Number series machine screws get their designations from wire gage sizes that they were made from. Which traces back to several different British thread systems waaayyy back in the day. While there is no hard and fast rule, the number series screws are sometimes referred to as ‘Machine screws’ while fractional series fasteners are referred to as ‘Bolts’. No one knows why, and Whitworth is dead. (Toolmaker humor).
All actual diameters of screws are a bit under nominal for external and a bit over size nominal for internal. This is the clearance needed to make a screw and nut fit together. This all applies to metric thread fasteners also. Thread fit classes are a story for another day.
Fondots@lemmy.world 5 days ago
I’m not sure if the wire gauge thing is right, unless you’re talking about a different system than I’m familiar with, because with wire gauge smaller number=bigger wire, and with screw sizes smaller number=smaller screw
Also just my 2¢ on “machine screw” vs “bolt” as a casual tinkerer with various things held together by different types of threaded fasteners.
Generally speaking if it’s got a hex head or nut that I’m using a wrench to tighten, it’s a bolt
If it’s got some sort of hole (or God forbid a slot) that I’m going to use some sort of a driver (for the purposes of this, an Allen “wrench” is a driver) to tighten, it’s a screw.
And of course everything gets really murky when we start talking about things like sheet metal screws, lag bolts/screws, masonry screws, etc.
jonfairbairn@hostux.social 5 days ago
@Fondots @Bluewing especially since the terminology is different in different dialects of English. I think what you call lag bolts might be coach bolts over here.
Fondots@lemmy.world 5 days ago
Lag bolts are more like a big wood screw, what you call coach bolts seem to be what we call carriage bolts