🔍 Key Concepts

• How screw thread designations are written, e.g., diameter – threads per inch – series – class • Difference between thread hand (right/left) and thread class (fit/allowance) • What the shaded / sectioned area in illustration C suggests about how tightly the male and female threads fit together

💭 Think About

• Look at the order of elements in 1/4-20 NC-2. Which part of that code is most likely to describe the quality of engagement between bolt and nut rather than their size or direction? • In the illustration, compare areas A, B, and C. Which appears to show the tightest or loosest engagement, and how might that relate to a numbered ‘class’ of something? • Ask yourself: if the thread were left-hand instead of right-hand, where in the designation would that usually be indicated, and would it normally be shown by a trailing digit alone?

✅ Before You Answer

• Verify what NC stands for in thread notation and what information it conveys by itself. • Confirm where right‑hand or left‑hand is normally indicated in a thread callout on engineering drawings. • Check a basic machine shop/fastener reference to see how classes 1, 2, and 3 are used in describing threads.