Description SLIDING RACK L-CLAMP HAVING A NON ROTATING SHAFT BACKGROUND OF THE INVENTION [ 1 ] 1. Field of the Invention [2] The present invention generally relates to L-clamps. Particularly, the present invention relates to a sliding rack L-clamp. [3] 2. Description of the Prior Art [4] A standard L-clamp or bar clamp has a straight bar attached to a fixed arm. A sliding arm slides along the straight bar and the outer end of the sliding arm comprises a threaded screw which comprises a handle on one end and a pad on the other end. When the handle is turned, the pad engages an object being clamped between the pad on the end of the threaded screw and the pad on the end of the fixed arm. However, as the screw is turned the pad at the end of the screw rotates generally causing marring of the object or walking movement or deflection of the screw on the object being clamped. [5] The related prior art includes U.S. Patent No. 6,367,790 (2002, Ocklenburg et al .) which discloses a sliding arm L-clamp comprising a bar, an elbow, and an arm. The movable arm extends from the bar, slides along the bar, and has an outer end which is threaded to a spindle carrying a handle or crank at one end and a first abutment pad at the opposite end. The fixed arm carries at its outer end a second abutment pad parallel with the first abutment pad. However, as the spindle turns, the abutment pad turns creating the possibility of marring a workpiece being held in the L-Oclamp. [6] U.S. Patent No. 5,893,553 (1999, Pinkous) discloses a C-shaped clamp having a pad that does not rotate when an elongated threaded rod with an extendible thread surface is screwed toward a ball jointed pad on the opposite end of the C-clamp device. The elongated threaded rod is attached to the top of a pad bracket and a bar means attaches to an end of the pad bracket perpendicular to the rod. The blocking rod prevents the pad bracket from rotating. However, this design does not support the exterior spindle and still allows for pad and spindle rotation, albeit limited. [7] U.S. Patent No. 5,405,124 (1995, Mayer) discloses a sliding arm C-shaped clamp bow with a nonOrotating pad or clamping body. A clamping spindle is threaded through a spindle nut at one end of the bow. The contact surface of the end of the spindle is secured within the clamping body so that as the contact surface of the spindle rotates within the clamping body, the surface clamping element remains stationary. The clamping body extends from the guide section along the clamp bow. However, this is a C-shaped clamp with a spindle and not an L-clamp and the exposed spindle is un¬ supported by an exterior housing. [8] Therefore, what is needed is a sliding arm L-clamp that does not rotate when being tightened. What is further needed is an L-clamp that prevents marring of a workpiece, walking movement, or deflection of a screw on a workpiece when tightening the L- clamp on a workpiece. What is still further needed is an L-clamp that provides fast clamping of a workpiece. SUMMARY OF THE INVENTION [9] It is an object of the present invention to provide a sliding arm L-clamp having a pad on the end of a cylindrical tube of the sliding rack that does not rotate when being tightened for holding a workpiece. It is another object of the present invention to provide a sliding arm L-clamp having a non-rotating shaft or inner tube to prevent marring of a workpiece, walking movement, or deflection of a screw on a workpiece when tightening the L-clamp on a workpiece. It is a further object of the present invention to provide fast clamping of a workpiece by sliding a rack along an L-clamp and turning a hex nut on one end of a cylindrical tube which turns a screw rod inside the tube thereby tightening a pad of the sliding rack quickly on the workpiece. [10] The present invention achieves these and other objects by providing a sliding rack L-clamp for clamping a workpiece having a first clamp means and a second clamp means. First clamp means has an L-shaped bar, and second clamp means has a rack, a cylinder assembly, a pad, a screw rod, and a linear actuating mechanism. The linear actuating mechanism is operably connected to the cylinder assembly. The L-shaped bar has an arm and a rail. The arm and rail form approximately a ninety degree angle. The arm has a flat portion on one end, i.e. the flat arm end, for contacting the workpiece. [11] The rack has an opening on a first end for slidably receiving the rail of the L- shaped bar therethrough. The cylinder assembly, which is mounted perpendicular to the rack on a second end of the rack and which is axially aligned with the flat arm end, includes an outer tube and a nonrotatable inner tube. The pad attaches to a first end of the inner tube opposite the flat portion of the arm of the L-shaped bar. [12] The linear actuating mechanism is operably attached to a second end of the inner tube for moving the pad back and forth when a torque providing means engages the linear actuating mechanism for clamping the workpiece. The linear actuating mechanism includes a screw rod, an end cap, a threaded insert, a hex nut, a spring pin, and an optional handle. The end cap attaches within the outer tube. The threaded insert has a first end attached, preferably by a press-Dfit, into the second end of the inner tube. The screw rod has a threaded portion and a rod end portion. The screw rod is threaded into the threaded insert with a major portion of the threaded portion of the screw rod extending through the threaded insert and into the inner tube. The end portion extends through the end cap. The hex nut receives the end portion of the screw rod and a spring pin passes through the hex nut and the end portion of the screw rod to secure the screw rod within the cylinder assembly. [13] The objects are further accomplished by an L-shaped bar having an arm and a rail forming approximately a ninety-degree angle, an adjustable sliding rack assembly having a rack with an opening in a first end for receiving the rail of the L-shaped bar and a cylinder assembly attached to a second end of the sliding rack, the cylinder assembly having a first end for contacting a workpiece as the sliding rack is moved toward the arm of the L-shaped bar, and a linear actuating mechanism attached to the second end of the cylinder assembly for extending and retracting the first end of the cylinder without rotating the first end of the cylinder. [14] Additional objects, features and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of the preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived. BRIEF DESCRIPTION OF THE DRAWINGS [15] The appended claims particularly point out and distinctly claim the subject matter of the present invention. The various objects, advantages, and novel features of the present invention will be more fully apparent from a reading of the following detailed description in conjunction with the accompanying drawings in which like reference numerals refer to like parts. [16] FIGURE 1 is a side elevational view of a sliding arm L-clamp having a non- rotating abutment pad according to the present invention. [17] FIGURE 2 is a side perspective view of an L-clamp having a non-rotating abutment pad according to the present invention showing workpieces of a block and a board, in phantom, held together by the L-clamp. [18] FIGURE 3 is a cross-sectional view of a cylinder of the L-clamp of Fig. 1. [19] FIGURE 4 is an end view of the outer tube of the cylinder of Fig. 3. [20] FIGURE 5 is an end view of an inner tube of the cylinder of Fig. 3. [21] FIGURE 6 is a cross-sectional view of an Acme nut of the cylinder of Fig. 3. [22] FIGURE 7 is a side elevational view of a screw rod of the cylinder of Fig. 3. [23] FIGURE 8 is a top view of a rack of the LD-clamp of Fig. 1. [24] FIGURE 9 is a top plan view of the L-clamp of the present invention. [25] FIGURE 10 is a front elevational view of the L-clamp of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT [26] The preferred embodiment(s) of the present invention is illustrated in Figs. 1-10. Figure 1 illustrates a side elevational view of a sliding rack L-clamp 10 having an L- shaped bar 11 and an adjustable rack assembly 15. Adjustable rack assembly 15 includes a rack 16, a cylinder assembly 18, a non-rotating pad 20, and a linear actuating mechanism 50 of which only a portion is shown as a hex nut 24 with an optional handle 26. L-shaped bar 11 has a rail 12 and an arm 14 forming a right angle or an L-shape. At the end of arm 14 on an inner side is a flat arm end 22. Rack 16 includes an opening 17 (not shown) at a first rack end 16a for receiving rail 12 of L- shaped bar 11. On the other end of rack 16, i.e. second rack end 16b, is cylinder assembly 18 having a pad 20 at one end. Cylinder assembly 18 includes an outer tube 34 and a nonrotatable inner tube or shaft 36. Pad 20 is attached to a first inner tube end 36a of inner tube or shaft 36, which extends from a first outer tube end 34a of outer tube 34 when a hex nut 24 located adjacent a second outer tube end 34b of outer tube 34 is rotated by a handle 26. Pad 20 preferably has a short cylindrical shaft (not shown) having external threads which screws into inner tube or shaft 36. [27] Optional handle 26 is slidably received through an opening in hex nut 24, and a cap 28 screws onto one end of handle 26 to retain the handle 26. The other end 29 of handle 26 is knurled so that it does not enter hex nut 24. Pad 20 does not rotate as it extends from cylinder assembly 18 from a proximal position adjacent outer tube end 34a to a distal position adjacent flat arm end 22 thereby avoiding marring the workpiece 30, walking on the workpiece 30, or causing any deflection of cylinder assembly 18 on the workpiece 30. When handle 26 is removed from hex nut 24, a power tool or a manual wrench may be used to turn hex nut 24. [28] Referring to Fig. 2, a perspective view of LD-clamplO having non-rotating pad 20 is shown clamping a first workpiece 30 to a second workpiece 32. Pad 20 contacts the first workpiece 30 and flat end 22 of arm 14 of L-shaped bar 11 contacts the second workpiece 32. Rail 12 is inserted in opening 17 of rack 16 and pad 20 on the end of shaft 36 has been extended by turning handle 26 to hold the first workpiece 30 in contact with the second workpiece 32. [29] Referring to Fig. 3, a cross-sectional view of the cylinder assembly 18 is shown. Cylinder assembly 18 includes an outer tube 34, shaft or inner tube 36, having pad 20 attached at first inner tube end 36a, and linear actuating mechanism 50 operably connected to outer tube 34 and inner tube 36. Linear actuating mechanism includes a threaded insert 38, preferably an Acme nut, a screw rod 40, hex nut 24, a spring pin 25a, an end cap 42, and optional handle 26. Screw rod 40 is rotated by hex nut 24 or by handle 26 passing through hex nut 24. Rack 16 is welded to the outer wall of outer tube 34 near first outer tube end 34a where pad 20 is located. [30] End cap 42 attaches within second outer tube end 34b of outer tube 34 by threading or press fitting. Threaded insert 38 has a first end 38a attached, preferably by a press- Dfit, into the second inner tube end36a of inner tube 36. Screw rod 40 has a threaded portion 41a and an end portion 41b. Screw rod 40 is threaded into threaded insert 38 with a major portion of threaded portion 41a of screw rod 40 extending through threaded insert 38 and into inner tube 36. End portion 41b extends through end cap 42. Hex nut 24 receives end portion 41b of screw rod 40 and spring pin 25 passes through hex nut 24 and end portion 41b of screw rod 40 to secure screw rod 40 within cylinder 38. [31] Referring to Fig. 4 and Fig. 5, Fig. 4 is an end view of outer tube 34 and Fig. 5 is an end view of inner tube 36 that is slidably received within outer tube 34. Outer tube 34 includes a notch or groove 35 along its entire length. Inner tube 36 includes a ridge or protrusion 37 on the outside of inner tube 36 along its entire length. Ridge 37 is configured to be slidably received within groove 35 of outer tube 34. [32] Referring now to Fig. 6, there is illustrated a cross-sectional view of threaded insert 38 shown in Fig. 3. Threaded insert 38 has a shoulder portion end 38a, a smaller diameter shaft portion 38b, and internal threads 38c. Internal threads 38c are preferably left hand threads. As previously disclosed, threaded insert 38 is preferably an Acme nut. Shaft portion 38b is preferably press Dfitted into the second inner tube end36b of inner tube or shaft 36, but may also be threaded, welded, pinned, and the like, to secure threaded insert 38 into shaft 36. Screw rod 40 is screwed into threaded insert 38. When screw rod 40 is rotated, threaded insert 38 rides along the threads of screw rod 40 causing inner tube or shaft 36, which is attached to threaded insert 38, to move in and out of outer tube 34 of cylinder assembly 18 by way of first outer tube end 34a. Inner tube 36 cannot rotate due to its outer ridge 37 lying within groove 35 of outer tube 34. [33] Figure 7 is a side elevational view of screw rod 40. Screw rod 40 includes threaded portion 41a and end portion 41b. As noted, threaded portion 41a extends over a major portion of screw rod 40. End portion 41b is shown having two levels of reduced diameters, a first reduced diameter 44 and a second reduced diameter 46. Second reduced diameter includes a through opening 48. First reduced diameter 44 extends through end cap 42 of cylinder 18. As previously disclosed, end cap 42 preferably screws into threads on the inside diameter of outer tube 34. Second reduced diameter 46 extends through end cap 42 and into the middle of hex nut 24. Hex nut 24 also includes a small cylindrical opening 25 that is aligned with a through opening 48 in second reduced diameter 44 of screw rod 40. A spring pin 25a is inserted into through opening 48 for retaining the hex nut 24 on end portion 41b of screw rod 40. [34] Referring to Fig. 8, a top view of rack 16 of L-clamp 10 is shown comprising opening 17 for receiving rail 12 of L-shaped bar 11 to pass through in order to facilitate quickly securing workpieces such as workpieces 30, 32 in Fig. 2. Rack 16 slides along rail 12 freely until clamping pressure is applied, at which time a binding of rack 16 and rail 12 prevents movement. The opposite end of rack 16 is attached to cylinder assembly 18 which houses inner tube or shaft 36 with pad 20 on first inner tube end 36a of shaft 36 for contacting workpiece 32. [35] Referring to Fig. 9 and Fig. 10, Fig. 9 is a top plan view of L-clamp 10 showing arm 14 of LD-shaped badl. Fig. 10 is a front elevational view of L-clamp 10 showing the end of arm 14 having flat end 22 for contacting workpiece 32 and pad 20 for contacting workpiece 30 as shown in Fig. 2. A front portion of rail 12 of L-Dshaped bar 11 is shown between flat portion 22 of arm 14 and pad 20. [36] Although the preferred embodiments of the present invention have been described herein, the above description is merely illustrative. Further modification of the invention herein disclosed will occur to those skilled in the respective arts and all such modifications are deemed to be within the scope of the invention as defined by the appended claims.