Fractional Inches to Millimeters: Machinist & Woodworking Best Practices
Everything you need to know about converting fractional inch drill bits and timber stock to metric with tolerance thresholds.
When manufacturing precision parts, transitioning between American fractional drill bits (1/64" through 1") and metric CNC tooling requires calculating both the nominal converted value and machinist tolerance errors. This tutorial covers fractional precision intervals down to 1/64" and how to minimize tooling deflection.
Key Takeaways
- Machining standard fractions operate on binary divisions: 1/2, 1/4, 1/8, 1/16, 1/32, and 1/64.
- 1/64" equals 0.015625 inches or exactly 0.396875 millimeters.
- CNC milling feedrates require true decimal or metric coordinates to avoid step cumulative positioning errors.
Binary Fraction Conversion Steps
To compute the exact millimeter value of any fractional inch: divide the numerator by the denominator, then multiply by 25.4.
mm = (Numerator / Denominator) × 25.4| Fraction | Decimal Inch | Exact Millimeters |
|---|---|---|
| 1/16" | 0.0625" | 1.5875 mm |
| 1/8" | 0.1250" | 3.1750 mm |
| 3/16" | 0.1875" | 4.7625 mm |
| 1/4" | 0.2500" | 6.3500 mm |
| 3/8" | 0.3750" | 9.5250 mm |
| 1/2" | 0.5000" | 12.7000 mm |
| 5/8" | 0.6250" | 15.8750 mm |
| 3/4" | 0.7500" | 19.0500 mm |
| 1" | 1.0000" | 25.4000 mm |
Frequently Asked Questions
What is the closest metric size to 1/4 inch?
1/4 inch is exactly 6.35 mm. The closest standard metric tool is 6 mm or 6.5 mm.
How does tolerance affect CNC machining?
Standard CNC tolerances are typically ±0.005" (±0.127 mm). High-precision aerospace tooling can require ±0.0002" (±0.005 mm).
Related Converters
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