Soldato
Shows how little I know about proper DIY. Self tapping screws have little drill bits on them right? I can buy some. Should I get a selection of sizes for wood?
But anyway, I just removed the screws my builder put in (he replaced some of the fencing) and reused them lower down. They needed to be drilled into the post, they wouldn't have gone in with just using the electric screwdriver I don't think?
You don't use self tapping screws for wood. They're usually used for metal, i.e screwing things to metal partitioning, or fixing sheet metal in place.
For wood you can usually just screw a woodscrew straight into softwood fence posts etc without pre-drilling. In hardwoods, or if you're screwing into internal joinery that will be on show, or a thin piece of wood that might split, you can pre-drill a hole before putting the screw in. I usually hold a drill bit up to the screw and find one that is the same diameter as the screw neck, but smaller diameter than the thread.
HSS drill bits are for use with metal and can also be used on wood. Don't use them on masonry, it'll destroy them. Cobalt just means it has had cobalt added to the alloy of the steel, which I believe allows them to drill harder metals.