Appearance
A manufacturer of automotive transmission shafts (Ø12 mm × 500 mm gun-drilled bores in SAE 8620H steel, 25,000 parts per year) was regrinding gun drills in-house on a standard universal tool and cutter grinder with manual indexing. The regrind process produced inconsistent geometry: point angle variation of ±5° (versus ±1° specification), gash depth variation of ±0.3 mm (versus ±0.05 mm), and edge hone variation of ±15 µm (versus ±5 µm). First-pass bore rejection from reground tools was 12% (versus 2% for new tools), and reground tool life was only 45% of new tool life (180 m versus 400 m). An audit revealed that the grinding wheel was dressed only once per 15 tools (versus the recommended once per 3–5 tools), the wheel grade was too hard (H versus J grade for carbide), and the operator lacked training on gun drill geometry. The company invested in a dedicated CNC gun drill grinding machine (€85,000) with automatic wheel dressing, in-process optical geometry measurement, and a geometry database for 24 drill sizes. After implementation: point angle variation ±1.5°, gash depth variation ±0.03 mm, edge hone variation ±3 µm, first-pass rejection from reground tools dropped to 2.5%, reground tool life increased to 85% of new (340 m versus 400 m), and annual tooling cost decreased by 58% (from €42,000 to €17,600).
Gun Drill Regrinding
Gun Drill Geometry Elements
| Geometry Element | Typical Range | Effect on Performance | Measurement Method | Tolerance |
|---|---|---|---|---|
| Point angle (outer angle) | 20–40° (steel), 15–25° (aluminum), 30–50° (hardened steel) | Determines chip formation and cutting force direction; lower angle reduces thrust force | Optical comparator or CMM at 20–50× | ±1° (precision), ±2° (production) |
| Inner angle (gash angle) | 15–25° relative to axis | Controls chip flow into flute; affects chip curling and evacuation | Optical comparator | ±1° |
| Clearance angle (primary) | 8–15° (steel), 6–10° (aluminum), 10–18° (hardened steel) | Prevents rubbing on flank face; too low causes wear; too high weakens cutting edge | Optical comparator or protractor | ±1° |
| Clearance angle (secondary) | 20–30° | Provides clearance behind primary land; prevents heel drag | Optical comparator | ±2° |
| Edge hone (K-land) | 5–30 µm (steel), 2–10 µm (aluminum), 10–40 µm (hardened steel) | Edge strength vs sharpness trade-off; larger hone for interrupted cuts | White light interferometer or optical comparator at 200× | ±5 µm (precision), ±10 µm (production) |
| Chip breaker (gash) depth | 0.3–1.5 mm (depends on feed rate and material) | Controls chip curl and break length; too shallow = long chips, too deep = weak edge | Depth micrometer or optical measurement | ±0.05 mm |
| Chip breaker width | 0.5–2.0 mm | Matches expected chip width | Optical measurement | ±0.10 mm |
| Flute geometry (radius, depth, helix) | Flute radius 40–60% of drill diameter | Chip evacuation capacity; must match chip volume | Profile projector or cast replica | ±0.10 mm |
Regrind Sequence
| Step | Operation | Equipment | Quality Check | Typical Stock Removal |
|---|---|---|---|---|
| 1 | Visual inspection and cleaning | Magnifying lamp, solvent bath | Identify damage (chipping, fracture, thermal cracks) | N/A |
| 2 | Flank face regrind (primary clearance) | CNC gun drill grinder or tool and cutter grinder with optical positioning | Point angle, clearance angle, edge condition (200× inspection) | 0.10–0.30 mm |
| 3 | Flank face regrind (secondary clearance) | Same setup as step 2 | Clearance angle, step between primary and secondary | 0.05–0.15 mm |
| 4 | Gash (chip breaker) regrind | CNC grinder with formed wheel or manual with fixture | Depth, width, position relative to cutting edge | 0.05–0.20 mm |
| 5 | Edge hone preparation | Diamond paste or fine wheel (600–1,200 grit) | Edge radius measurement at 200× | 2–10 µm removal |
| 6 | Flute polish (if needed) | Rubber bonded wheel or abrasive flow | Chip flow test; surface finish of flute < Ra 0.4 µm | 1–5 µm (polish only) |
| 7 | Coating removal (if re-coating) | Chemical stripping (caustic or acid) or mechanical (fine blasting) | Complete coating removal verified by 50× inspection | 5–15 µm (coating thickness) |
| 8 | Coating application (if re-coating) | PVD or CVD per coating type | Coating thickness, adhesion (Rockwell indentation test), coverage | 2–8 µm (PVD), 10–30 µm (CVD diamond) |
| 9 | Final inspection | Optical comparator, microscope at 200×, coordinate measurement | All geometry parameters within tolerance | N/A |
Grinding Wheel Selection for Gun Drill Regrinding
| Tool Material | Wheel Material | Wheel Grade | Grit Size | Bond | Coolant | Dressing Frequency |
|---|---|---|---|---|---|---|
| Carbide (tungsten carbide, 6–12% Co) | Diamond (resin bond) | J–K (medium-soft) | 400–800 grit (roughing), 1,000–2,000 (finishing) | Resin (for carbide); avoid vitrified bond for carbide | Water-miscible coolant at 5–8% concentration | Every 3–5 tools (roughing); every 5–10 tools (finishing) |
| Carbide + PCD (polycrystalline diamond) | Diamond (metal bond) | L–M (medium) | 200–400 grit (roughing), 600–1,000 (finishing) | Metal or vitrified | Water-miscible coolant; PCD requires lower temperature | Every 1–2 tools (roughing); every 3–5 tools (finishing) |
| High-speed steel (HSS) | CBN (cubic boron nitride) | J–K | 400–800 grit | Vitrified or resin | Straight oil or water-miscible (5–8%) | Every 5–10 tools |
| Cermet | Diamond (resin bond) | I–J (soft) | 600–1,200 grit | Resin | Water-miscible coolant | Every 3–5 tools |
BTA Head Reconditioning
BTA Head Components and Reconditioning
| BTA Head Component | Reconditioning Method | New Part Cost | Reconditioned Cost | Reconditioning Interval | Life Expectancy |
|---|---|---|---|---|---|
| Carbide insert (indexable) | Replace with new insert; no reconditioning | €5–25 per insert | N/A (replace only) | Every edge: 1–4 insert indexes per head | Per edge: 50–300 m (steel) |
| Insert pocket/seat | Clean and inspect; recondition if damaged | N/A (head body) | €15–40 per seat (laser cladding + machining) | Every 100–500 insert changes (or when seat damaged) | 1,000–5,000 inserts (head body life) |
| Guide pads (carbide) | Replace with new pads; grind OD to size after brazing | €15–60 per pad | N/A (replace only) | Every 20–100 bores (depending on pad wear) | 20–200 bores per set |
| Guide pad seats | Clean, inspect for galling; recondition if damaged | N/A (head body) | €20–50 per seat (grind + shim for position) | Every 5–10 pad sets (or when seat galling observed) | 500–2,000 pads (head body life) |
| Coolant holes | Clean with wire brush or abrasive flow | N/A | €5–15 per hole (cleaning) | Every insert change | N/A |
| Thread connections (BTA tube thread) | Inspect with thread gauge; replace or recut if damaged | N/A | €30–80 (thread recutting) | Every 50–200 insert changes | 200–1,000 changes (thread life) |
BTA Head Inspection Criteria
| Inspection Item | Method | Acceptance Criteria | Reject Criteria |
|---|---|---|---|
| Insert seat flatness | Surface plate with feeler gauge or CMM | < 0.010 mm deviation | > 0.020 mm deviation |
| Insert seat position (radial and axial) | CMM or optical comparator | ±0.025 mm from drawing position | > 0.050 mm deviation |
| Guide pad seat clearance (for brazed pads) | Feeler gauge | 0.02–0.05 mm gap for braze material | > 0.10 mm gap or < 0.01 mm (no braze flow) |
| Guide pad position (diameter and angle) | CMM or fixture with indicator | ±0.020 mm on diameter; ±0.5° on angle | > 0.050 mm or > 1.0° deviation |
| Coolant hole condition | Visual (borescope for internal holes) | Clean, no blockage, no edge damage | Blockage > 20% of cross-section; edge crumbling |
| Head body hardness | Hardness tester (Rockwell C) | HRC 55–62 (typical for head body) | < HRC 50 (wear accelerates) |
| Thread condition | Thread gauge (go/no-go) | Go gauge passes; no-go gauge stops within 2 turns | Go gauge fails; no-go gauge passes fully |
Economic Analysis
Regrinding vs New Tool Cost Comparison
| Tool Type | New Tool Cost | Regrind Cost (in-house) | Regrind Cost (outsourced) | Regrind Life (% of new) | Break-Even Regrinds |
|---|---|---|---|---|---|
| Gun drill, Ø5 mm × 300 mm | €85 | €12–20 | €25–40 | 75–90% | 3–5 regrinds |
| Gun drill, Ø12 mm × 500 mm | €140 | €18–30 | €40–60 | 75–90% | 4–6 regrinds |
| Gun drill, Ø20 mm × 1,000 mm | €250 | €25–45 | €55–85 | 70–85% | 4–7 regrinds |
| Gun drill, Ø35 mm × 1,500 mm | €500 | €40–70 | €90–140 | 70–85% | 5–8 regrinds |
| BTA head, Ø50 mm (3 inserts) | €350 | N/A (inserts replaceable) | €80–150 (seat recondition + pads) | 100% (with new inserts) | 5–10 reconditions |
| BTA head, Ø80 mm (4 inserts + 4 pads) | €650 | N/A | €150–280 (full recondition) | 100% (with new inserts/pads) | 5–10 reconditions |
| BTA head, Ø150 mm (5 inserts + 6 pads) | €1,200 | N/A | €280–500 (full recondition) | 100% (with new inserts/pads) | 5–10 reconditions |
Tooling Cost Per Hole: New vs Reground
| Scenario | Parts Per Year | New Tool Cost Per Hole | Reground Tool Cost Per Hole | Annual Savings | ROI Period for CNC Grinder (€85,000) |
|---|---|---|---|---|---|
| Ø5 mm × 300 mm, steel | 50,000 | €0.42 | €0.18 | €12,000 | 7.1 years |
| Ø12 mm × 500 mm, steel | 25,000 | €0.70 | €0.22 | €12,000 | 7.1 years |
| Ø12 mm × 500 mm, steel | 100,000 | €0.70 | €0.22 | €48,000 | 1.8 years |
| Ø20 mm × 1,000 mm, steel | 10,000 | €1.25 | €0.40 | €8,500 | 10.0 years |
| Ø20 mm × 1,000 mm, steel | 50,000 | €1.25 | €0.40 | €42,500 | 2.0 years |
| Ø35 mm × 1,500 mm, steel | 5,000 | €2.50 | €0.75 | €8,750 | 9.7 years |
| Ø35 mm × 1,500 mm, steel | 20,000 | €2.50 | €0.75 | €35,000 | 2.4 years |
FAQ
How many times can a gun drill be reground?
The number of possible regrinds for a gun drill depends on the drill diameter and the stock removal per regrind. A typical gun drill can be reground 4–8 times before the tool becomes too short for the application or the carbide tip is consumed. The limit is reached when: the total length reduction (approximately 0.3–0.5 mm per regrind for the cutting tip) exceeds the available carbide tip length (typically 3–8 mm for standard gun drills), or the overall tool length has been reduced by more than 3–5 mm, which may cause the tool to no longer reach the required bore depth or may change the flute chip evacuation characteristics. The practical rule is: small-diameter gun drills (Ø3–10 mm) can be reground 3–5 times because the carbide tip is shorter (3–5 mm); medium-diameter gun drills (Ø10–25 mm) can be reground 5–8 times (carbide tip 5–10 mm); large-diameter gun drills (Ø25–40 mm) can be reground 6–10 times (carbide tip 8–15 mm). The quality of the regrind also affects the number of regrinds — a regrind that removes more material than necessary (over-grinding) reduces the number of possible regrinds. Using a CNC gun drill grinder with precise stock removal control (typically 0.10–0.30 mm per regrind on the flank face) maximizes the number of regrinds achievable.
What is the most common mistake in gun drill regrinding?
The most common mistake is incorrect edge hone (K-land) preparation — either grinding the edge too sharp (no hone, or hone < 2 µm) or too heavy (hone > 30 µm for steel). An edge that is too sharp chips immediately on the first cut, particularly in steel with hardness above 25 HRC or in interrupted cuts. An edge that is too heavy (excessive hone) increases the cutting forces by 20–50%, causing higher thrust force, increased cutting temperature, and reduced tool life (40–60% of the life of a correctly honed tool). The correct edge hone for gun drilling steel is 10–20 µm (depending on hardness — harder steel requires a larger hone). The second most common mistake is incorrect point angle — a point angle that is too low for the material (e.g., 20° for hardened steel) causes the cutting edge to be too fragile, while a point angle that is too high (e.g., 40° for aluminum) increases thrust force excessively, causing bore straightness deviation. The third most common mistake is grinding burn — using a wheel that is too hard or a feed rate that is too high generates heat that can cause thermal cracks in the carbide tip (visible as fine cracks perpendicular to the cutting edge when examined at 50–100× magnification). Tools with grinding burn have 30–60% of expected tool life and should be rejected at inspection.
Can BTA heads be reconditioned, or should they be replaced entirely?
BTA heads can and should be reconditioned — the head body is a precision-machined component that costs 2–5× the cost of the wear parts (inserts and guide pads) and can last through 5–10 reconditioning cycles with proper maintenance. The reconditioning process for BTA heads includes: remove all worn inserts and guide pads; clean the head body in solvent and inspect all surfaces; inspect insert seats for damage (galling, deformation, erosion) — minor damage can be repaired by laser cladding and re-machining; inspect guide pad seats for galling and dimensional accuracy; repair or replace any damaged coolant holes; inspect thread connections (where the head attaches to the drill tube); install new inserts (indexable, no machining required) or braze new guide pads (requires OD grinding after brazing); and verify all dimensions (insert position, pad position, coolant hole alignment) on a CMM. The cost of a full BTA head reconditioning (new inserts, new guide pads, seat inspection and repair, thread inspection, and CMM verification) is typically 30–50% of the cost of a new head. The head body should be replaced when: thread damage cannot be repaired; insert seats are damaged beyond repair (galling has removed more than 0.2 mm of material); head body hardness has dropped below HRC 50 (from overtempering during brazing); or the head body has been reconditioned more than 10 times.
What inspection equipment is needed for in-house tool regrinding?
The minimum inspection equipment for quality tool regrinding includes: optical comparator (20–50× magnification, with X-Y digital readout or CNC measurement) for measuring point angle, clearance angle, gash geometry, and edge condition; stereo microscope (10–100× magnification) for edge quality inspection and damage detection; edge radius measurement system (white light interferometer or optical comparator at 200×, or replication method with measurement microscope) for edge hone verification; surface roughness tester (profilometer) for checking flute surface finish and flank face finish; coordinate measuring machine (CMM) or tool presetter for measuring overall tool dimensions, tip concentricity, and drill tube straightness; grinding wheel profiler or diamond roll dresser for maintaining wheel geometry; and coating thickness gauge (calotest or XRF) for verifying coating thickness on re-coated tools. The total investment for a basic inspection setup (optical comparator, microscope, profilometer, and tool presetter) is approximately €30,000–60,000 — a necessary investment for any company regrinding more than 500 tools per year in-house.
How do I decide between in-house regrinding and outsourcing?
The decision between in-house and outsourced regrinding depends on five factors: volume — companies regrinding more than 500–1,000 gun drills per year (or 200–500 BTA heads) should consider in-house regrinding, as the volume justifies the equipment investment; tool variety — companies using 10+ different tool sizes benefit from in-house regrinding (faster turnaround, no tool shipping, better control over each tool's regrind history); quality requirements — companies with tight tolerance requirements (IT6–IT7) benefit from in-house control over geometry and edge preparation; turnaround time — in-house regrinding allows same-day turnaround versus 3–10 days for outsourcing; and capital availability — the equipment investment for a CNC gun drill grinder (€40,000–120,000) plus inspection equipment (€30,000–60,000) and tooling (€5,000–15,000) requires capital that smaller companies may prefer to avoid. For volumes below 500 tools per year, outsourcing is typically more economical — the annual cost of outsourced regrinding at €25–60 per tool is €12,500–60,000, which is comparable to the annualized cost of in-house equipment (depreciation + labor + consumables) at 500–1,000 tools per year. For volumes above 1,000 tools per year, in-house regrinding is almost always more economical.
Disclaimer: The regrinding procedures, geometry specifications, and cost data presented in this article are based on published technical literature, tool manufacturer recommendations, and industry-reported experience with deep hole drilling tool reconditioning. Actual regrind quality depends on equipment condition, operator skill, grinding wheel selection, and coolant condition. Tool geometry should be selected based on the specific workpiece material, machine tool, and application requirements — the values provided are guidelines only. Coating removal and reapplication should be performed by qualified coating service providers with experience in the specific coating type. No guarantee of specific reground tool life, cost savings, or process performance is expressed or implied. All data is provided for informational purposes and reflects industry practices as of 2026.