Driver Bits Information
Driver bits apply torque to screws. More specifically, they are the interface that transfers torque from a tool, such as a screwdriver, t-handle, or drill, to the mating recess on top of a helically-threaded fastener. Driver bits are frequently interchangeable, come in variety of diameters and lengths, and are complemented by many accessories.
The tip of the driver bit inserts into a depression that has been machined into the top of screw head in a male tool / female fastener configuration. Female tool / male fastener combinations exist, but are less common. The rotary drive of the tool applies torque to the screw, which due to the threads machined along its shank gains traction against the workpiece. Wood screws have a pointed tip and sharpened threads to assist the assembly of items to wood substrates. Machines screws have a flat or tapered point and assemble pre-tapped workpieces, or pre-drilled substrates in union with a nut.
Slotted and Phillips drives are unquestionably the most common type of screw drive, while hex socket (Allen®), square (Robertson®), and hexlobe (Torx®) are also readily available. Square and hex drives are most common for bolts; most other bolt styles are incompatible with driver bits. Specialty styles are employed sparingly but offer unique attributes that make them suitable for limited applications. Some large-scale manufacturers may utilize proprietary drive types (such as Apple Inc.'s pentabular screw) that are not commercially available and are not listed below, though drives for these screws may be available depending on the market. Variants of drive types can sometimes be driven by the original bit.
Sockets are often used as bolt drivers in conjunction with an extension or adapter.
Image credit: Ramset
|Interfaces with a six-sided bolt head, the most common bolt head style; easiest bolt style to torque|
|Four-sided head; fairly common in old structures, but rarely used today; easiest bolt head to manufacture; first bolt head ever created|
Image credit: Surtools
|Five-sided head; turned by five-point specialty socket; drives are uncommon; prevents unauthorized access to utilities (i.e., fire hydrants)|
Image credit: Power Tool Centre
|Best for torqueing via screwdriver; simplest drive style to produce||
|Cams-out once screw is installed to prevent overtorqueing and damage to substrate; most common screw drive type||
Image credit: Steritool
|A tapered, square driver retains the screw on the bit until installed; screws can be torqued by an angled driver; self-centering; drive recess rarely strips; reduces product damage but does not cam out||
|Additional bit and screw contact points increase torque transmittance; long bit life; requires minimal downward pressure to drive screw; little to no cam out; angled-driver compatible||•External hexlobe (male fastener / female tool)|
|Hex key (Allen®)||
|Not equivalent to hex-headed bolt; a hexagon recess is machined into bolt head; efficient torque transfer; screws can have small or even no head (i.e., set screw); driver is cheap to produce; hex wrenches (viz., allen keys) are more common than screw or drill bits||
|A TA drive bit resembles a triangle; common on consumer items; TP3 drive bits feature a Reuleaux triangle instead|
Image credit: EHardware
|Essentially a Phillips driver style but with only three points; found on consumer electronics|
Image credit: Pan American Tool
|Three segmented slots that form a triangle with extended segments, which may or may not include a TA-style recess in the center; common on electronics and aerospace components|
|Offset cross (Torq-set®)||
Image credit: Pan American Tool
|Resembles a Phillips, but slots are offset from center; used on military aircraft such as the B-2||•ACR Torq-set|
Image credit: Tacoma Screw
|Drive geometry resembles hourglass (Type A) or butterfly shape (Type G); used to attach sheet metal which needs occasional removal; easily installed via power tools; applications include trailers, mobile homes, and farm equipment|
Image credit: Instructables
|Resembles a six-point Phillips with rounded arcs between points; resists camming out; common in high-torque automobile applications||•Ribe®|
Image credit: Max-Gain Systems
|Most efficient torque-transitive drive available as force is applied at 90° angles around axis; minimizes stripping and cam-out; useful for non-ferrous hardware; used in avionics, communications, and construction and agricultural equipment||•Four-pronged bristol|
Image credit: UK Tools
|Resembles a triple-square drive, but is not triple-square compatible; resists camming out; used primarily in auto engines|
Image credit: Home Depot
|Two round, opposing holes provide torque-transitive recesses; used in public fixtures to prevent tampering||•Tri-spanner (three holes)|
With the exception of slot, Phillips, and tri-point drives, any of the drive styles above can be turned into a security drive. These modified drives have a center pin reject in the middle of the screw to prevent common tools and mismatched driver bits from applying torque to the screw. While the fastener is by no means locked in position, the ownership of compatible tools is rare, and only authorized individuals with the prescribed instruments can manipulate these fasteners.
One-way drives are screw designs which guarantee the driver bit cams out if fastener removal is attempted. While a one-way screw can be inserted with a basic slotted screwdriver, it can only be removed by destroying the screw, destroying the workpiece, or drilling holes in the screw head to make it spanner-drive compatible.
- Drive guide: a hollow tube with an internal driver bit prevents screw wobble during installation.
- Socket adapter: a driver bit that attaches ratchet-driven sockets to a screwdriver, t-handle, or drill.
- Drive extension: a rigid extension of tool-grade material that increases the distance between the bit and torque driver; helpful for hard-to-reach screws and bolts.
- Flexible/pivot extension: a bit extension that transmits torque to bolts or screw that are inaccessible to typical tooling. Two styles are common: a long metallic hose with an internal, flexible shaft; and a dual-revolute joint that is most effective at short distances.
- Right-angle adapter: a rigid extension which transmits torque through bevel and miter gearing.
- Magnetizer: a strong magnet induces polarity in a driver bit made of non-magnetic material to retain a screw or bolt at the end of a bit.
All driver bits eventually wear out and become less efficient with repeated use. Manufacturers regularly produce driver bits in titanium, stainless steel, tool-grade steel (S2), high-speed steel (HSS), and chromium-vanadium-molybdenum (CVM) tool steel, as well as non-sparking metals for certain combustion risk applications.
S2 tool-grade steel is most common. The S represents a type of shock-resistant steel with an underlying silicon structure, and 2 represents the shock grade of the material (in this case being on the low-end of the shock-level scale: S1, S2, S5, S6, and S7). S2 steel is composed of 97% iron, 1.05% silicon, .4% to .44% carbon, and .5% or less of the following materials: vanadium, manganese, molybdenum, phosphorus, and sulfur. It has been also been heat treated to increases its hardness.
High-speed steels are also common and have largely replaced high-carbon steels as they can handle higher temperatures. Tools of this material typically are steel alloys containing chromium and vanadium, along with ratios of manganese, phosphorus, sulfur, and silicon.
Tool steels are often coated or finished to improve durability. The following are common finishes, along with their imparted properties.
- Zinc-plated: resists oxidation
- Nickel-plated: resists oxidation
- Titanium nitride: smooth bit finish
- Tungsten carbide: enhances grip between driver and bolt
- Diamond: ultra-high torque
- Ultra gold: damage evident
- Ultra silver: high corrosion resistance
- Visibility: colored coatings improve tool discernibility
Drive styles are manufactured in different sizes, each of which interfaces with individual sizes or a range of screw and bolt sizes. The following tables represent the common sizes of various screw/bolt drives, as available. Many drivers are denominated by the size of fastener they interface with; instances where this rule of thumb is not applicable are noted.
The following are attributes available in some driver bit products.
- Magnetic: the driver bit retains the screw thanks to the polarity imparted by its composition materials.
- Quick change: the driver bits support rapid insertion and extraction in the tooling.
- Double ended: the driver bit has interfaces as both ends, sometimes of a different style or size.
- Non-sparking: the driver material or coating does not spark if struck on metal or stone.
Only the most basic fastener driver styles have developed industrial standards, and some are linked below. Some driver styles, such as Robertson® and Torx®, are regulated by the patent or copyright holder, but that does not outrule counterfeit driver bits. In other cases, drive types are not manufactured in an array that requires regulatory standards (at least yet).
ISO 2351-1 - Assembly tools for screws and nuts - Machine-operated screwdriver bits - Part 1: Screwdriver bits for slotted head screws
ISO 2351-2 - Assembly tools for screws and nuts - Machine-operated screwdriver bits - Part 2: Screwdriver bits for cross-recessed head screws
ISO 2351-3 - Assembly tools for screws and nuts — Machine-operated screwdriver bits Part 3: Screwdriver bits for hexagon socket screws
ASME B18.3 - Socket Cap, Shoulder, Set Screws, and Hex Keys (Inch Series)