Description Non-Stop Fixture Technical Field The invention relates to a fixture of a lathe, in particular to an improved structure of a non-stop fixture. Background of the Invention Workpieces need to be fixed when machined on a lathe. Some people fix workpieces by a core camper. After one workpiece is completely machined, the lathe needs to be stopped to place another workpiece in position. In the prior art, the structure of the whole non-stop fixture device is complex and the manufacture cost thereof is high. Clamping of workpieces is not so stable, thereby affecting the machining precision of workpieces. Summary of the Invention The purpose of the invention is to provide a non-stop fixture, which is convenient for replacing workpieces, high in clamping and positioning precision, stable and reliable in working performance, and which effectively improves the working efficiency. The above technical purpose of the invention is mainly achieved by the technical solution as follows: a non-stop fixture is provided, comprising a main body and a core camper arranged on one end of the main body, the main body has a cavity, a compression spring and the core camper are provided in the cavity, a driving plate is mounted on an end of the main body, and a front end of the core camper is able to pass through a passageway on the driving plate to extend outside the main body. The core camper and the compression spring are creatively mounted together in the invention. Under the action of the compression spring, the core camper is able to shrink forward and backward. The core camper is inserted in the machining hole of a workpiece for conveniently clamping the workpiece. Therefore, the operation is quite convenient and the working efficiency is improved. Therefore, the invention has the following advantages: first, the core camper is able to shrink forward and backward, which is convenient for clamping and replacing workpieces, thereby improving the working efficiency; second, end teeth are provided, the end teeth may be in fitted connection with workpieces, thereby further increasing the friction force and making the clamping of workpieces firmer; and third, the clamping and positioning precision is high, the operation is stable and reliable, the clamping force of the core camper on workpieces may be changeable by a regulator, so that the application of the whole device becomes more flexible. Brief Description of the Drawings Fig. 1 is an exploded view of the invention; Fig. 2 is a structure diagram of a regulator of the invention; and, Fig. 3 is an enlarged view of an end face of a driving plate of the invention. Detailed Description of the Invention The technical solutions of the invention will be further described in details as below by embodiments with reference to drawings. Embodiment 1 As shown in Fig. 1, a non-stop fixture is provided, comprising a main body 1 and a core camper 2 arranged on one end of the main body 1, the main body 1 has a cavity 1 a, a compression spring 3 and the core camper 2 are provided in the cavity 1a, a driving plate 4 is mounted on an end of the main body 1, and a front end of the core camper 2 is able to pass through a passageway 4b on the driving plate 4 to extend outside the main body 1. A step 1c is provided on an end of the main body, and a tail end of the core camper 2 is provided with a bulge loop 2a which leans against an inner side of the step 1c. The cavity 1 a on the main body 1 is a through hole, a regulator 5 is arranged on a tail end of the cavity 1 a, and one end of the compression spring 3 acts on the regulator 5 and the other end thereof acts on the core camper 2. As shown in Fig. 2, the regulator 5 has a sleeve pillar 5a with an outer diameter smaller than that of the cavity 1 a, and one end of the compression spring 3 is sleeved on the sleeve pillar 5a. As shown in Fig. 3, multiple end face teeth 4a are arranged on a front end of the driving plate 4. The regulator 5 and the main body 1 are in threaded connection with each other. The driving plate 4 and the main body 1 are in threaded connection with each other. In use, the core camper 2 may be placed in an insert hole of a workpiece. The workpiece is clamped under the action of the compression spring. As the core camper 2 is able to shrink, replacing workpieces is quite convenient, so that the working efficiency is improved. In addition, the friction force may be further increased under the action of the end face teeth 4a on the front end of the driving plate 4 when the driving plate 4 comes into contact with workpieces.