Drill diameter for use prior to tapping screw threads
MOD ISO 2306:1972
全国刀具标准化技术委员会
本标准规定了用于正常旋合长度的丝锥攻丝前钻孔用麻花钻的直径。麻花钻直径近似等于螺纹公称直径（大径）减去螺距。
列出的钻头尺寸适用于下列螺纹:
——GB/T 193-2003的普通螺纹(粗牙和细牙系列):表1和表2；
——ISO 263:1973的英制螺纹(UNC和UNF):表3和表4；
——GB/T 7306.1-2000和GB/T 7307-2001的管螺纹：表5和表6。
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