Cutting press

Abstract

 The movement of the cutting head (18) is monitored by a linear potentiometer (LP) whereby the "stroke termination" position (die height) can be determined as one output value of the potentiometer and the "stroke termination signal" position can be determined from it by subtracting a difference value (ET) (representative of the characteristics of the press and in particular of its hydraulic circuitry) which is determined in a calibrating operation of the press. Once the difference value (ET) has been established, the "stroke termination signal" position is calculated each time the machine is set for a given die height. The depth of penetration of the die into the cutting pad can be separately adjusted by the operator, if desired, using a display (D1) and increment/decrement button switches (B11, B12). A similar system (D2, B21, B22) is provided for setting the "daylight", together with a system for calibrating the press for the return stroke independently of the cutting stroke. The press includes microprocessor control means.

Description

[0001] This invention is concerned with a cutting press comprising two platens constituting a cutting head and a cutting bed, drive means for effecting relative movement of approach between the platens thus to cause an operating stroke of the press to take place, and stroke control means, comprising a transducer device the output of which has a value which varies according to and proportionately with the distance between the platens, comparator means for comparing the output value with a comparison value, and signalling means operable to supply a "stroke termination" signal to the drive means when, in effecting an operating stroke, the output and comparison values match, wherein, in a stroke-setting mode of operation, the drive means causes relative movement of approach to take place between the platens under a reduced pressure and with the signalling means disabled, whereby for a given cutting die positioned between the platens the stroke termination position for that die can be determined.

[0002] In one such press (see GB-A2099750) the "stroke termination" signal is supplied to the drive means at the stroke termination position, as determined in the stroke-­setting mode of operation of the press. It will, however, be appreciated that the drive means does not cease its operation immediately upon the signal being supplied, bearing in mind the reaction times of valves and indeed the inertia of the system, as well as taking into account beam deflection or distortion when cutting under a load, so that the press further requires operator-actuatable setting means for varying the incidence of the "stroke termination" signal in order to produce the correct cutting effect. In general, furthermore, the desired setting is a matter of trial and error for each die, and in any event the machine has no clearly defined datum value (or "nil" position) for the setting means, which would enable the operator to set the stroke termination signal position according to previously determined settings.

[0003] It will of course be appreciated that, especially where a large number of cutting dies is being used in the course of a working shift, the need to set the stroke termination position empirically for such cutting dies is time-consuming and tedious, quite apart from any detrimental effect (in terms of material being cut) created by the need to set the stroke termination position by trial and error.

[0004] A further disadvantage of currently available cutting presses resides in that even where it may have been possible to achieve some regulation of the settings required for each cutting die in a given press, e.g. by the operator becoming accustomed to particular settings on a scale therefor, such settings become irrelevant if any maintenance work is carried out on the cutting press, thereby altering its characteristics, and moreover the settings applicable for any one press cannot be utilised on another press.

[0005] It is the object of the present invention to provide an improved cutting press wherein the setting operation is simplified such that, once a setting has been determined for a given die, that setting can be recalled reliably each time the die is used, regardless of the setting for any die previously used, and indeed regardless of change to the machine characteristics.

[0006] This object is resolved in accordance with the present invention, in a press as set out in the first paragraph above, by the provision of manually operable calibrating means whereby, in a calibrating mode of operation of the press, in which an operating stroke is effected at normal cutting pressure with a suitable cutting die positioned between the platens, the comparison value for that die can be varied in relation to the stroke termination position for that die, and by the provision of processor means which stores as a difference value the difference between the comparison value and the output value corresponding to the stroke termination position and, in any subsequent stroke-setting operation for a cutting die, applies the difference value to the output value corresponding to the stroke termination position of that die, thus to determine and set the appropriate comparison value for that die.

[0007] It will be appreciated that, by the operating stroke being effected, in the calibrating mode, at normal cutting pressure, the conditions of a cutting stroke are reproduced, so that in the press in accordance with the invention calibration takes place effectively under normal operating conditions and the machine characteristics which are exhibited during such normal operating conditions are thus taken into account in the calibrating mode. This is in contra-distinction to conventional stroke-setting arrangements, wherein the stroke termination position is determined under a reduced pressure. Furthermore, although the determination of the comparison value for the cutting die selected is a matter of trial and error, once the difference value for that die has been determined, effectively a datum value is fixed which is based upon the specific machine characteristics and which can then be applied to individual settings for individual dies.

[0008] In carrying out a calibration of the press, conveniently the engineer selects a cutting die having a small cutting length as the "calibration" tool; such a small cutting die will, for a given applied load, be driven further into the cutting pad than a cutting die of a longer cutting length. Similarly, the calibration tool is preferably located at a position of minimum deflection between the platens; in the case of a swing beam press such location is closely adjacent the column, while in a multi-pillar press of the type described e.g. in GB-A2099750 such location will be in the centre of the platens, at which location the distance between the platens is "measured" by the transducer devices. At such a location also, it will be appreciated, the die is likely to be driven more deeply into the cutting pad, for a given load, than in locations of greater deflection.

[0009] As already mentioned, the determination of the difference value as described above is a matter of trial and error, and consequently the engineer may effect a number of calibrating strokes before being satisfied that he has achieved the optimum position for the comparison value. When he is satisfied with the cutting conditions achieved, and thus with the difference value determined, the latter should be stored in the processor means and to this end preferably means is provided, operable in the calibrating mode of operation, for causing said difference value to be so stored.

[0010] The press in accordance with the invention preferably also comprises operator-actuatable setting means effective to enable the operator, without modifying the difference value as stored, to vary the comparison value for any die by applying an adjustment value to the comparison value. Thus, the difference value as stored remains as a datum value quite apart from any setting for an individual die.

[0011] Furthermore, for facilitating the setting for an individual cutting die, preferably a display is provided by which the adjustment value is indicated. In this way, the operator can readily make a setting repeatedly, once it has been determined, merely by reference to the display, thereby avoiding any further trial and error for setting to individual dies; what is more, such settings will be applicable not only when the press is in the condition in which the settings were made, but also after any maintenance or service thereof which can affect the machine characteristics, and indeed in other presses which have been similarly calibrated.

[0012] In practice, since, as already mentioned above, the penetration depth is likely to be at a maximum for the calibration tool, the calibration procedure is intended to ensure that any individual setting for a die will be to increase the penetration depth, rather than decrease it. Thus, conveniently the operator-actuatable setting means is not required to enable both positive and negative adjustment values to be applied as aforesaid, but only positive values. Thus, quite apart from minimising any damage to the cutting pad which could arise where a cutting die is driven too far thereinto, the setting means need only be of simple design and, used in conjunction with the display, can be handled without the possibility of operator error due to confusion as to whether the value displayed is positive or negative. Conveniently when a new setting has been made for a die as aforesaid, the operator has to confirm the setting before it becomes operative, and to this end preferably means is provided, operable other than in the calibrating mode of operation of the press, for causing such adjustment value to be stored by the processor means. Such adjustment value remains stored until it is varied by the use of the operator-actuatable setting means and a new adjustment value is stored in its place.

[0013] Conventionally in cutting presses, see again e.g. GB-A2099750, the drive means is also effective to cause relative movement of separation to take place between the platens, thus to cause a return stroke of the press to take place, and the comparator means of the stroke control means serves also to compare the output value with a further comparison value, the signalling means being operable to supply a "return stroke termination" signal to the drive means when, in effecting a return stroke, the output and further comparison values match, and further operator-actuatable setting means being provided for setting a return stroke termination position.

[0014] It may be desirable in such machines to provide a similar facility for setting the latter position accurately and consistently in the press, regardless of any variation of the machine performance characteristics which may arise due to servicing or maintenance, and indeed in other presses of the same type. To this end, therefore, in accordance with the present invention conveniently further manually operable calibrating means is provided whereby, in the calibrating mode of operation, with a cutting die positioned between the platens, the further comparison value can be varied in relation to the return stroke termination position as set, the arrangement being such that the processor means stores as a further difference value the difference between the further comparison value and the output value corresponding to the return stroke termination position and, in any subsequent stroke-setting operation for a cutting die, applies said further difference value to the output value corresponding to the return stroke termination position of the die, thus to determine and set the appropriate further comparison value for that die. As in the case of the calibration of the stroke termination position, furthermore, the means for causing a difference value to be stored is also effective for causing a further difference value to be stored by the processor means.

[0015] It will thus be appreciated that, by this arrangement, the return stroke termination position, and thus the position of maximum daylight between the platens can be accurately and indeed consistently determined.

[0016] Moreover, the further difference value again provides a datum value on the basis of which the operator can set the position of maximum daylight for any given die and, once determined, can return to that position reliably not only in the press in the condition in which the original setting was made, but also in the press after maintenance or service has been carried out (followed of course by re-calibration) and indeed in other calibrated presses of the same type.

[0017] It will thus be seen that the press in accordance with the invention as described above has the facility for close control both of the stroke termination position and, if desired, the return stroke termination position.

[0018] In another of its several aspects the invention provides a machine as set out in the first paragraph above, which machine is characterised by manual setting means, operable in a calibrating mode of operation, for enabling the distance to be determined through which, in an operating stroke at normal cutting pressure, the relative movement of approach continues between the platens after the signalling means has operated as aforesaid, such that, with a suitable cutting die placed between the platens, the operating stroke is terminated at the stroke termination position previously determined for that die, and by processor means whereby such distance is stored as a difference value and such difference value is thereafter applied, when a cutting die is presented to the press in a stroke-setting operation, to the output value corresponding to the stroke termination position of that die as determined in such operation, thus to determine and set the appropriate comparison value for that die.

[0019] There now follows a detailed description, to be read with reference to the accompanying drawings, of one cutting press in accordance with the invention. It will be appreciated that this cutting press has been selected for description merely by way of non-limiting example of the invention.

[0020] In the accompanying drawings:-

Fig. 1 is a side view of the cutting press in accordance with the invention;

Fig. 2 is a view of a control panel of said press; and

Figs. 3 to 5 are flow charts representing three modes of operation of said press, namely a calibrating mode, a setting mode and a cutting mode.

[0021] The cutting press in accordance with the invention is generally similar, except as hereinafter described, to the second of two presses described in DE-A3412488 and comprises a frame 10 having integral therewith a cutting bed 12 on which is carried a cutting pad 14. Also supported by the frame, on the upper surface thereof and rearwardly of the cutting bed, is an upstanding column 16 on which is supported, for swinging movement about the axis thereof, a cutting head in the form of a so-called swing beam 18. For ease of swinging the beam 18, and also to give more rigidity in the cutting operation, the beam is supported on the column by means of ball-bearing arrangements 20, as is described in greater detail in the aforementioned patent specification.

[0022] For effecting heightwise movement of the beam 18 in relation to the cutting bed 14, conventional hydraulically operated means (not shown) is provided, generally as described in DE-A3016527, the press also comprising a two-hand trip arrangement in the form of two spaced-apart manually operable switches 22 mounted in a conventional manner on the front edge of the swing beam 18. Operation simultaneously of the two switches 22 causes a cycle of operation of the press to be initiated, it being of course necessary, as is conventional, to maintain the two switches actuated until the stroke of the press is completed, and release of either one of said buttons during a stroke of the press being effective to return it to its initial, rest, position.

[0023] For monitoring the heightwise movement of the swing beam 18, transducer means in the form of a linear potentiometer (not shown but referenced LP) is provided towards the rear of the cutting head, the voltage across said potentiometer varying proportionately with the distance through which the cutting head is moved heightwise, and such voltage being monitored and constituting an output value of the transducer means. Thus, by comparing the output value of the linear potentiometer with a pre-set comparator value, the movement of the cutting head can be monitored and indeed a "stroke termination" signal can be generated when the values match. Such an arrangement is described in GB-A2099750.

[0024] Referring to Fig. 2, which shows a control panel of the cutting press in accordance with the invention, a selector switch SS1 is provided whereby, as indicated by the diagrams at the top left and right corners of the control panel, a "setting" or a "cutting" mode can be selected, indicator lights L1, L2 being provided for indicating which selection has been made. Beneath the switch and lights are arranged two display panels D1, D2 and associated with each are two contact buttons B11, B12, B21, B22, the use of which will be referred to hereinafter. At the bottom of the panel is a further, four-digit, display D3 with two further contact buttons B31, B32 arranged one at either side thereof. The various buttons are in many instances multi-purpose, depending upon the mode of operation selected.

[0025] In addition to the control panel, a further selector switch (not shown but referenced SS2) is provided, which is not normally accessible to the operator, and by which either a "normal" or a "calibrating" mode of operation can be selected; it will of course be appreciated that calibration is a task for a factory engineer, rather than the operator. Where a "normal" mode of operation is selected, switch SS1 is enabled for the selection of a "setting" or a "cutting" mode of operation.

[0026] The calibrating mode of operation, when selected, enables the press to be set up in accordance with its characteristics, in particular the response times of the various integers of the hydraulically operated means and the switches, so that a datum value can be provided in the machine control system, on the basis of which datum value the various settings effected during "normal" operating can be based. In this way, in the event of changes in the press characteristics, e.g. because of maintenance work on the press system, nevertheless the settings for individual dies can be maintained without re-setting; similarly where a factory uses two or more such presses, dies can be interchanged and the settings for such dies will remain constant between the various presses, regardless of difference in the individual characteristics of the presses.

[0027] The various integers of the press in accordance with the invention are controlled by microprocessor control means operating in response to signals received from both the control panel and the linear potentiometer. The microprocessor control means thus includes a memory in which data will be stored at individual accessible locations.

[0028] In carrying out a "calibrating" operation the operator first takes a cutting die which is of relatively short cutting length and places it on the cutting pad 14 at a position of minimum deflection (distortion) of the cutting head. In a swing beam press, this location would be found as close as possible to the column 16. By selecting a die of relatively short cutting length and placing it in a position of minimum deflection, it will be appreciated, a situation has been selected in which it is unlikely that any other die placed in any other location will be caused to penetrate the cutting pad 14 to a greater extent under the same operating conditions.

[0029] With selector switch SS2 thus switched to "calibrating" mode, thereby disabling selector switch SS1, there is then displayed in the left hand display panel D1 the letters "PR", indicating that a service programme is to be selected. Using button B21, B22, which serve to scroll up or down respectively digits displayed in display panel D2, the operator then selects the appropriate programme for setting the cutting stroke control; more particularly, this enables the operator to determine the datum value referred to above, and to this end in the display D3 will be indicated the letter "ET" and a two-digit number, representing a difference value (ET) between the "stroke termination" position for the particular die and the "stroke termination signal" position (i.e. the position of the swing beam, in the course of its heightwise movment, at which the hydraulically operated means must be signalled to terminate the stroke). It is thus this difference value which constitutes the datum value and represents the press characteristics, and which must be applied in every case to a "stroke termination" position value in order to set the "stroke termination signal" position for each die. It will of course be appreciated that the "stroke termination" position and also the "stroke termination signal" position are both read from the linear potentiometer LP and stored as output values thereof, the "stroke termination signal" position value constituting the comparison value (COMP) by which in subsequent "cutting" operations the output (LINPOT) of the linear potentiometer LP is compared, thus to terminate the cutting stroke at the appropriate time.

[0030] In using the press in accordance with the invention the operator may choose to measure the die height in a prior "setting" operating before initiating a "calibrating" mode; although the machine will not be properly set up at this stage, nevertheless some indication as to the "stroke termination" position can thus be determined; in general, prior to initiating a "calibrating" stroke, the operator would then, using button B22 (which decrements the value displayed at D3) decrement the value there displayed in order to ensure that the knife is not driven too far into the cutting pad 14. It will of course be appreciated that by decrementing the number displayed at D3, the penetration depth will be decreased, while by increasing the value of said number the penetration would be increased.

[0031] By a process of trial and error, the operator then carries out a series of operations in order to determine where in relation to the die height the stroke termination signal should be supplied; that is to say, by determining the desired stoke termination position for the particular die in terms of setting the stroke termination signal position, the difference value appropriate to the machine characteristics is established.

[0032] Turning to Fig. 3, step 50 indicates that the status of selector switch SS2 is interrogated to determine whether or not a "calibrating" mode has been selected. In the event that it has, at step 52 the penetration depth value (ET′) (which will be referred to hereinafter) is set to zero and at step 54 the comparison value for the "calibration" die is calculated on the basis of subtracting any difference value, which has been established e.g. in a previous calibration operation as aforesaid and stored, from the "stroke termination" position value of the die, as measured in the preceding setting operation.

[0033] The press is then ready for a calibrating operation and at step 56 the status of the two-hand trip arrangment 22 is interrogated, the software looping until an affirmative answer is obtained, whereupon at step 58 a solenoid (not shown, but referenced SOL1) is energised to switch a main valve of the hydraulically operated means for moving the swing beam 18 downwards, and at step 60 a regulator valve (not shown, but referenced RV2) is actuated to ensure that fluid under normal cutting pressure is supplied to the hydraulically operated means. The swing beam 18 is thus moved downwardly. During its downward movement firstly the status of the two-hand trip 22 is continuously monitored (step 62) and secondly the comparison value (COMP), as calculated in step 54, is constantly compared with the output value (LINPOT) of the linear potentiometer (step 64). In the event that the two-hand trip arrangement is released or in the event that the comparison value is found to equal the output value from the linear potentiometer, then at step 66 solenoid SOL1 is de-energised and a further solenoid (also not shown, but referenced SOL2) is energised to initiate a return stroke of the press. The return stroke of the press is in fact controlled in the same manner as in the "cutting" mode to be referred to hereinafter.

[0034] After each calibrating stroke has been effected, the operator will inspect the cutting die with a view of determining whether it has been driven too far into the cutting pad or insufficiently far, and upon establishing optimum cutting pad penetration, the difference value, as thus established and displayed by display D3, is then stored for application in each subsequent "cutting" operation, viz. by adding it to the "stroke termination" position value, as read by the linear potentiometer in a "setting" operation. It should be observed that button B32 on the control panel is a "confirmation" button and is contacted each time a new setting of any of the parameters of the press is made, whereby the new information is then stored in the memory of the microprocessor control means. The calibration mode is thus terminated.

[0035] For effecting a "setting" mode of operation (Fig. 4), at step 100 the selector switch SS2 is interrogated and, in the event that "NORMAL" has been selected thereby, the status of selector switch SS1 is interrogated (step 102) and if set to "SET" the "setting" mode is initiated.

[0036] In this mode of operation, firstly the penetration depth value (ET′) is set to zero (step 104) and at step 106 the status of the two-hand trip arrangement 22 is interrogated, the software looping until an affirmative answer is received. Thereupon, firstly solenoid SOL1 is energised (step 108) to initiate downward movement of the swing beam 18, but in this mode of operation a further regulator valve (not shown, but referenced RV1) is actuated (step 110) whereby the fluid is supplied to the hydraulically operated means at a reduced pressure. Thus, in a "setting" operation, the swing beam is moved downwardly into contact with the die placed on the cutting pad 14 but without sufficient pressure to drive the die into the cutting pad.

[0037] During the downward movement the two-hand trip arrangement is monitored (step 112) until it is no longer actuated, whereupon solenoid SOL1 is de-energised (step 114) thus terminating the supply of fluid under pressure to the hydraulically operated means. At step 116 the software awaits actuation of a "return" button 24, and, until that button is actuated, also the status of the two-hand trip arrangement 22 continues to be monitored (step 118); in the event of a re-actuation of said arrangment then the software returns to step 108 and downward movement of the swing beam is continued.

[0038] Upon actuation of return button 24, the output value (LINPOT) of the linear potentiometer LP at the time of such actuation is stored, thus constituting the "stroke termination" position (or die height) value and also at this time, by subtracting the difference value (ET) from the "stroke termination" position value, the comparison value (COMP) is obtained. Also at this time solenoid SOL2 is actuated (step 122), termination of the return stroke being thus initiated and continuing until terminated as will be hereinafter described with reference to the "cutting" mode of operation.

[0039] In carrying out a "cutting" operation, again at step 150 the setting of selector switch SS2 to NORMAL is monitored and at step 152 the setting of selector switch SS1 to CUT is interrogated. In the event of an affirmative answer for each, at step 154 a calculation is made based upon the comparison value which has just been stored by virtue of the previous "setting" operation, but adding a value set by the operator and relating to the depth of penetration required for the particular cutting operation.

[0040] In the "cutting" mode, the penetration depth value (ET′) is displayed in display D1, being a value between 0.0 and 9.9 mm, buttons B11 and B12 being utilised to scroll this value up or down respectively. When the value is set to zero, it will be appreciated, the maximum penetration of the cutting pad by the cutting die will be equivalent to the penetration set by the operator in the calibrating mode of operation. Where, however, a larger cutting die is used, or where the die is located at a distance where more deflection (distortion) of the swing beam may be expected, it may be that insufficient penetration is achieved in order to enable the die to be driven cleanly through the material to be cut. Therefore the facility for varying the penetration depth is provided. It will, however, be appreciated that once this penetration depth for the die has been determined it does not need to be re-set at any future time merely because of any change in the machine characteristics, provided of course that re-calibration of the machine has taken place. It is thus envisaged that the cutting die could in fact be marked with its desired penetration depth value for future reference. It will of course also be appreciated that the die can be transferred between different cutting presses which have been calibrated in the same manner as set out above, without any need for establishing by trial and error what penetration depth adjustment needs to be made.

[0041] When the calculation varying the comparison value by adding the penetration depth value (ET′) has been made at step 154, the software then awaits actuation of the two-hand trip arrangement 22 (step 156). Upon actuation, as in the case of the "calibrating" mode of operation, solenoid SOL1 is energised (step 158) and also regulator valve RV2 (step 160) whereby the hydraulically operated means is supplied with fluid under normal cutting pressure, thus to drive the swing beam 18 downwardly and effect a cutting stroke. Also as in the case of the "calibrating" mode, the status of the two-hand trip arrangement 22 is constantly monitored (step 162) but in this case the output value of the linear potentiometer is compared with the comparison value as modified by the penetration depth value (step 164).

[0042] Upon release of the two-hand trip arrangement 22 prior to the end of the cutting stroke, or upon the comparison value (COMP) as modified matching the output value (LINPOT) of the linear potentiometer, whichever occurs first, solenoid SOL1 is de-energised and solenoid SOL2 is energised (step 166) whereupon a return stroke of the swing beam 18 is initiated.

[0043] For controlling the return stroke of the press, and thus the so-called "daylight" between the cutting head and cutting bed, the operator can set the return stroke value using buttons B21, B22 to increment or decrement a value as displayed by the right-hand display D2. As in the case of adjustment of penetration depth value, this setting may be made when the machine is idle in either the "setting" or "cutting" mode of operation. The value of the return stroke can be set between 6 and 99 mms. Thus, at step 168 the output value of the linear potentiometer is compared with the pre-set return stroke value (step 168) and, upon matching, solenoid SOL 2 is switched off, thereby terminating the return stroke.

[0044] As previously mentioned, the return stroke value is utilised for controlling the return stroke of the press in each of its modes of opertion.

[0045] For calibrating the cutting press in respect of its return stroke, a further service programme may be selected in the same manner as described with reference to the other calibration operation, again using buttons B21, B22 for incrementing or decrementing the appropriate value. In this way, the "daylight" between the cutting head and cutting bed can be accurately set.

[0046] In a cutting operation, the number of cuts to be made may be pre-set using the display D3 and incrementing or decrementing each digit in turn using buttons B21, B22, and selecting each of the four digits in turn using button B32. B31 serves to activate the counter.

[0047] In operating the machine when the counter has been activated, the microprocessor control counts down the number of cuts and terminates the cutting operation when the pre-set number of cuts has been made. Thereafter, the operation of the press can be continued upon actuation of button B31.

[0048] It will thus be appreciated that, using the press in accordance with the invention, the setting of the cutting stroke for different dies, i.e. dies of different height and also dies of different size, can more readily be achieved without the need for a re-setting operation each time the dies are used. In this way, the productivity of the machine is enhanced and in addition, especially because of the ability to interchange cutting dies between different presses, a more versatile cutting system is produced.

Claims

1. A cutting press comprising two platens (12, 18) constituting a cutting head (18) and a cutting bed (12),
drive means for effecting relative movement of approach between the platens (12, 18) thus to cause an operating stroke of the press to take place, and
stroke control means, comprising a transducer device (LP) the output of which has a value which varies according to and proportionately with the distance between the platens (12, 18), comparator means (164, 168) for comparing the output value (LINPOT) with a comparison value (COMP), and signalling means (166) operable to supply a "stroke termination" signal to the drive means when, in effecting an operating stroke, the output and comparison values (LINPOT, COMP) match,
wherein, in a stroke-setting mode of operation, the drive means causes relative movement of approach to take place between the platens (12, 18) under a reduced pressure and with the signalling means (166) disabled, whereby for a given cutting die positioned between the platens (12, 18) the stroke termination position for that die can be determined,
characterised by manually operable calibrating means (B21, B22) whereby, in a calibrating mode of operation of the press, in which an operating stroke is effected at normal cutting pressure with a suitable cutting die positioned between the platens (12, 18), the comparison value (COMP) for that die can be varied in relation to the stroke termination position for that die,
and by processor means which stores as a difference value (ET) the difference between the comparison value (COMP) and the output value corresponding to the stroke termination position and, in any subsequent stroke-setting operation for a cutting die, applies the difference value (ET) to the output value corresponding to the stroke termination position of that die, thus to determine and set the appropriate comparison value (COMP) for that die.

2. A cutting press according to Claim 1 characterised by means (B32) operable in the calibrating mode of operation for causing the difference value (ET) determined as aforesaid to be stored by the processor means.

3. A cutting press according to either one of Claims 1 and 2 characterised by operator-actuatable setting means (B11, B12) whereby the operator can, without modifying the difference value (ET) as stored, vary the comparison value (COMP) for any die by applying an adjustment value (ET′) thereto.

4. A cutting press according to Claim 3 characterised by a display (D1) by which the adjustment value (ET′) is indicated.

5. A cutting press according to any one of the preceding Claims wherein the drive means is also effective to cause relative movement of separation to take place between the platens (12, 18), thus to cause a return stroke of the press to take place, and the comparator means (164, 168) of the stroke control means serves also to compare the output value (LINPOT) with a further comparison value (RETCOMP), signalling means (170) being operable to supply a "return stroke termination" signal to the drive means when, in effecting a return stroke, the output and further comparison values match,
and wherein further operator-actuatable setting means (B21, B22, D2) is provided for setting a return stroke termination position,
characterised by further manually operable calibrating means (B21, B22) whereby, in the calibrating mode of operation, with a cutting die positioned between the platens (12, 18), the further comparison value (RETCOMP) can be varied in relation to the return stroke termination position as set,
and in that the processor means stores as a further difference value the difference between the further comparison value and the output value corresponding to the return stroke termination position and, in any subsequent stroke-setting operation for a cutting die, applies said further difference value to the output value corresponding to the return stroke termination position of the die, thus to determine and set the appropriate further comparison value for that die.

6. A cutting press according to Claim 5 when tied directly or indirectly to Claim 2 wherein said means (B32) for causing a difference value (ET) to be stored is also effective for causing a further difference value to be stored by the processor means.

7. A cutting press according to any one of the preceding Claims characterised by manually operable selector means (SS1, SS2) for selecting any one of three operating modes, viz. cutting, setting-up and calibrating.

8. A cutting press comprising
two platens (12, 18) constituting a cutting head (18) and a cutting bed (12),
drive means for effecting relative movement of approach between the platens (12, 18) thus to cause an operating stroke of the press to take place, and
stroke control means, comprising a transducer device (LP) the output of which has a value which varies according to and proportionately with the distance between the platens (12, 18), comparator (164, 168) means for comparing the output value (LINPOT) with a comparison value (COMP), and signalling means (166) operable to supply a "stroke termination" signal to the drive means when, in effecting an operating stroke, the output and comparison values match,
wherein, in a stroke-setting mode of operation, the drive means causes relative movement of approach to take place between the platens (12, 18) under a reduced pressure and with the signalling means (166) disabled, whereby for a given cutting die positioned between the platens (12, 18) the stroke termination position for that die can be determined,
characterised by manual setting means (B21, B22), operable in a calibrating mode of operation, for enabling the distance to be determined through which, in an operating stroke at normal cutting pressure, the relative movement of approach continues between the platens (12, 18) after the signalling means (166) has operated as aforesaid, such that, with a suitable cutting die placed between the platens (12, 18), the operating stroke is terminated at the stroke termination position previously determined for that die,
and by processor means whereby such distance is stored as a difference value (ET) and such difference value (ET) is thereafter applied, when a cutting die is presented to the press in a stroke-setting operation, to the output value corresponding to the stroke termination position of that die as determined in such operation, thus to determine and set the appropriate comparison value (COMP) for that die.

Drawing