Method and apparatus for reducing cutting impact in a precision blanking press

Abstract

In a method and an apparatus for reducing the cutting impact in a hydraulically-driven precision blanking press, the force necessary to reduce the cutting impact as a counterforce is generated directly in the pressure chamber of the drive piston such that the counterforce acts directly on the cutting punch and so that the design of the press can be simplified, costs can be reduced, no additional external hydraulic-mechanical means for reducing the cutting impact are needed, and so that the loads on the press and the die can be reduced.

Claims

1. A method for reducing cutting impact in a precision blanking press equipped with a hydraulic main drive, wherein a main piston, which is guided inside of a main cylinder chamber of a base and which supports a table top, makes a stroke movement in rapid traverse mode between a lower dead point UT and an upper dead point OT and, in a power stroke, executes a cutting or shaping operation, wherein first and second pressure chambers of the main piston are acted upon by a working pressure of a hydraulic fluid from a hydraulic system, working pressure being predetermined by a central control system and generated by a hydraulic pump unit, the method comprising the following steps: a) detecting, with a path measuring unit, position of the main piston during the stroke movement thereof toward a fixed stop before the OT is reached, the path measuring unit being operatively associated with the main piston and being configured to acquire positional data of the main piston, and forward the data to a central control system for processing; b) continuously detecting with pressure sensors, working pressures in the first and second pressure chambers of the main piston, the first and second pressure chambers and the main piston being so configured that the first and second pressure chambers apply opposing forces to respective first and second working surfaces of the main piston, the first pressure chamber applying force to the first working surface of the main piston, toward the UT, and the second pressure chamber applying force to the second working surface, toward the OT, the pressure sensors being configured to detect the pressure values and send said values to the central control system; c) determining an increase in the working pressure and maximum pressure thereof in the second pressure chamber; d) determining a maximum force in the second pressure chamber from a product of the detected working pressure in the second pressure chamber and area of the second working surface of the main piston and measuring a decrease of said force; and e) adjusting the working pressure in the first pressure chamber with a tank valve associated with the first pressure chamber based on the determined maximum force and the decrease thereof according to step d), said adjustment being such that the working pressure in the first pressure chamber is increased to generate a force which counteracts the cutting impact as soon as the force maximum is exceeded and such that the working pressure in the first pressure chamber is maintained until the cutting process is finished.

2. The method according to claim 1, wherein the working pressure in the second pressure chamber of the main piston during the power stroke is adjusted using the central control system by way of a controllable proportional valve, at least one of the pressure sensors for pressure detection, at least one pressure limiting valve for limiting pressure of hydraulic fluid supplied to the second pressure chamber and a proportional valve configured to adjust flow volume of the hydraulic pump unit.

3. The method according to claim 1, further comprising adjusting position of the main piston with the central control system as the OT is approached by controllably reducing flow volume of the hydraulic pump unit before the OT is reached.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1, a schematic representation of the cutting force profile in relation to the cutting process according to the prior art;

(2) FIG. 2, a perspective view of a precision blanking press connected to the hydraulic system;

(3) FIG. 3, a perspective view of the base with table top;

(4) FIGS. 4a and 4b, perspective views of the base, showing the position of the fluid channels and the vent channel;

(5) FIG. 5, a section of the base with table top according to line A-A in FIG. 3; and

(6) FIG. 6 a schematic representation of the operational sequence of the method according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

(7) FIG. 1 shows the profile of the cutting force during shearing of a full-edged work piece according to the prior art (R.-A. Schmidt, Umformen and Feinschneiden (Forming and Precision Blanking), Carl Hanser-Verlag, 2007, pg. 144-157). As can be seen, the cutting process first passes through an area of elastic deformation of the material (Section I) and the press frame, in which the cutting force F.sub.S increases linearly without any plastic deformation of the material. As the punch penetrates further into the material, the material begins to flow plastically, wherein the cutting force F.sub.S continues to increase (Section III). In this area, two counteracting mechanisms affect the magnitude of the cutting force F.sub.S, namely the increase in cutting resistance due to cold-hardening of the material as the cutting punch penetration depth into the material increases, and the decrease in the cutting force F.sub.S due to the decrease in cross section of the rest of the material transferring the force. Until the maximum cutting force F.sub.Smax is reached, the cold-hardening portion dominates, and thereafter the decrease in the remaining cross section dominates so that the cutting force F.sub.S decreases again.

(8) The separation phase is characterized by the appearance of cracks in the material, which show that the capacity of the material to deform has been reached (Area III). The material breaks through and the cutting force F.sub.S drops abruptly, whereupon the cutting impact, in which the die and the press are suddenly released, takes place. The system (die, press) begins to oscillate in a manner similar to a released spring, the oscillatory characteristics depending on the resonant frequencies of the die and the press (Area IV). This is where the invention begins.

(9) FIG. 2 shows a perspective representation of a hydraulically-driven precision blanking press 1, the main drive 2 of which basically facilitating a stroke movement between a lower dead point UT and an upper dead point OT in the direction of the stroke axis HU upward from below. The press frame 3 of the press 1 comprises a top 4, a base 5, box-shaped hollow columns 6 and tie rods 7. The main drive 2 is supplied with hydraulic fluid by way of hydraulic lines 41 from a hydraulic system 24.

(10) As illustrated in FIG. 3, a table top 8 is disposed at the top side OS of the base 5, the table top supporting the bottom part of the die, which is not shown. Two opposing fast-acting cylinders 9 are disposed in the base 5 approximately in the middle, the cylinders being aligned parallel to the stroke axis HU, each of the cylinders having a fast-acting piston with a piston rod 10 connected to a support 11 which is attached to a side wall 12 of the table top 8, so that the table top 8 with the bottom part of the die can be raised in the OT direction or lowered in the UT direction in rapid traverse mode.

(11) FIGS. 4a, 4b and 5 show the spatial position of the fluid channels 19a though 19h and of the vent channel 33 in the base 5 in a transparent representation and in a sectional view according to line A-A of FIG. 3.

(12) A main cylinder chamber 13 is formed in the base 5, the axis HA of which lies on the stroke axis HU of the precision blanking press, and which holds the dual-acting main piston 14.

(13) The main piston 14 comprises a cylindrical shaft 15 which has upper and lower working surfaces 16a and 16b which protrude perpendicular to axis HA in the form of a discus edge, the surfaces subdividing the main cylinder chamber 13 into a first (upper) pressure chamber 17a and a second (lower) pressure chamber 17b with a short stroke height such that the base 5 is compact and has a low design height.

(14) The main cylinder chamber 13, and as a result the upper pressure chamber 17a, is sealed off pressure-tight by way of a cover 18 which is fastened to the base 5. The cover 18 is designed such that it also forms a fixed stop 42 for the working surface 16a at the upper dead point OT.

(15) First (upper) fluid channels 19a, 19b, 19c and 19d and second (lower) fluid channels 19e, 19f, 19g and 19h lead to the pressure chambers 17a and 17b of the main piston 14, the channels being disposed one atop of the other in the base 5 perpendicular to the stroke axis HU corresponding to the elevation of the pressure chambers 17a and 17b. Fluid channels 19a through 19d are connected to fluid channels 19e through 19h by way of one bypass channel 20, respectively.

(16) Moreover, a pressure-controlled proportional valve 21a, 21b, 21c and 21d is disposed in each of the second (lower) fluid channels 19e through 19h as a built-in valve, each of said valves closing the respective bypass channel 20 when the second pressure chamber 17b is acted upon by hydraulic fluid of a predetermined pressure. The lower fluid channel 19e is connected to a feed channel 23 that is disposed in the base 5 parallel to the stroke axis HU and a branch channel 25 which branches from the feed channel, the hydraulic system 24, which is not further illustrated, being connected to the branch channel.

(17) In addition, a vent channel 33 opens into the first pressure chamber 17a, with a tank valve 26 for opening and closing the vent channel 33 being disposed therein as a built-in valve. The tank valve 26 is in the open position when the hydraulic fluid located in the first pressure chamber 17a is displaced during the power stroke.

(18) When the pressure chamber 17b is acted upon by hydraulic fluid of a predetermined pressure via the branch channel 25 and the feed channel 23 with the first pressure chamber 17a vented via the vent channel 33 and the tank valve 26, the main piston 14 makes a corresponding stroke movement in the power stroke mode and initiates the cutting and forming process.

(19) A path measuring unit 28, for example in the form of an eddy current sensor, is provided for the main piston 14. The path measuring unit 28 can be disposed along the first (upper) pressure chamber 17a in the base 5. However, it is also possible to position the path measuring unit 28 along the shaft 15 without straying from the spirit of the invention. It is only important that the positional data for the main piston 14 can be continuously detected during the stroke movement thereof in the OT direction.

(20) The path measuring unit 28 transfers the positional data to the central control system 29 for further processing.

(21) The operational sequence of the method according to the invention is described with the aid of FIG. 6, which shows the hydraulic line 30 for the power stroke of the main piston 14 and the hydraulic line 31 for controlling the cutting impact.

(22) The hydraulic line 30 for the power stroke comprises a hydraulic pump unit 32 having at least one proportional valve for adjusting flow, at least one pressure regulating valve 34 for limiting the pressure of the flow stream and at least one pressure sensor 35 for pressure detection in order to limit the power output and forward the pressure value to the central control system 29 for purposes of operating the pressure limiting valve 34, a controlled built-in valve 36 for releasing the hydraulic fluid fed to the second (lower) pressure chamber 17b for purposes of the power stroke, a pressure sensor 37 for continuous detection of the working pressure P.sub.U in the second (lower) pressure chamber 17b and pressure chambers 17a and 17b of the main piston 14.

(23) The proportional valves 21a through 21d, which are disposed in the fluid channels 19e through 19h and which open or close the vent channel 33, are part of hydraulic line 31, as is a pressure sensor 38 which is associated with the first (upper) pressure chamber 17a and is used for continuously detecting the working pressure P.sub.O in the first (upper) pressure chamber 17a and for forwarding the detected pressure values to the central control system 29 for processing, an operational amplifier 39 connected both to the pressure sensor 37 for the second (lower) pressure chamber 17b and to the pressure sensor 38 for the first (upper) pressure chamber 17a, as well as to a pressure-controlled 4/3-way proportional valve 40 which holds a tank valve 26 open during the power stroke or through pressure regulation.

(24) The method according to the invention includes the following steps: a) detecting the position of the main piston 14 during the stroke movement thereof toward a fixed stop 42 long before the OT is reached, using a path measuring unit 28 associated with the main piston 14, the unit detecting the positional data of the main piston 14 and forwarding this data to the central control system 29 for processing; b) continuously detecting the working pressures P.sub.O and P.sub.U in the pressure chambers 17a and 17b of the main piston 14 by way of the pressure sensors 37 and 38, which detect the pressure values and send them to the central control system 29; c) determining the increase in working pressure P.sub.U and the maximum pressure thereof in the second pressure chamber 17b; d) establishing a maximum force in the second pressure chamber 17b from the product of the detected working pressure P.sub.U and a working surface 16b of the main piston 14, and measuring the decrease in said force; and e) adjusting the pressure in the first pressure chamber 17a by limiting the pressure of a tank valve (26) associated with the first pressure chamber based on the force maximum determined and the decrease thereof according to step d), said adjustment being such that the working pressure P.sub.O in the first pressure chamber 17a is increased accordingly to generate a force which counteracts the cutting impact as soon as the force maximum is exceeded and such that the pressure is maintained until the cutting process is finished.