Determining the exact size of a #21 drill bit requires understanding the numbering system used. Unlike metric or fractional drill bits, which use decimal or fractional inches for sizing, number drill bits follow a less intuitive system. This system, primarily used in the US, doesn't correlate directly to a specific diameter. Instead, the numbers represent a progressive series, with lower numbers indicating smaller diameters.
The Mystery of the #21 Drill Bit Size
Unfortunately, there isn't a universally agreed-upon conversion chart for number drill bits that's entirely accurate across all manufacturers. Slight variations can exist due to manufacturing tolerances. However, a commonly accepted approximation for a #21 drill bit is 0.238 inches (6.05 mm).
Keep in mind this is an approximate value. For precision work, always consult the manufacturer's specifications for the specific drill bit you are using.
Why the Inconsistency?
The historical development of the number drill bit system contributed to the lack of a straightforward conversion. The system emerged before standardized measurement systems were widely adopted, leading to inconsistencies that persist today.
How to Find the Precise Size
To ensure accuracy, always refer to these resources:
- The drill bit itself: Many drill bits have their size clearly marked on the shank (the unfluted part of the bit). Look for markings that indicate the diameter in either inches or millimeters.
- Manufacturer's specifications: If the size is not directly on the bit, consult the packaging or the manufacturer's website for detailed specifications.
- Drill bit size charts: While approximations exist, use them with caution. Remember the potential for small variations between manufacturers. Cross-referencing multiple charts can be helpful but never substitute for checking the bit itself or manufacturer's data.
Using the Right Drill Bit Size
Selecting the correct drill bit size is critical for various applications, ensuring clean holes and preventing damage to the material being drilled. Using a drill bit that's too small will result in a tight fit, and using one that's too large could lead to excessive material removal or structural weakening.
Beyond the Number System: Metric and Fractional Drill Bits
While number drill bits are common, many projects benefit from using either metric or fractional drill bits, which offer clearer size indications. These are often preferred for their consistency and ease of understanding.
In conclusion, while a #21 drill bit is generally considered to be approximately 0.238 inches (6.05mm), always verify the size using the drill bit itself or the manufacturer's specifications to achieve the best results. Remember to choose the appropriate drill bit type for your specific application to ensure accuracy and avoid damage.
