螺纹底孔直径一、螺纹底孔直径计算式螺纹底孔直径可参照表1、表2的公式计算，也可查表3至表11。表1加工米制普通螺纹底孔钻头直径D的计算公式适用范围1D=d–t式中d：螺纹的公称直径t：螺距（1）螺距t<1；（2）工件材料塑性较大；（3）孔扩张量适中。2D=d–(1.04～1.08)t（1）螺距t<1；（2）工件材料塑性较小；（3）孔扩张量较小。表2加工英制螺纹底孔钻头直径的计算公式(mm)螺纹公称直径(英寸)铸铁与青铜钢与黄铜3/16"～5/8"D=25.4·(d-1/n)D=25.4·(d-1/n)+0.13/4"～11/2"D=25.4·(d-1/n)D=25.4·(d-1/n)+0.2注：n—每英寸牙数·2·二、螺纹底孔直径表表3米制粗牙螺纹攻丝前用的钻头直径(mm)螺纹小直径级别公称直径螺距5Hmax6Hmax7Hmax5H、6H、7Hmin钻头直径11.21.622.53456810121416182022240.250.250.350.40.450.50.70.811.251.51.75222.52.52.530.7850.9851.3011.6572.1122.5713.3824.2945.0076.8598.61210.37112.13514.13515.64917.64919.64921.1521.3211.6792.1382.5993.4224.3345.1536.9128.67610.44112.21014.21015.74417.74419.74421.2522.6393.4664.3845.2176.9828.75110.53112.31014.31015.85417.85419.85421.3820.7290.9291.2211.5612.0132.4593.2424.1344.9176.6478.37610.10611.83513.83515.29417.29419.29420.7540.750.951.251.602.052.503.304.205.006.808.5010.212.014.015.517.519.521.0·3·表4米制细牙螺纹攻丝前用的钻头直径(mm)螺纹小直径级别公称直径螺距5Hmax6Hmax7Hmax5H、6H、7Hmin钻头直径2.530.352.2012.7012.2212.7212.1212.6212.152.65450.53.5714.5713.5994.5993.6394.6393.4594.4593.504.5068100.755.3387.3389.3385.3787.3789.3785.4247.4249.4245.1887.1889.1885.707.209.208101214161820222417.1079.10711.10713.10715.10717.10719.10721.10723.1077.2179.21711.21713.21715.21717.21719.21721.21723.2177.2179.21711.21713.21715.21717.21719.21721.21723.2176.9178.91710.91712.91714.91716.91718.91720.91722.9177.009.0011.013.015.017.019.021.023.01012141.258.85910.85912.8598.91210.91212.9128.98210.98212.9828.64710.64712.6478.8010.812.8121415161820221.510.61212.61213.61214.61216.61218.61220.61210.67612.67613.67614.67616.67618.67620.67610.75112.75113.75114.75116.75118.75120.75110.37612.37613.37614.37616.37618.37620.37610.512.513.514.516.518.520.5·4·螺纹小直径级别公称直径螺...
