Description Quick Loading/Unloading Drill Jig Technical Field of the Invention The invention relates to a quick loading/unloading drill jig. Background of the Invention When workpieces with central holes on two end faces are drilled, the workpieces are mostly clamped between a spacer and a drill plate and then drilled by a drill bit from a die hole on the drill plate. After one workpiece is drilled, the workpiece is unloaded; and then another workpiece to be drilled is placed well for drilling. In the prior art, when workpieces are loaded or unloaded, the drill plate and the spacer need to be separated from each other. Such drill jig has deficiencies that it is troublesome to load/unload workpieces and low in working efficiency. Summary of the Invention The purpose of the invention is to provide a quick loading/unloading drill jig which is convenient to load/unload workpieces and high in working efficiency. The technical purpose of the invention is mainly solved by the following technical solutions: a quick loading/unloading drill jig is provided, comprising a spacer and a drill plate. The drill plate is provided with multiple die holes. The spacer is provided with a central through hole. One end of a threaded body is connected to the drill plate, and another end thereof is located in the central through hole of the spacer and provided with a nut. An inner end of the nut is provided with a split washer, and the outer diameter of the split washer is greater than that of the nut but smaller than the inner diameter of the central through hole of the spacer. Wherein, the diameter of the central through hole of 1 the spacer is greater than outer diameter of a workpiece to be convenient for loading/unloading the workpiece. The outer diameter of the nut is smaller than the diameter of the central hole of the workpiece, while the outer diameter of the split washer is greater than the diameter of the central hole of the workpiece. Before drilling, one workpiece is sleeved on the threaded body through the central hole, with two ends of the workpiece against inner end faces of both the spacer and the drill plate, the split washer is inserted between the nut and an end face of the workpiece, and then the nut is screwed so that the workpiece may be fixed, and finally the workpiece is drilled by the drill bit from the die hole of the drill plate. After the drilling is completed, the machined workpiece may be taken out by slightly unscrewing the nut and pulling the split washer out, without taking the nut off from the threaded body. In this way, the loading/unloading speed of workpieces is largely accelerated, and the working efficiency is improved. Preferably, the inner end face of the spacer forms a lower positioning groove, and the inner end face of the drill plate forms an upper positioning groove which is coaxial with the lower positioning groove. The lower end of the workpiece is fitted with the lower positioning groove of the spacer, and the upper end of the workpiece is fitted with the upper positioning groove of the drill plate, so that a part of a workpiece to be conveniently drilled can be aligned with the die hole on the drill plate, and the loading/unloading efficiency of the workpieces is improved. Therefore, the invention is characterized in that the quick loading/unloading drill jig is convenient to load/unload workpieces and high in working efficiency. Brief Description of the Drawings Fig. 1 is a three-dimensional structure diagram of the invention; Fig. 2 is a bottom view of the Fig. 1; Fig. 3 is a top view of the Fig. 1; 2 Fig. 4 is a view of the Fig. 3 along line A-A; and, Figure 5 is an exploded structure diagram of the invention. Detailed Description of the Invention The technical solutions of the invention will be further described in details as below by specific embodiments with reference to drawings. Embodiment 1 As shown in Fig. 1, Fig. 2, Fig.3, Fig.4 and Fig.5, a round spacer 1 and a round drill plate 2 are arranged coaxially from top to bottom, an inner end face of the spacer 1 forms a lower round positioning groove 7, and an inner end face of the drill plate 2 forms an upper round positioning groove 8 which is coaxial with the lower round positioning groove 7. The drill plate 2 is provided with six die holes 3 on a same circumference having a same center as the drill jig, and drill sleeves 4 are arranged around the die holes 3. The spacer 1 is provided with a central through hole 5, and a counter bore 9 coaxial with the central through hole 5 of the spacer 1 is arranged on the drill plate 2. A head end of a bolt 10 is matched up with a step of the counter bore 9 of the drill plate 2, and another end of the bolt 10 is located in the central through hole 5 of the spacer and provided with a nut 11. An inner end of the nut 11 is provided with a split washer 12, and the outer diameter of the split washer 12 is greater than that of the nut but smaller than the inner diameter of the central through hole of the spacer. As shown in Fig. 1, Fig. 2, Fig. 3, Fig. 4 and Fig. 5, before drilling, a workpiece 13 is sleeved on the threaded body through the central hole, with two ends of the workpiece located in the lower positioning groove of the spacer and the upper positioning groove of the drill plate, respectively, the split washer is inserted between the nut and an end face of the workpiece, and then the nut is screwed so that the workpiece may be fixed, and finally the workpiece is drilled by the drill bit from the die hole of the drill plate. After the drilling is completed, the machined workpiece may be taken out by slightly unscrewing 3 the nut and pulling the split washer out. 4