Roughing and finishing require different characteristics in a machining centre, which often means that two machines are used for the respective operations. However, this approach reduces production flexibility, increases capital investment and consumes extra space on the shop floor. It can also cause inaccuracies due to re-clamping and introduces the expense and risk of damage associated with work-in-progress.
Contract machinist, Horst Klein GmbH, has found a way around these problems by installing a Roeders RXU 1200 3-axis vertical machining centre. Having rigid axes and precise, dynamic linear drives, the machine is capable of both rough- and finish-machining of almost any job. Availability of Roeders equipment in the UK is through Hurco Europe, High Wycombe, sole agent for the German manufacturer.
Horst Klein produces moulds and dies for forging, punching, plastic injection moulding and aluminium die casting firms mainly supplying the automotive industry. Short and reliable delivery times for new moulds, competitive prices and prompt service and repair are essential.
The RXU machine was purchased primarily to reduce throughput times for the production of forging dies measuring approximately 500 x 600 x 350 mm, which had been a particular bottleneck for Horst Klein. The dies had usually been produced in the traditional manner on two different machines.
Roughing was performed on a robust machining centre using up to 66 mm diameter, indexable insert cutters. Stringent requirements with respect to precision and surface quality made it necessary to perform subsequent pre-finishing and finishing operations on a Roeders RP 800 HSC machining centre using ball nose cutters of between 3 and 6 mm diameter. A further reason for using the RP 800 was that the high dynamics of the linear drives allowed economical machining of the free-form surfaces.
Throughput times were typically 31.5 hours per mould half, irrespective of whether a new mould was being produced from solid or a worn die was being reworked. As the shop operated a single shift, it was impossible to produce more than three to four mould halves per week, despite great care being taken to extend machine utilisation by maximising unmanned operation overnight and at weekends. It was also nearly impossible to produce an urgent replacement mould in an acceptable time frame.
So at the end of 2011, the company decided to acquire a new RXU 1200 machining centre from Roeders, which was put into service in February 2012. A notable characteristic is the quadroguide design of the Z-axis, which features a rectangular cross section fitted with guide rails on all four corners. A total of eight carriages arranged in a cuboid pattern optimise transmission of forces from the spindle to the Y-axis, which itself is connected to the massive machine frame by another set of eight carriages.
The resulting tripling of the rigidity of the Z-axis enables the machine to tolerate very high cutting forces. The 51 kW milling spindle is correspondingly strong, yet reaches 30,000 rpm for finishing operations and is equipped with an HSK F63 tool interface. Linear scales, cooling circuits for thermal stabilisation and spindle growth sensing and CNC compensation help to ensure high accuracy machining.
Horst Klein, managing partner of the job shop in Velbert, commented, “After only a few trial passes it became obvious that, despite the smaller tool diameter, the RXU 1200 is able to rough material from a die at virtually the same speed as the older machining centre previously assigned to the task.
"Both machines typically need around three hours to complete the roughing of, for example, a forging die half made from DIN 1.2714 tool steel hardened to 48 HRC.
"Even when performing this extremely tough job, the new Roeders machine does not appear to be near its limits, but runs astonishingly smoothly."
"However, the real capability of the RXU is evident during subsequent operations - roughing of rest material, pre-finishing and finishing - where the machine's superior rigidity make it possible to virtually halve the machining times previously required, irrespective of material hardness. So overall, a mould half now takes around 19 hours to machine."
“Another important advantage is that, with total processing times of less than 24 hours, we can now fully use a ghost shift, allowing us to double the number of tool halves produced."
The advantages are not limited to the reduction in machining times, but also include simplified in-house logistics and less non-productive times, as with the RXU, a mould can be processed from a raw block to a finished mould half in one set-up.
Another noteworthy feature of the installation is that workpieces are held by magnetic tables. The position and orientation of the part are determined automatically using a touch probe in the spindle. The operator simply has to put the workpiece approximately in position on the table and close the machine door.
The 1,300 x 1,100 mm table equipped with two magnetic clamps allows two typical mould halves to be loaded. With a suitably populated tool magazine, over a weekend the machine can produce two finished mould halves from raw material, completely unmanned. Tool management and sister tool replacement are carried out automatically by the control system. Moreover, as the high rigidity of the RXU minimises vibration, tool life is extended dramatically.
Mr Horst continued, “We are in the process of optimising our existing machining programmes to take full advantage of the features of the new system and further reduce machining times.
"For example, the RXU can tolerate substantially higher jerk (rate of change of acceleration) than our older machines, which makes it possible to achieve substantially higher feed rates, especially in corners and other areas where abrupt changes in direction occur while the tool is in cut.
"We confidently expect that, once we have completed this optimisation, we will be able to produce nine mould halves per week.
“Looking at the dimensional accuracy, deviations seldom exceed more than a few microns and the quality of machined surfaces is excellent.
"We can now accept larger jobs with very short delivery times or squeeze in rush orders between current jobs.
"The machine has been such a success that we are already considering buying another RXU 1200."