Guide to Number Drill Sizes for Machinists and Hobbyists

Have you ever faced the frustration of preparing to tap a hole, only to realize your drill bit is the wrong size? Whether it's too large, resulting in loose threads, or too small, making tapping impossible, selecting the correct drill bit is crucial—especially when working with number-sized (Number-size) drill bits. This guide will demystify the numbering system, ensuring you never face this dilemma again.

What Are Number-Sized Drill Bits?

Number-sized drill bits use a numerical system to denote their diameter, typically for smaller holes. The numbering ranges from 80 (smallest) to 1 (largest). Unlike metric (millimeter) or fractional-inch drill bits, the sizes don't follow an obvious pattern, making a reference chart essential for accurate selection.

Number-Sized Drill Bit Conversion Chart (80# - 1#)

Below is a detailed conversion table listing drill bit numbers alongside their corresponding diameters in inches and millimeters. Note that these are standard values; minor manufacturing variations may occur.

Why Use Number-Sized Drill Bits?

Despite their seemingly complex numbering system, number-sized drill bits offer unique advantages:

• Finer size gradations: They provide more precise diameter options, especially for small holes, compared to metric or fractional-inch bits.
• Historical relevance: Older machinery or standards may still reference number-sized bits, making familiarity with them essential for maintenance.
• Specialized materials: Certain hard alloys may require specific number-sized bits for optimal results.

How to Select the Right Number-Sized Drill Bit

Consider these factors when choosing a number-sized drill bit:

1. Technical specifications: Consult engineering drawings to determine the required hole size. Convert metric or inch measurements to number sizes if necessary.
2. Threading requirements: For tapping, match the drill bit to the thread’s recommended pilot hole size using a tap drill chart.
3. Material properties: Harder materials demand higher-quality bits with appropriate coatings. Adjust drilling speed and feed rate accordingly.
4. Precision needs: High-accuracy applications warrant premium bits and stable drilling equipment to minimize deviation.
5. Post-drilling processes: If reaming or boring is planned, select a slightly smaller bit to allow for finishing.

Common Applications of Number-Sized Drill Bits

These specialized bits are indispensable in:

• Precision instrument manufacturing: Watch components, optical devices.
• Electronics production: Circuit board via holes.
• Model making: Miniature parts for scale replicas.
• Jewelry crafting: Stone setting and delicate metalwork.
• Medical device fabrication: Surgical tool components.

Conclusion

Mastering number-sized drill bits empowers craftsmen and engineers to execute precise drilling tasks with confidence. By understanding the numbering system, consulting conversion charts, and applying selection criteria tailored to specific projects, professionals can optimize their workflow and achieve superior results.