普通螺纹是机械零件中常用的60°牙形螺纹,螺纹紧固件在工业上的应用很为广泛。普通螺纹旧精度标准(GB197-63)的级数较少,粗牙螺纹只有1、2、3三级,细牙螺纹只有1、2、2a、3四级,以往大多数螺纹均为3级,3级螺纹规定在图中不必注明其精度等级,因此螺纹旧精度的问题不突出。随着国际间技术交流和对外贸易的扩大,我国也按国际标准ISO重新制订了普通螺纹精度标准(GB Common thread is commonly used in mechanical parts 60 ° thread, thread fasteners in a wide range of industrial applications. Ordinary thread old precision standard (GB197-63) series less, coarse thread only 1,2,3 three, fine thread only 1,2,2a, 3 four, in the past most of the threads are 3 , 3 thread provisions in the figure do not have to indicate the accuracy level, so the problem of thread accuracy is not prominent. With the international exchange of technology and the expansion of foreign trade, China has also re-established the general thread accuracy standard (GB