In the world of precision CNC machining, details aren’t just technicalities—they are the difference between a successful project and a financial disaster.
At Yuhuan Hongqian Machinery, we often receive customer drawings that specify the Across Flats (AF) dimension for a hexagonal part but omit the Across Corners (AC) or the required Raw Material Diameter. While this might seem like a small omission, skipping this calculation is a hidden risk that can jeopardize your entire production run.The following drawing is one typical drawing that the customer provides across flats (AF) size ,but ignores the Across Corners (AC) size.
1. The High Cost of Ignoring the “Sharp Corners”
As a professional factory with over 10 years of experience and 80 sets of CNC machines, we’ve seen how ignoring the “sharp corners” of a hexagon leads to three major failures:
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Material Under-Calculation:
If the raw material (round bar) is selected based only on the hex Across Flats (AF) dimension, the first mistake already occurs at the raw material selection stage.From this carbon steel pipe fitting drawing, we can see that the hex size is 19 mm. Therefore, the Across Corners (AC) dimension should be approximately 21.939 mm.
This calculation usually requires the factory to determine the correct raw material size based on the drawing and their technical machining experience. This blog mainly introduce how to go through by Across Flats (AC ) to get Across corners (AC) size .
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The Pricing Trap: If a factory quotes based on undersized raw material, the price will appear artificially low. Once production reveals the error, the factory must re-purchase larger material, wasting both money and precious lead time.
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The Professionalism Gap: True professionalism means being more familiar with production realities than the client. Having to explain a basic calculation error mid-project damages trust. A professional factory should catch these issues during the quoting stage, ensuring a smooth, “no-surprises” experience for the customer.
2. Technical Analysis: How to Calculate Hex Corners
You don’t need a degree in geometry to ensure your parts are accurate. For any standard hexagon, there is a fixed mathematical relationship that every engineer should know.
The Golden Rule for Machinists:
To find the Across Corners (AC) dimension, simply multiply the Across Flats (AF) by 1.1547 (as defined in ISO 4032 Hex Nut dimensions).
𝐴𝐶=𝐴𝐹×1.1547
Example:
If your Hex requirement is 19mm (Across Flats):
19* 1.1547 = 21.939
This means you must use a raw material bar larger than 22mm to ensure the sharp corners are fully formed and the hexagon is perfect. the hex
3. Real-World Case Study
The following video shows another carbon steel pipe fitting example. The hex Across Flats (AF) size is 31.7 mm.
Based on the standard formula:
AC = AF × 1.1547
The theoretical Across Corners (AC) dimension should be:
31.7 mm × 1.1547 = 36.6 mm
However, according to the measurement shown in the video, the final hex corner size is approximately 36.2 mm.
The reason is that the hex flats are primarily designed to work with a wrench, so the exact sharp corner size is usually not a critical functional dimension. As long as the flats meet the required size and provide a good surface for wrench engagement, the part will function correctly.
Another important reason is that the hex corners normally include a small radius or chamfer to prevent sharp edges that could scratch or injure users during handling.
Therefore, in practical production, the actual hex corner dimension is often slightly smaller than the theoretical calculated value. The final size is usually adjusted according to the customer’s design requirements and functional needs.
4. Quick Reference Table for Procurement
| Hex Size (Across Flats) | Calculated Across Corners (min. Material OD) | Recommended Bar Stock |
| 12 mm | 13.86 mm | 14 mm or 15 mm |
| 14 mm | 16.17 mm | 17 mm or 18 mm |
| 17 mm | 19.63 mm | 20 mm |
| 19 mm | 21.94 mm | 22 mm or 23 mm |
| 24 mm | 27.71 mm | 28 mm or 30 mm |
Note: AC values calculated based on the standard hexagon formula AC = 1.1547 × AF (ISO 4032). Actual material selection may include additional machining margin.5. FAQ: Expert Insights on Hex Machining
Q: Why do manufacturers sometimes choose raw material slightly smaller than the calculated AC?
A: In practice, hex corners usually include a small radius or chamfer for safety and surface finish. Using slightly smaller material avoids overly sharp corners that can scratch hands or damage assembly tools. The final size is always adjusted according to customer requirements.
Q: Does the material (Stainless Steel vs. Carbon Steel) change the calculation?
A: The geometry remains the same (1.1547 ratio). however, for Stainless Steel, we factor in a larger material margin to handle surface scaling and ensure a cleaner finish on the sharp edges.
Conclusion: Partner with a Factory That Thinks Ahead
At Yuhuan Hongqian Machinery, we don’t just follow drawings—we analyze them for success. Our expertise in iron and stainless steel machining, combined with our rigorous pre-production checks, ensures that your hex nuts, fittings, and connectors are delivered perfectly, every time.
Don’t let a simple calculation error cost you time and money. Work with a professional partner who understands the geometry of success.