A gun drill reground on a manual toolpost grinder is a gamble — the geometry depends on the skill of the grinder operator, the quality of the setup, and the consistency of the grinding wheel. A gun drill reground on a CNC tool and cutter grinder is a precision tool — the geometry is programmed, the wheel path is controlled, and the result is repeatable within ±0.01 mm. The initial cost of CNC grinding is higher, but the consistency eliminates the variability that causes half the tool life problems in deep hole drilling.
Gun Drill Geometry for CNC Programming
Critical Geometry Elements
| Geometry Element | Description | Typical Value | CNC Programming Method |
|---|
| Outer cutting edge angle | Angle of outer cutting edge relative to drill axis | 30–40° | Programmed as angle relative to drill axis — ground on primary clearance face |
| Inner cutting edge angle | Angle of inner cutting edge — typically steeper | 20–30° | Programmed separately — meets outer edge at specified radius |
| Primary clearance angle — outer | Relief behind outer cutting edge | 8–15° | Wheel orientation relative to drill axis |
| Primary clearance angle — inner | Relief behind inner cutting edge | 10–20° | Wheel orientation — often larger than outer clearance |
| Secondary clearance angle | Additional relief behind primary clearance | 15–25° | Second pass with different wheel orientation |
| Chisel edge width | Width of chisel edge at drill center | 0.1–0.5 mm | Programmed width — ground by wheel position relative to center |
| Chip breaker geometry | Width, depth, position of chip breaker | Width: 0.5–3.0 mm — Depth: 0.2–0.8 mm | Ground with shaped wheel — or multiple passes |
| Coolant hole position | Position of coolant hole relative to cutting edge | Specified by drill manufacturer | Must not be damaged during regrind — programmed safe area |
Reference Systems
| Reference | Description | How Established | Tolerance |
|---|
| Drill axis | Centerline of drill body | Align drill body in grinder collet or chuck — use dial indicator | ±0.005 mm runout at support |
| Drill center | Point on drill axis at drill tip | Programmed from drill body reference — or measured by probe | ±0.01 mm |
| Coolant hole position | Location of coolant hole exit at drill point | Critical — must not be damaged — maintain position relative to cutting edge | ±0.1 mm |
| Cutting edge line | Line from drill center to outer edge | Programmed angle from reference plane | ±0.02 mm |
| Margin OD | Outer diameter at the margin | Reference for clearance measurements | ±0.005 mm |
Machine Requirements
CNC Grinder Specifications
| Requirement | Minimum Specification | Recommended Specification | Reason |
|---|
| Axes | 4 axes (X, Y, Z, rotary) | 5 axes (X, Y, Z, rotary, tilt) | 4 axes adequate for standard geometry — 5 axes needed for complex chip breaker and thinning |
| Spindle power | 3 kW | 5–10 kW | Gun drills require moderate stock removal — higher power for larger drills |
| Spindle speed | 3000–6000 RPM | 6000–10000 RPM | Diamond and CBN wheels require high surface speed |
| Wheel diameter | 100–150 mm | 150–200 mm | Larger wheel provides better clearance at the drill center |
| Coolant system | Through-spindle or external flood | Through-spindle + external high-pressure | Grinding coolant essential for heat control and wheel life |
| Workholding | Precision collet — 0.005 mm TIR | Hydraulic or shrink-fit collet — 0.003 mm TIR | Gun drill must be held with minimal runout — runout at the holder = runout at the point |
| Probing system | Not required | Touch probe for drill location | Reduces setup time — ensures consistent reference location |
| Software | Basic tool grinding | Dedicated gun drill grinding package | Reduces programming time — ensures correct geometry |
Grinding Wheel Selection
| Operation | Wheel Type | Grit Size | Bond | Concentration | Surface Speed (m/s) |
|---|
| Primary clearance — carbide | Diamond — resin bond | 200–400 | Resin | 75–100 | 15–25 |
| Primary clearance — HSS | CBN — vitrified bond | 170–270 | Vitrified | 100 | 25–35 |
| Secondary clearance — carbide | Diamond — resin bond | 400–600 | Resin | 75 | 15–25 |
| Secondary clearance — HSS | CBN — vitrified bond | 270–400 | Vitrified | 100 | 25–35 |
| Chip breaker grinding | Diamond — resin bond (shaped wheel) | 200–400 | Resin | 75 | 15–25 |
| Flute polishing (if needed) | Diamond — flexible | 600–1200 | Flexible | — | 10–15 |
| Steel shank — regrind area | Aluminum oxide | 60–120 | Vitrified | — | 25–35 |
Setup Procedure
| Step | Action | Detail | Verification |
|---|
| 1 | Clean gun drill — remove all coolant residue | Solvent clean — inspect tip and shank | No residue — no damage |
| 2 | Mount gun drill in workholding | Insert in collet or chuck — tighten to spec — do not overtighten (can bend drill) | Drill extends from holder by minimum 30 mm beyond the area to be ground |
| 3 | Align drill — concentricity | Dial indicator at two positions: near holder and near tip — adjust until both < 0.005 mm TIR | Runout at tip < 0.005 mm after all adjustments |
| 4 | Establish reference — Z-axis zero | Touch wheel to drill tip — set Z = 0 | Reference established |
| 5 | Establish reference — X-axis zero | Touch wheel to drill OD — set X = 0 | Reference established |
| 6 | Measure coolant hole position | Visual inspection or probe — compare to program assumed position | Coolant hole position within ±0.1 mm of program assumption |
| 7 | Dress grinding wheel | Diamond dresser — ensure wheel face is true and sharp | No loading — no glazing — sharp cutting surface |
| 8 | Load grinding program | Select program for drill diameter — material — geometry | Program matches drill specifications |
| 9 | Dry run (first drill only) | Run program without wheel contact — verify clearances | No interference — all moves clear by minimum 0.5 mm |
| 10 | Grind primary clearance | First grinding pass — per program parameters | Visual — adequate clearance — no burning |
| 11 | Grind secondary clearance | Second grinding pass | Visual — adequate clearance — smooth transition |
| 12 | Grind chip breaker (if required) | Separate program or integrated step | Chip breaker dimensions within ±0.05 mm |
| 13 | Inspect ground point | Offline measurement — optical comparator | All geometry within tolerances (see quality acceptance criteria) |
| 14 | Deburr if necessary | Fine diamond stone — hand deburr only | No burrs on cutting edges — no damage to ground surfaces |
Grinding Parameters
| Parameter | Carbide — Roughing | Carbide — Finishing | HSS — Roughing | HSS — Finishing |
|---|
| Wheel speed | 15–20 m/s | 20–25 m/s | 25–30 m/s | 30–35 m/s |
| Feed rate (roughing) | 0.5–1.0 mm/s | — | 1.0–2.0 mm/s | — |
| Feed rate (finishing) | — | 0.2–0.5 mm/s | — | 0.5–1.0 mm/s |
| Depth of cut (roughing) | 0.02–0.05 mm | — | 0.05–0.10 mm | — |
| Depth of cut (finishing) | — | 0.005–0.015 mm | — | 0.01–0.03 mm |
| Spark-out passes | 1–2 | 2–3 | 1 | 2 |
| Grinding coolant | Water-based 5–7% — high pressure | Same | Same | Same |
Inspection and Quality Acceptance
Geometry Verification
| Inspection Item | Measurement Method | Acceptance Criteria | Frequency |
|---|
| Outer cutting edge angle | Optical comparator — 20× | Within ±0.5° of specified angle | Every regrind |
| Inner cutting edge angle | Optical comparator | Within ±0.5° of specified angle | Every regrind |
| Primary clearance angle — outer | Optical comparator — or clearance gauge | Within ±1° of specified angle | Every regrind |
| Primary clearance angle — inner | Optical comparator | Within ±1° of specified angle | Every regrind |
| Secondary clearance angle | Optical comparator | Within ±1° — smooth transition from primary | Every regrind |
| Chisel edge width | Optical comparator — 50× | Within ±0.03 mm of specified width | Every regrind |
| Cutting edge condition | Microscope — 50–100× | No chipping — edge radius 5–15 µm | Every regrind |
| Coolant hole position | Optical comparator — or visual at 20× | Not damaged — opening fully exposed — within ±0.1 mm of intended position | Every regrind |
| Point symmetry | Optical comparator — compare left and right edges | Left and right edges within ±0.02 mm of each other | Every regrind |
| Surface finish — ground faces | Profilometer — or visual comparison | Ra < 0.4 µm — uniform grind pattern | Sample basis — setup verification |
Regrind Documentation
| Data | Recorded Value | Used For |
|---|
| Drill ID | Serial number — identification | Traceability |
| Drill diameter | mm | Size verification |
| Drill length before regrind | mm | Tracking regrind life |
| Drill length after regrind | mm | Stock removal per regrind |
| Material removal per face | mm | Regrind consistency |
| Grinding program used | Program number | Setup repeatability |
| Wheel used | Wheel specification — dressing frequency | Process control |
| Operator | Name — initials | Accountability |
| Inspection results | All geometry measurements | Quality verification |
| Acceptance — pass/fail | Pass — or rework — or scrap | Disposition |
Common Regrinding Errors
| Error | Symptom | Cause | Prevention |
|---|
| Asymmetric point | One cutting edge longer than the other | Drill not centered in workholding — reference zero not centered | Verify drill alignment ≤ 0.005 mm TIR before grinding — check center reference |
| Coolant hole damaged | Coolant hole partially ground away | Grinding wheel path too close to coolant hole — incorrect reference | Verify coolant hole position — adjust program if needed — use safe clearance |
| Burr on cutting edge | Raised edge — visible burr at 20× | Wheel too dull — feed too high — no spark-out | Dress wheel — reduce finishing feed — add spark-out passes |
| Burning — discoloration | Blue/brown discoloration on ground surface | Feed too high — depth too high — coolant inadequate | Reduce feed or depth — increase coolant flow — check wheel condition |
| Chisel edge too wide | Chisel edge exceeds specified width | Wheel did not reach drill center — program error | Verify wheel path reaches center — check program coordinates |
| Chisel edge too narrow | Chisel edge missing — center of drill has gap | Wheel over-traveled past center | Reduce wheel path at center — check program |
| Clearance angle incorrect | Cutting edge rubs — poor hole quality | Wheel orientation incorrect — program error | Verify wheel orientation setup — check program |
| Chip breaker out of position | Chip breaker not aligned with cutting edge | Program error — reference shift | Verify chip breaker position in program — check reference |
| Excessive stock removal | Drill length reduced too much | Too many regrinds — depth of cut too high | Track drill length — limit regrinds to specified maximum |
FAQ
What are the advantages of CNC regrinding over manual regrinding for gun drills?
The advantages of CNC regrinding over manual regrinding for gun drills are: consistency — a CNC grinder produces the same geometry on every regrind — the point angles, clearance angles, and chip breaker are identical within ±0.01 mm and ±0.5° from one regrind to the next. Manual grinding produces different geometry every time — the operator's technique, wheel condition, and visual judgment vary. This consistency translates directly to drilling performance: the same feed rate, the same hole quality, and the same tool life from every reground drill. Repeatability — a CNC-ground drill that produces good results can be set up again with the same program and produce the same results — a manual-ground drill that produced good results may not produce the same results on the next regrind because the geometry will be slightly different. Complex geometry capability — CNC grinders can produce complex chip breaker geometries, multi-angle clearance faces, and precise point thinning that are difficult or impossible to produce consistently on a manual grinder. Reduced operator skill requirement — CNC grinding does not depend on the operator's visual judgment and manual dexterity — the operator must set up the machine correctly and verify the results — but the grinding itself is automated. Documentation and traceability — CNC programs can be stored and recalled — regrind parameters are documented — every reground drill can be traced to its program and inspection results. The disadvantages of CNC regrinding: higher initial cost (CNC tool grinder costs $50,000–200,000+ vs $2,000–5,000 for a manual toolpost grinder). Higher setup time (5–15 minutes for CNC setup vs 2–5 minutes for manual setup — but the grinding itself is faster and more consistent). For a shop that regrinds fewer than 10–20 gun drills per week, the manual grinder may be more cost-effective — the consistency advantage of CNC is real but may not justify the investment for low volumes.
To set up a gun drill in a CNC tool grinder for regrinding: clean the gun drill thoroughly — remove all coolant residue, chip debris, and any grinding swarf from the previous regrind — dirty drills cause alignment errors and grinding wheel loading. Mount the drill in a precision collet or hydraulic chuck — use the highest precision workholding available (hydraulic or shrink-fit chucks provide the best concentricity — precision collets (ER collets or equivalent) are adequate — standard drill chucks are not acceptable for gun drill regrinding). Align the drill for concentricity — mount a dial indicator against the drill body near the workholding — adjust until runout is less than 0.005 mm TIR — then check the runout near the drill tip — it should also be less than 0.005 mm TIR. If the tip runout exceeds the near-holder runout, the drill is bent or the workholding is misaligned. Establish the reference positions — touch the wheel to the drill tip to set the Z-axis zero (this establishes the drill tip position for all geometry calculations). Touch the wheel to the drill OD near the tip to set the X-axis zero (this establishes the drill centerline). Verify the coolant hole position — visually inspect the coolant hole at the drill point — compare its position to the programmed coolant hole location — if the actual position differs from the program assumption by more than 0.1 mm, adjust the program or use a different program that matches the drill. Select and load the grinding program — the program must match the drill diameter, carbide grade, and specified point geometry. Dry run the program on the first drill of a batch — verify no interference between the wheel and the drill — check that all wheel movements stay within the safe area and do not approach the coolant hole. Correct setup is the most critical step in CNC gun drill regrinding — 80% of regrind quality problems originate from incorrect setup (misalignment, wrong reference, or wrong program selection).
How many times can a gun drill be reground?
The number of times a gun drill can be reground depends on: drill diameter — small diameter drills (under 10 mm) can typically be reground 3–8 times — medium diameter drills (10–25 mm) can be reground 5–15 times — large diameter drills (over 25 mm) can be reground 10–25 times. The larger the drill, the more material is available for regrinding. Stock removal per regrind — typical stock removal is 0.1–0.3 mm per face per regrind — if the removal is at the low end of this range (consistent with CNC grinding), more regrinds are possible — if removal is at the high end (more typical of manual grinding), fewer regrinds are possible. Drill length reduction — each regrind shortens the drill by the stock removal amount (0.1–0.3 mm × 2 faces = 0.2–0.6 mm total length reduction). The drill can be reground until the remaining length is insufficient to reach the required hole depth — or until the drill body at the point becomes too thin to support the cutting forces. Coolant hole proximity — each regrind moves the cutting edge closer to the coolant hole — when the distance from the cutting edge to the coolant hole is less than 0.5 mm, the next regrind may break into the coolant hole — at this point, the drill must be retired. Web thickness at point — as the drill is reground, the web thickness at the point decreases — when the web becomes too thin (typically less than 10% of drill diameter at the point), the drill may crack or break at the center during drilling. Tracking regrind count — maintain a regrind log for each drill — record the drill length before and after each regrind — when the length has been reduced by 3–6 mm (depending on drill diameter), inspect the coolant hole distance and web thickness at the point to determine if further regrinds are possible. The rule: stop regrinding when the coolant hole-to-edge distance is less than 0.5 mm — or when the web thickness at the point is less than 10% of the drill diameter — or when the drill is too short to reach the required hole depth.
What inspection is required after CNC regrinding a gun drill?
Required inspection after CNC regrinding a gun drill: point geometry measurement using an optical comparator — the most important inspection. Place the reground drill in the comparator at 20–50× magnification — measure: outer cutting edge angle (target ±0.5° of specified angle), inner cutting edge angle (±0.5°), primary clearance angle (visual check — or measure with clearance gauge), chisel edge width (±0.03 mm of specified width), and cutting edge symmetry (left and right edges should be within ±0.02 mm of each other). Cutting edge condition — inspect under a microscope at 50–100× magnification — verify: no edge chipping (any chip larger than 0.01 mm is cause for rejection — the chip will propagate during drilling), edge radius 5–15 µm (sharper edges are acceptable but more fragile — duller edges increase cutting forces), no burr on the cutting edge (a burr indicates the wheel is too dull or the finishing pass was inadequate). Coolant hole condition — verify: the coolant hole is not damaged (no part of the hole should be ground away — if the grinding wheel contacted the coolant hole edge, the coolant flow pattern will be disrupted — the drill should be rejected if the coolant hole is damaged), the coolant hole is fully open (no grinding debris blocking the hole — clear with a wire if needed). Surface finish — ground faces should have uniform finish with no burn marks (discoloration indicates thermal damage — the carbide may have micro-cracks — reject if burn is visible). Chisel edge condition — the chisel edge should be centered and symmetric (a chisel edge that is off-center will cause the drill to wander at entry — producing an oversize hole or poor hole start). Document all measurements on the regrind record — pass/fail decision based on all criteria — a drill that fails any inspection should not be used for production drilling.
Can CNC regrinding correct damage to a gun drill point?
CNC regrinding can correct most damage to a gun drill point — provided the damage is not too extensive. Damage that can be corrected by regrinding: chipped cutting edge (a chip or fracture on the cutting edge — the damaged area is removed by the regrind — provided the chip depth is less than the stock removal for the regrind (typically 0.1–0.3 mm per face). If the chip is deeper than the stock removal, the drill may need additional grinding passes to remove the damaged zone — this reduces the total number of regrinds available from the drill. Moderate edge wear (worn cutting edge with wear land up to 0.3 mm — a standard regrind removes the wear land and restores the original geometry — if the wear land exceeds 0.3 mm, additional stock removal may be needed). Built-up edge (material welded to the cutting edge — the regrind removes the BUE and the damaged surface beneath it — ensure all BUE is removed before the regrind is accepted). Minor chisel edge damage (minor chipping or deformation at the chisel edge — the regrind removes damaged material and restores the chisel edge width). Damage that may NOT be correctable by regrinding: coolant hole damage (if the coolant hole has been broken into by a previous incorrect regrind — the coolant flow pattern is disrupted — the drill should be retired — regrinding cannot restore the coolant hole position). Extensive edge chipping (chips deeper than 0.5 mm — the stock removal needed to clear the damage may reduce the web thickness below the safe minimum — check web thickness after regrinding — if web thickness is less than 10% of drill diameter, the drill should be retired). Cracked carbide tip (a crack in the carbide tip cannot be removed by regrinding — the crack will propagate during drilling — the drill must be replaced). Bent drill shank (a bent drill cannot be corrected by regrinding — the bend will cause runout at the point — replace the drill). The general rule: if the damage is within the normal stock removal range (0.1–0.3 mm per face), a CNC regrind can correct it. If the damage requires removal of more than 0.5 mm per face, the drill may be better retired — the risk of reduced tool life or drill failure outweighs the cost of a new drill.
CNC tool and cutter grinder regrinding produces gun drill points with consistent, repeatable geometry — the same angles, clearances, and chip breaker on every regrind. This consistency translates directly to predictable drilling performance, consistent hole quality, and maximum tool life. Set up the drill with concentricity within 0.005 mm TIR — select the correct grinding program for the drill diameter and geometry — verify all reference positions before grinding. Inspect every reground drill on an optical comparator — measure cutting edge angles, chisel edge width, clearance angles, and edge condition — document all measurements. Reject drills with coolant hole damage, cracks, or edge chipping that cannot be removed by normal stock removal. CNC regrinding is an investment in drilling consistency — the cost per regrind is higher than manual, but the value of consistent performance far outweighs the incremental cost. This article reflects industry practice as of 2026.
