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Both the tool and socket have a slight taper. The contemporary square drive screw has all but replaced the Robertson screw proper and is commonly referred to as a Robertson because it has practically identical drive dimensions and the same colour identification system, but the contemporary square drive socket has parallel sides rather than tapered.
Square head cap screws up to and including 3 ⁄ 4 inch (19.05 mm) have a head 1 ⁄ 8 inch (3.175 mm) larger than the shank; screws larger than 3 ⁄ 4 inch (19.05 mm) have a head 1 ⁄ 4 inch (6.35 mm) larger than the shank. [9] In 1919, Dyke defined them as screws that are threaded all the way to the head. [10] socket screw
Slot screw drives have a single horizontal indentation (the slot) in the fastener head and is driven by a "common blade" or flat-bladed screwdriver.This form was the first type of screw drive to be developed, and, for centuries, it was the simplest and cheapest to make because it can just be sawed or filed.
Width across flats is the distance between two parallel surfaces on the head of a screw, bolt or nut. The width across flats will define the size of the spanner or wrench needed. The width across flats will define the size of the spanner or wrench needed.
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
Thus they are instead given as basic dimensions. In modern practice basic dimensions have a rectangular box around them, or sometimes the word "BASIC". BC or B.C. bolt circle: BCD or B.C.D. bolt circle diameter: BHC bolt hole circle Same definition as the bolt circle diameter BHCS: button head cap screw: Like an SHCS but with a button head. BHN
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A metric ISO screw thread is designated by the letter M followed by the value of the nominal diameter D (the maximum thread diameter) and the pitch P, both expressed in millimetres and separated by a dash or sometimes the multiplication sign, × (e.g. M8-1.25 or M8×1.25).