GB1578051A - Press having thermal control - Google Patents

Description

(54) PRESS HAVING THERMAL CONTROL (71) We, TOYO SEIKAN KAISHA LIMITED, a Japanese Company, of Saiwai Building, 3-1 Uchisaiwaicho l-chome, Chiyoda-ku, Tokyo 100, Japan, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to adjusting the relative positions of mutually co-operable first and second dies in the operating system of a press. More particularly (but not exclusively) the invention relates to the operating system of a transfer press arranged to perform a scoring, rivetting, and/or drawing operations on thin sheet metal by pressing.

The gap between the closed dies or tools on a press is usually set to a chosen size while the press is stationary and cold. When the press is operated frictional forces, such as those arising between the moving and stationary parts of the press or between the dies and the workpiece, generate heat which causes these parts of the press and the dies to expand. The press frame is usually large in relation to the dies and moving parts and so remains relatively cool and dimensionally stable. The frictionally induced thermal expansion of the moving parts and the dies which are held in the frame, causes the preset gap between the dies to close to an extent which spoils the workpieces being produced. For example, aluminium can ends having a removable portion defined by a score line are stamped in a pair of dies to leave a residual metal thickness at the base of the score of the order of 0.004 inches.

Any greater thickness will make it more difficult to tear the removable portion out and any less thickness will weaken the can end. Therefore press operators have regularly to inspect the can ends produced to find out whether or not the residual metal thickness is correct. If the thickness is not correct, the press has to be stopped and readjusted by mechanical means, which is time consuming and costly in terms of lost production.

According to this invention there is provided a press having a frame, and an operating system formed of a pressureimposing system and a pressure-receiving system operably linked by said frame, said pressure-imposing system comprising a ram, at least one connecting rod connected to said ram and movable with respect to the frame to reciprocate the ram towards and away from the pressure-receiving system, a first die shoe mounted on said ram, a first die holder mounted on said first die shoe, and a first die carried by said first die holder, said pressure-receiving system comprising a second die shoe mounted on said frame, a second die holder mounted on said second die shoe, and a second die carried by said second die holder so that when the pressure-imposing system is moved to a limiting position adjacent the pressure-receiving system, an operating clearance is formed between opposed die surfaces of the first and second dies, the connecting rod and/or the die holder of the pressure-imposing system having thermal control means to thermally control its length so as in use of the press substantially to reduce changes in the operating clearance caused by temperature variations in the operating system.

Throughout this specification the expression "operating system" is used to denote the pressure-imposing system which delivers the working load, plus the pressurereceiving system against which the load acts.

Various embodiments of the invention will now be described by way of example and with reference to the accompanying drawings, in which: Fig. 1 is a plan view of a tear-open can closure; Fig. 2 is a cross-section of Fig. 1 along the line Il-Il; Fig. 3 is a magnified cross-sectional view of a score part; Figs. 4 to 6 are graphs showing the initial characteristics of H, SR, and RD respectively; Fig. 7 is an explanatory view of the operating system of a transfer press embodying this invention Figs. 8 to 15 are respectively detailed sectional views of the first to the eighth embodiments of this invention.

Transfer presses are used to work tooling adapted for performing sequential operations, such as scoring and drawing, which produce articles having a dimensional accuracy of the order of one micron. One such article, for example, is the tear-open can end shown in Figs. 1 to 3.

The easy-opening can closure 26 is of a kind which is torn open, along a score line 24 scored in its surface, by means of a pull tab 22 which is fixed by means of a cylindrical rivet part 20. The can end is made in a sequence of operations performed by a transfer press which carries out: a bubble step, in which a first drawing stage is effected on the rivet part 20 on the surface of the dish-shaped can closure 26; a button step, in which a second drawing process is carried out on the rivet part 20; a score step, in which a score line 24 is formed; a stake step, in which the pull tab 22 is fixed onto the rivet part 20; and a "C"forming step, which is the final finishing step. The workpiece is transferred between the above steps in rotary or linear manner.

The accuracy required for this can closure is that the limits of permitted error of the score residual at the score line 24, denoted SR in Fig. 3, are +0.005" in 0.004 inches, and the limits of permitted error of the rivet diameter dimension RD of the fixing head 20a of the rivet part 20 are +0.005 in 0.145 inches (diameter of rivet).

If the score residual SR is less than the lower specified limit, the score line 24 may be broken by shock in the score-forming operation. Even if the closure survives the closure manufacturing operations, bursting can easily occur along the score line 24 due to external pressure imposed upon the closure in the subsequent processing and handling operations, or due to internal pressure from the contents of the can to which the closure is fitted. On the other hand, if the score residual SR is greater than the upper specified limit, it may be difficult for the user to tear open the score line by means of the pull tab 22, or the tab may break.

If the rivet diameter RD is below the specified limit, when tearing the score line 24 open by means of the pull tab 22, the tab may come off the rivet part 20 so making it impossible to open the can by means of the pull tab 22. There is then no alternative but to use a separate can opening device.

Accordingly, for carrying out these operations by a press operation the working condition of the press must be strictly controlled, because even a slight variation of the working condition will lead to the production of unsatisfactory products. The operatives who supervise the transfer press must therefore exercise continuous vigilance to ensure that its working condition is controlled.

There are many different possible causes for a variation of the working condition. One of them is due to thermal variations which occur in the press during its operation.

Such thermal variations arise because, during continuous operation of the press, heat of sliding friction produced by the sliding parts and impact heat produced by the impacting parts gradually accumulates in the machine. For example, in the case of a crank press, the connecting rod (whose upper end is pivotted at and depends from a crank pin of a crank arm which extends outwardly from the main shaft of the crankshaft), the ram (which is fitted at the lower end of the connection rod), the upper die shoe (which extends from the lower surface of the ram), the upper metal die holder or holders (which is or are installed on the upper die shoe), and the upper metal die or dies (which is or are held by the upper metal die holder or holders) - which parts constitute the "pressure-imposing system" of the press -- cause a rise in temperature and consequent thermal expansion of the stroke operating system as a whole.

In addition to the pressure-imposing system, the operating system comprises the "pressure-receiving system"; this is formed of the lower metal die or dies facing the upper metal die or dies, and the lower metal die holder or holders which is or are mounted on the frame bed by the lower die shoe.

The thermal expansion of the operating system causes displacement of the operative positions of the associated upper and lower die surfaces during the press operation. This leads in particular to variation of the upper position-locating length of the lower dead point of the pressure-imposing system. In the case of the can closure 26, this results in the score residual thickness SR and rivet diameter RD being affected, so that it is necessary to stop the press immediately before reaching the stage at which the permitted limit value is exceeded to enable manual adjustment to absorb this variation of length to be made by removal or replacement of a shim from between the upper die shoe and the upper metal die holder. The adjustment process takes about 20--30 min, during which time production is stopped, resulting in the corresponding loss of production of several thousand closures.

Thus this alone causes considerable production losses.

In the operating system of all presses, a displacement difference C is observed between, on the one hand, the lower dead point Ha of the upper metal die surfaces (with respect to the lower die surfaces) at the time Ta of a commencement of operation and, on the other hand, the lower dead point Hb at a time Tb after the machine has warmed up.

The characteristic curve a for the position of the bottom dead point H with respect to time until the machine has warmed up is shown in Fig. 4, in which the relative related position of the lower dead point H of the upper metal die surface with respect to the lower metal die surface or, in other words, the clearance between the die surfaces at the bottom dead centre of the press is the ordinate, and the time T (min) is the abscissa. The time Tb required for the clearance to stabilize is about 30 min, after which it is approximately constant.

When a scoring operation and a staking operation are performed with a transfer press, in association with this initial movement characteristic curve a, there are corresponding initial variations of the score residual thickness SR and rivet diameter RD, as shown respectively in Figs. 5 and 6.

In Fig. 5, SR (y) is taken as the ordinate, and the time T (min) is taken as the abscissa, giving an initial characteristic curve p. In Fig. 6, RD (,u) is taken as the ordinate, and the time T (min) is taken as the abscissa, giving an initial characteristic curve y.

Since on initial operation of the transfer press the score residual thickness SR and the rivet diameter RD will not be within the permitted ranges Pl, P2 defined between the respective permitted upper and lower limits U, L until after the lapse of the initial warming-up times Tl, T2, on starting the press from cold it is necessary to discard about 1000--5000 easy-opening type can closures 26 every day.

The initial characteristic curve of SR is of similar form to the initial characteristic curve a of the lower dead point H of the upper metal die surface; in contrast, the initial characteristic curve y of RD is generally in inverse relation to the initial characteristic curve a. There is the same displacement difference, namely A, between the value of SR and RD after the transfer press has warmed up sufficiently, and SR and RD on initial operation, and this is equal to the displacement difference C of the position of the lower dead point H.

This is a problem which concerns expansion of the operating system as a whole, especially the upper positiondetermining length of the pressure-imposing system.

Thus, data are obtained beforehand, by test operation of the apparatus, in the manner described with reference to Figs. 4 to 6. The data includes the characteristic in the initial period and the characteristic during the progress of operation for variation with time of the operative positions of the upper and lower metal die surfaces in the operating system. The variation of extension due to natural expansion in the operating system during use of the press is predicted on the basis of these data. This enables corrective expansion by heating or contraction by cooling to be carried out at a suitable location of the operating system so that, during operation of the press, the operative positions of the upper and lower metal die surfaces are kept constant, thereby increasing the working accuracy.

This adjustment operation, by which the variation of the extension of the operating system is absorbed, may be performed by electric heating, radiant heating, flame, or cooling water, cooling oil, or cooling gas applied at a suitable location of the operating system. The treatment can therefore be carried out without stopping the operation of the press, so that there is no loss of productivity caused by loss of time in which the press is operating, and there is no adverse effect on other related production lines. The way to remote control and automatic operation is thereby opened.

Consequently, the inevitable waste of unusable article during operation and during the initial period of working of the press due to variation of the extension of the operating system is substantially eliminated, resulting in a considerable economic advantage.

Moreover, because the heating means or cooling means with which one or both of the metal die holder and connection rod of the stroke operating system are provided are selected from the electric heating action of a band heater or a rod-shaped heater, the radiative heating action of a hot-wire radiator, the contact action of hot air from a nozzle, or a flame, the action of cooling water, cooling oil, or cooling gas passing through a cooling passage provided therein, or the contact action of cold air from a nozzle, the apparatus can be made compact and maintenance and repair are facilitated.

An embodiment of the invention will now be described with reference to Fig. 7.

Referring to Fig. 7, the operating system 30 of a transfer press 28 consists of a vertically reciprocable pressure-imposing system 32, and a stationary pressure-receiving system 34. The systems 32, 34 are linked by frame 10.

The pressure-imposing system 32 consists of a crankshaft 36, connecting rods 44 which are pivotted at their upper ends to crank pins 42 of crank arms 40 which extend from the main shaft 38 of the crankshaft 36, a ram 50 which slides in guide portions 48 of the inside of uprights 46 and is pivotted to the lower ends of the connecting rods 44, an upper die shoe 52 which extends across the lower surface of the ram 50, upper metal die holders 54 which are arranged along the upper die shoe 52, and upper metal dies 56 which are held in the upper metal die holders 54.

The pressure-receiving system 34 consists of a lower die shoe 60 which is mounted on a bed 58 of the frame 10, lower metal die holders 62 which are arranged in line on the lower die shoe 60, and lower metal dies 64 which are opposite to the upper metal dies 56 and are held in the lower metal die holders 62.

Before operation of the transfer press 28, data is obtained relating to the variation with time of the operative positions of the die surfaces 56a of the upper metal dies 56 and the die surfaces 64a of the lower metal dies 64, which arises from the initial displacement difference C of the position of the lower dead point H of the stroke operating system 30, this data being obtained beforehand and solely relating to the variation of the upper positiondetermining length I from the axis Q of the main shaft 38 of the crankshaft 36 to the position of the lower dead point H of the die surfaces 56a of the upper metal dies 56.

Based on this data, expansion by heating is carried out by electric heating, radiant heating, hot air, or flame at one or more suitable locations of the pressure-imposing system 32, which may be the connection rods 44 or upper metal die holders 54. If desired, further corrective heating may be applied to the lower metal die holders 62 of the pressure-receiving system 34. The upper position-determining length 1 is thereby extended, enabling fine lowering adjustment to be carried out to the extent of one ,u unit on the position of the lower dead point H.

Thus the vertical clearance required for the press operation between the die surfaces 56a of the upper metal dies 56 and the die surfaces 64a of the lower metal dies 64 can be set beforehand to the correct value.

Furthermore, the die surfaces 64a may, if desired, be slightly raised by extension of the lower position-determining length d, that is, the length of the pressure-receiving system 34 from the upper surface M of the press bed 58 to the die surfaces 64a of the lower metal dies 64. Variation of the operative positions of the die surfaces 56a and the die surfaces 64a during the press operation can thereby be substantially controlled. The displacement difference C of the position of the lower dead point H of the initial characteristic curve a in Fig. 4 is thereby absorbed and nullified. Then, when operation is commenced, as the temperature of the machine gradually rises naturally due to the energy of the operational process, the temperature of corrective heating may be gradually reduced in accordance with calculations made taking into account the coefficient of thermal expansion of the material and the cross-sectional area of the regions which are subjected to corrective heating so as to correspond to the coefficient of this temperature rise. The loss of unusable, outof-specification articles which had previously to be reckoned with on starting up can thus be eliminated.

If during operation a natural rise in temperature of the operating system 30 or excessive corrective heating causes the temperature to rise excessively, resulting in one or both of the upper positiondetermining length I and the lower positiondetermining length d becoming elongated and thereby causing unacceptable variation of the operative positions of the die surfaces 56a, 64a of the upper and lower metal dies 56, 64 during stamping, the temperature of corrective heating may be lowered, or the corrective heating may be stopped altogether. If even this is insufficient, contraction by cooling may be carried out by applying cooling water, cooling gas, cooling oil, or cooling air at one or more suitable locations of the operating system 30, namely, at the connection rods 44 or the upper metal die holders 54 of the pressureimposing system 32, or at the lower metal die holders 62 of the pressure-receiving system 34, thereby shortening one or both of the upper position-determining length 1 and the lower position-determining length d and so permitting fine separation adjustment of the upper and lower relative related positions of the die surfaces 56a, 64a to the extent of one y unit. For clarity the upper and lower position-determining lengths 1, d are shown in Fig. 7 to be consecutive. In heating, there will be a small discontinuity between them.

If the operating system 30 falls excessively in temperature due to a natural fall in temperature or due to a low degree of corrective heating, causing one or both of the upper position-determining length 1 and the lower position-determining length d to shorten and so resulting in the operative positions of the die surfaces 56a. 64a becoming too far separated, the temperature can be raised by corrective heating at one or more suitable locations of the operating system 30, namely, the connection rods 44 or upper metal die holders 54 of the pressure-imposing system 32, or the lower metal die holders 62 of the pressure-receiving system 34, so as to expand one or both of the upper position-determining length I and the lower position-determining length d. Fine approach and return adjustment to the extent of one y unit can thereby be carried out until the operative positions of the die surfaces are correct. In all these cases, the operative positions of the die surfaces 56a, 64a can be adjusted so that they are always maintained at the set standard value during the press operation. By means of the foregoing, the loss of articles which do not meet the specified standards and the fall of production rate resulting from the requirement to stop the production line each time adjustment is carried out on the transfer press 28 can be totally eliminated.

The production of easy-opening type can closures 26 as shown in Figs. 1 to 3 using the transfer press 28 shown in Fig. 7 will now be described.

Before the commencement of operation, in order to carry out adjustment based on the statistical data in accordance with the graphs of Fig. 4 to Fig. 6, by absorbing the initial displacement difference C of the lower dead point H to an initial null value, the connection rods 44 and/or the upper metal die holders 54 of the operating system 30 are subjected to expansion by corrective heating to extend the upper positiondetermining length 1. The lower positiondetermining length d may likewise be adjusted by applying heat to the lower die holders 62. In this way fine approach adjustment of the operative positions of the die surfaces 56a of the upper metal dies 56 and the die surfaces 64a of the lower metal dies 64 may be achieved, enabling SR and RD to be set within the permitted ranges PI, P2. Operation is then commenced.

With the commencement of operation, i.e. within the regions Tl, T2, as the temperature of the operating system starts to rise naturally, either the temperature of corrective heating may be reduced corresponding to the variation of temperature rise, or the connection rods 44 and/or the upper metal die holders 54 and possibly also the lower metal die holders 62 may be subjected to contraction by gradual corrective cooling at a cooling temperature corresponding to the variation of temperature rise, thus adjusting to maintain constant at least the operative positions of the die surfaces 56a of the upper metal dies 56 and possibly also of the die surfaces 64a of the lower metal dies 64. SR and RD are thereby prevented from moving outside the permitted ranges Pl and P2.

Further, during the course of operation, if a natural rise in temperature or excess corrective heating causes the operating system 30 to rise to too high a temperature, thereby unacceptably extending the upper position-determining length 1 and thereby resulting in a slight approach movement of the operative positions of the die surfaces 56a. 64a during formation of the score line 24 or of the rivet part 20, so causing SR to pass the lower standard limit L or RD to pass the upper standard limit U, the temperature of corrective heating may be lowered. If this proves insufficient, the connection rods 44 and/or the upper metal die holders 54, and possibly also the lower metal die holders 62, may be subjected to contraction by corrective cooling, thereby shortening the upper position-determining length 1 and possibly also the lower positiondetermining length d. The operative positions of the die surfaces 56a of the upper metal dies 56 and the die surfaces 64a of the lower metal dies 64 can thereby be separated and returned to the correct value, returning SR and RD to approximately intermediate positions in the permitted ranges denoted Pl, P2, in Figs. 5 and 6.

If, due to a subsequent natural fall in temperature or to cooling provided, the operating system should fall excessively in temperature, thus causing one or both of the upper position-determining length 1 and the lower position-determining length d to shorten, thereby slightly separating the operative positions of the die surfaces 56a of the upper metal dies 56 and the die surfaces 64a of the lower metal dies 64 during pressing of the score line 24 and during drawing to form the rivet 20, causing SR to pass the upper standard limit U and RD to pass the lower standard limit L, the degree of corrective cooling may be decreased. If this proves insufficient, one or both of the connection rods 44 and the upper metal die holders 54, and possibly also lower metal die holders 62, may be subjected to expansion by corrective heating, thereby expanding one or both of the upper positiondetermining length 1 and the lower positiondetermining length d and so returning the operative positions of the die surfaces 56a of the upper metal dies 56 and the die surfaces 64a of the lower metal dies 64 to approach each other correctly, and returning SR and RD to approximately intermediate positions in the permitted ranges PI, P2.

By control of the process as above, it is possible to perform press forming of can closures 26 having SR and RD which are always within the permitted ranges Pl, P2.

A first embodiment of the invention is now described with reference to Fig. 7 and Fig. 8. Referring to those Figures, the transfer press 28 has a score stage SCS and a stake stage STS provided with upper metal die holders 54 round the outside of each of which is wound a band heater 66. The band heater is fixed in position by means of a stop screw 68; it is covered by a heater cover 70 which is fixed to the upper metal die holder 54 by means of a stop bolt 72.

In the drawings, 74 is a metal connector which is inserted into an attachment plug 76 through which electric current flows to the band heater 66, the current flowing through a cord 80 and connector 78 provided on the ram 50. The connector 78 is connected by a cord 86 to a voltage regulating device 84, which passes current through a cord 82 from an electric source (not shown). 88 and 90 are fixing bolts, and 92 is a shank which locates the die holder in the upper die shoe 52.

In the second embodiment of the invention, as shown in Fig. 9, a plurality of rod-shaped heaters 94 are embedded in each upper metal die holder 54 of the score stage SCS and the stake state STS.

In the third embodiment of the invention as shown in Fig. 10, passages 96 for circulating relatively cold fluid are provided in each upper metal die holder 54 of the score stage SCS and the stake stage STS.

In Fig. 10, a hose (100) is connected to a circulating pump (not shown) to supply cooling water, cooling oil, or cooling gas to the cooling passages 96 from which it is exhausted through a further hose 98.

In the fourth embodiment of the invention, as shown in Fig. 11, a hood-like cover 102 is fixed by means of stop screws 104 to each upper metal die holder of the score stage SCS and of the stake stage STS.

Inside the hood-like cover 102, and opposite the periphery of the upper metal die holder 54, there is provided a hot-wire radiator 106 which emits infra-red rays to heat the die holder 54.

In Fig. 11 a nozzle 108 delivers gas from a pipe 110 connected to a source of hot air, of combustible gas, or cold air, separately provided and not shown. This, gaseous thermal control of the die holder may be alternative or additional to the radiant heating derived from the radiator 106.

In the fifth embodiment of this invention as shown in Fig. 12, a band heater 114 fixed by means of a stop screw 112 is wound around the periphery of each connecting rod 44, and surrounded by a cover 116.

In the sixth embodiment of this invention, as shown in Fig. 13, a plurality of rod-like heaters 118 are embedded in each connecting rod 44.

In the seventh embodiment of the invention, as shown in Fig. 14, circulating passages 120 for a coolant fluid are provided in each connecting rod 44.

In the eighth embodiment of the invention, as shown in Fig. 15, a hood-like cover 122 is fixed to each connecting rod 44 by means of stop screws 124. On the inside of the cover 122, and facing the periphery of the connecting rod 44, there is fitted a hotwire radiator 126 which emits infra-red light, and an optional nozzle 128 which delivers any kind of gas, at a suitable temperature.

If desired, the lower metal die holders 62 of the score stage SCS and the stake stage STS may be provided and heating and/or cooling arrangements arranged and operating in the manner of the embodiments of Figs. 8 to 11.

The temperature of the electric or radiant heating in the first, second, fourth, fifth, sixth, and eighth embodiments is controlled by suitable operation of a voltage regulator 84, thus subjecting the stroke operating system 30 to corrective heating. When, in the third, fourth, seventh, and eighth embodiments, a liquid or gas is used as a heating or cooling medium, corrective heating or cooling is performed by bringing the medium into contact with the operating system 30 intermittently, the flow of heating or cooling medium being interrupted or restored as required to effect the temperature control.

Although the invention has been particularly described with reference to its application to a crank press, it is to be understood the invention is not limited to presses of this particular kind.

WHAT WE CLAIM IS: 1. A press having a frame, and an operating system formed of a pressureimposing system and a pressure-recei

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. which is wound a band heater 66. The band heater is fixed in position by means of a stop screw 68; it is covered by a heater cover 70 which is fixed to the upper metal die holder 54 by means of a stop bolt 72. In the drawings, 74 is a metal connector which is inserted into an attachment plug 76 through which electric current flows to the band heater 66, the current flowing through a cord 80 and connector 78 provided on the ram 50. The connector 78 is connected by a cord 86 to a voltage regulating device 84, which passes current through a cord 82 from an electric source (not shown). 88 and 90 are fixing bolts, and 92 is a shank which locates the die holder in the upper die shoe 52. In the second embodiment of the invention, as shown in Fig. 9, a plurality of rod-shaped heaters 94 are embedded in each upper metal die holder 54 of the score stage SCS and the stake state STS. In the third embodiment of the invention as shown in Fig. 10, passages 96 for circulating relatively cold fluid are provided in each upper metal die holder 54 of the score stage SCS and the stake stage STS. In Fig. 10, a hose (100) is connected to a circulating pump (not shown) to supply cooling water, cooling oil, or cooling gas to the cooling passages 96 from which it is exhausted through a further hose 98. In the fourth embodiment of the invention, as shown in Fig. 11, a hood-like cover 102 is fixed by means of stop screws 104 to each upper metal die holder of the score stage SCS and of the stake stage STS. Inside the hood-like cover 102, and opposite the periphery of the upper metal die holder 54, there is provided a hot-wire radiator 106 which emits infra-red rays to heat the die holder 54. In Fig. 11 a nozzle 108 delivers gas from a pipe 110 connected to a source of hot air, of combustible gas, or cold air, separately provided and not shown. This, gaseous thermal control of the die holder may be alternative or additional to the radiant heating derived from the radiator 106. In the fifth embodiment of this invention as shown in Fig. 12, a band heater 114 fixed by means of a stop screw 112 is wound around the periphery of each connecting rod 44, and surrounded by a cover 116. In the sixth embodiment of this invention, as shown in Fig. 13, a plurality of rod-like heaters 118 are embedded in each connecting rod 44. In the seventh embodiment of the invention, as shown in Fig. 14, circulating passages 120 for a coolant fluid are provided in each connecting rod 44. In the eighth embodiment of the invention, as shown in Fig. 15, a hood-like cover 122 is fixed to each connecting rod 44 by means of stop screws 124. On the inside of the cover 122, and facing the periphery of the connecting rod 44, there is fitted a hotwire radiator 126 which emits infra-red light, and an optional nozzle 128 which delivers any kind of gas, at a suitable temperature. If desired, the lower metal die holders 62 of the score stage SCS and the stake stage STS may be provided and heating and/or cooling arrangements arranged and operating in the manner of the embodiments of Figs. 8 to 11. The temperature of the electric or radiant heating in the first, second, fourth, fifth, sixth, and eighth embodiments is controlled by suitable operation of a voltage regulator 84, thus subjecting the stroke operating system 30 to corrective heating. When, in the third, fourth, seventh, and eighth embodiments, a liquid or gas is used as a heating or cooling medium, corrective heating or cooling is performed by bringing the medium into contact with the operating system 30 intermittently, the flow of heating or cooling medium being interrupted or restored as required to effect the temperature control. Although the invention has been particularly described with reference to its application to a crank press, it is to be understood the invention is not limited to presses of this particular kind. WHAT WE CLAIM IS:

1. A press having a frame, and an operating system formed of a pressureimposing system and a pressure-receiving system operably linked by said frame, said pressure-imposing system comprising a ram, at least one connecting rod connected to said ram and movable with respect to the frame to reciprocate the ram towards and away from the pressure-receiving system, a first die shoe mounted on said ram, a first die holder mounted on said first die shoe, and a first die carried by said first die holder, said pressure-receiving system comprising a second die shoe mounted on said frame, a second die holder mounted on said second die shoe, and a second die carried by said second die holder so that when the pressure-imposing system is moved to a limiting position adjacent the pressure-receiving system, an operating clearance is formed between opposed die surfaces of the first and second dies, the connecting rod and/or the die holder of the pressure-imposing system having thermal control means to thermally control its length so as in use of the press substantially to reduce changes in the operating clearance caused by temperature variations in the operating system.

2. A press according to claim 1, wherein the means to thermally control the length of

the connecting rod and/or the die holder of the pressure-imposing system comprises an electrical resistance heater, a radiant heater or means to deliver a heated fluid to the connecting rod or die holder.

3. A press according to Claim I, wherein the means to thermally control the length of the connecting rod and/or the die holder of the pressure-imposing system comprises conduits in the connecting rod and/or the die holder for flow of cooling medium therethrough.

4. A press according to any preceding claim, which includes further thermal control means to thermally control the length of the die holder of the pressurereceiving system, the first said thermal control means and the further thermal control means being operable to maintain substantially constant the operative positions of the said die surfaces of the first and second dies, despite the said temperature variations in the said operating systems.

5. A press according to any preceding Claim arranged as a transfer press having a plurality of said first die holders mounted on said first die shoe, and first dies carried by said first die holders, the press further comprising a plurality of said second die holders mounted on said second die shoe, and a plurality of said second dies carried by said second die holders for co-operation with said first dies.

6. A press according to Claim 5, wherein at least some, but not all, of said first die holders have said thermal control means associated therewith.

7. A press substantially as hereinbefore described with reference to any one of Figs.

8 to 14 of the accompanying drawings.