攻丝前钻底孔用钻头直径计算公式

攻公制螺纹时的公式

t<1mm时：

dｚ=d -t

t>1mm时：

dｚ= d - (1.04 ～1.06) t

式中 t : 螺距（毫米）；dｚ：攻丝前钻头直径（毫米）；

       d: 螺纹公称直径（毫米）。

公制螺纹钻底孔用钻头直径尺寸表

公称直径              螺距             钻头直径

1                    粗 0.25                  0.75                 

                        细 0.2                    0.8                    

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2                     粗     0.4                 1.6

                        细     0.25               1.75

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     3                粗      0.5               2.5

                       细      0.35              2.65

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     4                  粗       0.7                3.3

                        细         0.5               3.5

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      5                粗          0.8            4.2

                        细           0.5           4.5

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     6                cu          1               5

                      xi            0.75           5.2

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   8             cu            1.25              6.7

                 xi              0.75               7.2

                细            1                      7

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     10           粗           1.5                8.5

                     细          1.25             8.7

                                 1                 9

                                      0.75         9.2

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        12               粗 1.75            10.2                        

                             细 1.5            10.5

                                 1.25            10.7

                               1                  11

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      14               粗 2             11.9

                         细 1.5            12.5

                               1.25          12.7

                             1                 13

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      16              粗 2                13.9      （13.8 ； 14也行）  

                       细    1.5           14.5

                              1               15

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    18            粗 2.5 （螺距）               15.4   （钻头直径）

                   细 2                                  15.9

                      1.5                                 16.5

                   1                                        17

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    20       粗   2.5 （螺距）          17.4 （钻头直径）

              细   2                              17.9

                1.5                               18.5

                    1                               19

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   22          粗 2.5   （螺距）     19.4 （钻头直径 ）（19.3 ；19.5 都行）         

                 细 2                         19.9

                    1.5                        20.5

                  1                            21

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    24        粗   3 （螺距 ）         20.9 （钻头直径）

               细   2                           21.9

                    1.5                          22.5

                  1                              23

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    27         粗 3 （ 螺距）        23.9 （钻头直径） 24 （也行）

                细    2                         24.9

                     1.5                          25.5

                       1                          26        

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30          粗 3.5   （螺距）      26.3 （钻头直径） ；（ 26.4 也行    ）

               细 3                         26.9

                    2                        27.9

                   1.5                     28.5

                   1                        29

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