No compromise on quality

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No compromise on quality

With the new compact Heckert machining centres for pallets of 400 and 500 mm, Starrag has successfully set new standards in terms of output and quality characteristics. A key factor of these machining centres is their high degree of rigidity, which is a consistent feature of the framework assemblies and one that is being continuously optimised in the test field. The end users benefit from increased productivity and high-quality machining results.

Almost two years have passed since Starrag presented its horizontal machining centres, which were newly developed from scratch – the Heckert L40 (line module for 400 mm pallets) and Heckert H50 (four-axis for 500 mm pallets). Thanks to the modular concept, further variants followed within a short period of time, such as the five-axis X40 / 45 and the five-axis, multi-functional T45.

During the development of this line, the Chemnitz-based company concentrated first and foremost on high productivity per unit area. This approach was clearly a success, as demonstrated by a reduction of as much as 30 % in the space requirement of the machines and a 15 % increase in productivity. Secondly, the developers paid close attention to achieving a consistently high quality of mechanical engineering – an asset that users of Heckert machines have appreciated for many years.

Quality characteristic: “Rigidity”

The new compact machining centres boast a high level of performance, process reliability and long-term precision. As the basic requirement for achieving these characteristics, a high degree of rigidity constitutes an integral component of the design concept. Rigidity is a crucial quality characteristic, particularly in framework assemblies – i.e. in machine beds, columns, tables and rotary swivelling units – and is necessary not only for heavy-duty cutting, but also for the processing of demanding materials and high-performance excavation of lightweight materials, such as those commonly used in aircraft construction.

Designers and technologists at the Chemnitz site worked together closely during the development of the new framework assemblies, capitalising on their many years of experience as well as modern analytical methods. Numerous calculations and experiments played a role in finding the perfect composition of rigidity, weight and cost.

A focus on framework assemblies

Throughout these experiments, the primary focus was on rigidity. However, rigidity was never looked at in isolation, but always in connection with damping and other parameters. In roughing processes, for instance, proper damping ensures that deflections can be brought to a standstill in no time at all. For this reason, the designers decided that a temperature-stable and vibration-resistant mineral-cast machine bed would be the perfect base for a machine with overall rigidity.

When it comes to designing the columns, differentiation is required to achieve perfect results every time. That is why Starrag offers a weight-optimised version for highly dynamic machining, while a stronger column with a composite construction was developed for heavy-duty cutting. The welded steel casing is filled with a special concrete that ensures improved damping.

The rotary swivelling unit is another crucial component of the five-axis versions (X and T) of the new Heckert machining centre. Here, the developers have successfully transferred the A-axis concept, which has been tried and tested in the HEC centres, into the compact new machines – with impressive results. The double symmetrical mounting lends the cradle optimum stability, supported by precise, high-performance circular axes and high-precision pallets, so that workpieces can be processed dynamically and with a high level of precision.

Even the tool and collet chuck were designed with rigidity in mind. If the machining centre is required for a high level of cutting performance, Starrag recommends the Heckert H55, X45 and T45 versions, which are equipped with the HSK-A100 or HSK-T100 tool holder as standard. This is because the large planar support provides the necessary rigid tool connection.

Optimisation in the test field

Especially with new machines, such as the horizontal Heckert machining centre, it is important that the prototype or pre-line model demonstrates that the combination of all the framework assemblies is able to deliver the expected result. The Starrag plant in Chemnitz has a test field that is perfectly equipped for such optimisations – not only with regards to the necessary measuring equipment, but first and foremost in terms of the specialists required to perform a wide range of test tasks and evaluations of properties.

From the fields of mechanics, electronics and control technology among others.

Klaus Frost, an expert in experimental quality analysis, describes the procedure as follows: “The first practical proof of efficiency must be demonstrated by the machine during various performance processes. In this example, the cutting performance of the main drive was tested as far as the shut-down limit. In the current example, when cutting tempered heat-treated steel (C45), our new Heckert T45 achieved a cut volume of 1,200 cc/min – 300 mm above the swivel axis and without having to cool down. This represents a considerable cut output of 45 kW”. Frost continues: “This test is only successful for us, however, if we achieve a perfect surface with a low level of tool wear. A cutting performance that can be achieved with technology is ultimately meaningless if the result does not meet expectations, or if the time saved is offset by tool costs that are so high as to no longer be economical”. To achieve a successful overall result, the machine must be able to offer a high level of rigidity, both when moving and when stationary. This means that, after the cutting performance test, more tests take place. Firstly, static measurements are carried out on the pre-line machine. The flexibility of the modules is determined using measuring stands and up to 20 gauges, with static forces measured in every direction. This very quickly indicates whether the simulation carried out beforehand was accurate. Just as important, but far more complex, is the test performed on the machine while it is moving. Using a hydraulic exciter, the machine is subjected to infinitely variable frequencies of up to 500 Hz. Klaus Frost explains the benefits of this measure: “We get a clear picture of all relevant vibrations that occur when operating the machine and gain an understanding of their significance for the subsequent cutting process. The results obtained from the quasi-static tests are essential. Using dynamic measurement methods, we are able to determine which components are statically deformed down to the level of individual parts”. During this process, Frost and his colleagues log the results of several hundred measuring points. After this analysis is performed, the construction is always redesigned to incorporate the findings into the upcoming series production, thereby improving the machine further. The pre-line machine then goes to the customer for industrial testing.

The optimisation process, during which the rigidity and thermal symmetry of Heckert machines are improved, extends from the plant in Chemnitz to other product ranges. This process provides a verifiable group-wide value, whether the products in question are assemblies and machine beds from the ECOFORCE centres in Mönchengladbach or STC machines from Rorschacherberg.

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