US3835743A

US3835743A - Punch press workholder

Info

Publication number: US3835743A
Application number: US00367523A
Authority: US
Inventors: V Taylor; R Johnson; R Faison; W Choate; S Banachowski
Original assignee: Warner and Swasey Co
Current assignee: Murata Wiedemann Inc
Priority date: 1973-06-06
Filing date: 1973-06-06
Publication date: 1974-09-17
Anticipated expiration: 1991-09-17
Also published as: JPS571336B2; GB1470333A; JPS5031490A

Abstract

An improved punch press workholder has a relatively high flexural rigidity in the X and Y directions and a relatively low flexural rigidity in the Z direction. The high flexural rigidity of the workholder in the X and Y directions enables it to hold a work sheet accurately in place during punching operations. The low flexural rigidity in the Z direction enables the work sheet to assume a proper position relative to a die under the influence of forces applied to the work sheet by a punch. The workholder includes a gripper having an upper or movable pivot arm which is pivotally connected with a lower or fixed pivot arm. A resiliently yieldable connection is integrally formed with the fixed pivot arm and connects the fixed pivot arm with a rigid base. The resiliently yieldable connection has a generally rectangular cross-sectional configuration with a small dimension measured in a direction parallel to the Z axis and a relatively large dimension measured in a direction parallel to the X axis so that the yieldable connection has a high flexural rigidity in the X direction and a low flexural rigidity in the Z direction.

Description

United States Patent Taylor et al.

[451 Sept. 17, 1974 PUNCH PRESS WORKHOLDER [75] Inventors: Victor E. Taylor, Phoenixville;

Wallace R. Choate, Norristown; Stanley Banachowski, Collegeville; Robert W. Johnson, Philadelphia; Robert C. Faison, Ridley Park, all of Pa.

[73] Assignee: The Warner & Swasey Company, Cleveland, Ohio [22] Filed: June 6, 1973 [21] Appl. No.: 367,523

[52] US. Cl 83/146, 83/277, 83/412, 83/465, 83/566, 269/56 [51] Int. Cl B26d 7/02 [58] Field of Search 83/146, 277, 566, 458, 83/409, 412, 465; 269/56 [56] References Cited UNITED STATES PATENTS 3,230,806 l/l966 Rovoldt 83/146 3,460,4I5 8/1969 Philipp 83/277 X 3,613,491 lO/l97l Kahmann 83/l46 Primary ExaminerFrank T. Yost [57] ABSTRACT An improved punch press workholder has a relatively high flexural rigidity in the X and Y directions and a relatively low flexural rigidity in the Z direction. The high flexural rigidity of the workholder in the X and Y directions enables it to hold a work sheet accurately in place during punching operations. The low flexural rigidity in the Z direction enables the work sheet to assume a proper position relative to a die under the influence of forces applied to the work sheet by a punch. The workholder includes a gripper having an.

upper or movable pivot arm which is pivotally connected with a lower or fixed pivot arm. A resiliently yieldable connection is integrally formed with the fixed pivot arm and connects the fixed pivot arm with a rigid base. The resiliently yieldable connection has a generally rectangular cross-sectional configuration with a small dimension measured in a direction parallel to the Z axis and a relatively large dimension measured in a direction parallel to the X axis so that the yieldable connection has a high flexural rigidity in the X direction and a low flexural rigidity in the Z direction.

13 Claims, 7 Drawing Figures la: 60 i \\\\\\\v PMENTEBSEHTIQM sum 1 or 2 I-TG. l.

PUNCH PRESS WORKHOLDER BACKGROUND OF THE INVENTION The present invention relates to a workholder and more specifically to a workholder which may be utilized in a punch press and has a relatively low flexural rigidity in the vertical or Z direction to enable a work sheet to be raised to a stripper of the press during a return stroke of a punch.

Turret-type punch presses commonly have workholders to hold a work sheet in a predetermined position relative to a die during punching operations. Known workholders, such as the workholders disclosed in U.S. Pat. Nos. 3,174,747 and 2,701,017, have a very rigid construction. This rigid construction is necessary in order to accurately position the work sheet relative to the X and Y axes of the punch press and retain the work sheet against movement during punching operations.

Disengagement of the work sheet from the punch of a press is facilitated if the work sheet can move upwardly into engagement with a stripper under the influence of forces applied to the work sheet by the punch. Once the work sheet has engaged the stripper, continued upward movement of the punch disengages it from the work sheet. The relative high structural rigidity of known workholders in the vertical or Z direction has prevented or substantially retarded upward movement of the work sheet under the influence of forces applied to it by the punch. In addition, the relatively high flexural rigidity of known workholders in the vertical or Z direction prevents or retards downward movement of the work sheet toward a die during punching operations. This can result in misalignment of the worksheet and die.

SUMMARY OF THE PRESENT INVENTION The present invention provides a workholder which has a relatively high flexural rigidity in the X and Y directions and a relatively low flexural rigidity in the vertical or Z direction. The relatively low flexural rigidity of the workholder in the 2 direction enables a work sheet to be moved downwardly into engagement with a die during a working stroke of a punch. In addition the work sheet can move upwardly into engagement with a stripper under the influence of forces applied to the work sheet during a return stroke of a punch. The relatively high flexural rigidijy of the workholder in the X and Y directions enables it to accurately position and hold a work sheet during punching operations.

The workholders relatively low flexural rigidity in the Z direction is obtained by providing a resiliently yieldable connection between a gripper assembly and a rigid base of the workholder. The gripper assembly includes fixed and movable pivot arms having jaws which clampingly engage a work sheet. The fixed pivot arm is connected with the rigid base of the workholder by the resiliently yieldable connection.

The resiliently yieldable connection is integrally formed with the fixed pivot arm and has a small crosssectional area. This cross-sectional area has a very small height or Z axisdimension and a rather large base or X axis dimension. The small height or Z axis dimension results in a relatively low flexural rigidity in the vertical or Z direction. However, the large base of X axis dimension results in a relatively large flexural rigidity in the X direction.

To prevent the resiliently yieldable connection from being overstressed and plastically deformed, a stop arrangement is provided to limit the extent to which the connection can be flexed in the Z direction. To further protect the resiliently yieldable connection, a guard is provided to prevent the connection from being damaged by engagement with falling workpieces and other foreign objects.

Accordingly, it is the object of this invention to provide a new and improved workholder which is adapted to be utilized in a punch press and which has a relatively high flexural rigidity in the X and Y directions to enable it to accurately position and hold a work sheet during punching operations and a relatively low flexural rigidity in the vertical or Z direction to enable the work sheet to be moved in a vertical direction under the influence of forces applied to it by the punch during a work and a return stroke of the punch.

Another object of this invention is to provide a new and improved workholder having a pair of arms which are pivotally connected at a location intermediate their ends, a pair of work engaging jaws fixed to outer ends of the arms, an actuator for opening and closing the jaws, and a flexible connection which permits movement of the arms in the same plane as in which they are pivoted by operation of the actuator.

Another object of this invention is to provide a new and improved punch press for working on sheets of material and which includes a gripper assembly having clamp surfaces to grip a sheet of material and hold it against movement in a generally horizontal plane during a work stroke of a punch and to enable at least a portion of the sheet of material to move in a vertical or Z direction during either a work or return stroke of the punch and wherein the gripper assembly includes a resiliently yieldable connection which interconnects the clamp surfaces and a base portion of the gripper assembly to enable the clamp surfaces to move through a substantial distance in a vertical direction under the influence of forces applied to the sheet of material by the punch as the punch is moved through either a work or return stroke.

BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other objects and features of the present invention will become more apparent upon a consideration of the following description taken in connection with the accompanying drawings wherein:

FIG. 1 is partially broken away illustration of a turret punch press and depicting the relationship between a punch assembly, a work sheet support table, and a pair of workholders constructed in accordance with the present invention;

FIG. 2 is a plan view of one of the workholders of FIG. 1;

FIG. 3 is a sectional view, taken generally along the lines 33 of FIG. 2, illustrating a gripper assembly of the workholder in an open condition;

FIG. 4 is a partially broken away elevational view, taken generally along the line 4-4 of FIG. 2, illustrating the gripper assembly in a closed condition;

FIG. 5 is an enlarge elevational view depicting the relationship between a lower or fixed pivot arm, a resiliently yieldable connection, and a rigid base portion of the gripper assembly;

FIG. 6 is a sectional view, taken generally along the line 66 of FIG. 5, illustrating the relatively small cross-sectional area of the resiliently yieldable connection; and

FIG. 7 is a sectional view, taken generally along the line 77 of FIG. 5, illustrating the relatively large cross-sectional area of the fixed pivot arm.

DESCRIPTION OF A SPECIFIC PREFERRED EMBODIMENT OF THE INVENTION A turret punch press 10 is illustrated in FIG. 1 and includes a punch assembly 12 for operating on a work sheet supported in a horizontal plane on a table assembly 14. During punching operations, the work sheet is positioned and held against horizontal movement relative to the top of the table assembly 14 by a pair of identical workholders l8 and 20. The

workholders

18 and 20 have a relatively large flexural rigidity in the X directions, indicated by the horizontal arrow 24 in FIG. 1, and in the Y directions, indicated by the horizontal arrow 26 in FIG. 1. In accordance with the present invention, the workholders l8 and 20 have a relatively low flexural rigidity in the Z directions, indicated by the vertical arrow 28. This relatively low flexural rigidity in the Z direction enables the work sheet to be raised against a stripper 32 under the influence of forces applied to the work sheet by a punch 34 during a return stroke of the punch.

The punch assembly 12 is of a known construction and includes a guide 40 which defines a cavity in which the punch 34 is disposed. During a punching operation, the punch 34 is moved downwardly relative to the guide and cooperates with a die 42 to form a hole in a work sheet in a known manner. The relatively low flexural rigidity of the workholders l8 and 20 in the Z di rection enables the work sheet to move downwardly into engagement with the die 42 during a punching stroke. Upon completion of the work or downward stroke, the punch 34 is retracted or moved upwardly through a return stroke. As the punch 34 moves upwardly, frictional interference between the sides of the hole in the work sheet and the punch 34 cause the work sheet to be raised upwardly into engagement with the stripper 32. Continued upward movement of the punch 34 causes the stripper 32 to disengage the work sheet from the punch in a known manner. Although many different types of punch and stripper arrangements could be utilized, one suitable punch stripper arrangement is disclosed in US. Pat. No. 3,348,443.

The table assembly 14 is of known construction and supports the work sheet at die height during punching operations. The table assembly 14 includes a pair of horizontal stationary support surfaces 48 and 50 disposed on opposite sides of a disappearing table 52. The disappearing table 52 includes a plurality of rollers 54 which are disposed in a spaced-apart parallel relationship and are interconnected by chain links 60. The upper surfaces of the rollers 54 are tangent with the plane of the support surfaces 48 and 50 to continuously provide a movable support for the work sheet at die height. The construction of the table assembly 14 is the same as is disclosed in U.S. Pat. No. 3,395,596 and will not be further described herein to avoid prolixity of description.

A cross slide 64 is slidably mounted on a guide bar 66 which extends parallel to the X axis. The

workholders

18 and 20 are connected with the cross slide 64.

When the position of a work sheet gripped by the workholders 18 and 20 is to be changed to the X direction, the cross slide 64 is moved along the guide bar 66 by a drive screw (not shown). When the position of the work sheet is to be changed in the Y direction, a Y-axis drive screw 70 is operated to move a carriage 72 along guideways 73 in a direction either toward or away from the punch assembly 12. Suitable drive arrangements for the cross slide 64 and carriage 72 are disclosed in US. Pat. No. 3,436,998 and will not be further described herein. However, it should be noted that these drive arrangements hold the cross slide 64 and carriage against movement in the X and Y directions during operation of the punch assembly 12 to enable the workholders l8 and 20 to hold a work sheet in a predetermined position relative to the punch assembly.

The workholder 18 includes a gripper assembly 74 which is operated by a pneumatic actuator 76 (FIG. 2) to clampingly engage a work sheet 80 in the manner shown in FIG. 4. The gripper assembly 74 includes a pair of spaced-apart parallel lower or fixed pivot arms 84 and 86 (FIG. 2) to which an upper or movable pivot arm 88 is pivotally connected by a suitable pin 90. Upon operation of the piston and cylinder type actuator assembly 76 from the extended position of FIG. 3 to the retracted condition of FIG. 4, the movable arm 88 is pivoted in a clockwise direction (as viewed in FIG. 3) to clamp the work sheet 80 between upper and

lower jaws

94 and 96.

The upper jaw 94 is formed on the outer end of the movable arm 88. The lower jaw 96 is formed by a jaw plate 100 which extends between the

arms

84 and 86. Carbide inserts 102 may be provided on the upper and

lower jaws

94 and 96 to indent or dig into the work sheet 80 and provide a firm gripping action.

To enable the gripper assembly 74 to hold the work sheet 80 against movement in the X and Y directions during a punching operation, a rigid base 106 of the gripper assembly 74 is fixedly connected with the cross slide 64 by a suitable connection 108 (see FIG. 3). A rigid spacer bar 112 is fixedly connected with the base 106.

When the punch 34 (FIG. 1) is moved upwardly through a return stroke, the punch pulls the workpiece upwardly toward the stripper 32. Of course, once the workpiece has engaged the stripper 32, continued upward movement causes the workpiece to be disengaged from the punch 34. To enable the workpiece 80 to move upwardly under the influence of forces applied to it by the punch 34, the lower or fixed

arms

84 and 86 of the gripper assembly 74 are connected with the base portion 106 of the gripper assembly by resiliently yieldable connection 116 (see FIGS. 4 and 5). The connection 116 enables the workholder 18 to flex either upwardly or downwardly with the work sheet.

The resiliently yieldable connection 116 between the fixed pivot arm 86 and rigid base 106 has a relatively low flexural rigidity in the Z direction to enable the work sheet 80 to move upwardly into engagement with the stripper 32 under the influence of forces applied to the work sheet 80 by the punch 34 as it moves through a return stroke. Thus, as the punch 34 moves upwardly through a return stroke. it applies a force to the work sheet 80 urging it upwardly toward the stripper 32. At least a portion of this force is transmitted by the work sheet 80 to the closed gripper assembly 74. Although the fixed

pivot arms

84 and 86 are relatively rigid, the

low flexural rigidity of the resiliently yieldable connections 116 enables the

pivot arms

84 and 86 to flex upwardly with the work sheet. Thus, the arm 86 is flexed from the position shown in solid lines in FIG. 5 to the position shown in dashed lines in FIG. 5. The yieldable connections 116 also enable the

arms

84 and 86 to flex in a vertical direction during the punching stroke.

The relatively low flexural rigidity of the resiliently yieldable connection 116 results from the fact that the connection has a relatively small cross-sectional area (see FIG. 6) in a central plane which extends parallel to the X and Z axes and perpendicular to the Y axis. Thus, the flexural rigidity (BI) is determined by the product of Youngs modulus of elasticity (E) for the material of the yieldable connection and the moment of inertia of the cross-sectional area with respect to its neutral axis (I). Of course, Youngs modulus of elasticity will be the same for the yieldable connection 116 whether flexural rigidity about the X or Z axis is being determined. Therefore, the difference between the flexural rigidity of the resiliently yieldable connection 116 about the X axis and about the Z axis is determined by the difference in the moment of inertia of the yieldable connection about these axes.

The moment of inertia of the resiliently yieldable connection 116 in the Z direction (I is calculated about a neutral axis 118 (FIG. 6) by multiplying the base of the area times the cube of the height and dividing by 12. The size of the base in regard to the neutral axis 118 is the length of the surface designated 120 in FIG. 6. The size of the height in regard to the neutral axis 118 is the length of the surface designated 122 in FIG. 6.

The base 120 is of a relatively large length while the height 122 is of a relatively short length. Since the moment of inertia in the Z direction is a function of the product of the base 120 times the cube of the height 122. the yieldable connection 116 has a relatively small moment of inertia about the neutral axis 118. Therefore, the yieldable connection 116 has a relatively small flexural rigidity in the Z direction.

The flexural rigidity of the yieldable connection 116 in the X direction is equal to Youngs modulus of elasticity (E) for the material of the connection times the moment of inertia in the X direction. The moment of inertia in the X direction (I is calculated about a neutral axis indicated at 124 in FIG. 6. When the moment of inertia about the neutral axis 124 (It) is to be determined, the base is equal to the length of the surface 122 while the height is equal to the length of the surface 120.

The base dimension 122 in regard to the neutral axis 124 is relatively small while the height dimension 120 in .regard to the neutral axis 124 is relatively large. Therefore, the moment of inertia about the neutral axis 124 (I is a function of the product of the relatively small base dimension 122 times the cube of the relatively large height dimension 120. Thus, the moment of inertia about the neutral axis 124 (1;) is far greater than the moment of inertia about the neutral axis 118 since the height about the neutral axis 124 is far greater than the height about the neutral axis 118.

The yieldable connection 116 is integrally formed with the fixed pivot arm 86. Although the crosssectional area of the arm 86 may vary along its length, the cross-sectional area along line 77 of FIG. 5 is shown in FIG. 7. It should be noted that when the flexural rigidity in the Z direction is to be determined, the moment of inertia is calculated about the neutral'axis indicated at 130 in FIG. 7. The base dimension for this calculation will be determined by the length of the surface indicated at 134 in FIG. 7. Since this surface is a continuation of the surface and is of substantially the same size, the base dimension for purposes of calculating moment inertia about the neutral axis of FIG. 7 is the same as the base dimension for calculating the moment of inertia about the neutral axis 118 of FIG. 6.

The height for moment of inertia calculation about the axis 130 is determined by the length of the surface 138. As can be clearly seen from comparing FIGS. 6 and 7, the length of the surface 138 is far greater than the length of the surface 122. Therefore, the height utilized to determine the moment of inertia about the neutral axis 130 is far greater than the height utilized to determine the moment of inertia about the neutral axis 118. Accordingly, the resiliently yieldable connection 116 will have a far smaller flexural rigidity in the Z direction than the fixed pivot arm 86.

Although it is contemplated that the resiliently yieldable connection 116 could have many different crosssectional configurations of different sizes, in one specific preferred embodiment of this invention, the yieldable connection 116 had a surface 120 of a length of approximately 0.75 inches and a surface 122 of a length of approximately 0.032 inches. With these dimensions, the moment of inertia about the neutral axis 118 (I is equal to approximately 20.5 10"). The moment of inertia about the neutral axis 124 (I is far larger and equals approximately 11.25 (10 Since Youngs modulus of elasticity for the material of the yieldable connection 116 is constant, it can be seen that the flexural rigidity in the Z direction will be equal to 20.5 (10' E. This is considerably smaller than the flexural rigidity of the yieldable connection 116 in the X direction which would be approximately I 1.25 (10) E.

In the specific preferred embodiment of the invention for which dimensions are set forth above, the fixed pivot arm 86 has a much larger cross-sectional area than the yieldable connection 116. Thus, the surface 134 of the fixed pivot arm 86 had a length of approximately 0.75 inches. However, the surface 138 had a length of approximately 0.25 inches. Of course, the relatively large length of the surface 138 compared to the length of the surface 122 results in the fixed pivot arm 86 having a much greater flexural rigidity in both the X and Z directions than the yieldable connection 116. The flexural rigidity of the yieldable connection 116 in the Y direction is extremely high. This is because forces in Y direction load the yieldable connection 116 in either tension or compression.

Although specific dimensions have been set forth for one specific preferred embodiment of the invention, it should be understood that these dimensions are provided to facilitate understanding of the present invention and it is not intended to limit the present invention to these particular dimensions. In addition, it should be understood that although the yieldable connection 116 and fixed pivot arm 86 have been illustrated in FIG. 6 and 7 as having rectangular cross-sectional configurations, it is contemplated that they could have other cross-sectional configurations. It should be understood that although only the yieldable connection 116 between the fixed pivot arm 86 and rigid base 106 has been illustrated in the drawings, a yieldable connection of the same construction is provided between the fixed pivot arm 84 and the rigid base 106.

Although it is contemplated that the resiliently yieldable connections 116 could be formed in many different ways, in one specific preferred embodiment of the invention, the fixed

pivot arms

84 and 86, connections 116, and base 106 were cast as a one-pice unit. As originally cast, the connections 116 had the same cross sectional area and configuration as the fixed

pivot arms

84 and 86. The one-piece unit was then machined to form the connections 116 with a reduced cross sectional area. Thus, the base 106, connections 116, and fixed

pivot arms

84 and 86 are integrally formed. However, it should be understood that the resiliently yieldable connections 116 could have a construction other than the specific construction disclosed herein.

To prevent permanent or plastic deformation of the yieldable connection 1 16, a stop arrangement 144 (see FIGS. 2 and 4) is provided to limit the extent to which the connection can be deformed under the influence of forces applied to the work sheet 80 by the punch 34. Thus, when the resiliently yieldable connection 116 is in its normal or undeformed condition (illustrated in FIG. 4), a stop pin 146 is near the bottom of a slot 148 formed in a guard member 150. When the fixed pivot arm 86 is moved upwardly under the influence of forces applied to the work sheet 80 by the punch 84, the pin 146 engages a stop or end surface 152 of the slot 148 to thereby limit upward movement of the fixed pivot arm 86.

The guard 150 has an upper section 164 (see FIG. 2) which extends over the resiliently yieldable connection 116. The upper section 164 of the guard 150 protects the resiliently yieldable connection against being damaged by falling workpieces or other foreign objects. The inner end of the guard 150 is fixedly connected with the rigid base by suitable connections 166 (FIG. 4).

Although the stop arrangement 144 has been shown separately from the guard 150, it is contemplated that the guard 150 could be provided with a leading end surface 170 (see FIG. 2) which is spaced from the pivot arm 86 by a predetermined distance to prevent excessive deflection of the fixed pivot arm 86. While only the stop arrangement 144 and guard 150 associated with the fixed pivot arm 86 has been fully illustrated, it should be understood that a similar guard and stop arrangement is provided in association with the fixed pivot arm 84.

In view of the foregoing description, it can be seen that the workholder 18 has a relatively high flexural rigidity in the X and Y directions and a relatively low flexural rigidity in the Z direction. The relatively low flexural rigidity of the workholder 18 in the vertical or Z direction enables the work sheet 80 to be moved downwardly into engagement with the die 42 during a work stroke of the punch 34. In addition, the relatively low flexural rigidity of the workholder 18 in the Z direction enables the work sheet 80 to be moved upwardly into engagement with a stripper 32 under the influence of forces applied to the work sheet during a return stroke of the punch 34. The relatively high flexural rigidity of the workholder 18 in the X and Y directions enables it to accurately position and hold the work sheet during punching operations.

The relatively low flexural rigidity of the workholder 18 in the Z direction results from the yieldable connections 116 between the fixed

pivot arms

84 and 86 of the gripper assembly 74. The resiliently yieldable connections 116 are integrally formed with the

pivot arms

84 and 86 and have relatively smallcross-sectional areas. The cross-sectional configuration of the yieldable connections 116 is such that they have a very small height or Z axis dimension and a rather large base or X axis dimension. The small height or Z axis dimension results in a relatively low flexural rigidity in the vertical or Z direction. However, the large base or X axis dimension results in a relatively large flexural rigidity in the X direction.

We claim:

1. A workholder for a machine tool having a cross slide for positioning a workpiece, comprising in combination: first and second arm members pivotally joined intermediate their ends, with one end of one of said members being affixed to said cross slide; work engaging jaws affixed to the other ends of said first and second arm members in opposing relationship to each other; actuator means coacting with the first ends of said first and second arm members to pivotally open and close said jaws; and flexure means intermediate the pivot point and the first end of one of said arm members to permit movement of said arm members in the same plane as the pivoting movement.

2. Apparatus according to claim 1 wherein said flexure means is an integral part of said one arm member.

3. Apparatus according to claim 2 wherein said flexure means is an area of reduced cross-section.

4. A punch press for working on sheets of material, said punch press including table means for supporting a sheet of material, said table means including support surface means for supportingly engaging a major side surface of the sheet of material, punch means for forming openings in the sheet of material while it is disposed on said table means, die means for cooperating with said punch means, means for effecting movement of said punch means through a work stroke in a first direction transverse to said support surface means and for effecting movement of said punch means through a return stroke in a second direction opposite to said first direction, said punch means being effective to move at least a portion of the sheet of material in one of said directions during at least a portion of one of said strokes, stripper means for effecting disengagement of the sheet of material from said punch means during a return stroke by limiting movement of the sheet of material in the second direction, and gripper means for holding the sheet of material against movement in a direction extending generally parallel to said support surface means during work and return strokes of said punch means and for enabling at least a portion of the sheet of material to move in said one direction during at least one of said strokes of said punch means, said gripper means including clamp surface means for clampingly engaging the sheet of material during work and return strokes of said punch means, a base portion, resiliently yieldable connection means for interconnecting said clamp surface means and said base portion and for enabling said clamp surface means to move relative to said support surface means through a substantial distance in at least said one of said directions under the influence of forces applied to the sheet of material by said punch means.

5. A punch press as set forth in claim 4 wherein said clamp surface means includes first surface means for engaging one side of a sheet of material and second surface means for engaging another side of a sheet of material, said gripper means further including a first elongated support portion connected with said first surface means and a second elongated support portion connected with said second surface means, said first and second elongated support portions having a substantially greater resistance to deflection in said one of said directions than said resiliently yieldable connection means.

6. A punch press as set forth in claim 5 wherein said resiliently yieldable connection means is integrally formed with said first elongated member.

7. A punch press as set forth in claim 4 wherein said gripper means includes an elongated arm extending between said resiliently yieldable connection means and said clamp surface means, said arm having a first cross sectional area in a plane extending perpendicular to the longitudinal axis of said arm, said resiliently yieldable connection means having a second cross sectional area which is substantially smaller than said first cross sectional area.

8. A punch press as set forth in claim 4 wherein said clamp surface means includes a first surface means for engaging one side of the sheet of material and a second surface means for engaging an opposite side of the sheet of material, said gripper means further including a first elongated arm having an inner end portion connected with said resiliently yieldable connection means and an outer end portion connected with said first surface means, a second elongated arm connected at an outer end portion with said second surface means, and connection means for pivotally connecting said second arm with said first arm at a location intermediate said inner and outer end portions of said first arm.

9. A punch press as set forth in claim 8 wherein said inner end portion of said first arm is integrally formed with said resiliently yieldable connection means and has a cross sectional area which is substantially greater than the cross sectional area of said yieldable connection means.

10. A punch press as set forth in claim 4 further including stop means for limiting movement of said clamp surface means in the second direction under the influence of forces applied to the sheet of material by said punch means as said punch means is moved through a return stroke.

11. A punch press for working on sheets of material, said punch press including table means for supporting a sheet of material, punch means for forming openings in the sheet of material while it is disposed on said table means, means for effecting movement of said punch means through working and return strokes, and gripper means for holding the sheet of material during working and return strokes of said punch means, said gripper means including first and second arm members pivotally joined intermediate their ends, one end of one of said members being connected with said table, work engaging jaws affixed to the other ends of said first and second arm members in opposing relationship to each other, actuator means coacting with the first ends of said first and second arm members to pivotally open and close said jaws, and flexure means intermediate the pivot point and the first end of one of said arm members to permit movement of said arm members in the same plane as the pivoting movement.

12. Apparatus according to claim 11 wherein said flexure means is an integral part of said one arm member.

13. Apparatus according to claim 12 wherein said flexure means is an area of reduced cross-section.