Appearance
A manufacturer drilling Ø12 mm × 600 mm bores in 4140 steel (250 HB, 12 000 bores per year) was replacing gun drills every 100–150 bores at $280 per tool — annual tool cost approximately $25 000. Implementing in-house resharpening (Precihole TGM 1-32 grinding machine, D126 diamond wheel rough grinding, D46 diamond wheel finish grinding, five-axis geometry restoration, diamond fibre brush edge preparation) reduced per-sharpen cost to $18 and restored tool life to 85–95% of new-tool performance. Each gun drill could be resharpened 8–12 times before end of life, reducing annual tool cost from $25 000 to $9500 — a 62% reduction.
Gun Drill Geometry and Wear Patterns
Gun Drill Tip Geometry: Critical Angles and Their Functions
| Angle / Feature | Symbol | Typical Range | Function | Effect of Incorrect Geometry |
|---|---|---|---|---|
| Outer (cutting) point angle | φ₁ | 30–50° (measured from drill axis) | Determines chip formation mechanics; smaller angle reduces cutting forces per unit of feed but produces thinner, wider chips; larger angle produces thicker, narrower chips with better chip breakage | Too small: chip form too thin, difficulty with chip evacuation; too large: excessive cutting forces, bore deviation |
| Inner (centre) point angle | φ₂ | 20–35° (measured from drill axis, typically 10–15° less than φ₁) | Creates the chisel edge geometry that initiates drilling at the bore centre; negative rake on the inner cutting edge forces chip formation by crushing rather than shearing | Too large: poor centring, drill wander; too small: excessive thrust force, centre point chipping |
| Outer clearance angle (primary) | α₁ | 8–12° | Provides relief behind the outer cutting edge; prevents rubbing between the clearance face and the newly machined bore surface | Too small: rubbing, heat generation, rapid flank wear, burnishing marks on bore surface; too large: edge weakening, micro-chipping, reduced tool life |
| Inner clearance angle (primary) | α₂ | 12–18° | Provides relief behind the inner cutting edge; larger than α₁ because the inner edge has lower cutting speed (closer to centre) and requires more clearance to avoid rubbing | Too small: rubbing at centre, high thrust force, centre point overheating; too large: centre point weakness, chipping at entry |
| Secondary clearance (shoulder relief) | α₃ | 15–25° | Provides clearance behind the primary clearance on the outer cutting edge; reduces friction between the tool flank and bore wall | Too small: flank rubbing on bore wall, increased torque, bore surface damage; too large: excessive carbide removal reduces number of possible resharpenings |
| Front clearance (end face relief) | α₄ | 20–30° | Provides clearance on the end face of the tool, behind the cutting edge at the centre; allows coolant to reach the cutting point | Insufficient: coolant starvation at centre point, rapid centre wear; excessive: reduces support for centre point |
| Chip flute (gullet) radius | r | 2–6 mm (depends on drill diameter) | Determines chip curl radius and chip evacuation efficiency through the flute | Too small: chip jamming in flute; too large: weakens the drill cross-section at the tip, reduces torsional strength |
| Coolant hole offset | δ | 0.1–0.5 mm (offset from centre toward cutting edge) | Positions coolant hole for optimal delivery of coolant to the cutting edge-workpiece interface; offset toward the cutting edge ensures coolant reaches the high-temperature zone | Incorrect offset: coolant stream misses cutting edge, reduced cooling efficiency, premature flank wear |
Gun Drill Wear Patterns Requiring Resharpening
| Wear Pattern | Location | Cause | Visual Characteristics | Effect on Drilling Performance | Minimum Regrind Required | Maximum Material Removal for Regrind |
|---|---|---|---|---|---|---|
| Flank wear (primary) | Outer cutting edge clearance face | Abrasive wear from workpiece carbides; adhesive wear from chip flow | Uniform wear band on clearance face, typically 0.1–0.3 mm wide; visible as a bright, polished band adjacent to the cutting edge | Increased surface roughness (Ra increase 0.2–0.5 µm); gradual increase in thrust force (10–20%); possible bore diameter decrease (< 0.01 mm) | Remove to VB = 0 (clean up entire wear band) | 0.3–0.8 mm from tip face (depending on wear severity) |
| Flank wear (centre) | Centre point / inner cutting edge | Abrasive wear from chip rubbing at centre; thermal softening from high temperature at low cutting speed | Uneven wear at centre point (chisel edge); rounding of centre point; possible micro-chipping at centre | Increased thrust force (20–40%); poor centring; bore deviation 0.05–0.15 mm at depth; whining noise at drill entry | Remove to restore sharp centre point geometry | 0.3–0.6 mm |
| Crater wear | Rake face, behind cutting edge | Diffusion wear from chip flowing across rake face at high temperature (800–1000°C at tool-chip interface) | Crescent-shaped depression on rake face, 0.05–0.20 mm deep; visible as a dark, textured area behind the cutting edge | Chip form change (longer, tighter curl); reduced surface finish; risk of edge chipping if crater approaches cutting edge (crater-to-edge distance < 0.1 mm) | Not directly reground during resharpening — must monitor crater-to-edge distance; if < 0.1 mm, need regrind back past crater | Requires 0.1–0.4 mm additional face removal beyond flank wear regrind |
| Outer corner wear / rounding | Outer corner (intersection of cutting edge and bore wall) | Mechanical abrasion against bore wall; thermal softening from high sliding velocity at outer corner | Rounded or chamfered corner, 0.05–0.20 mm radius; visible as a bright, polished area at the outer corner | Diameter undersize (0.01–0.03 mm); burr formation at bore exit; poor surface finish at bore entry | Restore sharp outer corner geometry by regrinding tip face | 0.3–0.8 mm |
| Guide pad wear | Carbide guide pads (leading and trailing) | Adhesive wear — workpiece material transfer to pad surface; abrasive wear from embedded chips | Smooth, polished area on pad contact surface; possible scoring lines in the direction of drilling; leading pad wears faster than trailing | Increased surface roughness; torque increase (10–25%); possible bore diameter increase if pad wear is asymmetric (> 0.02 mm) | Guide pad re-honing recommended (not same as tip regrind); remove transferred material with diamond hand stone or fine diamond wheel | 0.01–0.03 mm removal from pad surface (re-hone, not regrind) |
| Chipping / micro-chipping | Cutting edge, at any location along the edge | Mechanical impact from interrupted cut, hard inclusion in workpiece, or chip jamming | Small (0.05–0.20 mm) chips missing from cutting edge; visible as irregular, sharp-edged notches in the otherwise straight cutting edge | Immediate degradation of surface finish (Ra increase 50–100%); localised increase in cutting forces; possible crack propagation from chip site | Remove material to below deepest chip; taper regrind to minimise carbide removal | 0.2–0.5 mm |
| Brazed joint degradation | Carbide-to-steel interface | Thermal cycling during drilling (alternating heating and cooling); coolant chemical attack on braze material | Visible crack or discolouration at the braze line; possible separation of carbide tip from steel shank | Catastrophic tool failure if undetected — carbide tip separates in bore | Tool is at end of life — cannot be resharpened; must discard | N/A — scrap tool |
Resharpening Equipment and Grinding Wheel Selection
Resharpening Machine Specifications
| Machine Model | Type | Spindle Power (kW) | Spindle Speed (rpm) | Grinding Wheel Ø Max (mm) | Axes | Typical Drill Ø Range (mm) | Integrated Coolant Filtration | Approximate Cost (USD) |
|---|---|---|---|---|---|---|---|---|
| Precihole TGM 1-32 | Tool and cutter grinder (manual or CNC) | 0.75 | 3000 | 125 | 5 (manual rotary axes + X/Y/Z) | 3–32 | Yes (paper filter, 10 µm) | $25 000–40 000 |
| Precihole TGM 2-32 | CNC tool grinder | 1.5 | 4000 | 150 | 6 (CNC: X, Y, Z, A, B, C) | 3–32 | Yes (paper filter, 10 µm) | $60 000–90 000 |
| Xuetai DRM-32 | CNC gun drill resharpening machine | 1.5 | 5000 (max) | 150 | 6 (CNC) | 3–32 | Yes (centrifugal + paper) | $50 000–80 000 |
| Xuetai DRM-50 | CNC gun drill resharpening machine | 2.2 | 4000 (max) | 200 | 6 (CNC) | 5–50 | Yes (centrifugal + paper) | $70 000–110 000 |
| Guhring DSS 125 | Dual-spindle tool grinder (dedicated gun drill) | 2 × 1.0 | 4500 (max per spindle) | 125 (per spindle) | 6 (CNC, dual independent spindles for rough + finish) | 3–32 | Yes (integrated, 5 µm) | $90 000–140 000 |
| Walter Helitronic / Vollmer | Universal CNC tool grinder | 3–8 | 6000–10 000 | 200–250 | 5–6 axes | 3–50 (with appropriate fixture) | Yes (high-pressure, 5 µm) | $150 000–300 000 |
Grinding Wheel Selection for Gun Drill Resharpening
| Wheel Specification | Abrasive Type | Grit Size | Bond Type | Concentration | Application | Material Removal Rate | Surface Finish Achieved | Wheel Life | Cost per Wheel |
|---|---|---|---|---|---|---|---|---|---|
| Diamond D126 | Synthetic diamond (monocrystalline) | D126 (118–138 µm, equivalent to 120/140 mesh) | Resin bond (phenolic or polyimide) | 75–100 | Rough grinding of carbide tip — initial material removal to remove wear; shaping the primary clearance angles | 1–3 mm³/s per mm of wheel width | Ra 0.8–1.5 µm (on carbide) | 500–1500 resharpenings per wheel | $150–300 |
| Diamond D64 | Synthetic diamond | D64 (50–63 µm, 230/270 mesh) | Resin bond | 100–125 | Finish grinding of carbide cutting edge — final clearance angle and edge preparation | 0.3–0.8 mm³/s per mm | Ra 0.2–0.5 µm | 1000–3000 resharpenings | $180–350 |
| Diamond D46 | Synthetic diamond | D46 (40–50 µm, 325/400 mesh) | Resin or hybrid bond | 100–125 | Fine finish grinding for precision cutting edge geometry; final pass on clearance faces | 0.1–0.3 mm³/s per mm | Ra 0.1–0.3 µm | 1500–4000 resharpenings | $200–400 |
| Diamond D20 | Synthetic diamond | D20 (16–25 µm, 600/700 mesh) | Resin bond | 125 | Ultra-fine finish grinding for PCD-tipped gun drills; final edge finishing | 0.05–0.1 mm³/s per mm | Ra 0.05–0.15 µm | 2000–5000 resharpenings | $250–500 |
| CBN B126 (for HSS drills) | Cubic boron nitride | B126 (118–138 µm) | Resin bond (vitrified for HSS) | 75–100 | Rough grinding of HSS gun drill tips; HSS grinding requires CBN (not diamond) because diamond reacts chemically with HSS at grinding temperatures | 2–5 mm³/s per mm | Ra 0.8–1.5 µm | 800–2000 resharpenings | $200–400 |
| CBN B64 (for HSS drills) | Cubic boron nitride | B64 (50–63 µm) | Vitrified bond | 100 | Finish grinding of HSS cutting edges; vitrified bond provides better shape retention for HSS grinding | 0.5–1.0 mm³/s per mm | Ra 0.2–0.5 µm | 1500–3000 resharpenings | $250–500 |
| Diamond fibre brush | Diamond-impregnated nylon fibres | D64–D20 (grit embedded in nylon filaments) | N/A (nylon brush) | N/A | Cutting edge preparation — edge honing and deburring after resharpening | N/A (edge preparation only) | Edge radius 3–15 µm | 5000–20 000 edges per brush | $50–150 |
Grinding Parameters for Gun Drill Resharpening
| Operation | Wheel Grit | Wheel Speed Vc (m/s) | Infeed per Pass (mm) | Cross Feed (mm/min) | Coolant | Cycle Time per Tool | Remarks |
|---|---|---|---|---|---|---|---|
| Rough grind — carbide tip | D126 (resin bond) | 18–22 | 0.03–0.08 | 200–500 | Water-based emulsion, 3–5% | 30–90 seconds | Maintain consistent infeed to avoid thermal cracking of carbide; reduce infeed for drills < 6 mm diameter |
| Finish grind — carbide clearance | D64 or D46 (resin bond) | 20–25 | 0.005–0.020 | 100–300 | Water-based emulsion, 3–5% | 60–180 seconds | Multiple light passes for final geometry; check angle with optical comparator after each tool |
| Edge honing (deburring) | Diamond fibre brush D64 | 10–15 (brush rotary speed) | 0.05–0.15 mm engagement | 2–5 strokes (manual or CNC) | None (dry) or light oil | 10–30 seconds | Stroke direction: from rake face toward clearance face; 3–15 µm edge radius target |
| Guide pad re-honing | Diamond hand stone D150 (coarse) or D64 (fine) | Manual (hand held) | 0.01–0.02 mm removal per pass | 5–10 strokes (manual) | Light oil for lubrication | 60–120 seconds | Remove transferred workpiece material from guide pad surface; do not change pad geometry |
| Coolant hole cleaning | N/A (not a grinding operation) | N/A | N/A | N/A | Compressed air (6 bar) + wire brush Ø1–3 mm | 10–20 seconds | Verify coolant hole is clear of grinding debris before returning tool to production |
Resharpening Procedure and Quality Control
Step-by-Step Gun Drill Resharpening Procedure
| Step | Operation | Machine Setup | Inspection Checkpoint | Acceptance Criterion | Common Errors |
|---|---|---|---|---|---|
| 1 | Visual inspection and wear assessment | Borescope or optical microscope at 10–20× | Measure flank wear width (VB), crater depth, edge chipping, guide pad condition | Assess if resharpening is feasible — reject if brazed joint cracked, carbide tip < 2 mm remaining length, or coolant hole obstructed | Attempting to resharpen a tool with brazed joint failure — carbide tip may separate during grinding |
| 2 | Coolant hole cleaning | Compressed air (6 bar) + wire brush | Blow compressed air through coolant hole; verify with feeler gauge (wire Ø0.5–1.0 mm) that hole is clear | Coolant hole clear for full length; no obstruction from chips or grinding debris | Missed coolant hole cleaning leads to drilling with restricted coolant flow — rapid tool failure |
| 3 | Outer clearance angle (α₁) grinding — rough | D126 wheel, 18–20 m/s | Optical comparator: measure α₁ relative to drill axis | α₁ = 8–12° (per drill specification); ±0.5° tolerance | Grinding too deep → carbide removal > 0.5 mm; angle incorrect → rubbing or edge chipping |
| 4 | Inner clearance angle (α₂) grinding — rough | D126 wheel, index drill | Optical comparator: measure α₂ relative to drill axis | α₂ = 12–18°; α₂ > α₁ by 3–6°; ±0.5° tolerance | Setting α₂ equal to α₁ → rubbing at centre point; setting α₂ too large → centre point weakness |
| 5 | Secondary clearance (α₃) and front clearance (α₄) grinding | D126 wheel, reposition drill | Visual inspection at 10–20×; confirm clearances are distinct from primary clearance angles | α₃ = 15–25°; α₄ = 20–30°; no intersection of secondary clearance with primary clearance edge | Inadequate secondary clearance → flank rubbing; excessive secondary clearance → unnecessary carbide removal reducing number of resharpenings |
| 6 | Finish grinding — all clearance angles | D64 or D46 wheel, 22–25 m/s | Optical comparator: verify all angles; surface finish inspection on clearance faces | All angles within ±0.3°; clearance face Ra < 0.3 µm; no grinding burn (discolouration) on carbide | Grinding burn (blue/brown discolouration) → thermal damage to carbide reduces tool life |
| 7 | Outer corner restoration | D46 wheel, light pass (0.005 mm) | Optical comparator: verify outer corner radius < 0.02 mm; corner position relative to drill OD | Outer corner restored to sharp edge; corner position at nominal drill diameter ±0.01 mm | Leaving rounded corner → bore undersize, burr formation; over-grinding corner → drill oversize, scrapped tool |
| 8 | Rake face inspection | Optical microscope at 10–20× | Measure crater-to-edge distance; inspect for chips or cracks | Crater-to-edge distance > 0.2 mm; no chips > 0.05 mm on cutting edge | Crater too close to edge (< 0.1 mm) → edge collapse risk; need additional regrind to remove crater zone |
| 9 | Cutting edge preparation (edge honing) | Diamond fibre brush D64; or diamond paste on soft wheel if controlled radius needed | Edge radius measurement (optical comparator at 50–100× or edge replication) | Edge radius 3–15 µm (material-dependent: 3–8 µm for Ti, 8–15 µm for steel); uniform radius along full cutting edge | Excessive honing (> 20 µm radius) → edge blunt → increased cutting forces; inadequate honing (< 2 µm) → edge chipping |
| 10 | Guide pad re-honing | Diamond hand stone D150 (coarse) → D64 (fine) | Visual inspection at 10–20×; surface finish measurement (if available) | Guide pad Ra < 0.2 µm; no transferred workpiece material visible; pad geometry unchanged | Removing pad material (changing pad height) → reduced pad support → bore deviation; must only remove transferred material, not carbide |
| 11 | Final inspection | Optical comparator + microscope | All angles, edge condition, surface finish, drill diameter at outer corner, coolant hole clearance | All parameters per drill specification; diameter within +0/−0.01 mm of nominal; Ra < 0.3 µm on clearance faces | Shipping tool with burrs on cutting edge → bore surface damage on first hole; missed coolant hole obstruction |
| 12 | Tool marking and record-keeping | Laser marker or etching pen | Record: regrind number, material removed (mm), angles measured, inspection results | Regrind count tracked per tool; tool discarded after 8–12 resharpenings or when carbide tip < 3 mm remaining | Losing track of regrind count leads to tools with insufficient tip length cracking in operation |
Quality Verification After Resharpening
| Inspection Parameter | Inspection Method | Instrumentation | Acceptance Criterion | Sampling Frequency | Corrective Action if Non-Conforming |
|---|---|---|---|---|---|
| Outer clearance angle α₁ | Optical comparison (shadow projection) | Optical comparator at 20–50×, with angle measurement overlay | Specified angle ±0.5° | 100% of resharpened tools | Re-grind if > 0.5° deviation; discard tool if deviation > 1.5° (excessive material removal would be required to correct) |
| Inner clearance angle α₂ | Optical comparison | Optical comparator at 20–50× | Specified angle ±0.5°; α₂ > α₁ by 3–6° | 100% | Re-grind; verify angle differential; if α₂ < α₁, regrind is mandatory (centre point will rub) |
| Secondary clearance α₃ | Optical comparison | Optical comparator at 20–50× | 15–25°; clearance depth 0.3–0.8 mm below primary clearance | 100% | Re-grind if inadequate clearance; if clearance depth > 1.0 mm, check remaining carbide length |
| Cutting edge radius | Optical measurement or replication | Comparator at 50–100×; or silicone replica + optical measurement | 3–15 µm (material-dependent) | Statistical sample (1 per 10 tools) | If edge radius > 15 µm: re-hone with lighter pressure; if < 3 µm: apply additional honing pass |
| Drill diameter at outer corner | Micrometer or laser micrometer | Mechanical micrometer (±0.001 mm) or laser scan micrometer (±0.001 mm) | Nominal diameter +0.000/−0.010 mm | 100% | Undersize: discard tool; oversize: re-grind outer corner (one additional light pass on D46 wheel) |
| Clearance face surface finish | Visual comparison (surface roughness comparator) or stylus profilometer | Surface roughness comparator (Ra standards, 0.1–0.8 µm); or stylus profilometer with 2 µm tip | Ra < 0.3 µm on clearance faces | 1 per 10 tools (process validation) | If Ra > 0.3 µm: add finish pass with D46 wheel; verify wheel condition (glazed or loaded wheel causes poor finish) |
| Tool concentricity (tip-to-shank alignment) | Precision V-block + dial indicator | V-block (matched to shank diameter) + dial indicator (±0.001 mm) | Runout < 0.01 mm at tip, measured 5 mm from outer corner | 100% of resharpened tools (mandatory) | If runout > 0.01 mm: check collet / workholding; re-grind tip centred; if runout > 0.02 mm: tool likely bent — discard |
| Coolant hole position | Optical inspection at tip face | Optical comparator at 20×, measure coolant hole centre offset from drill axis | Coolant hole centre within ±0.1 mm of nominal position; hole unobstructed | 100% | If coolant hole offset > 0.1 mm: verify drill has sufficient material to maintain hole integrity; if hole obstructed: clean with wire; if position shifts with resharpening (progressive drift), discard tool after specified max resharpenings |
| Guide pad surface condition | Visual inspection | Optical microscope at 10–20× | No transferred workpiece material; pad Ra < 0.2 µm; no scoring deeper than 5 µm | 100% | Re-hone with diamond stone if transferred material present; if pad scoring > 10 µm deep, tool is near end of life |
| Brazed joint integrity | Visual inspection + dye penetrant (PT) if suspect | Optical microscope at 10–20×; dye penetrant kit | No visible cracks, voids, or discolouration at braze interface | 100% (visual); PT if visual indicates possible crack | If crack detected at braze joint: discard tool — brazed joint cannot be repaired |
FAQ
How many times can a gun drill be resharpened, and what determines its ultimate end of life?
A carbide-tipped gun drill can typically be resharpened 8–15 times before reaching end of life, depending primarily on the original carbide tip length, the amount of material removed per resharpening, and the drill diameter. A new gun drill typically has a carbide tip length of 6–15 mm (measured from the tip face to the steel shank, proportional to drill diameter — approximately 1.0–1.5× the drill diameter for most standard tools). Each resharpening removes 0.3–0.8 mm from the tip face (to clean up flank wear and restore sharp geometry), meaning the carbide tip is consumed by 0.3–0.8 mm per resharpening. With a starting tip length of 10 mm on a Ø12 mm drill and 0.5 mm average removal per resharpening, the theoretical maximum is 20 resharpenings before the tip is completely consumed. In practice, the safe maximum is 8–15 resharpenings because: the braze joint between carbide and steel must maintain at least 1.5–2.0 mm of carbide between the cutting edge and the braze line to prevent heat from the cutting edge from degrading the braze material (braze softens above 400–500°C, while the cutting edge reaches 800–1000°C during drilling); the carbide tip's cross-section must remain sufficient to withstand cutting forces without fracturing (a carbide tip shorter than 2–3 mm is prone to cracking at the braze interface); the coolant hole position drifts toward the cutting edge as the tip is consumed (each resharpening reduces the distance from the coolant hole to the cutting edge by the amount of face removal, and when the hole-to-edge distance falls below 0.5 mm, the wall between the coolant hole and the cutting edge may fracture); and the guide pads also wear and cannot be ground back indefinitely without changing the drill's effective diameter or pad support geometry.
The end-of-life decision for a gun drill is based on five criteria, any one of which triggers retirement: remaining carbide tip length < 3 mm (measured from tip face to braze line); coolant hole wall thickness to cutting edge < 0.5 mm (risk of breakthrough during resharpening or drilling); drill diameter at outer corner more than 0.03 mm undersize from repeated resharpening; guide pad wear exceeding 0.1 mm reduction in pad height (pad height determines drill centring and bore diameter); and visible crack or degradation at the carbide-to-steel braze joint. In production environments, tool life tracking is essential — each gun drill should be serialised (laser-etched on the shank), and a resharpening log maintained with records of material removed per regrind, tool life achieved after each regrind, and reason for final retirement. The most reliable end-of-life indicator is the trend in tool life after resharpening: when the number of bores achieved drops below 60% of the new-tool baseline for two consecutive resharpenings, the tool should be retired regardless of remaining tip length, as further resharpenings will produce diminishing returns. The economic breakeven point between resharpening and replacement is typically at 8–10 resharpenings for standard carbide gun drills ($150–400 each) and 12–15 resharpenings for PCD-tipped or diamond-coated gun drills ($400–1200 each), assuming in-house resharpening cost of $15–30 per regrind. When outsourced resharpening is used ($30–60 per regrind), the economic breakeven decreases to 5–8 resharpenings.
What are the critical differences between resharpening a gun drill and resharpening a conventional twist drill?
Resharpening a gun drill is fundamentally different from resharpening a conventional twist drill in six aspects: geometry complexity, asymmetry, material combination, coolant hole, grinding technique, and quality verification. Gun drill geometry is asymmetrical — the cutting edge is on only one side of the drill, with the opposite side comprising a guide pad (or two guide pads) that do not cut but instead burnish the bore wall and guide the drill along its path. The gun drill has four distinct clearance angles (primary outer, primary inner, secondary shoulder relief, and front clearance) compared to a twist drill's two symmetrical clearance faces. Each gun drill clearance angle has a different function and a different angular range, and all must be ground independently. The asymmetry means the drill must be indexed (rotated to a specific angular position) for each grinding operation — typically requiring 8–12 indexing steps for a complete resharpening, compared to 2–3 steps for a twist drill. The material combination is another critical difference — a carbide-tipped gun drill has a carbide tip brazed to a steel shank. The grinding wheel must be selected for carbide (diamond wheels) for the tip, but the steel shank area (if it contacts the wheel during grinding) requires either CBN wheels or careful avoidance. Grinding a carbide-tipped gun drill with a CBN wheel on the carbide is ineffective (CBN is softer than diamond), and grinding with a diamond wheel on the steel shank causes excessive diamond wheel wear (diamond reacts chemically with steel at high temperature). The operator must ensure that the diamond grinding wheel contacts only the carbide tip, not the steel shank.
The coolant hole in a gun drill is a precision feature located as close as 0.5–1.5 mm from the cutting edge. During resharpening, material is removed from the tip face, reducing the distance from the coolant hole to the fresh cutting edge. The operator must monitor this distance and prevent breakthrough into the coolant hole — if the coolant hole is intersected during grinding, the tool is scrap. This requires precise control of the amount of material removed per resharpening (typically 0.3–0.8 mm) and verification of hole position after each regrind. The grinding technique for gun drills typically uses a five-axis CNC tool grinder or a dedicated gun drill resharpening machine, not the universal tool and cutter grinder commonly used for twist drills. The gun drill's carbide tip geometry is defined by the intersection of multiple ground surfaces at precise angles, and the reference datums (such as the drill axis and the centre line) must be accurately established and maintained across successive resharpenings. Quality verification for resharpened gun drills requires optical comparator measurement of all four clearance angles, drill diameter at the outer corner, coolant hole position, guide pad condition, and cutting edge radius — significantly more parameters than a twist drill's two-angle verification. A resharpened gun drill that meets geometric specifications should achieve 85–95% of new-tool tool life, while a poorly resharpened gun drill — with incorrect clearance angles, burrs on the cutting edge, or an obstructed coolant hole — can produce less than 20% of new tool life and may require early replacement. The higher complexity and quality requirements of gun drill resharpening justify the need for dedicated equipment and trained operators, and explain why many deep hole drilling shops outsource resharpening to specialised tool grinding companies.
What grinding wheel specifications are optimal for resharpening carbide gun drills, and how do they differ for HSS gun drills?
For resharpening carbide gun drills, the optimal grinding wheel specifications are diamond wheels with resin bond and grit sizes that match the operation (rough grinding vs finish grinding). For rough grinding — removing 0.2–0.5 mm of carbide to clean up flank wear — the recommended wheel is D126 grit (118–138 µm, equivalent to 120/140 mesh) with resin bond and concentration 75–100. D126 provides aggressive material removal (1–3 mm³/s per mm of wheel width) while producing a surface finish of Ra 0.8–1.5 µm on the clearance face, which is adequate for roughing. The resin bond provides a balance between wheel wear rate (self-sharpening through bond erosion exposes fresh diamond grit) and shape retention (resin bond wheels maintain their form well enough for clearance angle grinding). For finish grinding — the final pass that establishes the cutting edge geometry and clearance face finish — the recommended wheel is D64 (50–63 µm, 230/270 mesh) or D46 (40–50 µm, 325/400 mesh) with resin bond and concentration 100–125. D46 produces a surface finish of Ra 0.1–0.3 µm on carbide, which is critical for the clearance faces because a smoother clearance face reduces friction against the bore wall and improves chip flow. The higher concentration (100–125) in finish wheels provides more diamond particles per unit volume, producing a more consistent surface finish and longer wheel life between dressings. For PCD-tipped gun drills, which have a polycrystalline diamond cutting edge rather than carbide, an ultra-fine grit wheel of D20 (16–25 µm, 600/700 mesh) is recommended for final edge finishing, as PCD requires a finer finish than carbide to achieve its optimal cutting performance.
For resharpening HSS (high-speed steel) gun drills, diamond wheels must NOT be used — diamond reacts chemically with iron at grinding temperatures (above 400°C), causing rapid diamond graphitisation and wheel wear. Instead, CBN (cubic boron nitride) wheels are used. CBN is the second-hardest abrasive after diamond and is chemically inert toward HSS at grinding temperatures. For rough grinding HSS gun drills, B126 grit (118–138 µm) with vitrified bond is recommended. The vitrified bond provides excellent shape retention and thermal stability for HSS grinding, where the grinding temperature can reach 600–800°C. For finish grinding HSS, B64 grit (50–63 µm) with resin bond or vitrified bond produces surface finish Ra 0.2–0.5 µm on HSS. The grinding wheel speed for both carbide and HSS gun drill resharpening should be 18–25 m/s — higher than conventional twist drill grinding (10–15 m/s) because the small-diameter wheels (125–150 mm) require higher rotational speed to achieve adequate cutting velocity. Coolant is essential for both carbide and HSS grinding — a water-based emulsion at 3–5% concentration, delivered at 10–20 L/min with 10–20 µm filtration. Coolant prevents thermal cracking of the carbide (carbide is sensitive to thermal shock), prevents grinding burn on HSS, and flushes grinding debris from the wheel surface. The coolant must be directed at the wheel-workpiece interface, not just sprayed generally on the tool. Without adequate coolant, diamond wheels glaze (the bond erodes without exposing fresh diamond), CBN wheels load (swarf fills the pores between abrasive grains), and the carbide or HSS surface may develop grinding cracks that propagate during drilling.
What is cutting edge preparation (edge honing) and why is it important for resharpened gun drills?
Cutting edge preparation, also called edge honing or edge radiusing, is the process of applying a controlled micro-radius (typically 3–15 µm) to the sharp cutting edge after resharpening. The purpose of edge honing is to remove the microscopic burrs and grinding marks left by the grinding wheel on the cutting edge, and to create a stable, uniform edge geometry that resists micro-chipping during the initial stages of drilling. A freshly ground cutting edge, viewed under a scanning electron microscope at 500–1000×, is not a perfectly straight line but a series of microscopic peaks and valleys — burrs, grinding striations, and micro-notches — that act as stress raisers. During the first few seconds of drilling, these micro-defects cause localised edge chipping that degrades the cutting edge geometry and reduces tool life by 30–50% compared to a honed edge. Edge honing removes these micro-defects by rounding the edge to a controlled radius, distributing cutting forces over a larger area, and reducing the peak stress at the cutting edge. The optimal edge radius depends on the workpiece material: for titanium alloys (Ti-6Al-4V), which are sensitive to edge bluntness and work-harden easily, a light honing of 3–8 µm radius is optimal — sufficient to remove grinding burrs but not so large as to increase cutting forces. For steel alloys (4140, 4340, stainless steels), a larger radius of 8–15 µm provides better edge stability without excessive force increase. For hardened materials (> 40 HRC), a radius of 10–20 µm may be used, as the higher edge strength is needed to resist the higher cutting forces.
The edge honing methods available for gun drills include: diamond fibre brushes (the most common method — a brush with diamond-impregnated nylon filaments that is passed across the cutting edge, removing 1–5 µm of material and creating a uniform radius of 3–15 µm depending on brush grit, pressure, and stroke count); diamond paste on a soft grinding wheel (a felt or soft resin wheel charged with diamond paste, producing a controlled radius of 5–25 µm); silicone-based abrasive flexible wheels (abrasive-impregnated flexible rubber or silicone wheels that conform to the edge geometry, producing a radius of 5–20 µm); magnetic abrasive finishing (MAF — a developing technology using magnetic fields to guide abrasive particles across the edge, producing very consistent radius of 2–10 µm); and laser edge preparation (a specialised process using a low-energy laser to melt and re-solidify the edge to a controlled radius, used primarily for PCD tools). The diamond fibre brush method is preferred for production resharpening because it is fast (10–30 seconds per tool), does not require a separate machine (can be integrated into the resharpening sequence on the tool grinder), and produces a consistent, repeatable edge radius. The edge radius should be verified after honing using one of three methods: optical comparator measurement at 50–100× (direct measurement of radius on the projected edge profile); replication (silicone replica of the edge, sectioned and measured under optical microscope); or contact profilometry (special stylus with 2 µm tip traced across the edge). The acceptance criterion for resharpened gun drills is an edge radius of 3–15 µm with no burrs, no micro-notches > 5 µm deep, and a uniform radius (±2 µm along the entire cutting edge). The edge honing step is sometimes skipped in shops that outsource resharpening or treat it as optional — this is a false economy, as controlled edge honing typically improves tool life by 30–80% compared to unhoned resharpened tools, and the additional 30 seconds of processing time per tool is negligible.
What is the economic case for in-house gun drill resharpening versus outsourced resharpening versus replacement?
The economic case for in-house resharpening, outsourced resharpening, or replacement depends on four factors: annual tool consumption (number of gun drills used per year), tool cost per unit, resharpening cost per tool, and the number of resharpenings per tool before end of life. The analysis is best expressed as cost per drilling metre (or cost per bore), which levels variations in production volume. For a mid-size deep hole drilling shop using 200 gun drills per year at an average cost of $280 per drill, with each drill capable of 10 resharpenings achieving 90% of new-tool life between resharpenings, the three scenarios compare as follows. Replacement-only strategy: 200 new drills × $280 = $56 000 per year tool cost. Outsourced resharpening (at $45 per regrind, including logistics): 200 drills used initially, then each drill is resharpened 10 times, covering 11 life-equivalents per drill (1 new + 10 × 0.9 = 10 equivalent new lives). Annual tool replenishment = 200 / 10 = 20 new drills at $280 = $5600 (to replace tools that reach end of life). Total annual cost = $5600 (new tool replenishment) + [total annual resharpening count] × $45. Total annual resharpening count = (200 drills × 10 resharpenings) / 10 equivalent lives = 200 resharpenings per year. Total cost = $5600 + (200 × $45) = $14 600 per year. In-house resharpening (capital investment $60 000 for Precihole TGM 2-32, per-sharpen cost $18 for labour + wheel wear + coolant + power): annual new tool replenishment (same 20 drills) = $5600; annual resharpening cost = 200 × $18 = $3600; total annual operating cost = $9200. Capital cost amortised over 5 years = $12 000 per year. Total annual cost (year 1–5) = $9200 + $12 000 = $21 200. Total annual cost (after year 5, fully amortised) = $9200.
The breakeven analysis shows that: replacement-only is the most expensive strategy at all volumes above 50 tools per year; outsourced resharpening is the lowest-cost option in years 1–5 ($14 600 per year versus $21 200 for in-house); in-house resharpening becomes the lowest-cost option after capital amortisation (year 6+: $9200 versus $14 600 for outsourced). The cumulative cost over 10 years for each strategy: replacement = $560 000; outsourced = $146 000; in-house = (5 × $21 200) + (5 × $9200) = $152 000. Outsourced and in-house are nearly equivalent over 10 years, with the choice depending on capital availability and whether the shop has skilled operators to run the resharpening machine. At higher volumes (400+ tools per year), in-house resharpening becomes clearly superior because the operating cost scales linearly with volume while the capital cost is fixed. At lower volumes (< 75 tools per year), outsourced resharpening or even replacement may be more economical, as the in-house capital cost cannot be justified. For shops that already have a universal tool grinder (Walter, Vollmer, or equivalent), the incremental cost of adding gun drill resharpening capability (fixtures, diamond wheels, training) is typically $10 000–20 000, making in-house resharpening economical at any volume above 30 tools per year. The hidden benefit of in-house resharpening that is not captured in this cost analysis is quality control — in-house resharpening allows the shop to maintain tighter control over tool geometry, respond immediately to tool availability needs, and iteratively optimise cutting edge preparation for specific workpiece materials.
The information provided in this article is for general informational purposes only and does not constitute professional engineering advice. Always consult qualified tooling engineers, equipment manufacturers, and grinding wheel suppliers for specific gun drill resharpening applications. Data and parameter recommendations are based on published research and industry experience as of 2026.