To do the M4 screw hole, what size holes must be drilled first?
According to my experience in the industry, since you are an aluminum piece, you should use kerosene or kerosene and cutting fluid when you tap. This will make the tapping more smooth and protect the tap and thread. In addition, the general aluminum pieces should be steel wire screw, otherwise, the teeth are easily broken.
M4 pitch 0.7, the hole diameter of 3.4 (but also generally drill 3.2 aperture, mainly to see when tapping convenient), generally with twist drill (drill hole processing tool, from the solid material is the most widely used tool in machining tool)
