Head shapes for machine screws have been rationalized in BS EN ISO 1580 and BS EN ISO 7045. For the purpose of this British Standard, the generic term
'screws' applies to products which are threaded up to the head or having an unthreaded portion of the shank.
The length of the thread is defined as the distance from the end of the screw, and this includes any chamfer, radius or cone point, to the leading face of the nut which has been screwed as far as possible onto the screw by hand. Note on the illustrations which follow that in the case of the countersunk head types of screw, the length of the screw includes the countersunk part of the head. For pan and cheese head screws, the screw length does not include the head.
The Standard should be consulted for manufacturing dimensional tolerances, also concentricity tolerances for the heads of the screws.
The illustrations which follow show each of the screws and tables are also given showing the dimensions of regularly used sizes.
The sizes quoted in the tables are for screws manufactured in steel. Standard screws are also available in brass but generally the range is not quite so extensive.
For all of the machine screws illustrated here, the countersunk head types have an included angle of 90°.
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