Two stage punch press actuator with output drive shaft position sensing

A press for forming a workpiece with a tooling set includes an actuator assembly carrying a movable half or punch of the tooling set and having a two phase operation. In the first phase the actuator provides relatively low force in moving the punch to an intermediate position in close proximity to the workpiece. A sensor detects the position of the punch, and when the punch reaches the intermediate position provides a continue signal. The second phase of actuator operation is conditioned on occurrence of the continue signal, and uses normal high force to press the movable tooling half against the workpiece to complete the operation. The low force first phase allows obstructions of any kind to stop movement of the punch during the first phase before high force applied to the actuator may cause damage or injury to the obstruction. In a preferred embodiment the actuator assembly has hydraulic operation.

BACKGROUND OF THE INVENTION

Punch or stamping presses are a staple of manufacturing operations for products formed from workpieces such as metal sheets, rods, bars, etc. With properly designed tooling sets, punch presses can be used for a variety of manufacturing process operations including cutting, forming, drawing, shaping, and assembling. Punch presses come in sizes ranging from a meter or less in height to several meters in height, and can develop force ranging from hundreds to many thousands of kilograms.

The structure of a punch press includes a frame with a table and with a drive rod or shaft mounted on the frame. Force applied to the rod causes the rod to translate toward and away from the table. The table supports a fixed half of the tooling set called a die. The drive rod carries at an end adjacent to the die, a movable half of the tooling set called a punch and designed to closely mate or engage with the die. Punch presses are designed so that tooling can be easily replaced. An actuator mounted on the frame applies a large amount of force to the drive rod during a power stroke to move the drive rod and the punch carried by it toward the table. During each power stroke the actuator drives the punch toward the die to mate with the fixed die, with a workpiece between the punch and die. As the actuator forces the punch and die together, cooperating patterns in the punch and die bend, cut, draw, thin, etc. the workpiece as desired to create the intended product. Some tooling sets are designed with a number of stations so that the workpiece may be shifted sequentially to each of the stations between pressing events to complete the product.

The tooling set is made from tool steel or other hard, durable material. The tooling set must have precision dimensions and its halves are designed to mate with great accuracy as well as to operate without failure for many cycles under the high forces generated by the press. In fact, tool making is itself a recognized craft, with those having such skill in great demand. A tooling set must be designed to be compatible with the press for which it is intended. Design considerations include the amount of force each pressing operation requires and the amount of force the press can develop. By designing the tooling set for compatibility with the press and workpiece, a wide variety of products can be produced efficiently and economically.

The actuator traditional press designs use includes a heavy flywheel mounted for rotation on the frame in combination with a mechanical linkage and a clutch to develop and convert flywheel momentum to force applied to the drive rod. An electrical motor spins the flywheel up to a design speed. After the flywheel is spinning at its design speed, the operator engages the clutch, transferring the flywheel momentum to the mechanical linkage. The mechanical linkage applies the flywheel momentum to the drive rod to force the die halves to mate. On continuing rotation of the flywheel the linkage engages the drive rod to lift the punch from the die, allowing the operator to remove the finished workpiece. It is also possible to provide for a spring which is compressed during the power stroke, to provide for returning the drive rod once the clutch disengages.

The following is well known, but is helpful to clearly define a number of terms which will be frequently used hereafter, and to explain the basics of hydraulic cylinder operation. We use the term “hydraulic cylinder” or more conveniently, “cylinder” here to mean a hydraulic device for converting a flow of pressurized hydraulic fluid to linear mechanical motion, or for converting linear mechanical motion to a flow of pressurized hydraulic fluid. A cylinder comprises a housing having internal walls defining a cylindrical bore essentially closed at one end and open at the other, and with a port in the closed end through which pressurized hydraulic fluid flows. A piston which fits closely to and slides within the bore, defines a cylindrical pressure chamber between itself and the closed end of the bore. The pressure chamber is completely filled with hydraulic fluid. The volume of both the pressure chamber and the hydraulic fluid within the chamber changes as the piston slides within the bore. A piston rod is attached to the piston to transfer force between the piston and an external machine. When a cylinder operates in power mode, pressurized hydraulic fluid is forced into the pressure chamber through the port during a power stroke. During a power stroke, the piston slides linearly from a retracted to an extended position as pressurized hydraulic fluid flows into the chamber. A hydraulic pump of some kind provides the pressurized hydraulic fluid to the chamber.

Newer punch press designs use a hydraulic cylinder as the actuator, and the invention here forms an improvement to these hydraulic presses. The pump which supplies hydraulic fluid to the cylinder is attached to the frame along with the valves and other components of the hydraulic actuator system. Often the pump comprises a hydraulic cylinder operating in pump as opposed to force mode. The pump can draw its energy to operate from a compressed air source.

Of course, some mechanism must be provided for a hydraulic press to restore the piston to its retracted position after a power stroke. For hydraulic actuators, pneumatic or hydraulic pressure applied to the piston on the side opposite the pressure chamber can be used to force the piston to its retracted position. A spring can also be used to provide the retraction force.

In some designs the hydraulic “pump” for a hydraulic press's actuator comprises a so-called air over oil cylinder. An air over oil (AOO) cylinder has a piston which has a compressed air pressure chamber on one end and a hydraulic pressure chamber on the other end. High pressure air entering the air chamber drives the piston to force hydraulic fluid out of the hydraulic chamber and into the actuator hydraulic cylinder. By changing diameters of the pistons appropriately, the force provided by the compressed air can be greatly increased at the output of the hydraulic cylinder. An AOO cylinder-type hydraulic pump provides a moderate amount of high pressure hydraulic fluid inexpensively and with easily controlled pressure.

We find that traditional mechanical actuators have a number of problems in their operation. Among the problems are double strikes, faulty tool alignment, and operator risk. Double strikes for a mechanical press arise when a clutch improperly or unexpectedly applies force to the drive rod to mate the punch with the die without deliberately engaging the clutch. Typically, double strikes occur as the result of wearing or faulty adjustment of the clutch parts. Punch press clutches transfer large amounts of force and operate in dirty and otherwise hostile environments, so it is not surprising that the clutch mechanisms deteriorate with time. In the best of situations, proper maintenance prevents this deterioration, but in the real world proper maintenance does not always occur. And of course unseen and catastrophic failure of critical parts can also lead to double strikes.

Double strikes have the potential to be dangerous. If the operator's hand is between the punch and die for the purpose of removing the finished workpiece from the die, a double strike may smash the hand with obvious potential for serious injury. A less harmful scenario finds the operator's hand safely out of the danger zone but with the workpiece only partially removed or inserted. A double strike in this situation of course spoils the finished workpiece or workpiece blank, and may even damage the tooling.

Faulty tool alignment is a situation where the punch and die do not properly align. This usually also arises from wear or poor maintenance. The result is potentially to damage or even destroy the punch or die, or perhaps to damage the workpiece. Tooling under high loads has even been known to shatter causing broken parts to strike the operator. Even if there is no injury, the damage or destruction of a tooling set is quite enough harm to justify avoidance.

Operator risk occurs of course in the double strike situation as already mentioned. But even during normal operation, it is possible for an operator to carelessly leave her hand in the danger zone. Further, mechanical presses are extremely noisy, which has the potential for hearing damage to the operator. Ear protection reduces this possibility of harm, but makes it more difficult to speak to the operator, which has its own safety problems of course.

Hydraulic actuators have a number of advantages over mechanical actuators. First of all, there are fewer double strikes because the hydraulic and compressed air subsystems tend to deteriorate more slowly and less catastrophically. For example, a compressed air valve may fail by slowly leaking, which conceivably will give an operator time to shut down the press or at least remove her hand from the danger zone. However, the basic hydraulic actuator design does not absolutely preclude double strikes. For example, a malfunctioning compressed air valve can cause a double strike. Nor does a hydraulic actuator deal either with an operator's hand in the danger zone during normal operation, or with tool misalignment.

As to noise, the hydraulic press appears to be much more acceptable than the mechanical press. A hydraulic actuator is much quieter because the high force impact of a clutch arm striking a force-transferring surface on the drive rod is eliminated.

So the present state of the art is that hydraulic cylinder type actuators provide large forces inexpensively and somewhat more safely than mechanical actuators. For this reason they are becoming quite popular for presses. However, they (and mechanical actuators as well) still have significant disadvantages. The enormous forces which these presses apply to the workpiece have the potential to cause serious operator injury. A number of safety features have been devised to prevent operator injury. While these are usually effective, they tend to slow down production, are not always effective, or can even be defeated by careless or rushed operators. Accordingly, it is fair to say that presently available designs do not completely resolve punch press safety issues.

BRIEF DESCRIPTION OF THE INVENTION

We have invented an improvement to punch presses and other devices such as power operated clamps, which dramatically reduces the potential for injury or damage. Instead of trying to prevent the operator from placing himself within the zone of harm, we have devised a way to detect the presence of unexpected resistance or obstruction during an approach phase of the power stroke. When this unusual resistance or obstruction is detected, the press is prevented from completing the power stroke.

In its broadest embodiment, the invention forms a part of a press having a frame and a table mounted on the frame for supporting a die on which a workpiece is to be placed for forming. An actuator assembly carried on the frame includes a drive rod mounted to slide between a retracted position spaced from the table and an extended position spaced adjacent to the table. While the drive rod slides toward the extended position the drive rod applies force to a punch to press the punch against the workpiece and die to complete the forming operation. The actuator assembly comprises an actuator element having a low force mode of operation responsive to a start signal during which the actuator element applies low force to the drive rod. The actuator element also has a high force mode of operation responsive to a continue signal during which the actuator element applies high force to the drive rod. A position sensor is in operative connection to the drive rod, and provides the continue signal responsive to the drive rod achieving a preselected spacing from the table intermediate between the retracted and extended positions of the drive rod.

We implement our preferred version of the invention in a press having a conventional frame and a table mounted on the frame for supporting a workpiece. A hydraulic actuator is mounted on the frame for carrying and applying force to the punch. The actuator preferably comprises a hydraulic cylinder including an actuator piston sliding within an actuator bore, and an actuator piston rod attached to and projecting or extending from the actuator piston toward the table. The piston rod has an end for transferring force from the actuator piston to the punch and the workpiece. The actuator piston defines between itself and an end of the actuator bore an actuator pressure chamber. A fluid port in flow communication with the actuator pressure chamber allows pressurized hydraulic fluid to enter the pressure chamber. Pressure applied by pressurized fluid to the actuator piston causes the piston to slide between a retracted position with the piston rod end retracted from the table and an extended position with the piston rod end adjacent to the table.

A first fluid source supplies relatively low pressure fluid to the actuator's fluid port responsive to a start signal. A variety of devices such as conventional pumps and hydraulic cylinders can function as the first fluid source. In one preferred embodiment, a first hydraulic cylinder operated by compressed air serves as the first fluid source to provide the low pressure fluid. In this arrangement, selecting the cross section area of the piston in the first cylinder and adjusting the air pressure provided to the first cylinder controls the pressure of the fluid provided to the actuator cylinder.

The position sensor is operatively connected to the actuator piston rod end. The sensor provides the continue signal responsive to the actuator piston rod end achieving a preselected spacing from the table intermediate between the retracted and adjacent positions of the piston rod end. This spacing should be chosen to for the most part eliminate the existence of various types of obstructions to or resistance to the normal movement of the piston rod and the punch carried on it. The force generated by the low pressure hydraulic fluid during the first phase of piston rod movement must be great enough to reliably move the piston rod end toward the table and should be low enough to avoid serious injury or damage to any obstruction resisting movement of the punch during the first phase.

A second fluid source supplies relatively high pressure fluid to the actuator's fluid port responsive to the continue signal. One can see that the second fluid source does not supply high pressure fluid to the actuator fluid port unless the actuator piston rod end has reached the intermediate position. The ability of the rod end to reach this position strongly suggests that there is no obstruction or interference to the movement of the rod end.

The sensor can sense the position of the piston rod end in a variety of ways. The position of the piston rod end can be directly detected. It is also possible to detect the rod end position less directly, for example by measuring the position of the actuator piston. Our preferred first fluid source allows a different mechanism still for detecting position of the actuator piston rod end. This preferred first fluid source is a first hydraulic cylinder having a first piston sliding within a first bore and to which force is applied to pressurize fluid in the first cylinder's pressure chamber. This fluid is provided to the actuator fluid port and pressure chamber to create force on the actuator piston.

We have found there is a predictable and repeatable relationship between the positions of the first piston and the actuator piston. The change in volume of the first cylinder's pressure chamber as the first piston slides between preselected retracted and extended positions within the first bore exactly equals the change caused thereby in the volume of the actuator's pressure chamber. By coordinating the dimensions of the first cylinder and the actuator cylinder, the movement of the first piston between its retracted and extended positions can cause the actuator piston to shift from its retracted position to precisely its intermediate position.

We attach to the first piston a first shaft aligned with the movement of the first piston and projecting from the first bore. The first shaft moves with the first piston and reliably indicates position of the first piston. The sensor in this arrangement comprises a switch having a control arm in contact with the first shaft. The switch has a first conductive state responsive to a first position of the control arm, and a second conductive state responsive to a second position of the control arm. The control arm has the first position when the first piston is between its retracted and extended positions, and the second position when the first piston is at the extended position. The switch conducts the continue signal provided by an external source while in its second conductive state. If an obstruction prevents the first piston from reaching the extended position, the switch will not reach its second conductive state, and therefore the second, high force phase of the power stroke does not occur.

While our presently preferred embodiment uses hydraulic actuation, it is possible that mechanical, pneumatic, or even electrical actuation can be adapted to incorporate the method of our invention. Such an improved method is for operating a press apparatus having a frame and a table mounted on the frame for supporting a die of a tooling set on which may be placed a workpiece. A drive rod is mounted on the frame to move toward and away from the table between retracted and extended positions respectively. The drive rod has an end adjacent to the table for carrying a punch of a tooling set. The drive rod transfers force to the punch. This improved method comprises a first step of providing relatively low force to the drive rod responsive to a start signal. This low force urges the drive rod toward the table. The press apparatus senses position of the drive rod while the drive rod is receiving the low force and moving toward the table. The press apparatus provides a continue signal responsive to the drive rod achieving a preselected position between its retracted and extended positions. In responsive to the continue signal the press apparatus supplies relatively high force to the drive rod. This high force step completes the press operation and forms the workpiece according to the pattern in the tooling set.

An obstruction will prevent the drive rod from reaching the preselected position, which results in no continue signal occurring. If no continue signal occurs, the high force step will not occur. This prevents harm or injury arising from the presence of the obstruction.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention as shown inFIG. 1is substantially simplified as compared to a commercial embodiment. Referring toFIG. 1, the punch press shown there includes a C-shaped frame10including an upper arm11and a lower arm13. Lower arm13defines on its upper surface a table12for supporting a tooling half or die15. During an operating cycle of the press, a workpiece27to be formed rests on die15. Upper arm11supports an actuator element comprising a hydraulic cylinder16which is a part of a complete actuator assembly. Cylinder16comprises a housing18with an internal cylindrical (and usually circular cross section) bore22defined by cylindrical walls21and closed by an end wall20. A piston19is mounted to slide along a linear piston path within walls21as suggested by the double arrow shown. Piston19has a fluid-tight fit with walls21while sliding along the piston path. Piston19along with end wall20and side walls21collectively define a pressure chamber28. A fluid port at26allows pressurized fluid supplied by a pipe45to enter and exit from pressure chamber28. A piston rod23is connected to the bottom of piston19and is aligned with the piston path. The lower end of rod23supports a punch or movable die half14which is designed to mate in a predetermined manner with die15. Force applied to punch14drives it to mate with die15, and with workpiece27between punch14and die15as shown, workpiece27is changed to the shape dictated by the tooling.

The force applied to punch14is provided by cylinder16as it receives pressurized fluid at port26from pipe or line45. As a matter of notation or display, dashed lines such as line45denote hydraulic lines or pipes. Dotted lines shown in other Figs. denote pneumatic or compressed air lines or pipes. Readers should also note that the press shown inFIG. 1has been substantially simplified relative to a commercial embodiment which more closely resembles the press shown in the following Figs.

A single operating cycle or power stroke in our invention comprises two distinct phases, a low force first phase in which low pressure hydraulic fluid is provided at port26until punch14reaches what we call an intermediate position, and a second, high force phase where high pressure hydraulic fluid is applied to port26. The second phase is not permitted to start until the first phase has completed successfully.

Low pressure hydraulic fluid is provided on line48to a valve38by a hydraulic fluid source we call a low force actuator advance mechanism33, and which comprises another part of the actuator assembly. In many but not all cases advance mechanism33will comprise a low pressure pump. High pressure hydraulic fluid is provided by a high pressure hydraulic pump or fluid source35which comprises another part of the actuator assembly. High pressure fluid flows through line47to a valve42similar to valve38. Advance mechanism33and pump35can have a variety of structures. Where either provides positive pressure hydraulic fluid there are rotary pumps capable of providing fluid of adequate pressure. But for the relatively small amount of pressurized fluid which operates cylinder16, it is more efficient to use as a pump, a separate cylinder operating in a pump mode and to whose piston force is applied. A bicycle tire pump is an example of this type of pump. It is even possible to have a single high pressure pump which functions as both advance mechanism33and pump35, and whose high fluid pressure is dropped by a throttling valve of some type to provide the low pressure fluid on path48.

Valves38and42are operated in a way allowing the apparatus ofFIG. 1to operate in the mode implementing the invention. The open or closed state of valves38and42is preferably electrically or pneumatically controlled by signals carried by paths39and40which are applied to what is shown as “O” and “C” control points of valves38and42. The state of a valve38or42is dictated by the most recent signal applied to its O and C points. That is, each valve38and42operates in a way similar to that of an electronic flip-flop in that the current state of a valve38or42is set by the most recent control signal received at its O or C point. For example, if valves38and42are electrically controlled, a voltage pulse on path39causes valve38to open and valve42to close. A later similar pulse on path40causes valve38to close and valve42to open. We assume that operating or actuation time of valves38and42is small compared to the operations of cylinder16controlled by these valves. In point of fact, valves38and42can be replaced by switches or other controls which cause mechanism33and pump35to operate when the associated valve is to be opened. With such an arrangement, check valves or some other mechanisms on lines48and47are necessary to prevent backflow of pressurized fluid to either mechanism33or pump35from the other.

A position sensor25is operatively connected to punch14and the end of rod23to detect the position of punch14relative to die15. When piston19reaches a preselected position within bore22, sensor25provides a continue signal on path40. The preselected position of piston19corresponds to a preselected intermediate position of punch14relative to die15. InFIG. 1sensor25is shown adjacent to cylinder walls21so as to detect the position of piston19, which of course is directly connected to piston rod23and through it, to punch14. There are a variety of devices which can detect the position of piston19, the rod23end, and punch14. For the sake of generality we show a sensor25inFIG. 1which directly detects piston19position, but our preferred embodiment uses a different mechanism which we show inFIGS. 2-6. Showing sensor25as in the form ofFIG. 1makes the point that there are a variety of functionally equivalent solutions to detecting position of punch14, and more to the point, detecting when punch14reaches the intermediate position defined for it. The position-sensing mechanism chosen for a particular system should cooperate and integrate well with the other components of the system.

As mentioned above, the apparatus ofFIG. 1is substantially simplified in a number of ways so as to allow the invention to be described broadly. One of these simplifications is the absence of any means to reset the apparatus to the ready state shown in FIG.1. InFIG. 1, the press is ready for a complete operating cycle with both valves38and42closed and piston19in its retracted position. The position of piston19and the status of valves38and42change during an operating cycle, and these must be restored to the ready state prior to the start of an operating cycle. The reset functionality is not a part of the invention, and we expect a person of skill in the art to easily add suitable structure to implement the reset actions.

With the apparatus ofFIG. 1in the ready state, a START signal is applied to path39which begins an operating cycle. The START signal causes valve38to open and valve42to close initiating the start of the low force phase of the operating cycle. Low pressure fluid from mechanism33flows through valve38causing the pressure within pressure chamber28to allow piston19to move downward away from its retracted position and toward an extended position. This causes punch14to approach workpiece27and die15. During this phase of operation, piston19and punch14move with relatively low force. When punch14reaches the preselected intermediate position and piston19the corresponding position, sensor25provides a continue signal on path40. This preselected intermediate position of punch14occurs between the retracted position and the fully extended position of piston19and rod23. The continue signal causes valve38to close and valve42to open, starting the high force phase of the operating cycle. High pressure fluid flows through lines47and45to pressure chamber28causing piston19to advance toward its fully extended position with relatively high force, pressing punch14against workpiece27and die15and completing the operating cycle. Apparatus not shown detects when there is no further motion of piston19, at which time a signal is applied to close valve42. At this point the unshown reset mechanism causes piston19to return to its retracted position and the press to return to ready status.

The pressure of the hydraulic fluid provided by mechanism33must be great enough to assure that during normal situations, the force applied to piston19is sufficient to cause punch14to approach workpiece27and achieve the intermediate position. Further, the force should be low enough to prevent any serious injury or damage should there be an obstruction, perhaps a relatively fragile obstruction such as a finger, between punch14and die15. The pressure of the hydraulic fluid in line48should result in a total force advancing punch14in the approximate range of 50-100 lb. or 25-50 kg. This amount of force is adequate in most cases to overcome the friction in the system and move the piston19and punch14to the intermediate position, and yet not cause serious damage or injury to an obstruction such as a finger or misaligned die15or punch14. A 75 lb. (34 kg.) force applied by piston19is roughly equivalent to having one's finger stepped on, definitely uncomfortable but not likely to cause any serious injury. If the obstruction between the punch14and die15is the operator's finger for example, the finger gets no more than a painful pinch, rather than being severed or crushed.

The majority of the force for advancing a punch14having low mass will be provided by low pressure hydraulic fluid from mechanism33. When dealing with larger presses and heavier punches however, the weight of punch14may become significant in calculating the total force present during the approach phase. In these systems, the weight of punch14alone may generate force sufficient to cause punch14to move toward the intermediate position without any pressurized hydraulic fluid from mechanism33, in which case mechanism33need not pressurize fluid in line48. In fact, it is entirely possible for a very heavy punch14that mechanism33will have to operate in what we will call negative pressure mode to retard or oppose the punch-weight generated force with which punch14approaches die15. We include both negative pressure devices such as throttling valves and positive pressure pumps within the definition of mechanism33for generally describing our invention. When operating in negative pressure mode, mechanism33must maintain an appropriate constant negative pressure so as to limit force buildup on an obstruction which may be present. As a general rule, we prefer punch14to approach die15with as little force and speed as is needed to allow for reliable and suitably rapid and efficient operation.

Where the combined weight of punch33, rod23and piston19is so great that the resultant force urges punch14toward die15with excessive force, mechanism33can comprise a throttling valve. A throttling valve reduces the pressure of fluid flowing through it and meters the rate at which the fluid flows through it. Thus, a throttling valve serving as mechanism33provides reduced pressure fluid to the actuator's fluid port. This reduced pressure fluid applies force to the actuator piston opposing the weight carried by the actuator piston, thereby reducing the total force applied by punch14to any obstruction which might be present between punch14and die15.

On this point, it is usually preferable to move punch14rapidly during the approach phase so that time is not wasted, which would lengthen the entire operating cycle and reduce the production capacity of the press. Heavy punches14however, may require a slower approach speed to avoid injury or damage to an obstruction resulting simply from the substantial momentum inherent in a rapidly moving heavy mass. But in most cases, the approach phase is a small percentage of the entire time for a complete press cycle including loading and unloading the workpiece from die15. Slowing the speed of the approach phase by even 50-75% will not usually create an unacceptable delay. And we recommend skillful tooling design which may sometimes allow shifting of some of the mass from punch14to die15, reducing punch14momentum during the approach phase.

Another important parameter for realizing the safe operation of which our press system is capable, is selecting the intermediate position for punch14(at which the continue signal issues). The space or gap between punch14and die15when sensor25provides the continue signal should be small enough so that most or all of any potential obstructions prevent piston19from moving punch14to the intermediate position. In most cases, we feel that 0.25 in. or 1 cm. is a suitable gap between punch14and die15to define the intermediate position of piston19. This gap will cause almost any finger trapped between punch14and die15to prevent punch14from reaching its intermediate position, and thus will prevent from occurring, the high force phase which can cause serious injury.

One problem which may on occasion arise when practicing this invention, is dealing with a workpiece27which is so thick that when punch14is resting on workpiece27, the gap between punch14and die15is wide enough to allow a finger to intrude. Creative use of skirting forming a part of the punch14or die15, or even a dense foam band around the periphery of punch14or die15which is intended to contact an obstruction, may operate to stop punch14before it reaches its intermediate position.

FIGS. 2-4show a more complex version of the invention, and incorporate features which we prefer. These Figs. show the system in three stages of progression during an operating cycle. Similar or identical elements inFIGS. 2-4have reference numbers similar to those in FIG.1. The punch press itself ofFIGS. 2-4differs little from that of FIG.1. Only the mechanism for sensing that punch14has reached the intermediate position, and line26for restoring piston19and punch14to their retracted or ready position from their extended position are totally different from FIG.1. As previously mentioned, dashed and dotted lines respectively denote hydraulic and pneumatic lines.

FIGS. 2-4show pneumatically operated air over hydraulic cylinders serving as advance mechanism33and pump35, and these are indicated at33′ and35′. Each of the elements ofFIGS. 2-4are typically mounted on C frame10as indicated by the frame symbol at10on cylinders33′ and35′ as well as on hydraulic fluid reservoir50. Most of the remaining elements shown inFIGS. 2-4are also mounted on frame10. The presentation inFIGS. 2-4is intended to draw attention to the novel structure of the invention rather than the commercial form, although the commercial form has all of the elements shown. Anyone who is skilled in mechanical design can easily develop a suitable configuration for the elements shown inFIGS. 2-4. The description ofFIGS. 2-4which follows assumes that those skilled in the art are able to calculate pressures and forces generated in hydraulic and pneumatic systems. These have been a part of the art for a long time and the physics is not difficult, so this assumption is reasonable.

Power for operating the system ofFIGS. 2-4is provided by compressed air in our preferred embodiment. However, it is possible that certain lower force presses may permit use of electrical power for pressurizing the hydraulic fluid, perhaps with high pressure gear pumps or rack and pinion actuators. While we show the preferred compressed air operation, electrical power is a viable option as well, and may be considered to be interchangeable with compressed air power.

Compressed air is provided from a high pressure air source85typically not mounted on frame10. Output pressure of source85may be in the range of 100-300 psi. or 7 to 21 kg./cm.2. Flow of compressed air to the press system is controlled by an air valve88receiving a control signal on path87. For powering cylinder33′, high pressure compressed air has its pressure reduced to perhaps 30-100 psi. (2-7 kg./cm.2) through a throttling valve90which supplies an air line91. Air over hydraulic cylinder33′ is typical in having two pressure chambers, a pneumatic chamber55into which compressed air flows, and a hydraulic chamber58which provides pressurized hydraulic fluid, in the case of cylinder33′, at relatively low pressure. Compressed air flows from throttling valve90through line91into chamber55where it applies force to a piston57mounted for sliding within cylindrical bore59. As piston57is forced by air pressure within chamber55to move down and reduce the volume of chamber58, low pressure hydraulic fluid flows through line62to a port64of cylinder35's pressure chamber71.

High pressure hydraulic fluid is provided by a compound air over hydraulic cylinder35′ having a pneumatic pressure chamber80and a hydraulic pressure chamber71. Compressed air is provided on line83to chamber80. Compressed air piston77slides within wall75and provides force to connecting rod74as the compressed air in chamber80exerts force on piston77. Cylinder35′ is called compound because force generated by a large diameter pneumatic piston77is applied to a small diameter hydraulic piston67. Piston67slides within cylinder wall69with the force provided by piston77pressurizing hydraulic fluid within chamber71. Compressed air is provided typically at higher pressure to chamber80than to chamber55, perhaps at line pressure as shown inFIGS. 2-4. The product of the piston77face area and the chamber80pressure specifies the force applied to rod74and piston67. The force applied to rod23equals the force applied to rod74times the ratio between the area of the piston19face and the area of the piston face68. Controlling force applied to pressure chamber80allows reasonably adequate control of the force piston applies to punch23during the power phase.

One important feature of theFIGS. 2-4system is the mechanism for preventing backflow of high pressure hydraulic fluid from chamber71to chamber58. We prefer that port64be located within chamber71very close to face68of piston67when piston67is in its totally retracted position as shown in FIG.2. As piston67begins its power stroke, face68passes port64and wall66closes port64preventing backflow of hydraulic fluid into chamber58from chamber71.

We also use a novel mechanism to sense the instant when punch14reaches the intermediate position. There is a precise relationship between the position of piston57and the position of piston19during the approach or low force phase of an operating cycle. We find it convenient as well as extremely accurate to sense position of piston19by sensing position of piston57. While we show the sensing and control elements as pneumatic, electrical control is equally suitable. To provide the required control function, a sensing shaft52attached to the pneumatic side of piston57projects from the end56of cylinder33′. A pneumatic valve or switch92receives high pressure compressed air from line89and when conducting or open, allowing compressed air to flow to line83. Valve92has a control arm94which rides on or otherwise senses the presence of a preselected feature of shaft52, the shaft end in this embodiment. With shaft52in the position shown inFIG. 2, valve92is closed or non-conducting, preventing flow of compressed air through valve92. When shaft52advances to the position shown inFIG. 3, control arm94shifts due to sensing the shaft52end, allowing valve92to conduct or pass compressed air to line83.

Accurate measurement of punch14position requires a constant volume of hydraulic fluid in the system comprising compression chambers58,71, and28. Since a certain amount of hydraulic fluid tends to leak from the system during use, a reservoir50is provided from which replacement fluid flows through a check valve48between operating cycles to keep the system completely filled.

An operating cycle starts with the system in its ready state as shown inFIG. 2with pistons57,67,77, and19all in their retracted positions. Valve87is opened allowing compressed air to flow through throttling valve90where the pressure is dropped. The reduced pressure compressed air flows through line91to chamber55of cylinder33′. This causes piston57to move downwards toward its extended position as shown inFIGS. 3 and 4, enlarging pressure chamber55and shrinking pressure chamber58. Hydraulic fluid in chamber58flows through line62and port64to pressure chamber71, from where it flows to line45. From line45, the pressurized fluid flows through port29into chamber28, causing piston19to slide away from its retracted position toward die15. During normal operation piston57reaches the extended position shown inFIG. 3, at which point piston19, connecting rod23, and punch14all have reached their intermediate position. Simultaneously with punch14reaching the intermediate position, control arm94detects the end of sensing shaft52as piston57slides into chamber58. At this point valve92opens, allowing high pressure compressed air to flow to chamber80. Piston77slides rightwardly from its retracted position shown towards its extended position, sliding piston67rightwardly as well. A volume of high pressure hydraulic fluid flows from chamber71through line45to pressure chamber28. This constitutes the second, high force phase of an operating cycle. Piston19slides from its intermediate position toward its extended position under the influence of the high pressure hydraulic fluid flowing into pressure chamber28. The change in volume of chamber28during this second phase of the operating cycle very nearly equals the change in the volume of chamber71. Punch14is forced against workpiece27and die15to perform the machining of workpiece27and complete the operating cycle. The operating cycle is complete when piston19has reached its extended position and die14is stopped. Stopping of die14can be detected in a number of ways, for example by detecting air flow through line37from the chamber below piston19.

After the second, high force phase of the operating cycle is complete, the reset phase begins. As mentioned above, this is relatively well known. A simple mechanism for the reset function is providing compressed air to lines36and37to force pistons19and77to the retracted positions shown in FIG.2. We find that it is possible to restore piston57to its retracted position as well by continuing compressed air flow in line37after piston77has reached its retracted position where port64is uncovered.

Should punch14encounter an obstruction during an approach phase, the low force producing motion of punch14causes advance of punch14to stop, and before the intermediate position has been reached. The system essentially halts, frozen in that position. In this simplified position, the operator will have to remove the START signal from control path87. This closes valve88and removes power from cylinders33′ and35′. At this point the operator will be able to safely remove the obstruction whatever it is. Of course, inserting anything other than workpiece27in the space between punch14and die15is risky, and should not be done.

FIGS. 5 and 6show an integrated structure for cylinders33′ and35′ with pressure chambers58and71sharing a common wall70. Line62has been eliminated and port64is in wall70. In the ready position shown in FIG.5and which corresponds toFIG. 2, port64is uncovered by piston67. InFIG. 6, which corresponds toFIG. 4, the high force phase is complete, with port64covered by piston wall66shortly after piston67starts its motion.

FIG. 7shows some alternatives which may be useful in certain press systems. For situations where punch14may be able to provide with its own weight, all of the force required to advance itself toward die15without positive force from mechanism33, it is possible to add weights30to punch14. These weights30may be add-ons or even integral with the rest of punch14. The idea here is to weight punch14sufficiently to provide the preferred 50-100 lb. net amount of force for advancing punch14toward die15during the approach phase.

Another feature of this invention shown inFIGS. 7 and 8, solves a problem which arises with a relatively thick (tall) workpiece27. A thick workpiece27may create a situation where punch14contacts workpiece27before the possibility of an obstruction between punch14and die15has been completely eliminated. While it may not be possible or at least easily possible to detect the punch14-die15misalignment type of obstruction, it may be possible to detect presence of some types of objects forming obstructions when thick workpieces27are involved. Specifically, it may be possible to detect presence of a hand or finger29(FIG.7), in the space between punch14and die15. We do this by using a resilient strip31attached to the operating face of punch14. InFIG. 7, a single resilient strip31is shown in end view. InFIG. 8, three resilient strips31,31a, and31bare shown attached to the operating face of punch14along three sides thereof. Strip31should face the operator directly. This arrangement of strips inFIG. 8will detect most obstructions which are operators' hands or fingers and most likely to be inserted from the front or sides of punch14. For improved perspective, a representative pattern36of tooling is shown as well.

It is possible to mount strip31on the die15as well as on punch14. Functionally, both arrangements should be equivalent.

Should a finger or hand29be in the space between punch14and die15, at the start of an operating cycle, the approach phase will bring strip31into contact with the finger or hand29before punch14reaches its intermediate position. Strip31should have sufficient stiffness or density when pressing against a finger or hand to resist further motion of punch14and prevent punch14from reaching its intermediate position. High density foam or soft rubber such as that used in pencil erasers will often be suitable. Specifically, the material comprising strip31may preferably be of the type which will deflect approximately 0.01 to 0.1 in. (0.25 to 2.5 mm.) when pressing against 0.1 in.2of an obstruction surface with 50 lb. force. The important factor in this selection is to avoid serious injury to a finger or hand and yet be able to easily resist movement of punch14during the first phase of the operating cycle with little compression of the strip material.

In order to prevent substantial changes in the height dimension of a strip31during normal operation, a slot or groove32may be provided in die15with which strip31mates during the high power phase. Strip31will deflect slightly when encountering an obstruction but will still provide adequate resistance to further advancing of punch14. Most importantly, the pressure which is applied to a finger caught between the die15and strip31by even, say 100 lb. of force advancing punch14, will not cause serious injury.