Tap Limit for 3B Unified Threads

【英】困ったときの知恵袋詳細

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

We are planning to work on aircraft parts and we will tap 1/4-28 UNJF 3B and 5/16-24 UNJF 3B internal threads. I understand that a UNJF internal thread can be machined with a standard tap, if the pilot hole diameter is adjusted to the specified internal diameter size per the UNJ specification. When using 1/4-28 UNF and 5/16-24 UNF taps for Unified 3B class of internal screw threads, what is the recommended limit size of the tap for a 3B class internal screw thread?

The limit size for taps used to machine Unified 3B class of threads is recommended by ANSI. This is not an absolute matter because the processing dimensional difference varies depending on the tapping conditions and the taps size and type. It is only to be used as a guide.

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