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An external Torx screw has a projecting head in the shape of a Torx screwdriver bit (instead of a standard recessed cavity); a Torx socket is used to drive it. The external "E" Torx nominal sizing does not correspond to the "T" size (for example, an E40 socket is too large to fit a T40, while an E8 Torx socket will fit a T40 Torx bit [ 70 ] ).
Phillips screwdrivers come in several standard sizes, ranging from tiny "jeweler's" to those used for automobile frame assembly—or #000 to #4 respectively. This size number is usually stamped onto the shank (shaft) or handle for identification. Each bit size fits a range of screw sizes, more or less well.
Screwdriver bits in different sizes for Robertson screws. When Henry Ford tried the Robertson screws, he found that they saved considerable time in Model T production. When Robertson refused to license the design, Ford realized that the supply of screws would not be guaranteed, and chose to limit their use to his Canadian division. [4] [5] [6]
An External Torx version exists, also known as Inverted Torx, where the screw head has the shape of a Torx screwdriver bit, and a Torx socket is used to drive it. The external "E" Torx nominal sizing does not correlate to the "T" size, (e.g. an E40 socket is too large to fit a T40 Torx bit, while an E8 Torx socket will fit a T40 Torx bit [6]).
Example (inch, coarse): For size 7 ⁄ 16 (this is the diameter of the intended screw in fraction form)-14 (this is the number of threads per inch; 14 is considered coarse), 0.437 in × 0.85 = 0.371 in. Therefore, a size 7 ⁄ 16 screw (7 ⁄ 16 ≈ 0.437) with 14 threads per inch (coarse) needs a tap drill with a diameter of about 0.371 inches.
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.
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