Screwdriver - Blade Types

Blade Types

The tool used to drive a slotted screw head is called a "flat-blade", "slot-head", "straight", "flat", "flat-tip", or "flat-head" screwdriver. This last usage can sometimes be confusing, because the term "flat-head" is also used to describe a screw with a flat top, designed to be installed in a countersunk hole. Such a flat-headed screw may have a slotted, cross, square recessed, or combination head, causing further confusion about the terminology.

Among slotted screw drivers, there are a couple of major variations at the blade or bit end involving the profile of the blade as viewed face-on (from the side of the tool). The more common type is sometimes referred to as keystone, where the blade profile is slightly flared before tapering off at the end. To maximize access in space-restricted applications, the cabinet variant screwdriver blade sides are straight and parallel, reaching the end of the blade at a right angle; this is frequently used in jeweler's screwdrivers, among other applications.

Many textbooks and vocational schools instruct mechanics to grind down the tip of the blade, which, due to the taper, will increase its thickness and consequently allow more precise engagement with the slot in the screw. This approach is intended to create a set of graduated slotted screwdrivers that can be select fitted to a particular screw for a tighter engagement and to reduce the deformation of the screw head. However, many better-quality screwdriver blades have already been induction-hardened (surface heat-treated), and tip grinding after manufacture will compromise their durability. Thus, it is best to select a tip that was precisely made to fit properly to begin with, and to avoid weakening the factory heat-treatment.

Phillips screwdrivers come in several standard sizes, ranging from tiny "jeweler's" to those used for automobile frame assembly, or #00 to #3 respectively. This number is usually stamped onto the shank (shaft) or handle for identification. Each bit size can fit a range of screw sizes, more or less well. Each Phillips screwdriver size also has a related shank diameter. The driver has a 57° point and tapered, unsharp (rounded) flutes. By far the most commonly found size around the household, automobile, and office is the #2, —which fits computers, printers and photocopiers, light switches, carburetors, furniture, household appliances, door hinges, and so forth. The second most commonly seen household Phillips screw is the #1, which fits calculators, cameras, smaller toys, and cell phone sized devices. The #1 and smaller bits come to a blunt point, but the #2 and above have no point, but rather a nearly squared-off tip, making each size incompatible with the other. A "#2 x 6 Phillips screwdriver" designation as commonly seen in the tool catalogs describes a Number Two bit with a six-inch-long shank.

The rounded, tapered slots of the Phillips head were deliberately designed for the screwdriver to "cam out" (pop out) of the screw head recess under high torque on high-speed factory assembly lines. This prevents stripping damage to the screw threads, at the expense of possible damage to the recess in the screw head. Modern torque-limiting power driver tools for professional use eliminate overtorquing damage much more reliably. Market inertia and ignorance of the design's historical intent have led to widespread misapplication of Phillips head fasteners in applications where cam-out is not desirable. Some Phillips head screwdrivers have been manufactured with hardened "anti-cam-out" (ACO) grooves in an attempt to defeat this designed-in behavior.

Robertson, also known as a square, or Scrulox screw drive has a square-shaped socket in the screw head and a square protrusion on the tool. Both the tool and the socket have a taper, which makes inserting the tool easier, and also tends to help keep the screw on the tool tip without the user needing to hold it there. (The taper's earliest reason for being was to make the manufacture of the screws practical using cold forming of the heads, but its other advantages helped popularize the drive.) Robertson screws are commonplace in Canada, though they have been used elsewhere and have become much more common in other countries in recent decades. Robertson screwdrivers are easy to use one-handed, because the tapered socket tends to retain the screw, even if it is shaken. They also allow for the use of angled screw drivers and trim head screws. The socket-headed Robertson screws are self-centering, reduce cam out, stop a power tool when set, and can be removed if painted-over or old and rusty. In industry, they speed up production and reduce product damage. One of their first major industrial uses were the Ford Motor Company's Model A & Model T production. Henry Ford found them highly reliable and saved considerable production time, but when he couldn't secure licensing for them in the United States, limited their production use to his Canadian division. Robertson-head screwdrivers are available in a standard range of tip-sizes, from 1.77mm to 4.85mm.

Reed and Prince, also called Frearson, is another historic cross-head screw configuration. The cross in the screw head is sharper and less rounded than a Phillips, and the bit has 45° flukes and a sharper, pointed end. Also, the Phillips screw slot is not as deep as the Reed and Prince slot. In theory the different size R&P screws will fit any R&P bit size.

Pozidriv and the related Supadriv are widely used in Europe and most of the Far East. While Pozidriv screws have cross heads like Phillips and are sometimes thought to be effectively the same, the Pozidriv design allows higher torque to be applied than Phillips. It is often claimed that they can apply more torque than any of the other commonly-used cross-head screwdriver systems due to a complex fluting (mating) configuration.

Japanese Industrial Standard (JIS) cross-head screwdrivers are still another standard, often improperly referred to as "Japanese Phillips". Compatible screw heads are usually identifiable by a single raised dot or an "X" to one side of the cross slot. This is a screw standard throughout the Asia market and Japanese imports. The driver has a 57° point with a flat tip.