🔍 Key Concepts

• Standard machine screw designation format such as diameter - threads per inch - series - class of fit • Difference between thread series (UNC, UNF, UNEF) and class of fit (like 1A, 2A, 3A) • How the notation in the illustration groups letters and numbers: which part is size, which part is pitch, which part is tolerance/fit

💭 Think About

• Look at each portion of the callout 1/4-20 UNC-2. What does 1/4 represent, what does 20 represent, and what does the trailing -2 most likely represent? • Think about common screw markings you’ve seen: when the letters change from UNC to UNF, what actually changes about the screw? Is it the shape of the thread, how tight it fits, or how coarse/fine the pitch is? • Which element (letters vs trailing number) is normally used to specify how tight or loose the mating threads will be?

✅ Before You Answer

• Be sure you know that 1/4 is the nominal diameter in inches and 20 is the number of threads per inch • Confirm that classes of fit are usually written as numbers combined with a letter (like 2A, 2B), not as a three‑letter group like UNC • Recall that UNC, UNF, UNEF are standard designations that distinguish different series of Unified threads based mainly on pitch (coarse vs fine)