A factory processes a large number of aluminum parts for other units, among which M6, M8, M10, and M12 internal threads are required to be machined. Originally, threaded holes were machined on aluminum parts, bolts were assembled after tapping, and the users often reported that they were scrapped due to the thread “bad teeth” after they were put into use. In this case, our factory made reference to the relevant materials and did not find a reasonable process method. In practice, we used a lot of statistical data, plus repeated practice. Now we use the bolts to self-drill the new process after processing the bottom hole. , thus avoiding "bad teeth" and improving the connection strength.

Thread self-tapping is a chipless machining method that relies on the plastic deformation of the material to form a tooth pattern. During the self-tapping process, part of the workpiece is pressed by the crest of the thread, the material slides along the crystal plane of the lattice, and is filled into the tooth socket of the thread to form the same screw hole pattern as the thread profile, achieving thread coupling . However, if the size of the bottom hole is too small, self-drilling cannot be performed; the bottom hole is too large and the self-tapping teeth are not full. Based on the principle of multiple regression analysis, the following empirical formula for the determination of the internal hole bottom hole size is obtained by computer analysis:

d bottom=[(d 2 + d outside 2)/2-0.2109 t 2] 1/2

In the formula: d is the inner diameter of the thread; d is the thread outer diameter; t is the thread pitch; d is the bottom hole diameter.

For example, calculate by M8 tapped hole:

d inner=6.647d outer=8t=1.25

then:

d bottom=[(6.6472+82)/2-0.2109×1.252]1/2=7.3

So take d = 7.3-0.1

It should be pointed out that this empirical formula is only applicable to the bottom holes of internal thread M6, M8, M10, M12 of aluminum parts. Other internal threads were not analyzed and tested.

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