Appearance
A gun drill is a precision tool with multiple ground surfaces that must be held to tolerances of 0.01 mm or better. Reconditioning is not simply making the tool sharp again — it is restoring the complete geometry that determines hole size, surface finish, and straightness. The choice between sharpening and regrinding depends on the drill's condition, the hole specification, and the economics of the application.
Sharpening vs Regrinding: The Difference
Definition and Scope
| Aspect | Sharpening | Regrinding |
|---|---|---|
| Definition | Restores cutting edge on a drill that is still within diameter spec | Restores full geometry including diameter on a worn or damaged drill |
| Material removed | 0.02–0.05 mm per face | 0.05–0.15 mm per diameter |
| Diameter change | Negligible (< 0.01 mm reduction) | Intentional reduction (0.05–0.15 mm) |
| Surfaces ground | Only end cutting faces | End faces + OD clearance + guide pads |
| Chip breaker | Refreshed if part of end face | Ground to original specification |
| Time per drill | 5–15 minutes | 20–45 minutes |
| Cost per drill | $15–$40 | $40–$100 |
| Number of times possible | 4–8 per regrind cycle | 6–8 total before drill is consumed |
Tip: Sharpening and regrinding are complementary, not alternatives. A gun drill should be sharpened multiple times between regrinds. Sharpening 4–6 times followed by one regrind, then repeat, maximizes total tool life. Sharpening alone (without periodic regrinding) allows the drill diameter to drift undersize.
When to Sharpen vs Regrind
| Condition | Action | Reason |
|---|---|---|
| Normal wear, drill within 0.02 mm of nominal diameter | Sharpen | Removes worn cutting edge, restores sharpness |
| Flank wear < 0.15 mm | Sharpen | Wear land small enough to grind off on end faces |
| Minor edge chipping | Sharpen | Edge damage removed by end face grinding |
| Flank wear > 0.15 mm | Regrind | OD clearance needs restoration |
| Diameter below nominal by > 0.02 mm | Regrind | OD grind needed to restore diameter |
| Corner chipped or broken | Regrind | Corner geometry requires OD grind to restore |
| Guide pad worn or scored | Regrind | Guide pads must be re-ground |
| Thermal crack visible | Discard | Crack propagates, drill is unsafe |
| Diameter below minimum spec | Discard | No more carbide remaining |
The Sharpening Process
Step-by-Step Sharpening
| Step | Action | Equipment | Tolerance |
|---|---|---|---|
| 1 | Inspect drill condition | Microscope, 10–20× | Identify wear pattern |
| 2 | Clean drill thoroughly | Ultrasonic cleaner or solvent | Remove all coolant residue |
| 3 | Measure current diameter | Micrometer | Record to 0.001 mm |
| 4 | Set up in sharpening fixture | Gun drill sharpening fixture or CNC tool grinder | Concentricity within 0.005 mm |
| 5 | Grind primary facet | Diamond grinding wheel (D300–D600) | Point angle ±0.5° |
| 6 | Grind secondary facet | Same wheel, indexed position | Clearance angle ±0.5° |
| 7 | Refresh chip breaker | Form wheel or hand dress | Chip breaker depth ±0.02 mm |
| 8 | Deburr edges | Fine stone or brush | No burrs on cutting edges |
| 9 | Final inspection | Microscope, micrometer | Per geometry specification |
| 10 | Package for delivery | Individual tube or sleeve | Prevent edge damage in transit |
Sharpening Quality Checks
| Check | Acceptable Tolerance | Method |
|---|---|---|
| Point angle | ±0.5° of specification | Optical comparator or goniometer |
| Clearance angle | ±0.5° of specification | Optical comparator |
| Radiusing (if specified) | ±0.02 mm | Microscope with reticle |
| Edge condition | No burrs, uniform | Microscope 20× |
| Concentricity of point to shank | ±0.01 mm TIR | Dial indicator in V-block |
The Regrinding Process
Step-by-Step Regrinding
| Step | Action | Detail |
|---|---|---|
| 1 | Inspect drill condition | Check for cracks, excessive wear — discard if damaged beyond salvage |
| 2 | Measure all diameters | Tip OD, guide pad OD, shank OD |
| 3 | Grind OD clearance on tip | Restore clearance angle, remove wear land |
| 4 | Grind guide pad OD | Restore to nominal diameter |
| 5 | Grind end cutting faces | Primary and secondary facets per geometry spec |
| 6 | Refresh chip breaker | Restore original geometry |
| 7 | Blend all surfaces | Ensure smooth transitions |
| 8 | Deburr edges | Fine stone all edges |
| 9 | Final inspection | Full geometry check |
| 10 | Mark diameter on shank | New diameter after regrind |
Regrind Quality Standards
| Parameter | Standard Tolerance | Premium Tolerance | Measurement Method |
|---|---|---|---|
| Tip diameter | ±0.010 mm | ±0.005 mm | Micrometer |
| Guide pad diameter | ±0.010 mm | ±0.005 mm | Micrometer |
| Point angle | ±0.5° | ±0.25° | Optical comparator |
| Clearance angle | ±0.5° | ±0.25° | Optical comparator |
| Concentricity (tip to shank) | 0.010 mm TIR | 0.005 mm TIR | Dial indicator |
| Edge preparation | ±0.02 mm | ±0.01 mm | Microscope |
| Surface finish (ground surfaces) | Ra 0.2 µm | Ra 0.1 µm | Profilometer |
Service Provider Selection
Provider Evaluation Criteria
| Criterion | What to Look For | Verification Method |
|---|---|---|
| Grinding equipment | CNC tool and cutter grinder with diamond wheels | Facility visit or equipment list |
| Inspection equipment | Optical comparator, microscope, micrometer set | Equipment list |
| Quality certification | ISO 9001 or similar | Certificate |
| Turnaround time | 3–10 business days standard, rush available | Ask current customers |
| Geographic location | Local preferred for shorter shipping | Distance from your shop |
| Experience with gun drills | Specialist in deep hole drilling tools | Customer references |
| Documentation | Inspection report with each drill return | Sample report |
Questions to Ask a Potential Provider
| Question | Why It Matters |
|---|---|
| What grinding wheel grit do you use for gun drill carbide? | D300–D600 diamond is standard — finer grit for premium finish |
| Can you match original point geometry exactly? | Geometry changes affect hole quality |
| Do you provide inspection certificates? | Needed for quality traceability |
| What is your typical diameter loss per regrind? | Should be 0.05–0.10 mm |
| How many regrinds do you typically achieve per drill? | 6–8 is standard for quality drills |
| Do you sharpen in-between regrinds? | Yes — sharpening between regrinds extends total life |
Cost-Benefit Analysis
Cost per Hole Comparison
| Scenario | New Drill Cost | Cost per Regrind | Regrinds | Total Holes per Drill Life | Cost per Hole |
|---|---|---|---|---|---|
| New drill only, no reconditioning | $200 | — | 0 | 500 | $0.400 |
| New + sharpen only | $200 | $25 | 10 | 1,500 | $0.183 |
| New + sharpen + regrind cycle | $200 | $60 (regrind) + $25 (sharpening) | 6 regrinds + 30 sharpens | 4,000 | $0.096 |
| Discount reconditioning service | $200 | $35 (low quality) | 3 regrinds (poor quality) | 2,000 | $0.131 |
Value Analysis
| Service Quality | Cost per Hole | Tool Life per Regrind | Risk |
|---|---|---|---|
| Premium reconditioning | $0.08–0.12 | 90–100% of new tool life | Low |
| Standard reconditioning | $0.12–0.18 | 70–85% of new tool life | Medium |
| Low-cost / unverified | $0.18–0.25 | 40–60% of new tool life | High — may damage parts |
Tip: The lowest-cost sharpening service is rarely the most economical. A regrind that restores the drill to 90% of original performance at $60 is better value than a $35 regrind that only delivers 50% performance. Calculate cost per hole, not cost per regrind.
FAQ
What is the difference between gun drill sharpening and regrinding?
Sharpening restores the cutting edge geometry by grinding only the end faces of the drill. It takes 5–15 minutes, costs $15–$40, and does not change the drill diameter. Regrinding restores the complete geometry including the outer diameter clearance, guide pads, and cutting faces. It takes 20–45 minutes, costs $40–$100, and reduces the diameter by 0.05–0.15 mm.
How many times can a gun drill be sharpened before it needs regrinding?
A gun drill can typically be sharpened 4–6 times between regrinds, depending on the wear rate and material being drilled. Sharpening removes 0.02–0.05 mm from the end faces each time. After 4–6 sharpenings, enough material has been removed from the end faces that the OD clearance angle is no longer correct, and a regrind is needed to restore the full geometry.
How do I choose between sharpening and regrinding for my gun drill?
Choose sharpening if the drill diameter is within 0.02 mm of nominal and the wear is limited to the cutting edge (flank wear < 0.15 mm). Choose regrinding if the diameter has dropped by more than 0.02 mm, the guide pads are worn or scored, the corner is chipped, or the drill has been sharpened 4–6 times since the last regrind. If in doubt, measure the diameter and inspect the guide pads.
What quality standards should I expect from a gun drill regrinding service?
A quality regrind service should hold tip diameter to ±0.010 mm, point angle to ±0.5°, and concentricity to 0.010 mm TIR or better. Each reground drill should come with an inspection report showing these measurements. Ground surfaces should be free of burn marks and cracks. The service provider should use diamond grinding wheels on CNC tool grinding equipment.
Can a reground gun drill perform as well as a new one?
A properly reground gun drill from a quality service provider performs at 90–100% of new drill performance. The key factors are: using the correct geometry match to the original, maintaining concentricity, achieving the specified edge finish, and not exceeding the maximum number of regrinds. A regrind from a low-quality provider can perform at 50% or less — the difference is in the quality of the grinding and inspection equipment and the skill of the operator.
Gun drill reconditioning is an economic decision, not a maintenance task. The right balance of sharpening and regrinding, performed by a quality provider, reduces cost per hole by 50–75% compared to using only new tools. This article reflects industry practice as of 2026.