Not all regrinds are equal. I have learned the hard way that a bad regrind will cost you a broken drill and a scrapped part. A gun drill costs hundreds of dollars to replace, but a scrapped aerospace part can cost thousands. I inspect every reground drill before it goes into the spindle, and I reject about one in five regrinds from even my best sharpening vendors.
Here is my inspection routine. It takes ten minutes and has saved me from countless spindle crashes, broken drills, and rejected parts.
Visual Inspection Steps
I start every regrind check with the naked eye and a 10x loupe. Geometry errors that look small on the bench become catastrophic at the bottom of a deep hole.
Cutting edge condition. I look for chips, cracks, or burning on the carbide tip. A chipped edge will degrade surface finish immediately. Burning – a blue or discolored appearance on the carbide – means the grinder pushed too hard or lost coolant during sharpening. A burned tip has reduced hardness and will wear fast.
Tip face finish. The reground face should show uniform grinding marks in one direction. Cross-hatch or chatter marks mean the grinding wheel was dull or the setup lacked rigidity. These surface defects transfer to the hole wall.
Outer corner (OD corner). The corner where the cutting edge meets the drill diameter is the most fragile part of the tool. I verify it is sharp and intact. A rolled-over or chipped OD corner will cause oversize holes at the entry.
Wear pattern on the old grind. If the previous grind shows uneven wear – heavy on one side, light on the other – the drill geometry was off. I flag these for dimensional checks before accepting.
Flute and shank condition. I check the drill shank for galling, burrs, or damage from the previous setup. A damaged shank will not seat properly in the collet or bushing. The coolant hole entrance at the shank end must also be clean and free of debris.
| Inspection Check | Pass Criteria | Fail Criteria |
|---|---|---|
| Cutting edge chips or cracks | No visible chips, cracks, or missing carbide | Any chip larger than 0.1 mm or crack visible at 10x |
| Tip burning / discoloration | Uniform color, no blue or brown discoloration | Blue, brown, or black discoloration on carbide |
| Grinding finish quality | Uniform unidirectional grind marks | Cross-hatch, chatter, or uneven surface |
| OD corner condition | Sharp, intact corner with no rollover | Chipped, rolled, or missing corner |
| Shank condition | Smooth, clean, burr-free | Galling, dents, burrs, or debris in coolant hole |
| Coolant hole at tip | Fully open, centered on the relief face | Partially blocked, off-center, or deformed |
| Tip angle symmetry | Both clearance faces appear equal width | One side visibly wider – indicates asymmetric grind |
| Relief face flatness | No waviness when checked with a straight edge | Visible gap under a straight edge |
| Coating (if reground) | Uniform coating remnants or bare carbide clean | Peeling coating, inconsistent color, or burrs |
| Overall drill length | Within 1 mm of original length after accounting for regrinds | Shortened beyond usable range for the job depth |
Dimensional Checks
After the visual pass, I pull out the measurement tools. Visual inspection catches the obvious problems, but dimensional inspection catches the subtle ones that kill tool life slowly.
Tip Diameter
The tip diameter must be ground to the correct size relative to the drill body. A gun drill relies on the tip being slightly larger than the body – typically 0.02 to 0.08 mm larger – so the carbide tip cuts the hole and the steel shank clears without rubbing.
I measure the tip diameter with a micrometer calibrated to 0.001 mm. If the tip is undersized, the drill body will rub, generate heat, and break. If the tip is oversized, the hole comes in too large.
Clearance Angles
The primary clearance angle behind the cutting edge should be within 1 degree of the spec. Too little clearance creates rubbing that kills surface finish. Too much clearance weakens the cutting edge and can cause micro-chipping.
I check clearance with a toolmaker’s protractor or a dedicated clearance-angle gauge. The standard is 10 degrees, plus or minus 0.5 degrees, for most steel applications. Harder materials need less clearance; softer materials need more.
Coolant Hole Position
The coolant hole at the tip face must be open and centered on the relief face. If the regrind has shifted the hole off-center, coolant flow will be uneven. One side of the cutting edge gets more coolant and the other side runs hot.
I verify coolant hole position using a pin gauge and a loupe. The hole center should be within 0.1 mm of its design position relative to the cutting edge.
Measuring Tools I Use
You do not need a metrology lab to inspect a reground gun drill, but you need the right tools. Here is what I keep at the inspection bench:
- Outside micrometer, 0-25 mm range, 0.001 mm resolution. For tip diameter and drill body diameter measurements. I use a friction thimble for consistent contact pressure.
- Toolmaker’s protractor. For checking clearance angles and tip angles. A vernier protractor with 5-minute resolution is adequate for shop use.
- 10x and 20x loupe. For edge condition, coolant hole position, and surface finish checks. I prefer a LED-illuminated loupe for consistent lighting.
- Pin gauge set, 0.05 mm steps. For checking coolant hole diameter and verifying the hole is clear of debris or grinding swarf.
- Comparison microscope (optional). I use a 50x comparison microscope when qualifying a new sharpening vendor. It shows edge quality and clearance geometry that a loupe cannot resolve.
- Surface roughness comparator. Not a substitute for a profilometer, but adequate for a quick pass/fail on the reground carbide surface.
Acceptable Tolerances
Different drill diameters have different acceptable ranges. The table below shows the tolerances I use for inspecting reground gun drills.
| Drill Diameter (mm) | Tip Oversize (mm) | Primary Clearance (deg) | Tip Angle Tolerance (deg) | Coolant Hole Offset (mm) |
|---|---|---|---|---|
| 3 - 6 | 0.02 - 0.04 | 10 +/- 1.0 | +/- 1.0 | Max 0.08 |
| 6 - 12 | 0.03 - 0.05 | 10 +/- 0.5 | +/- 0.5 | Max 0.10 |
| 12 - 20 | 0.04 - 0.06 | 10 +/- 0.5 | +/- 0.5 | Max 0.12 |
| 20 - 30 | 0.05 - 0.08 | 10 +/- 0.5 | +/- 0.5 | Max 0.15 |
I tighten these tolerances by 50 percent for finishing operations where hole size tolerance is under 0.05 mm. For roughing, I use the standard ranges.
Regrind Quality Grading
I grade every reground drill into one of three categories:
- Grade A – Ready for spindle. Passes all visual and dimensional checks. Cutting edge is sharp, angles are within spec, coolant hole is centered. Goes straight to the tool crib.
- Grade B – Limited use. Passes dimensional checks but shows minor defects such as light grinding chatter or a slightly uneven relief face. I use these for roughing operations or non-critical holes where surface finish requirements are relaxed.
- Grade C – Reject. Fails one or more dimensional checks or has a visible defect that will affect performance. These go back to the vendor for re-grind or replacement.
I track grades by vendor. If a vendor sends more than 20 percent Grade C tools over a quarter, I schedule a quality visit to their facility. The regrind process itself may need adjustment.
When to Reject a Regrind
I reject a regrind immediately for any of these conditions:
- Cracked carbide. A crack in the tip will propagate under cutting load and cause a catastrophic failure. Do not run it.
- Burned tip (discolored). Grinding burn reduces carbide hardness. The edge will wear fast and may chip unpredictably.
- Coolant hole blocked or deformed. No coolant flow means no chip evacuation. The drill will clog, overheat, and break within seconds.
- Tip diameter below the low limit. Undersized tips cause body rubbing, oversize holes, and high spindle load. Not worth the risk.
- Asymmetric tip. If the two clearance faces are visibly different widths, the drill will cut unevenly and the hole will drift off-center.
A rejected regrind costs me the regrind fee plus the inspection time. That is still cheaper than a broken drill and a scrapped part.
The complete guide to gun drill geometry, including angles and coating selection, is covered in the Gun Drill Geometry Guide. If you are seeing inconsistent regrind quality from your vendors, the Inconsistent Regrind Quality troubleshooting guide walks through the common causes and fixes.
Key Takeaways
- Inspect every reground gun drill before putting it in the spindle. A ten-minute inspection saves hours of downtime and thousands in scrap.
- Use a two-pass approach: visual inspection with a loupe first, then dimensional checks with a micrometer and protractor.
- Grade every regrind as A, B, or C. Track vendor quality by grade percentage and escalate chronic problems.
- Reject any reground drill with cracked carbide, burned tip, blocked coolant hole, undersized tip diameter, or asymmetric geometry. These defects will cause tool failure.
- Maintain a tolerance table by drill diameter. The acceptable range for a 5 mm drill is not the same as for a 20 mm drill.
- Build a relationship with your sharpening vendors. Share your inspection data and grade distributions. The vendors who see your pass/fail data consistently improve their quality over time.