the hydraulic clamp of the present invention , generally designated 20 and best seen in fig1 and 2 , includes a cylinder 22 and piston 38 assembly , secured onto a cylinder base plate 24 by screws 26 , as shown in fig2 and 4 , which extend from and through countersunk drilled holes 25 in the opposite annular cylinder end surface 28 . an oil inlet 30 , seen in fig1 and 2 , extends inward and downward from the annular end surface 28 , providing a vertical oil passage 32 whose upper end , adjacent to the cylinder annular end surface 28 , is provided with an oil inlet seal 34 . an oil entrant passage 36 extends radially inward from the lower end of the vertical passage 32 to the piston 38 , now to be described . projecting outward from the cylinder annular end surface 28 , is a relatively large diameter piston rod 40 whose integral end is divided by a central slot 48 into two rod side segments 42 having , within the slot 48 , parallel slide surfaces 43 and opposite undercut horizontal faces 44 which give the central slot 48 a t - configuration conforming to that of the t - slotted bolster plate in which the clamp 20 is to be installed . a small detent , here a latching depression 46 , is formed at the center of the base of the t - configured slot 48 . except for the upper end portions of the piston rod 40 so described , the mechanism of the clamp 20 is conventional . the piston rod 40 is held in its outwardly projecting position by a compression spring 56 in a central cavity 58 in the piston bottom wall 60 . in this bottom wall 60 is an upward - extending alignment bore 62 , aligned with a lower bore 64 in the base plate 24 ; together they accommodate an alignment pin 68 which prevents the piston 38 from rotating within the cylinder 22 . at the juncture of the piston rod 40 with the piston 38 is a beveled annular piston face 50 . when no hydraulic pressure is exerted , its inner portion rests against a square - cut cylinder shoulder 52 , the bevel providing in effect a ring - like oil distribution groove 54 between the two surfaces . a conventional oil groove 66 is provided in the piston rod 40 at its juncture with the piston face 50 . conventional ring - like oil seals 70 are also provided above and below the beveled piston face 50 . to actuate the clamp 20 , oil passes through a tube 27 and oil inlet port 29 and then through the inlet 30 and through horizontal and vertical drilled passages 31 , 31 &# 39 ; to flow against the beveled piston face 50 ; its pressure provides a downward force , compressing the spring 56 and driving the piston rod 40 inward ; an air vent 72 permits this without build - up of air pressure . when the oil pressure is released , the spring 56 returns the piston rod 40 fully outward . hydraulic clamps 20 so assembled are mounted by embedding within a t - slotted surface against which a die shoe ( or other work member ) is to be held , as in the bolster plate 74 of fig6 - 8 . such t - slotted bolster plates may be affixed to both the bed and the ram of a punch press or other reciprocating machine tool ; or be merely used as portions of work - holding fixtures . the t - slots 76 extend inward from the plate 74 surface and have the same configuration as the t - slots 48 of the piston rods 40 . preferably two clamps are embedded in each selected t - slot at the spacing s shown in fig7 . as shown in fig3 and 5 for each clamp 20 embedded , a bore 78 whose diameter is large enough to slidably receive a piston rod 40 , is cut perpendicularly into the bolster plates 74 , centered along the centerline of a t - slot 76 . a counterbore 80 of a diameter large enough to accommodate its cylinder 22 is cut along the same centerline into that surface of the bolster plate 74 opposite to its t - slotted surface , leaving an annular shoulder 84 at the juncture of the bore 78 and counterbore 80 . the shoulder 84 is at such level that when the cylinder end surface 28 is fitted against the shoulder 84 and the piston rod 40 is fully projected outward , its t - slotted end 48 is continuous with the t - slot 76 of the bolster plate 74 and its oil passages 31 , 31 &# 39 ; communicate with oil inlet 30 of the clamp 20 . four bolts 82 , shown in fig5 sunk in the surface of the bolster plate 74 , extend through drilled holes 75 in the bolster plate 74 and aligned holes 77 in the wall of the cylinder 22 , to screw to the base plate 24 , attaching the cylinder 22 in place . a die set consisting of lower and upper dies , each attached to a die shoe , may be clamped as now to be described . using a typical die shoe 88 , in the embodiment shown in fig6 and 8 , its normally flat undersurface 90 is machined with a t - slot 92 of the same configuration as the piston rod t - slot 48 . latching depression detents 94 , like the detents in the piston rod t - slot described , are centered along the base of the t - slot 92 , at the spacing s shown in fig7 . to secure the die shoe 88 to a bolster plate 74 , a securing and force - transmitting member of substantially i - beam shape , here referred to as the i - beam 100 , is shown in phantom lines in fig6 . it includes a stem portion 102 terminating in opposite cap portions 104 ; each cap portion 104 has undercut flange surfaces 106 , preferably machined to mate with undercut faces such as the faces 44 of the piston rod 40 . the shape of the upper and lower halves of the i - beam 100 conforms to the t - slots 48 , 92 of the piston rod 40 and the die shoe 88 , so that the i - beam 100 may slide within each . its length is normally greater than the spacing s between the two clamps in a t - slot , and in any event preferably greater than the diameter of the bore 78 in which the piston rod 40 is accommodated . conventional upper and lower ball detents 108 , in registration with each other and illustrated in fig7 are centered along the centerline of the cap portions 104 at the same spacing s as the latching depressions 46 , 94 . in securing a die shoe 88 to a bolster plate 74 , one cap of an i - beam 100 is slid into the t - slot 92 of the die shoe 88 so that ball detents 108 engage the latching depressions 94 of the die shoe t - slot 92 . the opposite half of the i - beam 100 is then slid into the t - slot 76 of the bolster plate 74 , passing through and sliding along the parallel slide surfaces 43 of the t - slots 48 of its piston rods 40 , with its ball detents 108 engaging the piston latching depressions 46 . fluid pressure is applied , as heretofore explained , to draw the piston rod 40 inward , causing the flanges 106 of the i - beam cap portions 104 to press against the undercut faces 44 of the t - slots 48 . as the broken arrows in schematic fig8 show ; two clamping arrows c applied by the piston undercut faces 44 against the flanges 106 of the i - beam 100 , is applied , not locally , but as distributed clamping forces c along the undercut faces of the die t - slot 92 , over a broad area throughout the interior of the die shoe 88 . such forces , distributed along the interior of a die shoe 88 , instead of concentrated pinching forces along a die shoe edge , result in a better distribution of frictional resistance along the bolster plate 74 without die shoe distortion . when mating die sets are used in high speed punch presses as well as other forming operations , substantial sideward jarring forces may be applied repeatedly to the die shoes . using prior art edge clamping , no structural member resists such sideward jarring forces ; only friction is available . when the present invention is employed , such sideward forces , shown by the large sideward arrow s against the die shoe 88 of fig8 are transmitted by the i - beam 100 as distributed forces , shown by small sideward arrows s . these distributed faces are exerted against , and reacted by , the inner edge of the bolster plate t - slot 76 , over a length extending substantially beyond the t - slotted end of the piston rod 40 . such reaction by the t - slot of sideward forces transmitted through the i - beam 100 itself , substantially lessens the need for clamping friction between the die shoe 88 and the bolster plate 74 surfaces . by using identically spaced clamps in an upper bolster plate as well , such i - beams with complementary detents will locate and latch upper and lower shoes of die sets in precise alignment with each other automatically , each time they are installed in the machine tool . this invention thus eliminates the lengthy die - alignment steps , greatly reducing the time that a machine is out of service for die changes . after completing the use of a die set whose shoes are so equipped with t - slots and i - beams , and their removal from the bolster plates of a press , the i - beams are readily removed from the die shoes , which may then be compactly stacked for storage until re - use . an alternate configuration of a die shoe securing - and - force - transmitting member , useful when a lesser clamping force will suffice , is the t - bar 120 shown schematically in fig9 . it includes a stem 124 terminating in a cap portion 126 containing ball detents 108 &# 39 ; at the same spacing s . the t - bar 120 is affixed by a plurality of countersunk screws 122 to a grooved undersurface 90 &# 39 ; of a die shoe 88 &# 39 ; and then slid into a bolster plate t - slot 74 , passing through its piston rod t - slots 48 , so that its ball detents 108 &# 39 ; engage the piston latching depressions 46 . the downward pull of the clamps 20 is applied by the undercut faces 44 of the piston rods 40 , and is transmitted to its cap portion 126 in the same manner as with the i - beam embodiment . however , the clamping force in this instance is applied to the die shoe 88 &# 39 ; only by threads of the screws 122 . while in nearly every case the securing member , whether an i - beam 100 or a t - bar 120 , will be longer than the diameter of a piston rod 40 , such greater length is not fundamentally required by the present invention . since the rod side segments 42 are supported by the bolster plate bore 78 , so that they cannot bend outward , sideward forces applied by even a short attachment member are rigidly resisted by the bore 78 without relying on friction along the surface of the bolster plate 74 . conventional edge clamping lacks this advantage . another advantage is provided by the manner in which the clamps of the present invention are embedded within the t - slotted surfaces of the bolster plates . when the invention is not employed for clamping , the t - slotted surfaces are unencumbered and may be used , and brushed out or otherwise cleaned , as if no clamps were present . for shops equipped with air pressure lines , these may be used as a source of fluid pressure , and the manner of substitution of pneumatic cylinders will be obvious from this specification . as other modifications may be made in the constructions herein described and illustrated without departing from the scope of the invention , it is intended that all matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative rather than limiting .