30-Second Summary: Gundrills and BTA tools can be resharpened and recoated multiple times, dramatically reducing per-hole costs. A gundrill can typically be resharpened 8-10 times and deliver performance equal to a new tool when done correctly. Regrinding a gundrill costs 20-30% of a new tool, making it one of the highest-ROI activities in deep hole drilling. This guide covers when to resharpen, the regrinding process step by step, coating strategies after regrind, and when re-tipping is the better option.
How Many Times Can a Tool Be Resharpened?
High-quality gundrills can be resharpened 8-10 times over their life, with each resharpening restoring performance to near-new levels. BTA drill heads can be reground 5-8 times depending on wear patterns. The key is recognizing the right time to resharpen: too early wastes tool life, too late risks part scrap or tool breakage.
When to Resharpen
Watch for these indicators that a tool needs resharpening:
- Loss of tolerance or surface finish: Parts trending toward the upper tolerance limit or showing degraded Ra values (increase of 20% or more from baseline).
- Increased thrust and torque: Machine load meters show a sustained increase of 15-20% above baseline. Gradual increase is normal wear; a sudden jump indicates edge chipping.
- Irregular chip formation: Chips become segmented, stringy, or inconsistent in shape and size. For each material, learn what "good" chips look like and monitor for changes.
- Coolant pressure spikes: Indicates chip packing at the cutting zone or restricted flow through coolant orifices.
- Audible changes: Increased noise or vibration during cutting — often the first and most noticeable sign to experienced operators.
Regrinding Economics
| Item | New Tool Cost | Regrind Cost | Savings per Cycle |
|---|---|---|---|
| Gundrill (D=10 mm, carbide) | $120 | $25-35 | $85-95 |
| Gundrill (D=25 mm, carbide) | $250 | $50-75 | $175-200 |
| BTA drill head (D=50 mm) | $400-800 | $80-150 | $320-650 |
| BTA drill head (D=100 mm) | $1000-2000 | $200-400 | $800-1600 |
Over the full life of a tool (new + 8 regrinds for a gundrill), total cost with regrinding is roughly 30-40% of the cost of buying 9 new tools.
Regrinding Best Practices
| Step | Purpose | Typical Parameters |
|---|---|---|
| Inspect incoming tool | Assess wear pattern, measure remaining geometry, check for cracks | Visual + microscope inspection at 10-20x magnification |
| Primary facet grind | Restore cutting edge geometry, remove wear zone | Diamond or CBN wheel, 0.01-0.03 mm stock removal per pass |
| Inner relief grind | Maintain proper clearance angle behind cutting edge | 8-12° relief angle per tool spec |
| Front clearance grind | Provide clearance on the outer diameter corner | 2-5° front clearance |
| Oil dub-off (gundrill-specific) | Restore coolant outlet geometry for proper chip evacuation | Critical for chip flow; neglect causes 50% of regrind failures |
| Web thinning | Reduce thrust force and improve chip flow at the center | Thinned behind the chisel edge to reduce axial force by 20-40% |
| Edge preparation (K-land) | Bone cutting edge to prevent chipping at entry | 0.02-0.10 mm land width depending on material hardness |
| Runout compensation | Correct for grinding wheel wear and tool deflection | CNC compensation cycles; target < 0.005 mm runout |
Long-flute gundrills and BTA tubes require support with a steady rest during grinding to prevent deflection and ensure uniform stock removal along the entire cutting edge. Without support, the grinding force can deflect the tube, producing a non-concentric grind.
Coating Strategies After Regrinding
After regrinding, the cutting edge geometry is restored but the protective coating is removed from the ground surfaces. Two recoating strategies exist:
- Strategy 1 — Grind and recoat without stripping: Coat the reground area only. Faster turnaround (typically 2-3 days), acceptable for less demanding applications. The existing coating on unground surfaces remains intact. Risk of coating thickness variation at the boundary between old and new coating.
- Strategy 2 — Strip, regrind, then recoat: Chemically strip the entire coating, regrind all surfaces, then apply fresh coating. Preferred for high-performance drilling, critical tolerances, and difficult materials (titanium, superalloys). Yields consistent coating thickness across all surfaces. Longer turnaround (5-7 days) but better performance.
A drill can typically be recoated 5-10 times before stripping becomes necessary. Changing the coating type — e.g., upgrading from TiN to TiAlN or AlCrN during the recoating step — can actually improve performance over the original tool, especially if the original coating was selected for a different material.
Selecting a Regrinding Service
Quality of regrinding varies significantly between providers. Key evaluation criteria:
- Use the original manufacturer when possible — they have the original geometry specifications, grinding fixtures, and coating recipes.
- Request a sample regrind — have 3-5 tools reground and test them on your production parts before committing to volume.
- Verify geometry after regrind — measure point angle, relief angles, and tip diameter on a tool presetter or optical comparator.
- Check for edge quality under 10-20x magnification — look for micro-chipping, grinding burns (discoloration), or uneven edges.
Re-Tipping
When the carbide head of a gundrill or BTA drill has been reground to the point that insufficient carbide remains (typically after 8-10 regrinds), re-tipping is an option. This involves replacing the carbide cutting head on the existing steel shank or drill tube. Re-tipping saves up to 80% of the cost of a new tool and is available from most major tool manufacturers. The steel shank is inspected for straightness and wear before re-tipping — if the shank is damaged, a full new tool is required.
Gundrill-Specific Resharpening Sequence
For gundrills, the complete resharpening sequence on a CNC tool grinder should follow this order:
- Primary facet — grind the main cutting edge to restore sharpness
- Inner relief — grind the secondary clearance surface
- Front clearance — grind the outer diameter corner clearance
- Oil dub-off — reshape the coolant outlet for proper chip evacuation
The entire sequence should take under 10 minutes per tool once the grinder is set up. The oil dub-off step is the most critical — a poorly formed coolant outlet is the leading cause of chip evacuation problems after regrinding.