Appearance
A gun drill does not wear out — it dulls. And a dull gun drill reground correctly will outperform a new gun drill reground poorly. The difference between a drill that delivers 20 metres of hole per regrind and one that scrapes the bore after five is almost never the carbide grade. It is the quality of the regrind.
When to Regrind
Flank Wear Measurement (VB)
Tool wear on gun drills is measured by the flank wear width (VB) on the cutting edge:
| Drill Diameter | Maximum VB Before Regrind |
|---|---|
| < 15 mm | 0.2–0.4 mm |
| > 15 mm | 0.4–0.6 mm |
Regrind when VB reaches these limits. Continuing past these thresholds causes:
- Rapid acceleration of wear (the cutting edge breaks down exponentially after the wear land passes a critical width)
- Increased cutting forces that can separate the carbide tip from the steel shank
- Bore surface degradation
- Guide pad overheating and galling
Visual and Process Signs
| Sign | Indication | Action |
|---|---|---|
| Dull appearance on cutting edge | Wear approaching limit | Measure VB and schedule regrind |
| Surface finish degradation on hole | Edge breakdown | Immediate regrind — finish will worsen rapidly |
| Spindle power increase > 15% | Increased friction from worn edge | Regrind before overload causes tip fracture |
| Chip form changing (longer, stringier) | Edge geometry altered | Regrind — chip control depends on sharp edges |
| Audible change in cutting sound | Edge breakdown or chipping | Stop cut and inspect immediately |
Tip: Time-based regrinding — regrinding at a fixed interval based on metres drilled — is more reliable than waiting for visual signs of wear. Track metres per regrind for each drill diameter and material combination. When tool life per regrind drops by more than 20% from the established baseline, investigate the cause before regrinding again.
Gun Drill Geometry Overview
Key Angles
| Angle | Symbol | Function |
|---|---|---|
| Point angle | θ1 | Determines chip thickness distribution and centring ability |
| Primary relief angle | θ2 | Provides clearance behind the cutting edge — prevents rubbing |
| Secondary relief angle | θ3 | Additional clearance behind primary relief — reduces friction |
| Rotating angle | θ4 | Rotation applied between primary and secondary relief grinds |
| Honing angle | θh | Edge preparation angle — strengthens the cutting edge |
| Honing width | H | Width of the honed land on the cutting edge |
Angle Effects on Cutting
| Parameter | Effect of Increasing | Effect of Decreasing |
|---|---|---|
| Point angle (θ1) | Better centring, higher radial force | Reduced centring, lower radial force |
| Primary relief (θ2) | Less friction, weaker edge | Stronger edge, more friction |
| Secondary relief (θ3) | More chip clearance, weakened support | Stronger edge support, less clearance |
| Honing width (H) | Stronger edge, more cutting force | Sharper edge, less edge strength |
Regrinding Equipment
Diamond Wheel Specifications
| Operation | Wheel Type | Grit Size | Diameter |
|---|---|---|---|
| Flank grinding | Diamond cup wheel | 280–400 | 100–200 mm |
| Thinning | Diamond straight wheel | 280–400 | 100–200 mm |
| Finishing/polishing | Ceramic lap with spray diamond | 600 | — |
Grit selection guide:
- 280 grit — rough grinding, fast material removal
- 360 grit — general-purpose regrinding (Accu-Finish "Gator" wheel)
- 400 grit — finishing, better surface quality on cutting edge
- 600 grit — fine finishing, polishing
Sharpening Fixture
The Accu-Finish gun drill sharpening fixture is the industry standard for manual regrinding. Key features:
| Component | Specification |
|---|---|
| Slotted guide plate | Aligned to zero mark on table |
| Holder block | Clamps drill level |
| Angle gate insert | 60.1° (#001-328) |
| Flat gate insert | #011-329 (modifiable for different drill shapes) |
CNC Tool Grinders
For high-volume regrinding, CNC tool and cutter grinders offer automated, repeatable geometry:
| Machine | Key Feature | Best For |
|---|---|---|
| ANCA MX7 ULTRA | P-axis with Arobotech steady rest | Single-clamping regrind, high precision |
| ANCA TX7+ | Interchangeable P-axis | Mixed production, flexible |
| Cuttermaster GS series | Fixed-angle fixture | Manual regrind, small shops |
Step-by-Step Regrinding Procedure
Step 1: Pre-Grinding Inspection
- Measure VB to confirm regrind is needed
- Inspect for chips or cracks in the carbide tip
- Clean the drill thoroughly to remove coolant residue
- Check guide pad condition — pads should be replaced if worn beyond 0.1 mm
Step 2: Primary Relief Grinding
| Parameter | 3/4 Round Drill | 1/2 Round Drill |
|---|---|---|
| Down angle | 12° | 12° |
| Left table angle | 30° | 20° |
Grind until all wear has been removed from the cutting edge. The "Y" dimension (distance from primary relief to the cutting edge) should be:
| Drill Diameter | Recommended Y |
|---|---|
| 2.5–5 mm | 0.25–0.50 mm |
| 5–10 mm | 0.50–0.75 mm |
| 10–18 mm | 0.75–1.00 mm |
Warning: Grinding the Y dimension too large removes excessive carbide and reduces the number of regrinds achievable from the tool. Grinding it too small leaves wear on the edge. Stop grinding as soon as the wear痕迹 is completely removed.
Step 3: Inner Relief
| Parameter | 3/4 Round Drill | 1/2 Round Drill |
|---|---|---|
| Down angle | 12° | 12° |
| Right table angle | 20° | 15° |
| Target | Maintain D/4 offset | D/4 offset |
The inner relief establishes the centre offset (typically D/4 from the drill centreline). This offset creates the single-lip cutting action that defines gun drilling.
Step 4: Secondary Relief
| Parameter | Value |
|---|---|
| Down angle | 20° |
| Table angle | 30° left (3/4 round) or 20° left (1/2 round) |
| Y dimension after step | Maintain 0.2–0.8 mm from primary relief |
For Tungaloy drill types, secondary relief angle (θ3) ranges from -24° to -26° for DMX-F types, with the ridge line between primary and secondary relief brought to the drill centre.
Step 5: Front Clearance
| Parameter | Value |
|---|---|
| Down angle | 0° (centre) |
| Table angle | 0° (centre) |
| Target | Grind until the clearance face just reaches the bottom of the primary relief |
Step 6: Thinning
Thinning (cross-thinning or X-type) re-establishes the chip splitting geometry at the drill point:
| Drill Type | β1 | β2 | β3 | Notes |
|---|---|---|---|---|
| DSX, DSE | 140–145° | 38–42° | 108–112° | Standard cross-thinning |
| DSC-F | 132–138° | 38–42° | 108–112° | Leave R 0.05–0.1 mm |
| DSC | 150–156° | 40–50° | 100–105° | Leave 0.05–0.1 mm of secondary relief |
| DSC-FQ | 132–138° | 38–42° | 123–127° | Radius R1–5 |
Wheel: 280–400 grit diamond straight-type, 100–200 mm diameter.
Step 7: Honing
Honing strengthens the cutting edge by removing the microscopic irregularities left by grinding:
| Parameter | Specification |
|---|---|
| Honing angle (θh) | -20° |
| Honing width (H) | 0.03–0.10 mm |
| Rough honing | Electro-deposited diamond file, ~170 grit |
| Finish honing | Diamond hand stick, 400–600 grit |
| Effect on tool life | Proper honing can extend tool life up to 300% |
The honing width varies by application:
- 0.03–0.05 mm — fine finishing, light cuts
- 0.05–0.08 mm — general-purpose steel drilling
- 0.08–0.10 mm — interrupted cuts, tough materials
Step 8: Peripheral Chamfer
| Parameter | Value |
|---|---|
| Angle from drill axis | 10–20° |
| Width | Approximately equal to the Y dimension |
| Method | Hand rotate against the wheel |
The chamfer removes the sharp corner at the periphery. The chamfer must not enter the primary relief area.
Angular Specifications by Drill Type
Tungaloy Series
| Type | Point Angle θ1 | Primary Relief θ2 | Secondary Relief θ3 | Rotating Angle θ4 |
|---|---|---|---|---|
| DSX, DSE | -20° | -7° to -10° | — | 5° |
| DSC | — | -12° | — | — |
| DSC-F | -25° | -2° | — | — |
| DMX-F | -25° | -10° to -12° | -24° to -26° | — |
| DSS | -20° | — | — | — |
Note: For DSX and DSE types, rotate the drill by 5° (θ4) after primary relief, then grind secondary relief. For DMX-F and DSC-F types, grind secondary relief without rotating the drill.
Quality Verification
Critical Specifications After Regrinding
| Check | Specification | Method |
|---|---|---|
| Lip height difference | ≤ 0.02 mm | Dial indicator or sparkout grinding observation |
| Primary relief angle | Per drill type specification | Angular gauge or pre-set fixture |
| Secondary relief angle | Per drill type specification | Angular gauge |
| Centre offset (Y) | Per drill diameter (D/4 to D/3) | Calibrated magnifier or optical comparator |
| Honing width | 0.03–0.10 mm | Optical comparator (20–50× magnification) |
| Edge condition | No chips, cracks, or grinding burns | Visual inspection (10–20× magnification) |
| Thinning symmetry | Symmetrical both sides | Optical comparator |
Inspection Procedure
Visual inspection — 10–20× magnification. Check for:
- Chipped or fractured cutting edges
- Grinding burns (discolouration indicates overheating)
- Burrs on the cutting edge
Lip height measurement — The most critical quality parameter. Both cutting edges must be within 0.02 mm of each other. Uneven lip height causes:
- One-sided cutting load
- Oversize holes
- Rapid edge failure
Geometry verification — Check primary and secondary relief angles against specification for the drill type.
Test hole — Drill one test hole in the target material. Acceptable results:
- Surface finish within specification
- Chip form matches expected type (C-type or small broken chips)
- Diameter within tolerance
- Spindle power within 10% of baseline
Common Regrinding Errors
| Error | Cause | Consequence | Prevention |
|---|---|---|---|
| Asymmetric geometry | Uneven material removal during grinding | Oversize hole, deviation | Verify lip height after grinding |
| Burnt edge | Feed rate too high, insufficient coolant | Reduced tool life, surface degradation | Reduce feed, ensure coolant flow |
| Excessive honing | Honing width > 0.10 mm | High cutting forces, poor surface finish | Measure honing width optically |
| Insufficient relief | Relief angle too small | Edge rubbing, overheating | Verify relief angle before removing from fixture |
| Offset error | Inner relief ground incorrectly | Poor centring, hole deviation | Verify D/4 offset dimension |
| Chamfer entering primary relief | Chamfer ground too deep | Weak peripheral edge, chipping | Stop chamfer before primary relief zone |
| Coolant hole blockage | Grinding debris entering coolant passage | Coolant starvation at cutting zone | Clean coolant holes with compressed air after grinding |
Regrinding Frequency and Tool Life
Expected Regrind Cycles
| Drill Type | Typical Regrinds | Total Drilling Life per Tip |
|---|---|---|
| Standard carbide gun drill | 15–20 | 150–400 metres (steel) |
| Small diameter (< 6 mm) | 10–15 | 50–150 metres |
| Large diameter (> 20 mm) | 20–25 | 200–500 metres |
| PVD-coated gun drill | 8–12 | 100–250 metres |
Factors Affecting Regrind Life
| Factor | Effect on Regrinds per Tip |
|---|---|
| Material hardness | 20% reduction per 100 HB above 200 HB |
| Coolant condition | Up to 50% reduction with contaminated coolant |
| Edge preparation (honing) | Up to 300% improvement with proper honing |
| Grinding quality | Up to 100% variation between good and poor regrinds |
| Cutting parameters | 30% reduction if parameters exceed recommended range |
Tip: Track regrind number on each drill by marking the shank with a numbered tag. This allows correlation between regrind quality and tool life, and prevents drills from being reground beyond the safe number of cycles. Most carbide tip fractures after repeated regrinding are predictable — they occur because the tip has been reground to the point where insufficient carbide remains to withstand cutting forces.
FAQ
When should a gun drill be reground?
Regrind when flank wear (VB) reaches 0.2–0.4 mm for drills under 15 mm, or 0.4–0.6 mm for larger drills. Time-based regrinding (every 10–20 metres depending on material) is more reliable than visual inspection.
What diamond wheel grit is recommended for gun drill regrinding?
280–400 grit diamond cup wheels for flank grinding and 280–400 grit diamond straight wheels for thinning. Use 600 grit or a ceramic lap with spray diamond for finishing. Coarser grits remove material faster but leave a rougher edge.
What is the most critical quality parameter after regrinding?
Lip height difference — the two cutting edges must be within 0.02 mm of each other. Uneven lip height is the most common cause of oversize holes and poor straightness after regrinding.
What is the Y dimension in gun drill regrinding?
The Y dimension is the distance from the primary relief grind to the cutting edge. It should be 0.25–1.00 mm depending on drill diameter (larger drills = larger Y). Grinding Y too large wastes carbide and reduces the number of regrinds; too small leaves wear on the edge.
What is the centre offset and why does it matter?
The centre offset (typically D/4 from the drill centreline) creates the single-lip cutting action of the gun drill. It is established during inner relief grinding. An incorrect offset causes poor centring, hole deviation, and uneven cutting edge loading.
How many times can a gun drill be reground?
Standard carbide gun drills can be reground 15–20 times before the carbide tip is consumed. Small diameter drills (under 6 mm) achieve 10–15 regrinds. Track regrind count — tip fracture after repeated regrinding is predictable.
Can I hand-grind a gun drill?
No. Gun drill regrinding requires a dedicated sharpening fixture (such as the Accu-Finish system) or a CNC tool grinder. Hand grinding produces asymmetric geometry that will cause the drill to deviate immediately on entry.
What honing is required after regrinding?
Honing at -20° angle with a width of 0.03–0.10 mm. Rough hone with 170 grit diamond file, finish with 400–600 grit diamond stick. Proper honing can extend tool life by up to 300% by removing grinding irregularities and strengthening the cutting edge.
Does regrinding affect the coolant hole?
If the regrind removes more material than necessary, the coolant hole exit can become enlarged or distorted, reducing coolant velocity at the cutting edge. Always inspect the coolant hole opening after regrinding. Clean any debris from the coolant hole with compressed air.
How do I verify regrinding quality?
Inspect under 10–20× magnification for edge chips, grinding burns, and burrs. Measure lip height difference (≤ 0.02 mm). Check the Y dimension against specification. Finally, drill a test hole and verify surface finish, diameter tolerance, and chip form.
Conclusion
Gun drill regrinding is a defined procedure that must be followed in sequence: primary relief, inner relief (establishing D/4 centre offset), secondary relief, front clearance, thinning, honing, and peripheral chamfer. The equipment requirements are specific — diamond cup wheels (280–400 grit), a dedicated sharpening fixture or CNC tool grinder, and optical measurement capability for lip height verification (≤ 0.02 mm tolerance). A gun drill properly maintained through this procedure will deliver 15–20 regrinds and hundreds of metres of drilling life. A drill reground with incorrect angles, uneven lip height, or missing honing will fail within the first few metres — and the failure will be attributed to the tool, not to the regrind quality that actually caused it.