The meaning of UNC and UNF thread specifications

【英】困ったときの知恵袋詳細

Cutting TapsRoll (Forming) Taps Unified Threads Standard (Screw Threads, Center Holes)

What do the letters UNC and UNF represent when I see them on a drawing? I need to produce a component with a 1/4-20UNC and a 1/4-28UN thread.

UNC and UNF are symbols of Unified Screw Thread Series.
The Unified Thread Series was agreed on by the United States, Great Britain and Canada in 1949 to obtain interchangeability. Recently, modifications have been made to the original 1949 specifications for ease of use.
The Unified Thread Series is classified into coarse threads, fine threads and extra fine threads much like Metric screw threads.

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