英制螺紋系列代號說明
嚴格地說，UN、UNR、UNC、UNF、UNS以及UNEF是螺紋系列的規定代號，而不是某些英文詞組的縮寫：
統一螺紋：Unified Screw
Threads( 由英國(United Kingdom)、加拿大(Canada)和美國(United
States)於948年共同制定；
UN：外螺紋設計牙型根部可平可圓(A flat root contour is specified ,but it
is necessary to provide for some threading tool crest wear, hence a rounded root
contour cleared beyond the 0.25p flat width of the Basic Profile is
optional.)
UNR：外螺紋設計牙型根部必須為圓弧(In order to reduce the rate of threading tool
crest wear and to improve fatigue strength of a flat root thread, the Design
Profile of the UNR thread has a non-reversing continuous curved root tangent to
the thread flanks at a depth of 0.625 H.)
UNC：粗牙系列(Coarse-Thread
Series)；
UNF：細牙系列(Fine-Thread Series)；
UNEF：超細牙系列(Extra-Fine-Thread
Series)；
UNS：特殊螺距系列(Selected Combinations和 Other Threads of Special
Diameters, Pitches, and Lengths of Engagement)；
另外還有恒定螺距系列(Constant Pitch
Series)，有8UN、12UN、16UN等等，不一一列舉。
