Special report:
BLECH In Form 01/2004

Versatility as a plus in modern press manufacture

A company that wants to subsist in the tight market of hydraulic presses must be able to elaborate solutions exceeding what is usual. Such solutions lend themselves in the field of process integration. The example of a custom-made hydraulic gap press elucidates how modern press manufacturers align new ideas with time-tested design concepts thus giving their machines a versatility not known so far in sheet metal processing.

 


Figure 1, Jack of all the trades:

Hydraulic 1 MN gap press for forming and separating processes from deep-drawing and punching up to deburring

Users of press technology are nowadays more than ever confronted with one fundamental economic problem: If they must invest in new machine tools for the production of innovative products but these new resources are not completely utilized, then amortization of this investment is rather difficult. An equally serious imponderability is that it cannot be exactly anticipated how long these products will maintain their ground on the market and/or when new or revised workpieces must be manufactured in addition. In order to avoid outsourcing of the products to external service providers and nonetheless be in the position to invest in efficient machine tools, the market nowadays offers universal hydraulic presses that are able to manufacture a wide range of products or unite several work processes in one press.

 

 

Numerous procedures technically realized in a press

As far as the application described hereafter is concerned, the customer's requirement was to perform different work processes with one only machine tool. The user was looking for a universal deep-drawing press that not only realizes the classic forming technique but should also be able to punch, bend, stamp, size, straighten, insert, border, cut and deburr. Because of the great variety of parts the component parts should be loaded manually. Thereupon, the press manufacturer Röcher, Netphen-Deuz, designed a hydraulic open front press that exactly meets the defined requirements (figure 1). The machine tool with the designation – REP 100.01 SZ – has a maximum pressing force of 1,000 kN and a maximum drawing force of 400 kN. The manufacturer regarded these two technical parameters as most important characteristic values of the performance of the machine tool to be sufficient for the planned product spectrum.

 


Figure 2. Perfect guidance:
Bronze strips that move on hardened tracks hold the ram in its correct position (guided press carriage principle)

In most designs the active, movable components of a hydraulic open front press are guided by the cylinder and by additional round guidance assemblies. Especially in case of presses with long strokes, this may have an adverse effect, as it is difficult for the guide rods to compensate eccentric loads in the lower stroke range. In order to avoid this disadvantage, the design of this universal press is based on a time-tested basic machine version with guided press carriage. As regards this version, the ram glides on bronze strips which for their part move on hardened tracks (figure 2). Bending up can be reduced and possible eccentric loads absorbed by these guidance assemblies and a robust, rigid frame construction.


 

Difficult processes well in hand with optimized parameters

In order to remove the workpieces easily and nevertheless efficiently from the workspace, a hydraulically controlled part ejector is integrated in the ram that is driven by a separate drive pump. The advantage of separate drive pumps for the individual hydraulic functions is a higher precision and a clearer hydraulic control than is the case with one-piece units.

 


Figure 3. Process at a glance:

The force behaviour during processing is represented in form of a force-way-diagram.

All forces and positions can be preselected via the machine control. The positions are optionally programmable with a teach-in function. The pressing and drawing forces are degressively and progressively variable via the stroke for best possible arrangement of the drawing processes under optimized conditions. Consequently, even difficult drawing processes can be realized with parameters tuned to each other. In order to control the process, the force behaviour is displayed in form of a force-way-diagram on the screen (figure 3). Beyond this, it is possible to document all screen contents using a commercial printer.

If a machine tool shall be used flexibly and economically, two criteria are above all decisive: On the one hand the ram speed must be sufficiently high, and on the other hand the setting times must be reduced to a minimum. In order to meet the second part of these requirements, great importance was attached to the tool management when designing the universal press. So all tool-specific machine parameters are stored under a tool name or tool number. Each tool must only be adjusted once. When using the tool several times, the data is then retrieved from the memory of the control. This minimizes setting times and avoids part errors.

 


Figure 4. Flexible:
During punching, the drawing pad can be moved back hydraulically thus clearing the pass-through hole

 

Cold or hot forming is possible

In order to ensure that the punching processes are performed environment-friendly and with high quality, the machine tool is equipped with a hydraulic snap-through dampening. When performing punching work, the drawing pad arranged underneath the machine table can be moved back hydraulically so that the pass-through hole is clear during punching, and a chute, collecting bin or conveyor belt can be placed under this hole (figure 4). As the forced sequence of a process not always permits the maximum working speed, the press was so laid out that the speed can be infinitely preselected via the control.

This is especially important in case of deep-drawing or bending processes. Straightening is accomplished using a sensitive electronic hand lever control. Depending on the movement of the lever, the cam either moves down or up. To accomplish this, low speed values are provided so that the operator is in the position to carry out the process sufficiently exact. The ram speed is controlled proportionally to the hand lever movement.

 


Figure 5. Keep cool:

A thermal protection plate with additional cooling (at the bottom of the ram) makes the press a work tool for hot forming.

A further advantage of the universal press: It is laid out for hot and cold forming. For hot forming, it is only necessary to fit a thermal protection plate with additional water cooling to the ram (figure 5). In order to ensure the exact positioning of the ram, the manufacturer of the universal press has not fitted depth stops that are frequently used by press operators and need to be adjusted accordingly when making or reworking tools. In the universal press the fixed stops are replaced by a stroke limitation (figure 6) integrated in the cylinder and adjustable via a motor. The position of the mechanical stroke limitation can be preselected via the press control.



Figure 6. Rapidly positioned:

The stroke limitation with motor adjustment in the cylinder replaces fixed stops.

For loading operations, the operating modes can be activated with the dual palm button control, light curtain, with or without jogging operation or pedal switch. Although the press owner presently uses the machine only in the manual loading mode, the automatic operating mode is nevertheless prepared so that automation is possible later on.

The electric control is based on a "Siemens Simatic S7" and is operated by means of an industrial PC "Siemens 'PC 670". All standard functions exist, e.g. preselection of the positions, preselection of the forces, force or position dependent changeover, force holding times, pressure relief control for tools with spring pad, high-speed feature or stroke counter. A steplessly acting mechanical ram locking aims at maximum safety in case of possible service work on the tool.

As in case of failure of universal core machines no replacement machines are available, a robust, durable design is of great importance. For this reason, the universal press described here is equipped with a compact hydraulic unit. In the event of a failure, the control transmits an error diagnosis so that the defect can be quickly localized. Alternatively, the machine tool can be completed with a remote service connection to the manufacturer so that a rapid error diagnosis through specialists is possible. The short-term availability of the manufacturer's service staff is a prerequisite for exhausting the profitability resources of such a hydraulic press.


 

Processing in a whisper is the declared target

The noise level of machine tools for the processing of sheet metal is nowadays an import factor for the assessment of a press. Hydraulic systems are often a disturbance, especially when a machine is located in a space where many persons are permanently staying but the noise level is otherwise not high. Special measures taken for sound insulation on the press reduce the undesired disturbance variables. Use of hydraulic pumps with low pulsation flow, a hydraulic pump arrangement outside the hydraulic oil tank and sound insulating measures (figure 7) reduce e.g. the noise level to a minimum.



Figure 7. Please be quiet:

The arrangement of the pump outside the hydraulic oil tank is only one of the measures taken for sound reduction.

In summary it may be said that hydraulic presses for versatile use can provide considerable potentials to the user in order to improve the economic efficiency of his sheet processing processes. A machine concept as the one described offers numerous future possibilities to manufacture supplied parts of great variety and with high quality requirements in a safe process. This ensures long-term utilization of such investment intensive machine tools.