Special report:
BLECH In Form 02/2006

Quality as a competitive factor

Hydraulic Presses

The more specialised the application, the more specialised the machinery. As simple as this wisdom may seem, that's just how sophisticated the consequences arising from it are for the design of the modern press. If precision forming processes can be designed, then additional processing, reworking or imposition can mostly be avoided. Such precision demands multi-dimensional thinking in order to design safe processes for wholly diverse requirements.

The typical
characteristic of presses and other forming machinery is its capability to ensure a precise, reproducible and stable forming process according to the specific range of requirements. This capability demands that the subsequent requirements are already taken into account as precisely as possible during development and construction of the system. For example it is generally recognised that with hydraulic systems, after applying the ram with the upper tool to the component, a pressure peak is created which can lead to defective processing for critical processes and sensitive components. In order to eliminate these defects, hydraulic presses can be equipped with an instantaneous pressing force controller which controls the pressing force throughout the forming process to within ± 0.5 percent of the nominal force, thus preventing pressure peaks during application. The production process is checked by automatic monitoring of all relevant process parameters in the press control system. Certain forming processes require precise positioning control, for example positioning and testing of high-precision helical springs. In this case the operator can freely select all ram positions on the press control system, which are then achieved by the machine to the appropriate accuracy. When testing and positioning the helical springs the force is additionally monitored, using a precision pressing force measuring system, so that the entire process can be operated and controlled by a concisely and simply arranged machinery control system.

 


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Always in position:
hydraulic presses for 
accurate positioning and 
testing of helical springs
Maintains a constant thickness:
this press stamps to a thickness
tolerance of less than 2 µm. During
the downstroke of the ram the 
workpiece is measured and the 
lower switching point is adapted to suit
Deals with off-centre loadings:
Press with hydraulic-electronic 
synchronisation control

 

Precision-stamping of control edges for all tolerances
When manufacturing servo-control components several control edges have to be stamped onto the upper and undersides of the workpieces to a depth tolerance of maximum 0.002 mm. The stamping depth is related to the applicable surface area of the workpiece. All workpieces for processing are prefabricated with a thickness tolerance of maximum 0.2 mm - a significantly larger tolerance than the permissible deviation in depth of the stamping. In this case, since the material is not of 100% uniform composition, a stamping process could be used that is only controlled according to the force. A special press and related stamping tool was designed for this purpose. The special feature of this machine is that the workpieces are measured during the downstroke of the ram and the lower switching point of the press is adapted to these parameters. The lower ram position, and thus the stamping depth, is determined by a solid positive stop which is automatically brought to the correct measurement by the press control system. The stop is arranged to the right and left of the tool and can be adjusted by means of an electric servo-drive. The force exerted on the workpiece is precisely defined so that the deflection of the machine remains reproducible. This means that the pressing ram, the stop device and the pressing bench form one system with a high degree of spring-stiffness. Elastic or temperature-dependent deformations in the machine tool frame or in the pressing cylinder have no effect on the stamping result.

All components support synchronisation control
For processes with extreme off-centre loadings and precision tools, parallelity must be guaranteed during processing in order to ensure a stable process and to minimise wearing of the tools. To this purpose hydraulic presses are often equipped with hydraulic-electronic synchronisation control. The special feature of one variant of this design principle is that control is not effected via the two main cylinders, but via the four cutting impact damping cylinders. These cylinders have a minimal fluid volume so that a high degree of control accuracy is achieved. That means that this is a passive control in the form of a superimposed position-pressure control. Since, on the related presses, progressive compound tools of up to 2 m length are used for very thin semi-finished components, the desired result can only be obtained if, in addition, any sideways displacement between the ram and the bench is prevented. This is in turn facilitated by a high-precision ram guide - to be precise a twelve-fold pre-tensioned roller guide - which has the advantage over a sliding guide that it does not need the same clearance between the press frame and the ram. Furthermore the press frame must be designed such that it supports the other "precision-trimmed" press components as well as possible. In order to achieve that, the assistance of FEM programmes can be called upon during the design phase.

 

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Precision and speed at the
same time:

Press with roll feed. The ram
is positioned with the aid of
a high-speed controller
Pressure on demand:
Press with variable drawing pad-internal ram

 

Additional rams guarantee precision
Precision processes often have to be implemented in combination with a high cycling rate. If accurate positioning is necessary at the lower switching point then it is often sufficient to have a positive stop on which the ram bottoms out. This provision can either be made in the tool or a motor-actuated adjustable positive stop can be provided in the press. - One solution that often represents the most cost efficient for many tools, particularly since the height can be easily adjusted using the press control system and the settings can be saved in the tool data set. If, however, intermediate positions have to be achieved with precision, a stop at the lower switching point will no longer suffice. In this case the ram is positioned using a quick-acting controller, so that both high precision as well as high cycling frequencies can be achieved. In many deep-drawing processes the drawing pad force can be changed according to the travel, so that an optimum drawing process is achieved. If, however, the difference between minimum and maximum drawing pad force is very great, a problem arises: For minimal drawing pad force the hydraulic system has to operate at extremely low hydraulic pressure. If the ratio of drawing force to nominal drawing pad force is too high and the resulting accuracy is insufficient, then you can only approach an optimum system status by the use of a special design. The accuracy of the drawing pad force must always be considered in dependence of the nominal pressure. If, for instance, the drawing pad force is less than 5% of the maximum possible drawing pad force, then process accuracy can not be guaranteed. You can remedy this by fitting an additional small ram in the drawing pad. Since a smaller ram area therefore becomes available, the hydraulic pressure is increased to a value that will permit a more accurate process. In order also to be able to change between smaller and greater forces during the process however, when switching over between the two ram surfaces, there must be no undefined status, no pressure peaks and no undesired reduction in drawing pad force created as a result. The hydraulic system must be designed in accordance with these switching-over conditions and must facilitate soft merging of the two systems.

Quality is the criteria for designing the press
It is self-evident that increasingly specific forming processes to varying requirements will affect the design of machine tools and will especially necessitate integration of quality assurance-related components. Therefore it is currently even more important than ever that the machine tool manufacturer is concerned with the production process and its requirements when consulting with the client and that these requirements are evaluated correctly and in sufficient detail. Apart from the reliability, maintainability, consequential costs, energy costs and cycling frequencies, the quality and construction of the machine tool have a significant influence on the production process itself. Ultimately, the quality of a press will be reflected in the quality of the product generated by using it.