Master Small Hole Boring: How to Achieve a Mirror Finish with Korloy PC9030
Internal boring, especially in small diameters, is one of the most challenging tasks for any CNC machinist. Between limited space for chip evacuation and the natural lack of rigidity in long boring bars, achieving a high-quality surface finish can feel like a battle. However, with the right combination of tooling geometry and material-specific grades, you can achieve consistent, high-precision results.
The Secret Weapon: Korloy PC9030
When working with stainless steel or small-diameter boring, the Korloy PC9030 grade is widely considered the industry standard. Why? Because it addresses the two biggest enemies of internal turning: Heat and Adhesion.
4 Steps to Better Internal Surface Finish
1. Control the "Built-Up Edge" (BUE)
In small holes, heat builds up quickly because there is less space for coolant to reach the cutting edge. Stainless steel then tends to weld itself to the insert, leading to a torn, rough surface finish.
The PC9030 Advantage: Its ultra-smooth PVD coating acts like a non-stick surface, preventing material from sticking even in high-heat internal environments.
2. Choose the Smallest Possible Nose Radius
In internal boring, radial cutting forces are the primary cause of chatter (vibration). A larger nose radius (like R0.4 or R0.8) increases these forces.
Pro Tip: Use an R0.2 radius (like the CCMT060202-HMP). The smaller contact area significantly reduces radial pressure, allowing for stable cutting even with longer overhangs.
3. The 3:1 Ratio Rule for Rigidity
To avoid vibration marks, your boring bar's "overhang" should ideally not exceed 3 times its diameter (3xD). If you must go longer, you must compensate by:
- Reducing your cutting speed (Vc).
- Increasing the feed rate slightly to "bury" the tool and stabilize the cut.
- Using a sharp, positive geometry insert like the HMP chipbreaker.
4. Optimize Chip Evacuation
In a small hole, there is nowhere for chips to go. If you "re-cut" a chip, it will scratch the wall of your part.
Solution: Ensure your coolant is aimed directly into the hole to flush chips out. The HMP chipbreaker on the PC9030 is designed to curl chips tightly, making them easier to eject from narrow bores.
Recommended Setup for Stainless Boring
| Parameter | Target Setting |
|---|---|
| Insert Grade | Korloy PC9030 (PVD) |
| Chipbreaker | HMP (High Precision) |
| Cutting Speed (Vc) | 100 - 150 m/min (Stainless 304) |
| Feed Rate (f) | 0.05 - 0.10 mm/rev |
Conclusion
Precision boring doesn't have to be a gamble. By using a dedicated stainless steel grade like PC9030 and choosing a small nose radius, you can achieve tolerances and finishes that were previously impossible.
