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Side and end view of a 4-fluted countersink. The fluted countersink cutter is used to provide a heavy chamfer in the entrance to a drilled hole. This may be required to allow the correct seating for a countersunk-head screw or to provide the lead in for a second machining operation such as tapping.
The recess in the mating part must be designed to accept the particular plow bolt. ASME B18.9 standard recommends a No. 3 head (round countersunk head square neck) plow bolts and No. 7 head (round countersunk reverse key head) plow bolts for new designs. The necessary dimensions for the head styles can be found in the standard. [13] [14] [15]
The minimum limits for external (or bolt, in ISO terminology), and the maximum limits for internal (nut), thread sizes are there to ensure that threads do not strip at the tensile strength limits for the parent material. The minimum limits for internal, and maximum limits for external, threads are there to ensure that the threads fit together.
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.
The major diameter is given by 6p 1.2, [4]: 12 rounded to two significant figures in mm and the hex head size (across the flats) is 1.75 times the major diameter. BA sizes are specified by the following British Standards: BS 57:1951 — B.A. screws, bolts and nuts
Even though #14 and #16 screws are still available, they are not as common as sizes #0 through #12. [citation needed] Sometimes "special" diameter and pitch combinations (UNS) are used, for example a 0.619 in (15.7 mm) major diameter with 20 threads per inch. UNS threads are rarely used for bolts, but rather on nuts, tapped holes, and threaded ODs.
Hanger bolt - Bolt that has no head, machine threaded body followed by a wood threaded screw tip. Allow nuts to be attached to what is really a screw. Hex bolt - Bolt with a hexagonal head and threaded shank. Section immediately under head may be unthreaded for fastening thicker materials. J bolt - Bolt shaped like the letter J, used for tie downs.
The countersunk head has a Pozidriv recess. A self-tapping screw is a screw that can tap its own hole as it is driven into the material. More narrowly, self-tapping is used only to describe a specific type of thread-cutting screw intended to produce a thread in relatively soft material or sheet materials, excluding wood screws .