The pressing of sheet metal parts in a pressing tool with a moveable and a fixed tool half is usually done in a number of successive stages. In the first stage, in which the basic shape of the part is created, it is desirable to optimise the flow of material in the pressing tool. The material flow is largely controlled by means of clamps and more precisely by the force with which the clamp applies the sheet metal against one of the tool halves.

In this context, applying the clamp by means of springs, such as gas-filled springs, arranged between the tool halves, or by means of an air cushion available on certain presses, is known in the art. A contact pressure is hereby obtained which increases more or less sharply with increasing compression of the springs/the air cushion.

US-A-5, 100, 113 discloses a pneumatically acting arrangement, which when pressing gives an application force, which varies non-linearly in the course of the pressing process. This known arrangement constitutes a part of the press, in which the press- ing tool is fitted in order to produce a desired sheet metal part. This means that with each change of pressing tool the application force gradient must be adjusted with reference to the current pressing tool. This adjustment requires skilled personnel and takes time, which could otherwise be spent in production. As examples of the prior art, reference is also made to DE 19521525 Al and US 4111030 A.

DE 19521525 Al relates to a system for applying hold-down forces in drawing dies for sheet metal forming by using nitrogen-filled cylinders, the working pressure chamber in each nitrogen gas cylinder being connected by way of a feed line to a pressure source. The technical problem is to produce a design for the nitrogen gas cylinder, which permits extensive, random changes in and variations of the curve for the hold-down force characteristic. The chamber of the nitrogen gas cylinder on the piston rod side is of gas-tight design and is connected to the feed line by way of a line that contains a throttle.

US 411 lO10 A relates to a press pad comprising a piston in a cylindrical chamber.

The head end of the chamber is designed to connect with a compressed fluid, such as nitrogen gas. The rod end of the cylinder chamber is designed to connect to a pure air source, preferably a low-pressure pure air feed.

The object of the present invention is to produce an improved arrangement of the type referred to in the introductory part. According to the invention this is achieved by the features specified in the characterising part of Claim 1.

A number of advantages ensue from the invention. Since the clamping function fol- lows each tool and does not have anything to do with the actual press, existing presses can be used without having to invest in the conversion of these. Nor is in- vestment in new presses required in order to achieve the required function. A further advantage is that the force distribution of the clamping function is well defined and predictable and in the main needs to be adjusted once.

The invention will be explained in more detail below with reference to the drawings attached, in which Figure 1 shows an example of the curve of a desired application force as a function of the movement of a moveable tool half. Figure 2 illustrates in diagrammatic form the construction of a clamping arrangement according to the in- vention, and Figure 3 is a diagram of the clamping arrangement acting in association with the tool halves. Figure 4 illustrates an alternative embodiment of the clamping arrangement in Figure 2.

In Figures 2 and 3,1 generally denotes a gas-filled spring comprising a cylinder 1 c, with ends la, lb. A piston 2, which is sealed off from the wall of the cylinder lc by a seal 3 and which divides the interior of the cylinder into two gas-filled chambers 4 and 5, is axially moveable in the cylinder 1 c. The piston 2 is connected to a piston rod 6, at the outer end of which a sheet metal clamp 7 is arranged. The piston rod 6 runs through an opening in the end la sealed off by a seal 8.

The chambers 4 and 5 are connected by means of a duct 9, into which a pressure- limiting valve 10 and in parallel therewith a reciprocating valve 11 are connected. In the duct 9, a non-return valve 12 is connected in parallel with the pressure-limiting valve 10 and the reciprocating valve 11. The pressure-limiting valve 10 can be ad- justed to open at different pressures depending on the desired application force curve.

In the embodiment shown in Figure 2 the opening pressure of the pressure-limiting

valve is determined by what gas pressure is applied to the valve. In order to prevent the occurrence in the upper chamber 5 of a pressure lower than atmospheric pressure, which may be damaging to the spring 1, the adjustment of the valve 10 is designed so that it can not be set to a higher opening pressure than the pressure prevailing in both chambers 4,5 of the spring before the operating cycle commences. In this position the same pressure prevails in both chambers. An alternative to this arrangement, for example, is to monitor and control the various pressure levels by means of pressure sensors, a solenoid valve and a control system.

The function of the non-return valve 12 is to allow the gas to flow from the upper chamber 5 to the lower chamber 4, should the pressure in the upper chamber 5 ex- ceed that in the lower chamber 4. This arrangement makes it possible for the spring to return even if the reciprocating valve 11 is kept closed, which may be desirable from the point of view of safety. This arrangement also permits a (virtually) total evacuation of the lower chamber 4 of the spring, even if the reciprocating valve 11 should be kept closed. This, too, may be desirable from the safety point of view.

Should it still be desirable to actively control the return travel of the spring 1, the non-return valve 12 is omitted, which means that the return travel of the spring 1 will be dependent upon control of the reciprocating valve 11.

Figure 3 shows a diagram of fixed and moveable tool halves 13 a and 13b respec- tively of a sheet metal forming tool. 14 denotes a block containing the pressure-lim- iting valve 10, the reciprocating valve 11 and the non-return valve 12.15 denotes a pneumatic limit position valve, which is designed, when acted upon by a part 16 connected to the tool half 13b, to control the reciprocating valve 11 by way of a line 17, in order to open/close the duct 9 between the chambers 4,5 at a predetermined point in time during the downward movement of the tool half 13b, in order thereby to influence the magnitude and curve of the application force, as will be explained later with reference to Figure 1. It will be obvious that there is a refilling fitting, not shown in the drawing and connectable to the spring 1, by means of which fitting gas is delivered to the spring at a pressure, which according to the invention lies in the interval 25-500 bar, preferably in the interval 70-400 bar. A pressure regulator is also advantageously connected to the spring 1 for setting a suitable pressure differential between the chambers 4 and 5 of the spring 1. According to the invention the gas is N2, in order that it will not react with lubricant present in the spring 1. An important characteristic of the invention will also be apparent from Figure 3, namely that the opposite end of the spring 1 to the sheet metal clamping arrangement 7 is applied

against the tool half 13a. It will be realised that the end of the spring 1 to which the sheet metal clamping arrangement 7 is to be fitted and the tool half to which the other end of the spring 1 is to be applied can be freely selected according to which is most suitable. It will also be realised that there can be multiple springs 1 between the tool halves 13a, 13b in order to achieve the desired application and predetermined mate- rial flow in the course of the pressing process.

Figure 4 shows an embodiment of the spring according to the invention, in which the components of the block denoted by 14 in Figures 2 and 3, that is the pressure-limit- ing valve 10, the reciprocating valve 11 and the non-return valve 12 are arranged at the bottom of the spring 18. The line 9 from the chamber 4 is a pipe, which is sealed off by a seal 2a from the piston 2, opens into an axial duct 6b connecting with the chamber 4 by way of bores 6a in the piston rod 6, and which is connected in parallel to the above-mentioned components. In terms of function, there is no difference be- tween the embodiments shown in Figures 2 and 4.

The function of the arrangement according to the invention will now be explained in more detail with reference to Figures 1, 2 and 3. Figure 1 shows spring force as a function of the time/travel of the moveable tool half 13a. In an initial position 0 the tool halves 13a, 13b are separated, but the spring 1 exerts a certain force Fo. When the moveable tool half 13a begins to move downwards, the spring force increases approximately linearly up to a value Fi determined by the pressure-limiting valve 10, at which, at a point in time tl, the pressure differential between the chambers 4,5 has attained the value set on the valve 10. The spring force Fi then increases slightly to the point in time t2, when the tool half 13a has been moved by the length of travel determined by the limit position valve 15 in Figure 3. As previously explained, the valve 15 causes the reciprocating valve 11 to open, so that the pressure differential between the chambers 4,5 is equalised. As a result, the spring force diminishes with a certain delay to a value F2, which has been attained at time t3, and thereafter increases somewhat.

As indicated by a dashed curve 19, it is naturally possible, if technical reasons con- nected with the pressing so dictate, to close the reciprocating valve 11 again during the pressing cycle, thereby restoring the spring force Fl. The person skilled in the art will also appreciate that by controlling the sensitivity of the pressure-limiting valve 10 by connecting up multiple pressure-limiting valves sensitive to different differen- tial pressures, it is possible to achieve further variations in the spring force.