Appearance
A new gun drill costs $200–$1,200 depending on diameter and length. A regrind costs $30–$150. The economics are compelling — but only if the reground tool performs like new. A poorly reground gun drill will not cut straight, will not hold tolerance, and will often break on the first hole, taking the workpiece with it. The geometry of a gun drill is deceptively complex: an asymmetrical point, four distinct clearance angles, two guide pads with specific relief, and a subtle back taper along the entire shank. Regrinding is not sharpening — it is re-creating a precision cutting geometry within 0.01 mm of the original, and it requires a 5-axis CNC grinding machine with appropriate software. This article covers what happens when a gun drill goes back to the grinder, what can go wrong, and how to ensure the reground tool performs as well as the original.
When to Regrind vs. Replace vs. Re-tip
Decision Matrix
| Condition | Regrind | Re-tip | Replace |
|---|---|---|---|
| Flank wear within regrind limit (< 0.30 mm) | Yes | No | No |
| Flank wear exceeds regrind limit (> 0.30 mm) | No | Yes | Maybe |
| Tip chipped but carbide intact | Yes (if within limit) | If beyond limit | No |
| Tip fractured / missing carbide | No | Yes | No |
| Shank bent or damaged | No | No | Yes |
| Diameter reduced beyond tolerance | No | Yes (oversize regrind) | Maybe |
| Multiple regrinds already performed | No | Yes | No (if re-tip possible) |
| Guide pads worn beyond relief limit | No | Yes (re-pad) | Maybe |
Typical Regrind Life
| Drill Diameter | Typical Regrinds | Diameter Loss per Regrind | Total Diameter Loss |
|---|---|---|---|
| 3–6 mm | 3–5 | 0.02–0.04 mm | 0.10–0.20 mm |
| 6–12 mm | 5–8 | 0.03–0.05 mm | 0.15–0.40 mm |
| 12–25 mm | 8–12 | 0.04–0.08 mm | 0.35–0.80 mm |
| 25–40 mm | 10–15 | 0.05–0.10 mm | 0.50–1.50 mm |
After the maximum number of regrinds, the drill is either re-tipped (new carbide tip brazed onto the existing shank) or scrapped.
The Gun Drill Geometry That Must Be Reproduced
Key Geometrical Features
| Feature | Function | Criticality |
|---|---|---|
| Inner cutting edge | Cuts centre region of hole | Defines centring ability |
| Outer cutting edge | Cuts outer diameter region | Defines hole size |
| Apex (intersection of inner and outer edges) | Divides chip load | Must be on centreline |
| Flank clearance (primary) | Prevents rubbing on cutting face | 8–15° typical |
| Secondary clearance | Provides relief behind primary | 15–25° typical |
| Inner clearance (gash) | Chip flow from inner edge | 20–30° typical |
| Shoulder dub-off | Relief at outer corner | 15–25° typical |
| Guide pads (narrow) | Bore guidance, burnishing | OD relief 0.005–0.015 mm |
| Guide pads (wide) | Primary bearing surface | OD relief 0.010–0.025 mm |
| Back taper | Reduces friction along shank | 0.005–0.020 mm per 100 mm |
| Margin | Cutting edge support | 0.3–0.8 mm width |
| Coolant hole position | Coolant delivery to cutting edge | Must not be obstructed |
The Four Clearance Angles
A gun drill has four distinct clearance angles that must be regenerated during regrinding:
| Clearance | Location | Typical Angle | Regeneration Method |
|---|---|---|---|
| Primary flank | Behind outer cutting edge | 10–15° | Z-axis + C-axis interpolation |
| Secondary flank | Behind primary, higher angle | 20–25° | Same as primary but different C-axis orientation |
| Inner clearance (gash) | Inner cutting edge relief | 25–30° | C-axis rotation + Z-axis feed |
| Shoulder dub-off | Outer corner of drill | 18–22° | B-axis tilt + X-axis feed |
Each clearance is defined by a combination of the drill's rotation angle and the grinding wheel's position. The 5-axis machine controls all four clearances in a single clamping setup.
Point Geometry
| Parameter | Typical Range (General Steel) | Notes |
|---|---|---|
| Outer cutting edge angle | 30–35° | Measured from drill axis |
| Inner cutting edge angle | 20–25° | Steeper than outer for centring |
| Apex offset from centre | 0.0–0.05 mm outward | Prevents centre interference |
| Point angle (included) | 110–140° | Sum of outer + inner angles × 2 (approximate) |
The Regrinding Process
Step 1: Inspection and Cleaning
Before regrinding, each drill is inspected:
| Check | Acceptance Criterion | Action if Failed |
|---|---|---|
| Shank straightness | ≤ 0.02 mm TIR over 100 mm | Straighten or reject |
| Remaining carbide length | Sufficient for geometry (≥ 2.0 mm) | Re-tip instead of regrind |
| Guide pad condition | No galling, scoring < 0.02 mm deep | Re-pad if damaged |
| Coolant hole obstruction | Clear, no chips | Clean ultrasonically |
| Flank wear width | < 0.30 mm | Re-tip if exceeded |
| Previous regrind count | < maximum per diameter | Re-tip if at limit |
Step 2: 5-Axis CNC Grinding
Modern gun drill regrinding is performed on 5-axis CNC tool grinders:
| Machine Parameter | Typical Specification |
|---|---|
| Axes | X, Y, Z (linear) + A (rotation), C (swivel) |
| Axis resolution | ≤ 0.0001° (rotary), ≤ 0.001 mm (linear) |
| Spindle power | 7.5–15 kW |
| Spindle speed | 8,000–24,000 RPM |
| Clamping | Precision collet or hydraulic chuck |
| Coolant during grinding | High-pressure oil or emulsion |
| Wheel type | Diamond (resin bond) for carbide, or vitrified CBN for steel |
Grinding sequence:
- Primary clearance grind — Z-axis feed with drill rotating to C-axis position for primary flank angle
- Secondary clearance grind — Same position, higher C-axis rotation
- Inner clearance (gash) grind — Drill rotated, wheel feeds along inner edge path
- Shoulder dub-off grind — B-axis tilt, wheel feeds radially at outer corner
- Guide pad relief — Both pads ground to OD + relief specification
- Edge preparation — Micro-brush or fine diamond wheel to remove burrs
Step 3: Edge Preparation
After grinding, the cutting edge requires preparation:
| Method | Grit | Edge Radius Produced | Application |
|---|---|---|---|
| Diamond filament brush | #400–#800 | 0.005–0.015 mm | General purpose |
| Silicon carbide flap wheel | #320–#600 | 0.010–0.025 mm | Steel, cast iron |
| Nylon abrasive wheel (Al₂O₃) | #240–#1500 | 0.002–0.020 mm | Aluminium, non-ferrous |
| Diamond paste on felt | #1000–#3000 | 0.001–0.005 mm | Precision, small diameters |
The edge hone is critical for gun drill performance — too sharp and the edge chips immediately; too blunt and the cutting forces increase, causing BUE and poor surface finish.
Step 4: Quality Inspection
| Inspection | Method | Acceptance Criterion |
|---|---|---|
| Point geometry | Optical profile projector (20–50×) | Inner and outer angles within ±0.5° of spec |
| Clearance angles | Optical comparator or CMM | Within ±1° of spec |
| Cutting edge radius | Confocal microscope or replication | Within ±0.005 mm of target |
| Concentricity | V-block + dial indicator (near tip) | ≤ 0.01 mm TIR |
| Concentricity | V-block + dial indicator (10 mm back) | ≤ 0.02 mm TIR |
| Guide pad OD | Micrometer (at tip, mid, and 50 mm) | Within ±0.005 mm of spec |
| Back taper | Micrometer at 100 mm intervals | 0.005–0.020 mm per 100 mm |
| Surface finish (flank) | Profilometer | Ra ≤ 0.2 µm |
| Coolant hole clear | Air blow or wire gauge | Obstruction-free |
Step 5: Test Drilling
For critical applications, a test hole is drilled:
| Check | Acceptance Criterion |
|---|---|
| Hole diameter | Within tolerance |
| Surface finish | Ra within spec |
| Straightness | Within spec (check with stepped mandrel) |
| Chips | Correct form (C-shaped, consistent) |
| Coolant pressure | Stable, no fluctuation |
| Cutting sound | Steady, no squeal or chatter |
Re-Tipping
When the carbide tip is too short for further regrinds (carbide remaining < 2.0 mm at the apex), or when the tip is fractured, re-tipping is the solution.
Re-Tipping Process
| Step | Description |
|---|---|
| 1 | Remove remaining carbide — grind out the old tip, typically 5–10 mm from the drill end |
| 2 | Prepare the shank slot — machine a transverse slot to receive the new tip |
| 3 | Braze new tip — silver solder with copper shims between carbide and steel |
| 4 | Centreless grind OD — bring the tip diameter to nominal |
| 5 | Grind flutes — flute through both steel shank and new carbide tip in one pass |
| 6 | Grind point — standard clearance and angle grinding |
| 7 | Final inspection — full geometry check as per regrind |
Re-Tipping Limitations
| Factor | Limitation |
|---|---|
| Diameter range | Typically Ø6 mm and above (smaller diameters are too fragile) |
| Shank condition | Shank must be straight and undamaged |
| Maximum re-tips per shank | 2–3 (each re-tip shortens the overall drill) |
| Cost vs. new | 40–60% of new drill cost |
| Turnaround | 2–5 business days (vs. 2–6 weeks for new) |
WARNING
Re-tipping is only economical when the shank is in good condition. A bent shank cannot be straightened reliably — the residual bending will cause the reground or re-tipped drill to walk. Always check shank straightness before authorising a re-tip. If the shank runout exceeds 0.05 mm TIR at 100 mm from the tip, replace the drill.
In-House vs. External Regrinding
Comparison
| Factor | In-House Regrinding | External Regrinding |
|---|---|---|
| Capital investment | $90,000–$250,000 (5-axis grinder) | None |
| Setup time | 15–30 minutes per drill type | 2–5 days shipping + processing |
| Cost per regrind | $15–$50 (amortised) | $30–$150 |
| Quality control | Direct operator control | Depends on service provider |
| Turnaround | Same day | 3–10 business days |
| Expertise required | High — specialised programmer | None (provided by service) |
| Minimum viable volume | > 500 regrinds per year | Any volume |
When to Regrind In-House
| Condition | Recommendation |
|---|---|
| > 500 regrinds/year | Strong economic case for in-house |
| 200–500 regrinds/year | Consider shared machine or JV with another shop |
| < 200 regrinds/year | External regrinding is more economical |
| Critical geometry requirements | In-house provides better quality control |
| Remote location / long shipping times | In-house may be necessary |
Common Regrinding Defects
| Defect | Cause | Symptom | Correction |
|---|---|---|---|
| Apex not on centreline | Incorrect clamping or wheel positioning | Drill walks, hole wanders | Re-grind point, check clamping |
| Asymmetric clearance angles | C-axis calibration error | Tool cuts oversized, one side loads | Re-calibrate C-axis, re-grind |
| Insufficient back taper | Back taper not ground or worn off | Tool jams in deep holes | Regrind back taper on OD |
| Excessive edge hone | Too much edge preparation | Increased cutting forces, BUE | Reduce honing time |
| Burr on cutting edge | Inadequate post-grind deburring | Rough surface finish | Deburr with fine diamond wheel |
| Guide pad relief too high | Excessive relief angle ground | Poor guidance, hole oversize | Re-pad or new tool |
| Coolant hole restricted | Grinding debris in coolant hole | Coolant pressure drop, chip packing | Ultrasonic clean after grinding |
| Regrind burn (thermal damage) | Wheel too hard or feed too fast | Discoloured carbide, reduced edge life | Reduce feed, use softer wheel |
| Profile mismatch (single-pass error) | Flute wheel not aligned to tip geometry | Chip flow disrupted | Re-setup, verify alignment |
Regrind Interval Management
Tracking System
A production regrind management system records:
| Field | Purpose |
|---|---|
| Tool ID | Unique identifier (laser-marked or RFID) |
| Drill diameter and type | Identifies regrind program |
| Total holes per regrind cycle | Measures tool life |
| Cumulative regrind count | Tracks when re-tip or replace is due |
| Measured flank wear at removal | Verifies regrind trigger threshold |
| Hole diameter of last acceptable part | Process capability verification |
| Coolant pressure at tool change | Baseline for next insert |
| Reason for removal (planned worn vs. broken) | Failure analysis input |
Regrind Interval Guidelines
| Material | Regrind Trigger (Flank Wear) | Expected Holes Between Regrinds |
|---|---|---|
| Low-carbon steel | 0.20–0.25 mm | 200–500 |
| Alloy steel (4140, 4340) | 0.15–0.20 mm | 100–300 |
| Stainless steel (304, 316) | 0.12–0.18 mm | 50–200 |
| Aluminium (6061) | 0.15–0.25 mm | 500–2,000 |
| Titanium (Ti-6Al-4V) | 0.10–0.15 mm | 20–80 |
| Inconel 718 | 0.08–0.12 mm | 5–30 |
| Cast iron | 0.20–0.30 mm | 300–800 |
Regrind Quality Verification Protocol
| Frequency | Check | Method |
|---|---|---|
| Every regrind | Point geometry | Optical projector — verify inner/outer angles |
| Every regrind | Concentricity | V-block + indicator — ≤ 0.01 mm TIR |
| Every regrind | Coolant hole clear | Air blow test |
| Every 5th regrind | Back taper measurement | Micrometer at 100 mm intervals |
| Every 10th regrind | Test hole drilling | Diameter, surface finish, straightness |
| Every re-tip | Full geometry certification | All above + CMM report |
FAQ
Q: How many times can a gun drill be reground? A typical carbide gun drill can be reground 5–12 times depending on diameter. Smaller drills (3–6 mm) allow 3–5 regrinds; larger drills (12–25 mm) allow 8–12 regrinds. Each regrind removes 0.02–0.08 mm from the diameter.
Q: How do I know when a gun drill cannot be reground and must be re-tipped? When the remaining carbide at the apex is less than 2.0 mm, or when the flank wear exceeds 0.30 mm. At this point there is insufficient carbide to regenerate the full point geometry.
Q: What is the cost difference between regrinding and buying a new gun drill? A regrind costs $30–$150 depending on diameter and complexity, compared to $200–$1,200 for a new drill — a savings of 70–85%. However, each regrind reduces the drill diameter, so the drill must be tracked to ensure hole size remains within tolerance.
Q: What machine is required for gun drill regrinding? A 5-axis CNC tool and cutter grinder with control resolution of ≤ 0.0001° on rotary axes and ≤ 0.001 mm on linear axes. Machines from Schneeberger (norma NGC), Schütte (325linear), and Chinese manufacturers (Xuetai) are commonly used.
Q: What are the four clearance angles on a gun drill? Primary flank (behind outer cutting edge, 10–15°), secondary flank (behind primary, 20–25°), inner clearance or gash (inner cutting edge relief, 25–30°), and shoulder dub-off (outer corner relief, 18–22°).
Q: Can a reground gun drill perform as well as a new one? Yes, when properly reground on a 5-axis CNC machine with the correct geometry parameters. A reground drill is capable of identical hole quality and tool life to a new drill, provided the geometry is reproduced within 0.01 mm of the original spec.
Q: What causes a reground gun drill to drill oversized holes? The most common cause is asymmetric clearance angles from improper C-axis calibration during regrinding. The drill loads unevenly, pushing it off-centre. Other causes include incorrect guide pad relief and insufficient back taper.
Q: How is back taper verified after regrinding? Measure the drill diameter with a micrometer at intervals of 100 mm from the tip. The diameter should decrease by 0.005–0.020 mm per 100 mm along the length. If the back taper is worn off or not reground, the drill will jam in deep holes.
Q: What is the re-tipping process for gun drills? The worn carbide tip is ground off, a new carbide blank is silver-brazed to the steel shank (with copper shims to manage thermal expansion mismatch), the assembly is centreless ground to diameter, flutes are ground through both carbide and steel in one pass, and the point geometry is ground.
Q: Should I regrind gun drills in-house or send them out? If your shop uses more than 500 regrinds per year, in-house regrinding on a 5-axis CNC grinder is economically justified. Below 200 regrinds per year, external regrinding services (like Mollart or regional tool grinders) are more cost-effective. In-house regrinding offers same-day turnaround and direct quality control but requires significant capital investment and specialised programming expertise.