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One advantage over the Phillips drive is that one driver or bit fits all screw sizes. It is often found in marine hardware and requires a Frearson screwdriver or bit to work properly. The tool recess is a perfect, sharp cross, allowing for higher applied torque, unlike the rounded, tapered Phillips head, which can cam out at high torque. It was ...
Sometimes a Green Robertson or Torx drive is used instead. All three patterns may also be combined with a slot for a flat-blade screwdriver. Usually they are provided with a 1/4 in (0.25 inches (6.4 mm)) flanged hex head. Non-standard metricized 5.5 millimetres (0.22 in) flanged hex heads can also be encountered.
One aspect of this method of sizing is that the size increment between drill bits becomes larger as bit sizes get smaller: 100% for the step from 1/64 to 1/32, but a much smaller percentage between 1 47/64 and 1 3/4. Drill bit sizes are written as irreducible fractions. So, instead of 78/64 inch, or 1 14/64 inch, the size is noted as 1 7/32 inch.
The middle size is the No. 30 size, and all numbers with the 30 in them have a chuck shank size of 9/32" (7mm) diameter. The biggest size is the No. 31, and all numbers with 31 in them have a chuck diameter of 5/16" (8mm) diameter. A '1' in front of the number denotes a return spring fitted.
Common sizes include T10, T15, and T25, while T35 and T47 tend to see specialized use. Only the proper driver can drive a specific head size without risk of damaging the driver or screw. The same series of Torx drivers is used to drive SAE, metric and other thread system fasteners, reducing the number of bit sizes required.
The internal-drive square socket for screws (as well as the corresponding triangular socket drive) had been conceived several decades before the invention of the Robertson screw and driver. An earlier patent covering both square-socket- and triangle-socket -drive wood screws, U.S. patent 161,390 , was issued to Allan Cummings of New York City ...