BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention concerns punch presses and more particularly a drive for a punch press ram which is coupled to punching or forming tools. 
     2. State of the Art 
     Punch presses for forming features or punching holes in sheet material workpieces involve a ram which is coupled to a forming or punching tool and drives the tool through a sheet material workpiece. Typically, the ram is driven by a crank mechanism, rotated by an AC electric motor. A flywheel may be interposed, with a clutch-brake controlling the connection of the flywheel to the crank mechanism, so that the stored energy in the rotating flywheel can be utilized to drive the ram. 
     A need for more sophisticated punching and forming motions has now been recognized as controlling the ram velocity to minimize punching noise, sometimes varying the velocity over the course of a punching stroke. As another example, the punch cycle time can be reduced by varying the speed of the ram over the punching cycle with a rapid ram advance and retraction combined with slow speed during actual punching. 
     Variable stroke control is also useful, as when forming louvers, and to minimize tooling set up. 
     In order to accomplish such sophisticated and flexible control over the ram motion, servo controlled hydraulic drives for rams have been developed, which allow a good deal of control over ram motion, such as variable speed and stroke punching. 
     See as examples of such hydraulic press ram drives, U.S. Pat. No. 4,823,658 issued on Apr. 25, 1989 for &#34;Punch Presses&#34;, U.S. Pat. No. 4,208,935 issued on Jun. 24, 1980, for a &#34;Control System for Hydraulic Press&#34;; U.S. Pat. No. 5,031,431 issued on Jul. 16, 1991, for a &#34;Method and Device for Controlling the Stroke of a Press Machine&#34;. 
     However, hydraulic equipment generally suffers from the disadvantages of increased cost and maintenance requirements, operates more slowly, and requires auxiliary cooling equipment. 
     Electrical servo motor ram drives have also been proposed, as described in U.S. Pat. No. 5,279,197 issued on Jan. 18, 1994, for a &#34;Punch Press&#34;; and U.S. Pat. No. 5,289,096 issued on Feb. 22, 1994, for a &#34;Press Machine Stroke Operation Mechanism and Operation Control Method Therefor&#34;. Those patents describe a ball screw toggle mechanism for converting the rotary output of an electrical servo motor to a reciprocating ram motion. 
     The cost of a ball screw mechanism sufficiently sturdy to withstand the high level punching forces encountered in metal working would add substantially to the cost of a press. 
     The ball screw toggle mechanism described in those patents also requires a rapid motor reversal at bottom dead center, placing excessive demands on the mechanical and electrical components, and slowing the cycle time. 
     U.S. Pat. No. 5,279,197 also describes an electrical servo drive motor combined with a slider crank mechanism. However, in that combination, the servo motor is driven in a single direction, thus requiring the motion of a complete crank revolution to complete a punching stroke. The slider crank mechanism is also not suited to withstand the high forces required for metal working. 
     Accordingly, it is the object of the present invention to provide an electric servo motor punch ram drive mechanism which is capable of easily generating the high tonnage requirements for metal working, does not require rapid motor reversal when the punch is fully advanced, minimizes cycle time and improves flexibility in controlling ram velocities and stroke distances. 
     SUMMARY OF THE INVENTION 
     The present invention comprises the combination of a reversible electrical servo motor with an eccentric crankshaft ram drive. The reversible electrical servo motor drives the crankshaft eccentric through reduction gearing. A servo control allows various oscillating modes of the motor to carry out punching or forming cycles. The crankshaft may oscillate the eccentric through a programmed angular increment to carry out a predetermined punching stroke of a ram from a hover height defined by an initial angle of the crankshaft to a ram end position defined by the advanced angular position of the crankshaft. The reversing electric servo motor oscillates the crankshaft between these positions during execution of a partial stroke punching program. 
     During normal stroke punching, the motor oscillates the crankshaft between end positions on either side of bottom dead center to carry out punching from each end position, so that motor reversal does not occur until after completion of the retraction stroke. 
     The oscillating modes minimize cycle times by minimizing the motion required. 
     The motor can also rotate the crankshaft continously in a single direction for maximum stroke motion required for particular forming operations, or combined with synchronized table incremental feed to carry out nibbling very efficiently. 
     The reversible electrical servo motor can be programmed for any desired punch velocity acceleration, deceleration, positioning, dwell, etc. The punch velocity can be varied over the punch stroke, as can be done with the hydraulic actuators but without the operational disadvantages of hydraulic equipment. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a fragmentary partially sectional view through a portion of a punch press incorporating an electrical servo motor eccentric journal crankshaft drive combination according to the present invention, using a linear transducer to generate ram position feedback signals. 
     FIG. 1A is a side elevational view of the crankshaft eccentric journal and coupled punch ram mechanism shown in FIG. 1, in the fully raised condition. 
     FIG. 1B is a side elevational view of the components shown in FIG. 1A shown in the full down condition. 
     FIG. 2 is a fragmentary, partially sectional view of an alternate embodiment of the punch press shown in FIG. 1, with a rotary encoder used to generate ram position feedback signals. 
     FIGS. 3A-3I are diagrammatic representations of crankshaft angular positions through which the crankshaft eccentric journal is rotated or oscillated by the electrical servo motor drive. 
    
    
     DETAILED DESCRIPTION 
     In the following detailed description certain specific terminology will be employed for the sake of clarity and a particular embodiment described in accordance with the requirements of 35 USC 112, but it is to be understood that the same is not intended to be limiting and should not be so construed inasmuch as the invention is capable of taking many forms and variations within the scope of the appended claims. 
     Referring to the drawings, and particularly FIG. 1, a portion of a press frame 10 is shown which rotatably mounts a crankshaft 12 between housing plates 14A, 14B. The crankshaft 12 includes an eccentric journal 16 which receives a bushing 18 carried in a bore in the upper end of a pitman arm 20. 
     The lower end of the pitman arm 20 is connected with a fulcrum pin to a press ram 22 slidable in a guide bushing 24 fitting in a sleeve 26 mounted to lower plate 28 of the press frame 10. 
     The lower end of the press ram 22 is adapted to be coupled to the upper end of each of any of a series of punches in an upper turret 32 by means of a T-slot in the manner well known in the art. 
     A sheet material workpiece W held on a table 11 which has a positioning drive 13 which moves the table 11 to carry the workpiece to position a particular location on the workpiece W at the punching station. The workpiece W is punched (or formed) by downward stroking of the ram 22 forcing the punch or forming tool 30 through the workpiece W. A mating die in a lower turret (not shown) receives the punch tool 30 after passing through the workpiece W. 
     The stroking of the ram 22 occurs by rotation of the crankshaft 12, the ram 22 fully elevated when the eccentric journal 16 is rotated to its highest point (FIG. 1A) and fully depressed when the eccentric journal 16 is rotated to its lowest point at bottom dead center (FIG. 1B). 
     According to the concept of the present invention, the crankshaft 12 is driven by a reversible electrical servo motor 34 mounted on a pedestal 36 mounted alongside the side plate 14B. Reduction gearing 38, 40 drivingly connects a crankshaft extension 42 and the output shaft 44 of the electrical servo motor 34 so that rotation of the output shaft 44 of the servo motor 34 causes a lesser rotation of crankshaft 12. 
     Activation of the electrical servo motor 34 is controlled by the punch program controls 46 to cause a predetermined ram motion. Such program controls are generally well known and may take the form of a suitable programmed microprocessor, as described in U.S. Pat. No. 4,823,658 described above. 
     A linear transducer 48 is operatively connected to the ram 22 as shown, generating position feedback signals which are used in the controls 46 to achieve a desired ram position and motion. 
     FIG. 2 shows an alternate arrangement for generating ram position feedback signals. A pulley 50 is fixed to a second extension 52 of the crankshaft 12A. A rotary encoder assembly 54 is mounted to press frame side plate 14A on a bracket 56. A second pulley 58 is mounted to an encoder shaft 60 rotatably mounted in a bearing mount 62 supported by bracket 56. 
     A rotary encoder 64 generates feed back signals corresponding to the degree of rotation of the encoder shaft 60, in turn rotated by belt 66, so that these signals correspond to the angular position of the crankshaft 12A, which in turn corresponds to the position of the ram 22. 
     According to the concept of the invention, the electrical servo motor 34 is operated by the controls 46 so as to either rotate or oscillate the crankshaft 12 to carry out punching, forming, or nibbling. 
     In the full rotation mode as indicated in FIGS. 3A-3C, the crankshaft 12 rotates in one direction to carry the ram between fully raised to fully lowered positions. 
     The full stroke may be carried out in discrete rotations to carry out forming operations requiring maximum stroking, such as when forming tall louvers. 
     The motor 34 can be continuously rotated to conduct nibbling, by program controlling the table drive 13 to incrementally advance the table 11 in synchronism with the crankshaft rotation to very efficiently punch small portions progressively along a line of advance of the workpiece W. 
     FIG. 3A shows the crankshaft 12 rotated to an angular position corresponding to a fully raised position, in which the punch tool 30 is located at a maximum height above the workpiece W prior to a punching stroke. 
     A full punch stroke is accomplished by rotation of the crankshaft 12 to bottom dead center as shown in FIG. 3B, the crankshaft then rotated to the angular position to return the ram 22 to the fully raised height as shown in FIG. 3C. 
     The electrical servo motor 34 is preferably normally operated to oscillate the crankshaft 12 between angular positions on either side of bottom dead center corresponding to punching from the &#34;hover height&#34; to the fully down stroked position of the punch tool 30. 
     The &#34;hover height&#34; is the programmed clearance height to which the punch tool 30 is partially retracted during program controlled movement of the workpiece W between punching strokes. This ram height is below the fully retracted position of the ram 22, assumed prior to initiation of a program, in order to shorten the punching cycle time. 
     For a programmed stroke less than full stroke, the crankshaft 12 is rotated forward through the angle α, and past bottom dead center by the angle α, as shown in FIG. 3E driving the ram 22 through a complete punching stroke, less than a full ram stroke undergone. 
     This eliminates the need for reversing the servo motor 34 at bottom dead center, and allows a gradual stop before executing the next cycle. 
     The crankshaft 12 is then oscillated by being rotated reversely to the angular position α on the other side of bottom dead center (bdc) as shown in FIG. 3F. 
     FIGS. 3G-3I show a program motion where a full down stroke is not executed. The servo motor 34 is oscillated between a retracted position α degrees before dead center to an advanced position β degrees before dead center. 
     In both oscillating modes, crankshaft rotation necessary to carry out the stroke is minimized to save cycle time. In the normal mode, rapid motor reversal is avoided. 
     The electrical servo motor 34 can execute programmed accelerations, decelerations, dwell periods, rapid approach, slow punch, fast strip, etc. as desired by the programmer. Thus, the sophistication allowed by the hydraulic actuators is achieved without being burdened with the disadvantages of hydraulic equipment as described above.
A punch press ram drive is described in which a reversible electrical servo motor is coupled to a crankshaft having an eccentric journal driving a pitman arm connected to the ram. The electrical servo motor is operated by program controls so as to either cause rotation or oscillation of the crankshaft to drive the ram through various programmed stroking modes, in which punch position, velocity, acceleration, varying over the course of a stroke may be programmed.