Appearance
A gun drill cutting edge that is too sharp — a perfect intersection of rake and clearance faces with zero radius — will have the lowest cutting forces and the best surface finish for approximately 3–5 holes. After that, the unsupported edge begins to micro-chip, the micro-chips propagate into macro-chipping, and the drill needs regrinding. A controlled edge hone of 0.02–0.05 mm radius removes the fragile micro-edge that would otherwise fracture under cutting loads, distributing the cutting forces over a larger, more stable area. Edge preparation is not a manufacturing afterthought — it is the final quality control step that determines whether the gun drill will achieve its designed tool life.
Edge Preparation Types
Preparation Method Comparison
| Type | Geometry | Typical Radius/Width | Material Removal | Process Control | Best Application |
|---|---|---|---|---|---|
| Radius hone | Smooth radius on edge | 0.005–0.15 mm radius | 0.5–5 µm edge removal | Moderate — brushing/drag | General gun drilling — consistent edge strengthening |
| T-land (chamfer) | Angled flat on edge | 0.05–0.30 mm width — 15–30° angle | 10–50 µm | High — grinding or CNC | Heavy cutting — interrupted cuts — hard materials |
| Waterfall hone | Radius blended into rake face | 0.01–0.08 mm radius | 0.5–3 µm | Low — abrasive flow | Fine finishing — low cutting forces |
| T-land + radius hone | Chamfer with subsequent radius | 0.05–0.20 mm T-land + 0.01–0.05 mm radius | 15–55 µm | Very high — multi-step | Premium gun drills — maximum life |
| Micro-blast | Random edge rounding | 0.003–0.015 mm radius | 0.3–2 µm | Low — blast parameters | Thin coatings — fine edge treatment |
Hone Radius Selection by Drill Diameter
| Drill Diameter (mm) | Hone Radius — Steel (mm) | Hone Radius — Stainless (mm) | Hone Radius — Cast Iron (mm) | Hone Radius — Aluminum (mm) | Hone Radius — Superalloys (mm) |
|---|---|---|---|---|---|
| 2–4 | 0.005–0.010 | 0.008–0.015 | 0.003–0.008 | 0.005–0.010 | 0.010–0.020 |
| 4–8 | 0.010–0.020 | 0.015–0.025 | 0.008–0.015 | 0.008–0.015 | 0.020–0.035 |
| 8–15 | 0.015–0.030 | 0.020–0.040 | 0.010–0.020 | 0.010–0.020 | 0.030–0.050 |
| 15–25 | 0.020–0.040 | 0.030–0.050 | 0.015–0.025 | 0.015–0.025 | 0.040–0.070 |
| 25–40 | 0.030–0.050 | 0.040–0.065 | 0.020–0.035 | 0.020–0.030 | 0.060–0.100 |
Preparation Methods and Process Control
Method Comparison
| Method | Radius Range (mm) | Cycle Time per Drill | Consistency (CV%) | Equipment Cost | Operator Skill | Production Volume |
|---|---|---|---|---|---|---|
| Abrasive nylon brush | 0.008–0.050 | 5–15 seconds | 15–25% | Low | Low | High — mass production |
| Drag finishing | 0.010–0.080 | 30–120 seconds | 10–20% | Moderate | Low | High — batch processing |
| CNC honing (robot) | 0.005–0.150 | 10–40 seconds | 5–10% | Very high | High | Medium-high — precision |
| Micro-blasting | 0.003–0.020 | 5–20 seconds | 20–35% | Moderate | Moderate | Medium — coating prep |
| Manual honing (oil stone) | 0.010–0.100 | 30–120 seconds | 30–50% | Very low | Very high | Low — regrind — repair |
| Laser edge preparation | 0.005–0.100 | 5–20 seconds | 3–8% | Very high | Moderate | Medium — high precision |
FAQ
What edge preparation is recommended for gun drill regrinding?
For gun drill regrinding, a light radius hone of 0.005–0.020 mm is recommended — depending on the drill diameter and application. Manual honing with a fine-grit silicon carbide or diamond stone (600–1000 grit) applied lightly to the cutting edge at the correct angle is the most common regrind edge preparation method. The hone should be applied to both cutting edges equally — an uneven hone creates cutting force imbalance that causes hole size variation. The honing stroke should be short (3–5 mm), light (barely perceptible pressure), and directed from the rake face toward the clearance face — never into the cutting edge. Over-honing during regrind is a common problem — removing more than 0.02 mm of edge material during preparation can reduce tool life by altering the effective rake angle and increasing cutting forces.
How does edge hone radius affect coating performance?
Edge hone radius directly affects coating performance in gun drills. A sharp edge (radius below 0.005 mm) causes coating thickness to build up at the edge during the PVD or CVD coating process — creating a localized stress concentration in the coating that initiates micro-cracking at the edge during the first cutting engagement. A controlled hone radius of 0.010–0.030 mm allows the coating to deposit with uniform thickness over the edge, eliminating the stress concentration and significantly improving coating adhesion. The optimum edge radius for coated gun drills is typically 0.015–0.025 mm — this radius provides the best balance of coating adhesion (good), edge sharpness (acceptable), and edge strength (improved). Drills with radii above 0.050 mm show reduced coating benefits because the increased edge radius requires higher cutting forces that can exceed the coating's load capacity.
What is the difference between a T-land and a radius hone?
A T-land (also called a chamfer) is an angled flat surface ground onto the cutting edge at a specific angle (typically 15–30°) and width (typically 0.05–0.30 mm), creating a defined transition between the rake and clearance faces. A radius hone is a rounded profile that smoothly blends the rake and clearance faces without a defined angle or flat. The T-land provides stronger edge reinforcement than a radius hone of equivalent material removal, making it preferred for heavy roughing and interrupted cuts. The radius hone provides a smoother cutting action with lower cutting forces, making it preferred for finishing operations and materials prone to work-hardening. Some premium gun drills combine both — a T-land for edge strength followed by a light radius hone to remove the sharp transition at the T-land intersection.
How do you measure the edge hone radius on a gun drill?
Edge hone radius measurement methods include optical comparison (comparing the edge profile against calibrated standards or shadowgraphs at 50–200× magnification — fast and practical for shop floor use), contact profilometry (drawing a stylus across the edge to generate a profile trace — accurate to ±0.001 mm but requires the drill to be sectioned or measured at the edge extremity), non-contact profilometry (white light interferometry or laser scanning confocal microscopy — accurate to ±0.0005 mm without contact — best for quality certification), and edge replication (taking a plastic replica of the edge and measuring the replica — useful for in-process measurement without removing the drill from the machine). For production verification, optical comparison against a calibrated standard at 100× magnification is the most practical method — providing adequate accuracy (±0.005 mm) for most applications with minimal setup time.
Can too much edge hone reduce gun drill performance?
Yes — excessive edge hone significantly reduces gun drill performance. A hone radius exceeding 0.10 mm on a 10 mm gun drill increases cutting forces by 15–25% compared to a 0.02 mm hone, requiring more spindle power and generating higher cutting temperatures. The increased force pushes the drill tip against the hole wall, increasing friction on the guide pads and accelerating pad wear. The effective rake angle becomes more negative as the hone radius increases relative to the feed per tooth — at some point, the cutting edge stops cutting efficiently and begins plowing the material, generating excessive heat that softens the carbide binder and accelerates flank wear. The maximum recommended hone radius for gun drills is approximately 3–5% of the drill diameter — above this ratio, the performance degradation outweighs the edge-strengthening benefit.
Disclaimer: The edge preparation parameters and recommendations provided in this article are general guidelines based on industry-standard practices for gun drill manufacturing and reconditioning. Optimal edge preparation varies with carbide grade, coating type, workpiece material, cutting parameters, and coolant conditions. Edge preparation should be verified through application testing under actual production conditions. The authors and publisher assume no liability for any damages or losses arising from the use of this information — always follow original equipment manufacturer guidelines for your specific equipment. Content is for informational purposes only and does not constitute professional engineering advice. Verify all parameters with qualified personnel before implementation as of 2026.