Hydraulic press brake tool holder

Information

  • Patent Grant
  • 6564611
  • Patent Number
    6,564,611
  • Date Filed
    Monday, July 30, 2001
    23 years ago
  • Date Issued
    Tuesday, May 20, 2003
    21 years ago
Abstract
Press brake tool holders suitable for releasing and securing press brake tools in response to applied fluid pressure. One press brake tool holder includes a horizontally elongated body having a cam shaft bore disposed longitudinally therethrough, and receiving a slidably and sealingly mounted cam shaft therein. The cam shaft can have at least one axial camming surface, having a large outer diameter region axially tapered to a small outer diameter region, and in contact with a cam follower pin slidably disposed in a cam follower pin bore transversely disposed through the body. The cam follower pin can bear against a pivotally mounted clamp disposed about the body. In response to applied fluid pressure, the camming surface can slide axially, thereby increasing the effective outer diameter as seen by the cam follower pin, thereby urging the cam follower pin outward and against the upper portion of the pivotally mounted clamp, and closing the lower clamp portion about a press brake tool. One press brake tool has a pair of opposed cam shaft segments with the pressure-bearing faces disposed near each other in the center. Another press brake tool holder has a single cam shaft with multiple camming surfaces disposed over the length.
Description




FIELD OF INVENTION




The present invention is related generally to press brakes which can be used to bend and form sheet metal. More specifically, the present invention relates to press brake tool holders.




BACKGROUND OF THE INVENTION




Press brakes commonly are equipped with a lower table and an upper table, one of which, commonly the upper table, is vertically movable toward the other table. Forming tools are mounted to the tables so that when the tables are brought together, a workpiece between the forming tables is bent into an appropriate shape. It is common for the upper table to include a male forming tool having a bottom workpiece-deforming surface of a desired shape, such as a right angled bend, and for the bottom table to have an appropriately shaped and aligned tool, such as a “V” shaped working surface, so that when the tools are brought together, a workpiece between the tools is given an appropriate bent shape. The forming tools commonly are horizontally elongated so that workpieces of various widths can be accommodated.




It often is necessary to exchange forming tools when a different bending operation is to be performed. The forming tools mounted to the upper table of a press brake often are not easily replaced. Forming tools usually are held by a clamp of a tool holder to the horizontally elongated bed of the upper table. Once the clamp has been loosened, the forming tool can, in some instances, be removed downwardly, and in others, must be removed by horizontally sliding it from the clamp. If a long forming tool is to be replaced, it becomes difficult to slide the forming tool from its clamp because of the proximity of neighboring clamps and forming tools which may themselves have to be removed in order to complete the tool exchange process.




Long forming tools can be quite heavy. When a holder is loosened to the point that the tool can be removed by moving it downwardly, care must be taken to prevent the tool from slipping immediately from the clamp.




Several press brake holders have been devised in an effort to facilitate the exchange of one forming tool for another. For example, Treillet, U.S. Pat. No. 4,993,255 discloses a tool holder that is attached by means of a C clamp to the bed of the upper table. Through the use of a camming mechanism, the upwardly extending shank of a forming tool is captured between a pivotable clamp and a portion of the holder, the shank and clamp having cooperating surfaces enabling the tool to be readily inserted in the holder. A locking cam is employed to lock the clamp against the forming tool. Kawano, U.S. Pat. No. 5,513,514, U.S. Pat. No. 5,511,407, U.S. Pat. No. 5,572,902, and European patent publication 0 644 002 A2 show tool holders of the same general type in which a pivoting clamp is employed to receive the shank of a tool between it and the mounting plate of the holder. In each of these patents, the holder is equipped with a threaded mechanism operated by a lever that pivots from side to side to lock and unlock the clamp, force being transmitted from the lever to the clamp via a spring structure.




Kawano, U.S. Pat. No. 6,151,951, discusses a tool holder having multiple hydraulically actuated pistons to transmit the clamping force of hydraulic fluid to a tool clamp. The multiple pistons are displaced outwardly to force the tool clamp shut.




U.S. Pat. No. 6,003,360 (Runk et al.), herein incorporated by reference in its entirety, provides an improved press brake tool holder. The tool holder includes a clamp which opens to a position allowing manual removal of the tool while not allowing the tool to fall. The clamp is controlled with a manual lever.




What would be desirable are clamps more suitable for remote and/or powered operation. Clamps suitable for hydraulic control would be advantageous.




SUMMARY OF THE INVENTION




The present invention includes a press brake tool holder for mounting to a press brake tool having a mounting shank. The tool holder is adapted to be controlled by a fluid pressure source, for example, by hydraulic fluid. One press brake tool holder includes a body having a support plate, and a clamp having upper and lower portions pivotally attached between the upper and lower portions to the body to enable the tool mounting shank to be clamped between the clamp lower portion and the body support plate. In one tool holder, the body has a cylindrical bore formed therein, and has a fluid entry port for delivering pressurized fluid to a fluid entry location within the bore. An elongate shaft having a camming surface along its length can be disposed within the bore and be slidable axially within the bore in response to fluid delivery under pressure into the bore. The camming surface can extend between a larger outer diameter region and a smaller outer diameter region, and is preferably continuously tapered in between. The body can have a cam follower engagable with the tapered camming surface and movable in response to axial movement of the shaft, so as to force the clamp to pivot with respect to the body. The pivoting clamp thereby forces the lower portion of the clamp toward or away from the support plate, to clamp or unclamp the tool mounting shank.




Some tool holders have a biasing element mounted at the end of the bore to urge the shaft toward the fluid entry location. The tapered camming surface can be oriented so as to encounter and bear against the cam follower either as the shaft slides toward or away from the biasing element, depending on the embodiment. In one tool holder, the elongated shaft has at least two axially spaced tapered camming surfaces, and the body includes at least two cam followers engageable respectively with the tapered camming surfaces.




Other tool holder embodiments include two of the elongate shafts slidably disposed within the bore, and have the fluid entry port positioned to deliver pressurized fluid between the shafts to urge them in opposite directions. The body can have at least two cam followers engageable respectively with the tapered camming surfaces of the elongated shafts. The tool holder can have biasing elements mounted at the ends of the bore to urge the shafts toward each other. The tapered camming surfaces can be oriented so as to encounter and bear against the cam followers either as the shafts slide toward or away from the biasing elements, depending on the embodiment.




The cam shaft camming portion can have a larger outer diameter region and a smaller outer diameter region joined by a tapering surface. The effective outer diameter of the tapered surface in contact with an axially stationary cam follower is increased by forcing the cam shaft to travel axially against a biasing spring, thereby forcing the cam follower outwardly. The cam shaft thus can be displaced longitudinally by the application of hydraulic pressure against a pressure-bearing, slidably sealed face of the cam shaft and by action of the biasing spring.




One embodiment of the invention includes a pair of identical symmetrically, opposed cam shafts or cam shaft segments having pressure-bearing faces disposed near each other within the cam shaft bore, and in communication with the pressurized fluid source. In response to applied fluid pressure, the twin cam shafts can be forced apart from each other and toward respective biasing springs, thereby increasing the effective outer diameter of the camming regions as presented to the cam followers. In response to the increased effective outer diameter of the camming regions, the cam follower pins are pushed outward, thereby pushing against an upper portion of the pivotally mounted clamp, and closing the lower portion of the clamp against the press brake tool shank. In response to the loss of fluid pressure, or a significant reduction in pressure, the effective outer diameter of the cam shaft camming region can be decreased. This allows the cam shaft follower pin to travel inward toward the axis of the cam shaft. This allows the clamp upper portion to travel inward, and the clamp lower portion to travel outward and away from the tool shank. In a preferred embodiment, when the clamp lower gripping portion travels outward, the tool shank is still held by a lip or notch, placing the tool into a position where it can be manually removed, yet not allowing the tool to fall under gravity prior to the manual removal.




In another embodiment of the invention, the cam shaft includes a single shaft segment having at least two camming surfaces. In this embodiment, a pressurized fluid source can be used to apply pressure to one end of the cam shaft, thereby causing the cam shaft and all camming surfaces thereon to move axially through the cam shaft, causing the cam follower pins to move in response.




Some embodiments of the invention are configured such that the cam follower pins apply force above the clamp pivot pins. Some embodiments of the invention increase the effective camming region outer diameter in response to applied pressure, while others embodiments have the reverse camming surface slope. In particular, some embodiments increase the camming surface effective outer diameters by decreasing the applied fluid pressure. Some embodiments of the present invention cause the clamps to fail open under loss or reduction of fluid pressure, while others cause the clamps to fail shut under the loss or reduction of fluid pressure. The present invention provides hydraulically operated press brake tool holders which can be operated by foot switches or automatically controlled hydraulic switches.











BRIEF DESCRIPTION OF THE DRAWINGS





FIG. 1

is a perspective view of a brake press tool holder, including a body, and a clamp having opposed clamp elements holding a brake press tool;





FIG. 2

is an exploded, perspective view of another embodiment of a brake press tool holder having two, symmetrical, opposed cam shaft segments adapted to be forced apart under fluid pressure for the purpose of forcing cam follower pins outward against pivoting, enclosing tool clamping elements;





FIG. 3

is a transverse cross-sectional view taken through

FIG. 2

, illustrating the pivotally mounted and spring biased clamp elements coupled to a body; and





FIG. 4

illustrates yet another tool holder embodiment having a single cam shaft with two camming surfaces disposed thereon.











DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS





FIG. 1

illustrates a tool holder


10


, illustrated holding a tool


28


. Tool


28


includes a lower workpiece encountering edge


38


, which can be used to form sheet metal pieces into the desired shape. Tool holder


10


includes generally a horizontally elongated body


12


and a mounting plate


16


. Mounting plate


16


has a clamp attached thereto, including opposed clamp elements


42


and


40


. Clamp elements


40


and


42


are pivotally attached to mounting plate


16


about parallel spaced pin apertures


47


. Body


12


includes a lower support plate portion


14


, having a bottom edge


26


. Mounting plate


16


includes elongated slots


144


, which are used to couple the mounting plate to a wedge


132


using threaded bolts


142


disposed through elongated slots


144


. Clamp element


42


may be seen to be divided into a first clamp unit


44


and a second clamp unit


46


by a center thin slot


94


.




Clamp element


42


may also be seen to include a plurality of downwardly extending finger elements


114


for resiliently gripping tool


28


by a tool shank portion


30


. Clamp element


42


also includes a longitudinal slot


112


which can also impart resiliency to clamping tool shank


30


. Downwardly extending finger elements


114


are separated from each other by a plurality of spaced, parallel, thin slots


110


. Clamp element


42


is divided by pin apertures


47


into an upper clamp portion


66


and a lower clamp portion


68


. Lower clamp portion


68


includes, in the embodiment shown, an inwardly turned lip


116


having an upwardly facing shoulder


118


. Clamp element


42


may be seen to pivot about pin apertures


47


, causing clamp lower portion


68


to grip tool shank


30


using inwardly turned lip


116


. Inwardly turned lip


116


forces tool shank


30


against body support plate portion


14


. Tool holder body


12


includes an end cap


70


capping a horizontal bore


72


(shown in phantom) which contains a cam shaft (not shown in FIG.


1


). In some embodiments, cam follower pins (not shown in

FIG. 1

) push outwardly on upper clamp portion


66


, thereby causing clamp lower portion


68


to close about tool shank


30


. In these embodiments, clamp lower portion


68


may be biased by springs to open outwardly from tool shank


30


, thereby releasing or placing the tool in a position where it can be removed. In a preferred embodiment, the cam shaft slides axially within cam shaft bore


72


, causing the cam follower pins to push outwardly on clamp upper portion


66


.





FIG. 2

is an exploded, perspective view of another tool holder


200


, illustrating, a preferred embodiment of the invention. Tool holder


200


includes generally a body


201


, a first clamp element


206


, a second opposing clamp element


208


, a wedge


204


, and a cam shaft bore


222


through body


201


for receiving a cam shaft


300


. As used herein, “camshaft” refers to an axially slidably shaft having a camming surface including a larger outer diameter region and a smaller outer diameter region and having a tapered, preferably conical surface therebetween. In a preferred embodiment, the camming surface is axially and conically tapered rather than stepped.




Wedge


204


is coupled to body


201


by threaded bolts


220


extending through elongated slots


221


through body


201


. First clamp element


206


may be divided into a clamp upper portion


212


and a clamp lower portion


214


by horizontally spaced pin recesses


240


, which can receive rounded head pins


241


. Horizontally aligned apertures


242


are disposed in body


201


, for receiving rounded head pins


241


. Apertures


242


include an enlarged end opening portion


244


and an undercut slot portion


246


.




First clamp element


206


may be seen to include a plurality of compression springs, including upper compression springs


250


and lower compression springs


252


. The compression springs can operate to bias first clamp element


206


away from body


201


.




Each clamp member unit half can be provided with horizontally spaced pins


241


, each pin having a threaded end that is received in a threaded bore formed in the clamp. Each pin has an enlarged, rounded head. As shown in

FIG. 2

, body


201


includes the plurality of horizontally aligned apertures


242


. Each aperture forms a horizontally elongated slot having the enlarged end opening


244


and the undercut slot portion


246


. The enlarged, rounded heads of the pins are sized so as to be received through the enlarged end openings


244


of the slots. When in this position, the individual clamps are slid horizontally toward the center line of the tool, the enlarged, rounded heads sliding into engagement with the undercut surfaces


246


of the slots. In this manner, each clamp half can be released from the clamping tool by sliding the clamp horizontally away from the midline of the tool until the pins become aligned with the enlarged openings


244


, whereupon the clamp halves can simply be removed from the support plate and mounting plate, respectively.




The series of compression springs


250


are mounted between the confronting surfaces of the clamp and the support plate or mounting plate, respectively, so that when the enlarged, rounded ends of pins


241


are received within the undercut surfaces of the slot


246


, the springs


250


and


252


tend to push the clamps away from the body such that each clamp is tethered to the body of pins


241


.




First clamp element


206


may also be seen to include a longitudinal recess


230


within first clamp element


206


. Longitudinal recess


230


can be substantially circular in profile and have therein a tool shank gripping member


236


, including a tool gripping lip


238


. Tool gripping number


236


may be rotatable about its longitudinal axis. Body


201


may also be seen to have a cam follower pin bore


282


for receiving a cam follower pin or element


280


within.




Cam shaft


300


includes, in the embodiment shown, a first cam shaft segment


302


and a second cam shaft segment


304


. Cam shaft segment


302


and cam shaft segment


304


are alternatively referred to herein as first cam shaft


302


and second cam shaft


304


, respectively. In the embodiment illustrated, first cam shaft


302


and second cam shaft


304


are identical, and the various elements of the cam shafts may be described with reference to either first cam shaft


302


or second cam shaft


304


. Cam shafts


302


and


304


may be held in place within cam shaft bore


222


by end caps


306


. First cam shaft


302


may be seen to be biased away from end cap


306


by a biasing spring


308


. First cam shaft


302


and second cam shaft


304


have a gap


312


therebetween for receiving a pressurized fluid. The first and second cam shafts also include a face


310


for receiving the fluid pressure and sealing rings


313


for sealingly containing the pressurized fluid. The pressurized fluid is preferably a hydraulic fluid. First cam shaft


302


may be seen to include generally a substantially constant outer diameter cam shaft portion


314


extending to a camming surface portion


320


, which includes a larger diameter region


316


and a smaller diameter region


318


. Cam shaft


302


abuts biasing spring


308


at an enlarged end region


322


.




A pressurized fluid source is illustrated generally by a pressurized fluid conduit


340


, having an elbow shape and containing a pressurized fluid lumen


342


within. Pressurized fluid source


340


may be used to supply pressurized fluid through an entry port in body


201


to enter cam shaft gap


312


. The pressurized fluid forces first and second cam shafts


302


and


304


apart from each other and against biasing springs


308


. As the cam shafts travel toward their outer biasing springs, the effective outer diameter of the cam shaft, as seen by the cam follower pins, increases from smaller diameter region


318


to larger diameter region


316


. Cam shaft follower pin


280


, received within cam shaft follower pin bore


282


, travels over the camming surface


320


. In some embodiments, at one extreme of travel, cam shaft follower pin


280


rests upon a sloped, tapered larger outer diameter region such as region


316


. In other embodiments, cam shaft follower pin


280


rests upon a constant larger outer diameter region such as region


314


. Resting upon a constant outer diameter region can eliminate any axial component of force transmitted by the follower pin to the cam shaft. As the effective outside diameter of the cam shaft increases, cam follower pin


280


is forced outward against first clamp element


206


. In one embodiment, cam shaft follower pin


280


is forced against first clamp element


206


upper portion


212


, thereby forcing lower portion


214


closed to grip a tool shank.




Similarly, when a reduced pressure, or even a vacuum, is applied through pressurized fluid source


340


to enter cam shaft gap


312


, the cam shafts travel inward, bringing faces


310


toward each other, as a result of the force applied by the biasing springs


308


. This changes the effective diameter of the cam shaft by decreasing the effective outer diameter, thereby allowing the cam shaft follower pins to travel inward into body


201


, thereby causing first clamp element


206


upper portion


212


to travel inward toward body


201


. The forcing of first clamp element


206


against cam shaft follower pin


280


can be caused by biasing springs


252


in some embodiments.





FIG. 3

illustrates a transverse cross-sectional view through brake press tool holder


200


of FIG.


2


. Elements identically numbered with respect to

FIG. 2

are similar in FIG.


3


and need not be reintroduced. Rounded head pins


241


may be seen to be received within aperture


242


and aligned with pin recesses


240


in first clamp element


206


and second clamp element


208


. Clamp biasing springs may be seen to be mounted within upper spring receiving pockets


251


and lower spring receiving pockets


253


. Cam shaft follower pin


280


may be seen to be disposed against first clamp element


206


upper portion


212


. Rotatable tool gripper


236


may be seen to include gripping notched portion


238


therein. In

FIG. 3

, cam shaft


300


may also be seen to be slidably disposed within cam shaft bore


222


.





FIG. 4

illustrates another embodiment of the invention in a tool holder


400


. Tool holder


400


is similar in many respects to tool holder


200


of

FIGS. 2 and 3

, but having a differing cam shaft design and fluid entry port location. Tool holder


400


includes a tool holder body


401


, having a cam shaft bore


403


longitudinally disposed therethrough. Cam shaft bore


403


is in fluid communication with pressurized fluid source


340


, which may be coupled to one end of cam shaft bore


403


. An O ring


406


is illustrated near cam shaft pressure-bearing face


408


, which may be described as the proximal-most face of cam shaft


402


. A shaft region


410


proceeds distally to a first camming region or surface


412


, which includes a proximal, larger outer diameter, region


414


tapering axially to a smaller outer diameter more distal region


416


. Cam shaft


402


proceeds still more distally to a general midshaft substantially constant outer diameter portion


418


, and proceeding further distally to a second camming surface or region


420


, including a larger outer diameter region


422


, tapering axially to a smaller outer diameter


424


, which proceeds distally to a cam shaft distal-most end portion


426


. End portion


426


bears against biasing spring


308


which seats against end cap


306


.




Tool holder


400


operates using pressurized fluid, as did tool holder


200


. Pressurized fluid, for example, hydraulic fluid or high pressure (pneumatic) air, may be supplied through pressure source


340


, into cam shaft bore


404


, against O ring


406


and pressure-bearing face


408


, thereby forcing cam shaft


402


distally against biasing spring


308


and end cap


306


. As cam shaft


402


is forced distally, the effective outer diameter of camming regions


412


and


420


increase. The increased effective outer diameter forces cam follower pin


280


outward against the upper portion of clamp element


206


, thereby forcing the clamp lower portion to close about the tool shank.




Various other embodiments of the invention may be described with reference to the previously discussed figures without requiring substantially duplicative figures. In one aspect of the invention, the placement of cam follower pin


280


relative to rounded head pivot pins


241


may be varied between embodiments. Placing the cam follower pin above the pivot pins can provide an embodiment in which the applied fluid pressure causes the clamp to grip the brake press tool, while the loss of pressure causing the releasing of the tool. In a preferred embodiment, the loss of pressure, whether intentional or unintentional, places the tool in a position where the tool can be removed by hand rather than causing the tool to drop. Such an operation is described in U.S. Pat. No. 6,003,360. In another embodiment, the cam follower pin is disposed beneath the pivot pins, thereby creating an embodiment in which the fluid pressure causes the jaws to open, and the loss of fluid pressure allows a clamp biasing spring to close the clamp about the tool.




Another aspect of the invention may be described with respect to FIG.


4


. In some embodiments, the slope of the camming surface is reversed with respect to that illustrated in cam shaft


402


in FIG.


4


. In particular, the larger outer diameter portion is distally further from the pressure bearing face than the smaller outer diameter portion. With respect to the slope of the camming surface, this may be illustrated by viewing cam shaft


402


as inserted into cam shaft bore


404


such that end


426


serves as the pressure-bearing face, rather than pressure-bearing face


408


. In this configuration, the effective outer diameter of the cam shaft decreases as the cam shaft travels distally against spring


308


, thereby allowing cam follower pin


280


to travel inward as pressure is applied through fluid pressure source


340


. This allows the upper portion of clamp element


206


to travel inward, thereby allowing the lower tool gripping portion to travel outward, putting the clamp into a position where the tool shank can be removed. In this embodiment, the loss of fluid pressure, either intentional or unintentional, can cause the cam shaft to travel away from biasing spring


308


, thereby increasing the effective outer diameter of the cam shaft, thereby forcing the cam follower pin outward, and causing the clamps to fail in a closed position. In a similar manner, the camming surface slopes of cam shaft


300


in

FIG. 2

may also be reversed.




The present invention has been described with respect to specific examples and embodiments of the invention for the purposes of illustration above. The scope of the present invention is described in the claims set forth below.



Claims
  • 1. A holder for mounting to a press brake a press brake tool having a mounting shank, the holder comprising a body having a support plate, a clamp having upper and lower portions and pivotally attached between said upper and lower portions to the body to enable the mounting shank of the tool to be clamped between said lower portion of the clamp and said support plate, the body having formed therein a cylindrical bore and having a fluid entry port for delivering pressurized fluid to a location within said bore, an elongated shaft slidable axially within said bore in response to the delivery of fluid under pressure into the bore and having a camming surface along its length, said camming surface including a first, larger outer diameter region having an outer diameter greater than a second, smaller outer diameter region, said body including a cam follower engagable with said camming surface and movable in response to axial movement of the shaft to force the clamp to pivot with respect to the body, thereby forcing the lower portion of the clamp toward or away from said support plate to clamp or unclamp the mounting shank of the tool.
  • 2. The press brake holder of claim 1, wherein the camming surface is tapered between the larger outer diameter region and the smaller diameter region.
  • 3. The press brake holder of claim 2, wherein the camming surface is continuously and uniformly tapered between the larger and smaller diameter regions.
  • 4. The press brake tool holder of claim 1 including a biasing element mounted at the end of said bore to urge said shaft toward said bore location.
  • 5. The press brake tool holder of claim 4 wherein said camming surface is oriented to encounter and bear against said cam follower as said shaft slides toward said biasing element.
  • 6. The press brake tool holder of claim 4 wherein said camming surface is oriented to encounter and bear against said cam follower as said shaft slides away from said biasing element.
  • 7. The press brake tool holder of claim 1 wherein said elongated shaft has at least two axially spaced camming surfaces, and wherein said body includes at least two cam followers engagable respectively with said tapered camming surfaces.
  • 8. The press brake tool holder of claim 1 including two of said elongated shafts slidable within said bore and wherein said entry port is positioned to deliver pressurized fluid at a location between said shafts to urge them in opposite directions, and wherein said body includes at least two cam followers engagable respectively with said camming surfaces of the elongated shafts.
  • 9. The press brake holder of claim 8 including biasing elements mounted at the ends of said bore to urge said shafts toward each other.
  • 10. The press brake tool holder of claim 9 wherein said camming surfaces are oriented to encounter and bear against said cam followers as said shafts slide away from each other.
  • 11. The press brake tool holder of claim 9 wherein said camming surfaces are oriented to encounter and bear against said cam followers as said shafts slide toward each other.
  • 12. The press brake tool holder of claim 1, wherein the clamp is biased to clamp the tool mounting shank.
  • 13. The press brake tool holder of claim 12, wherein the clamp has an upper portion and a lower portion, wherein the cam follower bears against the upper portion to urge the lower portion to clamp the tool mounting shank.
US Referenced Citations (11)
Number Name Date Kind
4497194 Martin et al. Feb 1985 A
4506538 Jones, Jr. Mar 1985 A
4545232 Martin et al. Oct 1985 A
4895014 Houston Jan 1990 A
4993255 Treillet Feb 1991 A
5065610 Yonezawa Nov 1991 A
6000273 Stover Dec 1999 A
6003360 Runk et al. Dec 1999 A
6151951 Kawano Nov 2000 A
6446485 Tarasconi Sep 2002 B1
6450004 Edmondson et al. Sep 2002 B1
Foreign Referenced Citations (3)
Number Date Country
2250606 Jun 1975 FR
986568 Aug 1988 SU
1569073 Jun 1990 SU