Deep hole drilling tools — gun drills and BTA drill heads — represent a significant capital investment. A single solid carbide gun drill can cost several hundred dollars, and a BTA drill head considerably more. Yet many shops discard these tools at the first sign of wear, unaware that proper regrinding can restore 80–90% of original performance through multiple cycles.
This guide covers the complete regrinding process for deep hole drilling tools. You will learn the critical geometry parameters that must be maintained, the step-by-step regrinding sequence used by tool manufacturers, inspection criteria to verify quality, and the economic factors that determine whether to regrind or replace.
Understanding Deep Hole Drilling Tool Geometry
Before discussing the regrinding process, it is important to understand what makes deep hole drilling tools different from standard twist drills.
Gun Drill Geometry
A gun drill has a single cutting lip, not two like a twist drill. This asymmetric design creates the self-piloting action that gives gun drilling its characteristic straightness. The critical geometric features of a gun drill tip include:
- Outer cutting edge — the primary cutting lip on the outside diameter
- Inner cutting edge — the secondary cutting lip near the center
- Apex (chisel edge) — the point where inner and outer cutting edges meet
- Primary relief (clearance) angle — the first relief behind the cutting edge, typically 10°–12°
- Secondary relief angle — the second, steeper relief behind the primary, typically 20°–27°
- Working approach angle — the angle of the cutting edge relative to the drill axis, typically 25° (outer) and 35° (inner)
These angles are not arbitrary. They are calculated based on the drill diameter, workpiece material, and expected cutting conditions. A study on Φ10 mm gun drills specified the following standard geometry: inner and outer rake angle at 0°, first outer clearance angle at 10°, second outer clearance angle at 25°, inner clearance angle at 15°, outer working approach angle at 25°, and inner working approach angle at 35°.
BTA Drill Head Types
BTA drill heads come in two primary configurations, and the regrinding approach differs for each:
- Brazed carbide heads — Carbide cutting tips and guide pads are brazed onto a steel body. The entire head is ground to final diameter after brazing, achieving tight tolerances. These are regrindable, typically 5–8 times depending on diameter and operating conditions.
- Indexable heads — Carbide inserts are mechanically clamped onto the tool body. When cutting edges wear, inserts are indexed or replaced rather than reground. The tool body itself can last indefinitely with proper maintenance.
For diameters under approximately 20 mm, brazed heads dominate because there is insufficient space for mechanical clamping. Above 20 mm, both types are common, with indexable heads preferred for larger diameters where insert replacement is more economical than regrinding.
When to Regrind
Timing is critical in tool regrinding. Regrind too late and excessive wear may require removing more material, shortening tool life. Regrind too early and you waste usable tool life.
Signs That Regrinding Is Needed
- Visible wear flats on the cutting edge exceeding 0.15–0.2 mm
- Increased surface roughness or chatter marks in the bore
- Changes in chip shape or color (blue chips indicate overheating)
- Increased cutting forces or spindle load
- Hole diameter drifting toward the low end of tolerance
- Penetration rate dropping by 10–15% at constant feed
Regrind Frequency Guidelines
The number of regrinds possible depends on the tool type, cutting parameters, and workpiece material:
- Brazed carbide gun drills (solid): Typically 3–5 regrinds before the head length is reduced below minimum. Some manufacturers mark regrind limits with cooling grooves — when the groove disappears, the tool is spent.
- Brazed BTA heads: 5–8 regrinds are typical, limited by guide pad height wear and carbide tip length.
- Brazed disposable heads (small diameters): Some small-diameter brazed heads are designed as consumables and should not be reground. Check with the manufacturer.
Performance After Regrinding
A properly reground tool typically achieves about 80% of its original performance. With optimal regrinding technique, coating removal and reapplication, performance can reach 90–100% of new. Each subsequent regrind removes slightly more carbide, and tool life gradually decreases.
An economic rule of thumb: if the remaining usable tool life after regrinding is less than 30% of a new tool’s life, it is cheaper to discard and replace than to regrind.
Step-by-Step Gun Drill Regrinding Procedure
The following regrinding sequence is based on manufacturer specifications from Mitsubishi Carbide and industry best practices. A universal tool grinder with a diamond wheel is the standard equipment, though dedicated gun drill sharpening machines are also available.
Equipment Requirements
- Universal tool grinder or dedicated gun drill sharpener
- Diamond grinding wheels: #200 grit for roughing, #400 grit or finer for finishing
- Collet chuck or dedicated gun drill fixture
- Dial indicator or height gauge for lip height measurement
- Diamond files (#400 and #1500 grit) for honing
- Coolant system (emulsion at approximately 1 MPa pressure)
Step 1: Inspection and Preparation
Before grinding, inspect the cutting edge for wear, chipping, or damage. If the carbide is chipped, remove the damaged segment using a green silicon carbide (GC) wheel before proceeding with diamond wheel grinding.
Clean the drill thoroughly. Debris inside the coolant hole can affect grinding accuracy and chip evacuation after regrinding.
Step 2: Primary Relief Grinding
Primary relief restores the cutting edge and removes wear from the flank face.
- Mount the drill in a collet chuck with the main cutting edge parallel when viewed from the drill point.
- Set the point angle to 140° (standard for most gun drills; some profiles use 145°).
- Set the swivel angle to 20°.
- Set the primary relief angle by inclining the drill:
- Standard solid carbide: 10°
- ZET1 type: 10°–12°
- MVS long/pilot drills: 6°–8°
- Grind in passes of 0.01–0.03 mm depth.
- Index 180° and grind the opposite edge.
- Finish with 2–3 spark-out passes at 0.01 mm depth with slow traverse.
- Measure lip height difference — must be within 0.02 mm.
The primary relief length (Y dimension) should be proportional to drill diameter: approximately 0.25–0.5 mm for small drills (Φ3–5 mm), 0.5–1.0 mm for medium drills (Φ10–20 mm).
Wheel specification: Diamond cup wheel, #200 grit for roughing, #400 grit for finishing.
Step 3: Secondary Relief Grinding
Secondary relief provides additional clearance behind the primary relief to prevent rubbing.
- Increase drill inclination to create a steeper angle:
- ZET1 type: 20°–22°
- MVS type: 23°–27°
- MAE/MAS type: 25°
- Grind both sides following the same procedure as primary relief.
- Create the ridge line — a visible line will appear where primary and secondary relief surfaces meet. Adjust the drill’s rotational position so this ridge line is parallel to the main cutting edge.
- Continue grinding until the ridge lines from both sides align to form a straight central point.
The offset distance between the two ridge lines (V dimension) should be controlled by drill diameter. For MVS type drills, this ranges from 0.05 mm for Φ3 mm drills to 0.35 mm for Φ12–14 mm drills.
Step 4: Thinning (Web Thinning)
Thinning reduces the chisel edge width, lowering thrust forces and improving centering action.
X-thinning (most common for gun drills):
- Set the work head horizontal (drill axis parallel to the table).
- Rotate the drill 45°–70° counterclockwise (viewed from the cutting edge), depending on drill type.
- Set the open angle to 35°–40° from the drill axis.
- Grind slowly with controlled depth using a mechanical stop.
- Offset the cutting edge 0.05–0.1 mm from the drill axis.
Wheel specification: Diamond straight-type wheel, #200 grit.
Step 5: Honing (Edge Preparation)
Honing is the final regrinding operation. It creates a micro-bevel on the cutting edge that improves edge strength and extends tool life by preventing micro-chipping.
- Honing angle: 25°–30° from the cutting edge.
- Honing width by drill diameter:
- Φ3–5 mm: 0.03–0.05 mm
- Φ5–10 mm: 0.05–0.10 mm
- Φ10–20 mm: 0.10–0.15 mm
- Grit progression: Start with #400 diamond file, finish with #1500 hand lapper.
- Apply homogeneously to the entire cutting edge.
Optional chamfer: For some applications, a 0.03–0.11 mm × 45° chamfer at the drill periphery corner can be added using a diamond file. The chamfer must not cross into the honing width or primary relief area.
BTA Drill Head Regrinding
BTA drill head regrinding follows a similar sequence but with important differences due to the multiple cutting edges and guide pads.
Brazed BTA Head Regrinding
- Inspect guide pads — these wear with use and their condition affects hole size and surface finish. Verify pad height against minimum specification.
- Grind the cutting edges — all cutting tips must be ground to the same height within 0.02 mm. The angular relationship between tips must match the original design.
- Restore the chip former geometry — the chip breaking groove must be maintained. Grinding too deep into the chip former changes chip shape and can cause chip packing.
- Check the coolant hole openings — ensure they are clear after regrinding.
- Verify concentricity — the drill head must run true within 0.01 mm TIR.
Guide Pad Restoration
Guide pads are critical to the self-piloting action of BTA tools. After several regrinds, the carbide guide pads may need replacement. This is a specialized operation:
- Brazed pads are removed by heating the steel body to soften the braze.
- New pads are brazed in position.
- The head is then ground to final diameter.
Pad wear is often the limiting factor for total regrind cycles. The usable pad length determines the maximum number of regrinds before pad replacement or head disposal is needed.
Coating Considerations
Most modern deep hole drilling tools are coated with PVD (Physical Vapor Deposition) coatings such as TiAlN, TiN, or AlTiN. Regrinding removes the coating from the flank faces, exposing the uncoated carbide substrate.
Performance Impact
- Without recoating: The reground tool will have reduced wear resistance compared to a new coated tool. Tool life may be 50–70% of new.
- With recoating: The reground tool can achieve 90–100% of new tool life.
Decoating Options
Before recoating, the old coating must be removed. Two approaches exist:
Decoating before regrinding — The coating is stripped chemically or via the PLATIT CT20 process before regrinding. This avoids damage to the reground geometry and ensures optimal adhesion of the new coating. The CT20 system uses a wet chemical process that stops automatically at a TiN adhesion layer, preventing cobalt leaching from the carbide substrate.
Overcoating after regrinding — The worn tool is reground at functional surfaces only, then the entire drill is coated again without stripping. This is less expensive but can cause tolerance buildup over multiple cycles. A drill can generally be overcoated 5–10 times before dimensional issues arise.
Cobalt Leaching Risk
The primary risk when decoating carbide tools is cobalt leaching — the removal of cobalt binder from the carbide surface layer. This leaves a porous surface that new coating cannot properly adhere to. Cobalt leaching can be caused by:
- Chemical stripping solutions left in contact too long
- Water-cooled grinding with inadequate coolant concentration
- Grinding too aggressively with a blunt wheel
Proper process control during both decoating and grinding is essential to preserve the carbide substrate.
Quality Inspection After Regrinding
Every reground tool should pass these checks before returning to production:
Dimensional Checks
| Inspection Item | Acceptance Criteria |
|---|---|
| Lip height difference (axial runout) | ≤ 0.02 mm |
| Primary relief angle | ±1° of specification |
| Secondary relief angle | ±1° of specification |
| Point angle | ±2° of specification |
| Centering / apex offset | ≤ 0.02 mm from center |
Visual Checks
- No remaining wear or damage on the cutting edge
- No grinding burrs or burns (blue discoloration indicates overheating)
- Clean, sharp cutting edge
- Even honing along the entire cutting edge
- Coolant hole openings clear of debris
Functional Checks
- Run a test part in the same material at standard parameters
- Verify hole diameter, surface finish, and straightness
- Monitor chip shape — it should match the pattern from a new tool
- Check for unusual vibration or chatter during the test cut
Common Regrinding Mistakes
Unequal Lip Heights
The most common regrinding defect. If the two lips are not equal in height, the drill will produce oversized holes, poor surface finish, and may walk off-center. Always measure lip height after grinding and correct if out of tolerance.
Incorrect Relief Angles
- Too much relief — creates a fragile cutting edge that chips easily, especially at hole exit.
- Too little relief — causes rubbing, overheating, and increased thrust forces.
Insufficient Stock Removal
Failing to remove all worn carbide leaves residual damage that acts as a stress raiser, leading to premature failure. When in doubt, grind until all visible wear marks are gone, then make one additional pass.
Overheating During Grinding
Grinding with excessive depth of cut, worn wheels, or insufficient coolant can cause thermal cracks in the carbide. These cracks propagate during drilling and cause sudden tool failure. Keep grinding depth to 0.01–0.03 mm per pass with adequate coolant flow.
Inconsistent Apex Offset
Research on gun drilling of Inconel 718 found that inconsistent apex offset between successive regrinds is a major cause of hole straightness deviation. When a reground drill contacts the previous hole bottom, unbalanced cutting forces cause the hole to deviate toward the thin wall. Maintaining consistent apex offset from one regrind to the next is critical for straightness.
Incorrect Gun Drill Head Profile Selection
Different workpiece materials require different gun drill head profiles. Regrinding must maintain the original profile type:
- Profile G (Universal) — Standard for most materials, high precision
- Profile E — Alloys and stainless steel, eliminates tool sticking
- Profile A — Cast iron and aluminum, cross drilling
- Profile C — Titanium, high-temperature alloys, angled entry
Regrinding should restore the original profile geometry, not change it.
When to Replace Instead of Regrind
Regrinding is not always the right choice. Replace the tool when:
- Remaining carbide tip length is less than the drill diameter after multiple regrinds
- Guide pad wear exceeds the minimum allowable height
- Chip formers are ground too deep to maintain proper chip breaking
- Cracking or micro-chipping extends beyond what grinding can remove
- The cost per regrind (including recoating) exceeds 50% of a new tool’s cost at the expected remaining life
Regrinding Equipment Options
Dedicated Gun Drill Sharpening Machines
Manufacturers such as Peiping Precision (PP-600), AGrade Carbide (GD-600), and various Chinese manufacturers produce machines designed specifically for gun drill regrinding. Common specifications:
- Diameter range: Φ3–40 mm
- Grinding wheel: 125 × 50 × 31.75 mm diamond wheel
- Motor power: 0.55–0.75 kW
- Wheel speed: 2800–3600 RPM
- Weight: 70–190 kg depending on model
The PP-600 uses a 6-pawl chuck that allows all six face angles to be ground in a single clamping, improving consistency.
Universal Tool Grinders
Most general-purpose tool and cutter grinders can be adapted for gun drill regrinding with a suitable fixture. The Accu-Finish fixture and similar devices provide the necessary multi-axis control for grinding the various clearance angles. These fixtures typically use a V-groove positioning system with adjustable rotational controls.
CNC Tool Grinders
5-axis CNC tool grinders offer the highest precision and consistency for gun drill regrinding. They can store regrind programs for different drill sizes and profiles, eliminating operator variability. These machines are typically justified by high-volume regrinding operations.
Summary
Tool regrinding is a cost-effective way to extend the life of deep hole drilling tools when done correctly. The key principles to remember:
- Follow the correct sequence — primary relief → secondary relief → thinning → honing
- Maintain precise angles — relief angles, point angles, and lip height must be within tight tolerances
- Inspect thoroughly — every reground tool needs dimensional and visual inspection before use
- Consider recoating — a reground tool with new coating performs nearly as well as a new one
- Know when to stop — excessive regrinding reduces tool life and hole quality
For a comprehensive overview of related deep hole drilling topics, see the Complete Guide to Deep Hole Drilling Troubleshooting and the Speeds and Feeds Reference for Deep Hole Drilling.