Screwdrivers are a type of hand tool used for the insertion and removal of screws. They are available in a multitude of variations of head bit to correspond to the correct screw drive. So screwdriver have multiple extra heads.
There are a lot of different kinds of screwdriver — at least a dozen distinct types. But unless you work on cars, cell phones, or, say, in the aerospace industry, you really need to know only five of them.
What are the heads of screwdrivers called?
Drive Tip or Bit. The screw head is where the force is applied. The various types of screw heads are designed to accept different driving instruments to distribute the force effectively.
The standard slotted head is best used with a flat screwdriver, while the cross-shaped indention is best suited to a Phillips screwdriver. A Robertson screw has a square-shaped indention and requires a matching driver head.
While screw heads have evolved to include expanded and combined head designs, most drill bit sets come with multiple bits and bit guides to show you the head design that will suit the bit. Match the drill bit to the screw head size and design and you’ll have a much easier time attaching the fastener without stripping the head.
Why do screwdrivers have different heads?
Same reason there different shapes and sizes of screws. It’s because there is no single design of screw or screw head which satisfies all requirements. Even when the Phillips design came out there was almost immediate competition from posi-drive and others (most of which have since disappeared) because the Phillips design leaves a lot to be desired, such as the bit rising in the slots when the screw gets tight. Some designs, such as allen heads, have been around a lot longer and for most purposes are very superior but even then, a countersunk allen head of necessity has a rather small socket which creates other problems, such as a tendency to break the keys. Also, allen heads and other socket designs are mostly only used in hardened materials, which resist the tendency of the socket to deform under pressure.
The reason for the different styles is cost and torque.
The slotted head screws are cheap and easy to make. But they’re completely useless for powered screwdrivers and you can’t put much torque on the screw without it either slipping out or stripping the head (and marring the surface of whatever you’re screwing).
Phillips (plus/cross) screws are self-centering, making powered screwdrivers possible. They’re somewhat more expensive to produce than slotted-head. They tend to ‘cam-out’ easily under torque, making it hard to apply much torque. I’ve heard they were designed that way to prevent overtightning. However, it’s not good for exposed fasteners to look stripped.
Robertson-head (square) and allen-head (hexagon/hex) fasteners can handle more torque than phillips-head fasteners, but are more expensive. Because the bottom of the hole is flat (unlike the pointed end of the phillips), there’s more contact area and so it’s less likely to cam-out. The robertson-head is cheaper than the allen-head, but the allen-head has six points of contact rather than 4, making it less prone to rounding out the hole.
The Torx-head fasteners solve the problem of rounding/stripping by having the flat bottom of the robertson/allen that reduces cam-out, but it has much better contact with the driving bit to prevent stripping the head. The points of the ‘star’ on the driving bit engage the recesses on the screw at nearly right angles, so it has a very positive contact. Torx is becoming more and more popular because of that, particularly in assembly-line work.
Because they’re less likely than a phillips to be damaged when tightening, the allen (internal hex) heads are often used for exposed (‘decorative’) fasteners on ‘some assembly required’ furniture. It’s also very cheap to make the allen keys, so they usually include one with the fasteners.
Once a company invents a new screw, some third party will start selling tools to remove them. But for a time, any new screw design means the only people who can open your parts are “certified” workshops you’ve given tools to. Personally I see this a lot in electronic equipment, where the original manufacturer wants some aspect of design kept private or doesn’t want after-market mods made to things they sell. Nintendo is particularly active in inventing new screws.
The full list of reasons of multiple extra heads
- Design improvements (a “good thing” on the road to the world having the best screws)
When people come up with improved designs, those start to get used.
- Network effects (a “bad thing”)
Some people will stick with older, inferior designs that have many users, since those will be easier to find and they will already have the screwdrivers and bits for them. Also…
- Economy of scale (a “bad thing”)
Gives an advantage to more established designs (even if they are terrible), as production of those will be large-scale and efficient, so new designs (better or worse) will tend to be more expensive.
- patenting (and other business reasons that have nothing to do with design) (a “bad thing”)
As Chris Cudmore points out in a comment to a different answer here, this is the reason Phillips screws are more widespread than Robertson (square) screws, despite Robertson ones being generally better (far more difficult to strip the heads. …and what was Ford’s first choice again? …exactly.).
- Desire to have some things be tamper-proof (mostly a “bad thing”)
- Different applications (mostly a “good thing”)
To end on a positive note after all of that, the different applications of screws really do leave room for multiple “best” designs. The points about wet applications and applications with small countersink-type heads (which can only fit much smaller bit sizes of the grippier designs) are great.
How many different types of screws (phillps/flat/hex/star/etc)?
There are a lot of different types of screws. We dug in and did piles of research and put together this well organized and categorized screw buying guide setting out and showing you every possible type of screwhead available for each type of job, material, etc.
Are there more heads of screwdrivers in the future?
Examples of security screws include 3-prong Torx variants, and Allen heads with a raised dot in the middle of the screw head to stop standard Allen wrenches from fitting.
If you’d like to see a much more industrial version, find a car which has been booted and look at the bolt. I’ve seen circular bolt heads with an off-center circular hollow inside. No shoulders or slots to grab, you have to have a very specific wrench. And, of course, there are security lug nuts for car tires.
We currently have so (relatively) few mainstream designs of screws is simple practicality. Nobody, merchant or user, wants to stock the massive number of screws we would need to have to use the best possible design for every possible application, so we compromise and use ones that are adequate in the majority of cases.