ROLL-FORMING MACHINE ROLLING GAP ADJUSTMENT STRUCTURE BACKGROUND OF THE INVENTION (a) Field of the Invention 5 The present invention relates to a roll-forming machine adjustable to fit different thickness of sheet materials for the formation of different components selectively and more particularly, to a roll-forming machine rolling gap adjustment structure, which allows quick adjustment of the gap between an upper shape-forming wheel set at an upper shaft and a 10 lower shape-forming wheel set at a lower shaft to fit different thicknesses of sheet materials. (b) Description of the Related Art A conventional roll-forming machine uses multiple forming roll sets for the formation of components from sheet materials. Each forming roll 15 set comprises left and right racks respectively affixed to the machine base, an upper shaft and a lower shaft respectively pivotally supported between respective bearing blocks of the left and right racks at different elevations, an upper shape-forming wheel set fixedly mounted on the upper shaft, and a lower shape-forming wheel set fixedly mounted on 20 the lower shaft. As the gap between the upper shape-forming wheel set and the lower shape-forming wheel set is not adjustable, the upper shaft or the lower shaft may be forced to deform when the upper shape forming wheel set and the lower shape-forming wheel set are rotated to ram a different thickness of sheet material, affecting the processing 25 quality. When a trouble occurred, the machine must be shut down for examination and troubleshooting, affecting the production. Therefore, it is desirable to provide a roll-forming machine rolling gap adjustment structure that eliminates the aforesaid drawbacks. SUMMARY OF THE INVENTION 30 The present invention has been accomplished under the circumstances in view. It is therefore the main object of the present invention to provide a roll-forming machine rolling gap adjustment structure, which facilitates fine adjustment of the rolling gap, preventing accidental deformation of machine parts and assuring a high level of 35 processing quality and system durability. According to one aspect of the present invention, the rolling gap adjustment structure comprises a gap adjustment member rotatable to rotate a screw rod in moving an upper bearing block upwards or 1 downwards for allowing the gap between an upper shape-forming wheel set at an upper shaft and a lower shape-forming wheel set at a lower shaft to be adjusted to fit different thicknesses of sheet materials, and a compression spring for supporting a bottom locating block and prevent 5 accidental deformation of the upper shaft or the lower shaft during gap adjustment. According to another aspect of the present invention, the associating upper bearing block and the upper shaft are moved with the associating screw rod and screw nut to carry the associating bottom locating block 10 upwardly in compressing the compression spring or downwardly to release the compression spring during rotation of the upper shape forming wheel set and the lower shape-forming wheel set to ram a sheet material, avoiding accidental deformation of the upper shaft or the lower shaft during gap adjustment and assuring a high level of processing 15 quality and system durability. According to still another aspect of the present invention, a spring tension adjustment member is provided for rotation by the user to adjust the tension of the compression spring to fit different requirements. BRIEF DESCRIPTION OF THE DRAWING 20 FIG. 1 is a plain view of a roll-forming machine according to the present invention. FIG. 2 is enlarged view of a part of FIG. 1, illustrating the arrangement of one forming roll set. FIG. 3 is a side plain view of one forming roll set in accordance 25 with the present invention. FIG. 4 is a sectional view in an enlarged scale of one forming roll set in accordance with the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, a roll-forming machine in accordance with the 30 present invention is shown comprising a machine base 1, a guide block unit 2 provided at the front top side of the machine base 1 for guiding a sheet material to the machine base 1 for processing, forming roll sets 3, speed reducers 5, couplings 6 respectively coupled to the speed reducers 5, and motors 50 for rotating the speed reducers 5. The roll-forming 35 machine gap adjustment structure of the invention is adapted for adjusting the gap between the upper shape-forming wheel set 35 and lower shape-forming wheel set 38 of each forming roll set 3. The number of the forming roll sets 3 is 15 numbered from the 1st through 2 the 15th. Each forming roll set 3 comprises: left and right racks 31 respectively affixed to the machine base 1 with fastening members 41 (see FIG. 2), each having two openings 311 5 disposed at different elevations, an upper bearing block 32 and a lower bearing block 33 respectively mounted in the two openings 311, and a top locating block 312 located on the top side thereof; an upper shaft 34 pivotally mounted between the upper bearing blocks 32 of the left and right racks 31 and having an upper gear wheel 10 341 fixedly mounted on one end thereof (see FIG. 1); a lower shaft 37 pivotally mounted in the lower bearing blocks 32 of the left and right racks 31 and coupled to one respective speed reducer 5 through one respective coupling 371 (see FIG. 1) and having a lower gear wheel (not shown) fixedly mounted on one end thereof and 15 meshed with the upper gear wheel 341 for synchronous rotation; an upper shape-forming wheel set 35 fixedly mounted on the upper shaft 34; and a lower shape-forming wheel set 38 fixedly mounted on the lower shaft 37; 20 wherein: each forming roll set 3 further comprises: a screw rod 30 vertically mounted in each of the left and right racks 31 on the middle and threaded through the associating top locating block 312 and connected to the associating upper bearing block 32 by a 25 fastening member 74 (see FIG. 2); a bottom locating block 70 set in each of the left and right racks 31 and having an axle hole 702 cut through top and bottom sides thereof and a locating groove 7011 located on the top side 701 thereof; a screw nut 7 comprising a screw hole 73 threaded onto the screw 30 rod 30 at each of the left and right racks 31, a tubular shank 71 surrounding the screw hole 73 and inserted upwardly through the axle hole 702 of the bottom locating block 70 and an axle hole 3121 on the associating top locating block 312, an outer thread 711 spirally extending around the periphery of the tubular shank 71 and a head 720 35 extending around the periphery of the bottom end of the tubular shank 71 and stopped at the bottom side of the bottom locating block 70 (see FIG. 4); a compression spring 80 set around the screw nut 7 in between the 3 top locating block 312 at each of the left and right racks 31 and the associating bottom locating block 70, having opposing top end 801 and bottom end 802 thereof respectively positioned in a locating groove 3122 at the bottom side of the associating top locating block 312 around 5 the respective axle hole 3121 and the locating groove 7011 at the top side 701 of the bottom locating block 70; a spring tension adjustment member 8 stopped at the top side of the top locating block 312 at each of the left and right racks 31 and having an inner thread 81 threaded onto the outer thread 711 of the screw nut 70 10 at the screw rod 30 at each of the left and right racks 31; and a gap adjustment member 9 stopped at the top side 82 of the spring tension adjustment member 8 at each of the left and right racks 31 and having an inner thread 91 threaded onto the associating screw rod 30. When rotating the gap adjustment member 9 clockwise or counter 15 clockwise, the associating screw rod 30 is rotated, causing the associating upper bearing block 32 to be moved upwards or downwards with the associating screw rod 30 to adjust the gap between the upper shape-forming wheel set 35 at the upper shaft 34 and the lower shape forming wheel set 38 at the lower shaft 37 to fit different thicknesses of 20 sheet materials. Further, during rotation of the upper shape-forming wheel set 35 and the lower shape-forming wheel set 38 to ram a sheet material, the associating upper bearing block 32 and upper shaft 34 are moved with the associating screw rod 30 and screw nut 7 to carry the associating bottom locating block 70 upwardly in compressing the 25 compression spring 80 or downwardly to release the compression spring 80, avoiding accidental deformation of the upper shaft 34 or the lower shaft 37 during gap adjustment and assuring a high level of processing quality and system durability. When wishing to adjust the tension of the compression spring 80, rotate the spring tension adjustment member 8 to 30 achieve the adjustment. In conclusion, the invention provides a roll-forming machine gap adjustment structure, which has the following features and advantages: 1. Simply by rotating the gap adjustment member 9 to rotate the associating screw rod 30 in moving the associating upper bearing block 35 32 upwards or downwards, the gap between the upper shape-forming wheel set 35 and the lower shape-forming wheel set 38 is adjusted to fit different thicknesses of sheet materials. 2. During rotation of the upper shape-forming wheel set 35 and the 4 lower shape-forming wheel set 38 to ram a sheet material, the associating upper bearing block 32 and upper shaft 34 are moved with the associating screw rod 30 and screw nut 7 to carry the associating bottom locating block 70 upwardly in compressing the compression 5 spring 80 or downwardly to release the compression spring 80, avoiding accidental deformation of the upper shaft 34 or the lower shaft 37 during gap adjustment and assuring a high level of processing quality and system durability. 3. Rotating the spring tension adjustment member 8 achieves 10 adjustment of the tension of the compression spring 80 to fit different requirements. 5