Number drill sizes are a specific system used to designate the diameters of drill bits, particularly smaller sizes that fall between standard fractional inch measurements.
Drill bits come in various sizing systems, including fractional inch (like 1/8", 1/4"), metric (like 3mm, 6mm), letter gauge (A to Z), and number gauge. The number drill bit gauge sizes range from size 80 (the smallest) to size 1 (the largest) followed by letter gauge size A (the smallest) to size Z (the largest). This means number sizes cover a range of very small diameters, starting even smaller than the smallest letter sizes.
Understanding the Numbered System
Unlike fractional or metric sizes where the number directly relates to the dimension (e.g., 1/4 inch, 6 mm), the numbers in the number gauge system are arbitrary identifiers. A smaller number corresponds to a larger drill bit diameter.
- Size 80: The smallest number drill size, with a very fine diameter.
- Size 1: The largest number drill size, bridging the gap before the letter sizes begin.
This system provides finer increments between sizes than standard fractional bits, which is crucial for applications requiring precise hole diameters.
Where Number Sizes Fit
The complete sequence often looks like this:
- Number Sizes: #80 (smallest) up to #1 (largest)
- Letter Sizes: A (smallest) up to Z (largest)
- Fractional Sizes: Starting from sizes slightly larger than Z (e.g., 17/64", 9/32", etc.)
Example: A #60 drill bit is smaller than a #30 drill bit. A #1 drill bit is larger than both and is followed by the 'A' size bit.
Common Uses
Number drill sizes are frequently used in:
- Metalworking: Creating tap drill holes for smaller machine screws where precise threading is required.
- Jewelry Making: Drilling fine holes in metal or other materials.
- Electronics: Drilling holes in printed circuit boards (PCBs).
- Model Making: For detailed work requiring very small holes.
Choosing the correct number drill size is essential for tasks like tapping threads, ensuring the tap has the proper amount of material to cut into for a strong thread. Reference charts are commonly used to find the corresponding number size for specific tap sizes or desired hole diameters.
Quick Reference: Number Size Examples
To illustrate the concept of smaller numbers indicating larger sizes:
| Number Size | Approximate Decimal Equivalent (inches) |
|---|---|
| #80 | 0.0135 |
| #60 | 0.0400 |
| #30 | 0.1285 |
| #1 | 0.2280 |
| A | 0.2340 |
(Note: Decimal equivalents are approximate and can vary slightly by manufacturer)
Understanding number drill sizes is key for precision work where standard fractional sizes are too coarse. They represent a system that allows for a wider range of incremental sizes for specialized applications.
