter parts." So why have machines above 32 mm capacity only arrived now and not years ago? Mr Totty says the sole reason for not going above 32 mm capacity in the past was to limit the inertia of the bar moving through the guide bush, bearing in mind that the stock is gripped in a collet, rather than more securely in a chuck, as on a fixed-head lathe. Moreover, magazines on sliding-heads are rarely of the short variety, but nearly always feed 3 or 4 m bars to achieve long periods of unattended running, so each length of stock is heavy. In Star GB's view, developments on many fronts over the past decade have made it possible to add another 6 mm to the maximum capacity, which adds 41% to the weight of any given bar. "First, the quality and straightness of stock has improved significantly, as bar is supplied from a smaller pool of reliable firms," says Mr Totty. "Secondly, smaller motors of higher power driving the axes of the ST-38 limit the additional size that the machine needs to be, in terms of footprint and weight. Bearings are also more capable, as regards limiting vibration, which is essential for achieving top machining accuracy and surface finish. Thirdly, modern tooling for sliding-head turn-milling is also playing its part. Heavy cuts can be taken in difficult materials, without stressing machine elements, causing vibration or imparting undue longitudinal force on the bar, which could otherwise cause it to slip in the collet." THE PATH TO PROGRESS Since 2007, Citizen has produced a range of machines to support different customer requirements, in terms of batch sizes and part complexity, so an increase in material size to 35 mm follows a progressive development programme. "This trend is important, as users have been requesting the ability to achieve the same advantages on bar sizes over 32 mm in tandem with machine developments, such as providing additional axes, tooling availability, reductions in non-cutting time and continuous control and software improvements," says Mr Bryant. Due to the configuration of the sliding-head machine, the distance between the guide bush and headstock is an important factor in determining bar capacity, as the material can influence machining performance and surface finish, due to vibration as the bar rotates. "Smaller diameter bar will often bend in transport, handling and storage, and even flex within the machine headstock," Mr Bryant explains. "However, smaller diameter material will often tend to straighten as it's progressively fed through the headstock, but, as bar size becomes greater, so its stiffness increases by a ratio equal to a factor of twice the diameter. This means significant engineering development had to be carried out to ensure process stability is maintained. For instance, to accommodate 35 mm bar, Citizen has strengthened the bar gripping mechanism and headstock, while the inbuilt rigidity of the machine provides a turn-mill platform matched to accommodate this size of material." Star's ST-38 also has features designed specifically to accommodate larger bar. For example, although modelled on the triple-turret ECAS-32T (now superseded), the new machine has been made nearly 10% heavier, at 6,250 kg, to withstand the higher forces produced. Also, a chiller unit for the coolant has been integrated, the rationale being that heavier cuts would otherwise generate more heat in the working area, leading to thermal growth. THE OUTCOME So, are all the development efforts paying dividends? Is there high uptake for sliding-heads over 32 mm capacity? "In the case of all Japanese-built machine tools, the strength of the yen has tended to suppress sales of higher end machines in European markets," says Mr Totty. "Star GB, along with other subsidiaries in the west, has been affected similarly. ST-38 sales in the UK match those in Europe, once the relative size of the markets is taken into account. In Mr Totty's view, mainstream uptake of Star's larger capacity sliding-head will come when the economy is fully recovered and manufacturers have greater confidence to invest in top-end machine tools. Incidentally, he thinks it unlikely that sliding-head lathes will be developed for handling stock of diameter larger than 38 mm, in the short term. A 4 m length of steel bar weighs around 35 kg. Rotating it at up to 7,000 rpm in an ST-38 and feeding it in an out of the machining area at 30 m/min during rapid traverse would seem to be close to the limit, given that the bar is clamped in a collet. He does foresee, however, 38 mm technology applied to other 32 mm sliding-head lathes in the Star range, notably the SV-32, which has a mixture of turret-mounted and platen tooling. John Stretton, UK account manager for Tornos, has his own thoughts on why sales of existing +32 mm capacity models have been sluggish. "In general, there is a lack of spindle power and collet gripping force, thus making it difficult to machine efficiently against 34 and 42 mm fixed-head models, which are also able to machine harder materials, such as stainless steel 316, with standard ISO cutting tools," he says. Other detracting factors cited by Mr Stretton include: small working area and swarf catching area; poor availability of collets and guide bushes larger than 32 mm; inability to run on soluble oil; and long bar ends, because of the sliding-head principle, thus equating to a material saving using fixed-heads. "It has always been the request of sliding-head customers to overcome the above issues," says Mr Stretton. "There are many parts produced from 34, 35 and 36 mm bar stock which are required for the automotive, hydraulic and fitting industry together with motor shaft requirements (long parts above 300 mm in length). Many of these components require heavy metal removal, both externally and internally. With the new Tornos Delta 38 mm machine launched at EMO last month, these points are now resolved." Image: Tornos' machine offers 38 mm bar capacity Box item Also available... Other large format sliding-head models currently available in the UK include the Maier range from Simon CNC Services (01246 224111), where the B, C, D and E series all offer 36 mm capacity in both main and sub-spindles, as do the higher specification F2 and F4 series machines. Tsugami is another manufacturer with +32 mm choices available, such as the BH38E – a 38 mm capacity model with opposed gang-tool slides and 12-station turret. Available from Warwick Machinery (01926 497806), it offers 34 tools and double-spindle configuration to allow for heavy cutting forces. Furthermore, the main spindle and guide bush are integrated into a single structure – hence emulsion type water coolant can be used, lowering the risk of oily smoke and fire. Remnant length is also reduced to minimum of 150 mm (plus component). The servo driven turret is indexed in 0.3 sec and mechanical shock is minimised by the use of a non-lift mechanism. Independent drive to the working power tool increases accuracy and lessens thermal distortion. First published in Machinery, October 2011