Reading thread numbers (or thread callouts) involves understanding the different components that specify the thread's characteristics, such as diameter, pitch, and length. Think of it as a coded message describing the exact size and type of a screw or bolt.
Here's a breakdown of how to decipher a typical thread callout:
1. Understanding the Basic Components
A typical thread callout looks something like this: 1/4-20 x 1 or M6-1.0 x 20. These examples tell you everything you need to know about that particular threaded fastener. Let's decode each part:
- Major Diameter: This is the first number in the callout (e.g., "1/4" or "M6"). It indicates the nominal diameter of the thread. This is the largest diameter of the screw thread.
- Threads per Inch or Pitch:
- Threads per Inch (TPI): For Unified National Coarse (UNC) or Unified National Fine (UNF) threads (typically inch-based), the second number indicates the number of threads per inch (e.g., "20" in "1/4-20" means 20 threads per inch).
- Pitch: For metric threads (designated by "M"), the second number indicates the pitch of the thread, which is the distance between adjacent threads in millimeters (e.g., "1.0" in "M6-1.0" means a pitch of 1.0 mm).
- Length: This is the number that follows the "x" symbol and indicates the length of the threaded portion of the fastener (e.g., "1" in "1/4-20 x 1" means 1 inch long, and "20" in "M6-1.0 x 20" means 20 mm long).
2. Example Decoded
Let's break down the examples from above:
- 1/4-20 x 1:
- 1/4: Major diameter is 1/4 inch.
- 20: There are 20 threads per inch.
- x 1: The threaded length is 1 inch.
- M6-1.0 x 20:
- M6: Metric thread with a 6 mm major diameter.
- 1.0: The pitch is 1.0 mm (the distance between threads).
- x 20: The threaded length is 20 mm.
3. Additional Information You Might Encounter
- Thread Class: Sometimes, you'll see a thread class designation (e.g., "2A" or "6g"). This indicates the tolerance and fit of the thread. These classifications define the acceptable range of deviations in thread dimensions and ensure proper assembly and function.
- Thread Direction: While most threads are right-handed, a left-handed thread will be indicated with "LH" after the thread size (e.g., "1/4-20 LH").
4. Table Summary
| Component | Inch (UNC/UNF) Example | Metric (M) Example | Description |
|---|---|---|---|
| Major Diameter | 1/4 | M6 | Nominal diameter of the thread |
| TPI/Pitch | 20 | 1.0 | Threads per inch / Distance between threads |
| Length | 1 | 20 | Length of the threaded portion |
| Thread Class | 2A | 6g | Tolerance and fit of the thread |
| Thread Direction | LH | LH | Left-handed thread indicator (if applicable) |
By understanding these components, you can accurately read and interpret thread numbers, ensuring you select the correct fasteners for your applications. Referencing standards like those from ANSI (American National Standards Institute) can also be helpful for understanding thread specifications in more detail.
