公制細螺底孔一般是指用丝锥攻内螺纹之前所预制的孔的直径,一般指加工件的内置圆孔，最底或最面上的圆形孔洞。丝锥在攻螺纹的过程中，切削刃主要是切削金属，但还有挤压金属的作用，因而造成金属凸起并向牙尖流动的现象，所以攻螺纹前，钻削的孑匕径（即底孔）应大于螺纹内径。底孔的直径可查手册或按下面的经验公式计算：脆性材料（铸铁、青铜等）：钻孔直径dO=d（螺纹外径）-1.1p（螺距）塑性材料（钢、紫铜等）：钻孔直径dO=d（螺纹外径）-p（螺距）公制粗螺紋規格M1.0X0.25M1.1X0.25M1.2X0.25M1.4X0.3標準徑2級牙鑽孔徑最大最小0.750.7850.7290.850.8850.8290.950.9850.9291.101.1421.075規格標準徑2級牙鑽孔徑最大最小M1.0X0.20.800.8210.783M1.1X0.20.900.9210.883M1.2X0.21.001.0210.983M1.4X0.21.201.2211.183M1.6X0.351.251.3211.221M1.7X0.351.351.4211.321M1.8X0.351.451.5211.421M2.0X0.41.601.6791.567M2.2X0.451.751.8381.713M2.3X0.41.901.9791.867M2.5X0.452.102.1382.013M2.6X0.452.202.2382.113M3.0X0.52.502.5992.459M3.0X0.62.402.4402.280M1.6X0.21.401.4211.383M1.7X0.21.451.5001.460M1.8X0.21.601.6211.583M2.0X0.251.751.7851.729M2.2X0.251.951.9851.929M2.3X0.252.052.0612.001M2.5X0.352.202.2212.121M2.6X0.352.202.2462.186M3.0X0.352.702.7212.621M3.5X0.33.203.2213.121M3.5X0.62.903.0102.850M4.0X0.73.303.4223.242M4.0X0.73.253.3263.1065M4.5X0.73.803.8783.6685M5.0X0.84.204.3344.134M5.0X0.94.104.1703.930M6.0X1.05.005.1534.917M7.0X1.06.006.1535.917M8.0X1.26.806.9126.6475M9.0X1.27.807.9127.6475M10X1.58.50&6768.376M11X1.59.509.6769.3765M4.0X0.53.503.5993.459M4.5X0.54.004.0993.959M5.0X0.54.504.5994.459M5.5X0.55.005.0994.959M6.0X0.75.305.3785.1885M6.0X0.55.505.5505.400M7.0X0.76.306.3786.1885M7.0X0.56.506.5506.400M8.0X1.07.007.1536.917M8.0X0.77.307.3787.1885M8.0X0.57.507.5207.400M9.0X1.08.00&1537.917M12X1.710.3010.4410.10M9.0X0.78.30&378&188516512.2111.83M10X1.2M14X2.012.000558.80&912&64714.2113.83M16X2.014.0005M10X1.09.009.153&91715.7415.29M10X0.7M18X2.515.504459.309.3789.18817.7417.29M20X2.517.5044M10X0.59.509.5209.40019.7419.2910.15M22X2.519.5044M11X1.010.0039.91721.2520.75M11X0.710.3710.18M24X3.021.0022510.308824.2523.7510.6710.37M27X3.024.0022M12X1.510.506626.7726.21M12X1.210.9110.64M30X3.526.5011510.8027公制細螺紋規格標準徑2級牙鑽孔徑最大最小M12X1.011.0011.15310.917M12X0.511.5011.52011.400M14X1.512.5012.67612.376M14X1.013.0013.15312.917M15X1.513.5013.67613.376M15X1.014.0014.15313.917M16X1.514.5014.67614.376M16X1.015.0015.15314.917M17X1.515.5015.67615.376M17X1.016.0016.15315.917M18X2.016.0016.21015.835M18X1.516.5016.67616.376M18X1.017.0017.15316.917M20X2.018.0018.21017.835M20X1.518.5018.67618.376M20X1.019.0019.15318.917M22X2.020.0020.21019.835M22X1.520.5020.67620.376M22X1.021.0021.15320.917M24X2.022.0022.21021.835M24X1.522.5022.67622.376M24X1.023.0023.15322.917M25X2.023.0023.21022.835M25X1.523.5023.67623.376M25X1.024.0024.15323.917M26X1.524.5024.67624.376M27X2.025.0025.21024.835M27X1.525.5025.67625.376M27X1.026.0026.15325.917M28X2.026.0026.21025.835M28X1.526.5026.67626.376M28X1.027.0027.15326.917M30X3.0M30X2.0M30X1.5M30X1.0M32X2.0M32X1.5M33X3.0M33X2.0M33X1.5M35X1.5M36X3.0M36X2.0M36X1.527.0027.25226.75228.0028.21027.83528.5028.67628.37629.0029.15328.91730.0030.21029.83530.5030.67630.37630.0030.25229.75231.0031.21030.83531.5031.67631.37633.5033.67633.37633.0033.25232.75234.0034.21033.83534.5034.67634.376
