Metric and Inch Threads公制与英制螺纹

Same shape, different sizes牙型相同，尺寸不同

ISO metric threads (ISO 68-1) and Unified inch threads (ASME B1.1) share the same basic 60° profile. What differs is how the size and pitch are specified: metric threads are sized in millimetres with the pitch in millimetres, while inch threads are sized in inches with the number of threads per inch (TPI).ISO 公制螺纹（ISO 68-1）和统一英制螺纹（ASME B1.1）的基本牙型相同，牙型角都是 60°。区别在于尺寸的表示方法：公制以毫米表示直径和螺距；英制以英寸表示直径，用每英寸牙数（TPI）表示螺距。

Reading a metric thread designation公制螺纹标记怎么看

• M10 — metric thread, nominal diameter 10 mmM10：公制螺纹，公称直径 10 mm
• × 1.25 — pitch 1.25 mm (fine). For the coarse pitch the pitch is usually omitted: “M10” means M10 × 1.5.× 1.25：螺距 1.25 mm（细牙）。粗牙通常省略螺距，“M10”即 M10 × 1.5。
• 6g — tolerance class. Lower-case letters are for external threads (bolts), capitals for internal threads (nuts): 6g / 6H is the standard fit for general fasteners.6g：公差带。小写字母表示外螺纹（螺栓），大写字母表示内螺纹（螺母）；6g / 6H 是一般紧固件的标准配合。

Reading an inch thread designation英制螺纹标记怎么看

• 5/16 — nominal diameter 5/16 inch (about 7.94 mm)5/16：公称直径 5/16 英寸（约 7.94 mm）
• 18 — 18 threads per inch (pitch ≈ 25.4 / 18 = 1.41 mm)18：每英寸 18 牙（螺距 ≈ 25.4 ÷ 18 = 1.41 mm）
• UNC — Unified coarse series; UNF is the fine seriesUNC：统一粗牙系列；UNF 为细牙系列
• 2A — tolerance class. “A” is external, “B” internal; class 2A / 2B is the normal commercial fit, 3A / 3B is tighter.2A：公差等级。A 表示外螺纹，B 表示内螺纹；2A / 2B 为一般商用配合，3A / 3B 精度更高。

Sizes below 1/4 inch use gauge numbers, e.g. #10-24 UNC. The “UNR” series requires a rounded root on the external thread, which improves fatigue strength; most rolled inch bolts today are UNR.1/4 英寸以下用号数表示，如 #10-24 UNC。UNR 系列要求外螺纹牙底为圆弧，可提高疲劳强度；目前滚压成型的英制螺栓大多是 UNR。

Why the tolerance class matters for coated parts为什么镀层件要关注公差带

External tolerance classes such as 6g (metric) and 2A (inch) include a small allowance — a built-in clearance that leaves room for a thin coating such as electroplated zinc. Classes without allowance (metric 6h, 4h; inch 3A) leave no room for coating.外螺纹公差带 6g（公制）和 2A（英制）带有一个小的“基本偏差”，也就是预留的间隙，可容纳电镀锌这类薄镀层。没有基本偏差的公差带（公制 6h、4h，英制 3A）则没有给镀层留空间。