Gun Drill Regrinding and Recoating Best Practices

Gun drills are precision tools with a significant upfront cost. Proper regrinding and recoating extends their useful life across multiple cycles, reducing cost per hole and maintaining consistent hole quality. This guide covers when to regrind, how many regrinds each drill type can accept, geometry restoration, coating considerations, and the economics of regrinding vs replacement.

When to Regrind

Regrinding too early wastes useful tool life. Regrinding too late risks poor hole quality or tool breakage. The optimal point is when tool wear reaches measurable thresholds but before damage occurs.

Wear Land Measurement

The flank wear land (VB) is the most reliable indicator for regrind timing:

Drill Type	Regrind at VB (mm)	Replace at VB (mm)
Brazed tip gun drill	0.15–0.20	0.30
Solid carbide gun drill	0.10–0.15	0.25
Indexable gun drill	N/A (change insert)	—
PCD-tipped gun drill	0.10	0.15

Signs Regrinding Is Needed

Symptom	Likely Wear Condition
Surface finish degrades (Ra increasing > 50% of baseline)	Flank wear > 0.15 mm
Hole diameter trending toward upper tolerance limit	Wear on the outer corner
Cutting edge chipping visible (under 10× loupe)	Edge micro-chipping — regrind immediately
Spindle load increasing (10–15% above baseline)	Flank wear or BUE on cutting edge
Exit burr size increasing	Edge dulling
Chip shape changing (from C-shape to longer curling chips)	Cutting edge geometry degraded

Measurement Method

Clean the drill tip with solvent
Use a toolmaker’s microscope or 20× optical comparator
Measure the flank wear land at the outer corner (most critical), the nose, and along the cutting edge
Record three measurements: VB_max, VB_avg, and distance from corner
Regrind when VB_max exceeds the thresholds above

How Many Regrinds Per Drill

The number of regrinds a gun drill can accept depends on the drill type and the amount of material removed per regrind.

Drill Type	Typical Regrinds	Notes
Brazed tip	3–5 regrinds	Carbide tip length limits total regrinds. Each regrind removes 0.15–0.25 mm from the face
Solid carbide	7–10 regrinds	Longer carbide section allows more regrinds. Each regrind removes 0.10–0.20 mm
PCD-tipped	2–3 regrinds	PCD layer is thin (~0.5 mm). Regrind removes 0.05–0.10 mm
Indexable (insert type)	N/A	Replace inserts, not the tool body

Material Removal Per Regrind

Regrind Pass	Material Removed (mm)	Purpose
Light (normal wear)	0.10–0.15	Restore sharp edge, maintain hole size
Medium (minor chipping)	0.15–0.25	Remove chipped area, restore corner
Heavy (significant wear)	0.25–0.35	Rare — indicates regrind interval too long

Tracking Regrind Life

Keep a log for each drill:

Drill ID: GD-412 (Brazed, Ø8.5 mm)
Date        Holes Since  Total Holes   VB (mm)   Action
2026-01-15  —            0             0.00      New
2026-02-20  320          320           0.18      Regrind #1
2026-03-18  310          630           0.20      Regrind #2
2026-04-22  305          935           0.19      Regrind #3
2026-05-25  290          1225          0.22      Regrind #4
2026-06-28  280          1505          0.18      Regrind #5
2026-07-30  270          1775          0.32      Replace — tip length insufficient

Observation: This drill provided 1,775 holes across 5 regrinds with consistent performance. Holes per regrind gradually declined from 320 to 270 as the carbide tip shortened, but quality remained acceptable through regrind #5.

Geometry Restoration

What Regrinding Must Restore

Geometry Feature	Why Important	Target After Regrind
Nose radius	Controls hole size, surface finish	±0.01 mm of original
Lip height	Ensures balanced cutting (single-lip tool)	±0.005 mm
Face angle (grind angle)	Chip formation, cutting forces	Original specification ±1°
Relief angles (primary + secondary)	Clearance, edge strength	Original specification ±0.5°
Outer corner radius	Smooth entry, surface finish	0.05–0.15 mm (material-dependent)

The Single Most Critical Measurement: Lip Height

For single-lip gun drills, lip height accuracy is the most critical parameter. If the cutting edge height is incorrect, the drill will not cut on-center and hole straightness will suffer.

Lip Height Error	Effect
< 0.01 mm	Acceptable — minor effect on hole size
0.01–0.03 mm	Hole 0.02–0.05 mm oversize; reduced straightness
> 0.03 mm	Drill wanders off-center. Hole may be out of tolerance

Requirement: Gun drill regrinding services must use a dedicated gun drill grinding machine (e.g., Christen, Ewag, or Walter) with proper lip height control. Standard tool and cutter grinders cannot hold lip height tolerance for gun drills.

Nose Grind Selection by Material

Material	Recommended Nose Grind
Steel (low/medium carbon)	N-8 (standard conical) or N-4 (double facet)
Alloy steel	N-8 or N-4
Stainless steel	N-8 with polished face (reduce BUE)
Cast iron	N-8 (sharp edge)
Aluminum	N-2 (high positive rake, polished)
Titanium	N-8 with T-land (reinforced edge)
Superalloys (Inconel, etc.)	N-8 with T-land and 0.10 mm hone
Hardened steel (HRC 40+)	N-4 or negative land

Coating Removal and Recoating

When to Recoat

Condition	Recommendation
Original coating still intact on non-worn areas	Do not recoat — coating removal process may damage substrate
Coating worn off at cutting edge only	Regrind only — new coating on the reground surface is optional
Coating completely worn or damaged	Consider recoat if tool has regrind life remaining
Material change (new material requires different coating)	Recoat may be worthwhile

Coating Removal Methods

Method	Suitable For	Risk to Substrate
Chemical stripping	TiN, TiAlN, AlTiN	Low (correct chemical bath)
Stripping + regrind (combined)	All coatings	Low (regrind removes the layer)
Regrind only (remove coated surface)	All coatings	None (coating removed by grinding)

Note: For gun drills, regrinding removes the worn coating from the cutting surfaces. Recoating the reground surfaces can extend tool life, but the coating thickness (2–5 µm) adds to the lip height. Adjust regrind dimensions to compensate if recoating.

New Coating Options (2025)

Coating	Material Groups	Benefit for Reground Drills
Nano-Tip multilayer (Dormer Pramet)	P, M, K	Thermal stability for steel and stainless
DPA74S / DPX74-M (CERATIZIT)	P, K	High performance for deep drilling cycles
BALINIT PERTURA (Oerlikon)	P, M, S	Nano-layer TiAlN — successor to FUTURA
AlTiN HiPIMS (Walter)	S, H	High-temperature stability for superalloys
AlCrN	M, S	Oxidation resistance for stainless and nickel alloys

Regrind Service vs In-House

Factor	In-House (Dedicated Grinder)	External Regrind Service
Capital investment	$50,000–$150,000 (Christen, Ewag)	$0
Turnaround time	Same day	5–15 business days
Quality control	Full control	Varies — requires qualification
Minimum batch	1 drill	Typically 5–10 drills
Cost per regrind	$15–$30 (labor + wheel wear)	$25–$50 (service fee)
Best for	High-volume production, many drills	Low-volume, occasional regrinds

Qualifying an External Regrind Service

Before sending drills to a new service, request a test regrind on 3 drills:

Provide the drill’s original specification sheet (nose grind angle, lip height, relief angles)
Ask for measurement reports (before and after) including lip height
Run the reground drills on a test part (20 holes)
Measure hole diameter, surface finish, and straightness
Compare to baseline performance with new drills

Cost Per Hole Analysis

Example: Ø8 mm Brazed Gun Drill

Factor	New Drill ($180)	5th Regrind ($35)
Tool cost	$180.00	$35.00
Holes per edge	320	270
Cost per hole (tool)	$0.56	$0.13
Savings vs new	—	77%

Cumulative Cost Over Drill Life

Cost per hole as drill progresses through regrinds:
New:    $0.56 per hole
R1:     $0.13 per hole
R2:     $0.13 per hole
R3:     $0.13 per hole
R4:     $0.13 per hole
R5:     $0.13 per hole

Total holes: 1,775
Total tooling cost (new + 5 regrinds): $180 + 5 × $35 = $355
Average cost per hole: $355 ÷ 1,775 = $0.20

If replaced as new each time (no regrinding):
Cost per drill: $180 ÷ 320 holes = $0.56 per hole
Without regrinding, total cost for same hole count = $995

Regrinding saves approximately 64% in tooling cost over the life of this drill.

Common Regrinding Mistakes

Mistake	Consequence	Prevention
Grinding burn (blue discoloration)	Micro-cracks in carbide, premature failure	Use adequate coolant during grinding; reduce infeed
Incorrect lip height	Hole wandering, oversize	Use dedicated gun drill grinder with lip height control
Excessive material removal	Reduced total regrinds, early tool replacement	Regrind at VB = 0.15–0.20 mm, not 0.30+ mm
Uneven relief angles	Unbalanced cutting forces	Verify with optical comparator after regrind
Recoating without removing damaged coating	Coating adhesion failure	Strip old coating before applying new
Over-polishing the flute	Chip flow disrupted, packing risk	Maintain original flute surface finish

Summary

Gun drill regrinding is a cost-effective practice that can reduce tooling cost per hole by 60–80% over the life of a drill. Brazed tip drills accept 3–5 regrinds; solid carbide drills accept 7–10. The key to successful regrinding is lip height control (±0.005 mm) using a dedicated gun drill grinding machine. Regrind at flank wear of 0.15–0.20 mm — too early wastes useful life, too late risks tool breakage. Recoating is optional and most beneficial when changing to a material-matched coating. For a complete tooling overview, see deep hole drilling tooling guide. For cutting tool materials, see cutting tool materials guide.