The correct screwdriver to use depends on the type of slot or recess in the head of the screw or bolt, and how accessible it is.
Most screwdrivers can't grip as securely as spanners, so it's very important to match the tip of the screwdriver exactly with the slot or recess in the head of a fastener. Otherwise the tool might slip, damaging the fastener, and worse still, you.
When using a screwdriver, always check where the screwdriver blade can end up if it slips off the head of the screw. A screwdriver can't tell the difference between a piece of steel, and a piece of you.
The most common screwdriver has a flat tip, or blade, which gives it the name, blade screwdriver. It's easy to see the blade should be almost as wide as the slot in the fastener so that twisting force applied to the screwdriver is transferred right out to the edges of the head where it has most effect.
Not so easy to see, is that the blade should be a snug fit in the slot of the screw head. Then the twisting force is applied evenly along the sides of the slot. This guards against the screwdriver suddenly chewing a piece out of the slot, and slipping, just when most force is being exerted.
If viewed side-on , the blade should taper until the very end where the tip fits in the slot. If the tip of the blade isn't clean and square, it should be reshaped.
When you use a blade screwdriver, make sure you support the shaft with your free hand as you turn it. This helps keep the blade square on to the slot and centered. Slipping screwdrivers are a common source of damage and injury in workshops.
A screw or bolt with a star-shaped recess needs a Phillips or a Pozidrive screwdriver. The star-shaped slot holds the tip of the screwdriver securely on the head.
The Phillips tip fits a tapered recess while the pozidrive fits into slots with parallel sides in the head of the screw. Both a Phillips or Pozidrive head can be pushed and twisted with more confidence, but again the screwdriver must be the right size. This is simplified with these 2 types of screwdriver because 4 sizes are enough to fit almost all fasteners with this sort of screw head.
The Allen key is designed to be a snug fit in screws with a socket head.
The socket and the key are hexagonal in shape, and there's a correct sized key for every socket, so Allen keys come in sets. They can come in either the metric or Imperial system, and are categorized in millimeters or fractions of an inch, according to the distance across opposite flats of the hexagon.They give the best grip on a screw or bolt of all the drivers, and their shape makes them good at getting into tight spots.
The offset screwdriver fits into spaces where nothing else will, and where there's not much room to turn it. The two tips look identical but one's at 90 degrees to the other. This is because sometimes there's sometimes only room to make a quarter turn of the driver. That is why the driver has two blades on opposite ends so that offset ends of the screwdriver can be used alternately.
The ratchet is a popular screwdriver handle that usually comes with a selection of flat and Phillips tips. It has a ratchet inside that turns the blade in only one direction depending on how the slider is set. When set, a screw can be undone without removing the tip of the blade from the head of the screw.
Equally, when set in the opposite direction screws can be inserted just as easily.
This is an impact driver . A screw or a bolt that’s rusty or over tightened needs a tool that can apply more force than the other members of this family.
The impact driver takes a variety of tips. Choose the right one for the screw head, fit the tip in place, and then tension it in the direction it has to turn. A sharp blow with the hammer breaks the screw free, and it can be unscrewed.