What does 4-40 mean when Im looking to buy some machine screws?
Major and minor diameters are only useful numbers if you’re cutting threads on a lathe. (Or, I suppose, if you’re making a tap or die.) For most purposes, the clearance and tap-drill diameters are what you’re interested in. Note that screws develop their maximum holding power with three full threads of engagement. Extra length of engagement does not provide any increase in strength. To learn all there is to know about screws, fasteners, and metalworking, get a used copy of Machinery’s Handbook. And if you are creating a tapped hole for a #6 screw, where you get to choose the thread pitch, for God’s sake, use a 6-40 tap and screw. 6-32 is the worst thread form on the planet, and responsible for more broken taps and stripped threads than any other.
