How to Read a Worn Gun Drill: Visual Failure Analysis from the Shop Floor
Every gun drill that comes back to the regrind station tells a story. The wear pattern is the drill’s way of telling you what went wrong down in the hole — coolant pressure, chip packing, wrong feed, you name it. I have looked at thousands of pulled drills over the years, and almost every failure mode leaves a signature you can learn to read in about thirty seconds under magnification.
This guide is for the operator or setup person standing at the regrind bench. I am going to walk through the most common wear patterns I see, what they actually mean, and what to do about them before the next drill goes in.
I have covered the full breakage taxonomy in Gun Drill Breakage Analysis and the systematic wear classification system in Gun Drill Tool Wear Patterns. This article sits between them — it is the practical visual guide you keep at the grinder.
Flank Wear Patterns and What They Mean
Flank wear is the most common thing you will see on a used gun drill. It shows up as a flat, shiny land on the clearance face behind the cutting edge. But not all flank wear is created equal.
A uniform wear band of even width across the cutting edge is normal. The drill has been cutting steadily, the coolant has been doing its job, and the edge has worn down gracefully. I expect to see this on every drill that has run a reasonable number of holes.
What I worry about is uneven flank wear. If the wear band is wider on one side of center than the other, something is out of alignment. The drill is cutting more on one edge, and that imbalance will eventually snap the tip or steer the hole off-course.
Heavy flank wear with galling — where the carbide looks smeared or torn rather than polished — points to insufficient coolant flow at the cutting edge. The heat has softened the binder and the carbide has started to deform plastically. This is the pattern I see most often when someone has dialed coolant pressure down to save pump wear.
Chipped Cutting Edge: Causes by Damage Type
A chip in the cutting edge is never normal, but the shape and location of the chip tells you the root cause.
Small edge fractures (micro-chipping) under 0.3 mm are usually caused by interrupted cuts — cross-holes, keyways, or stringy chip re-cutting. I see this a lot in hydraulic manifold work where a gun drill has to pass a intersecting port. The solution is to reduce feed by about twenty percent at the intersection, or pre-drill the cross-hole.
Large corner breaks that take out a chunk of the outer corner are almost always caused by chip packing at the exit. The drill pushes through the bottom of the hole, the remaining web can’t clear chips, and the accumulated debris snaps the corner off. Increase the chip-cleaning dwell before breakthrough.
Center-web chips are a different animal. When I see the center of the drill missing, it means the chip splitter or the center notch has lost its geometry during a previous regrind. The drill is not splitting chips anymore, so a long ribbon wraps around the tip and breaks the center out. Check your regrind setup — the center notch height is probably wrong.
Crater wear at the rake face that looks like a scooped-out divot close to the cutting edge is chemical wear. It happens when the workpiece material reacts with the carbide binder at high temperature. I see this most in titanium and high-nickel alloys. There is no visual fix — you need a different carbide grade or a coating.
Built-Up Edge Diagnosis
Built-up edge (BUE) shows up as a lump of workpiece material welded to the cutting edge. It looks like a blob of metal stuck to the tip, and it is often the same color as the chips in the chip pan.
BUE is a temperature problem. The cutting edge is running too cold for the material being cut. The work-hardened layer of the chip pressure-welds itself to the carbide instead of sliding off. I see this most with low-carbon steels and aluminum at low surface speeds.
The fix is almost always speed, not feed. Increase the surface speed by ten to fifteen percent and the frictional heat will rise enough to stop the welding. If you cannot increase speed — spindle-limited — try a coated drill or increase coolant concentration to improve lubricity.
A secondary cause of BUE that I see overlooked is a dull edge on a fresh regrind. If the reground edge has a measurable radius (more than 0.02 mm), it will generate enough pressure to cold-weld soft materials before the edge has a chance to cut cleanly. Inspect every reground drill edge under 20x magnification before it goes back in the spindle.
Corner Wear vs Center Wear
A gun drill cuts across a wide radius, from the center to the outer corner, and the wear rate is never the same across that whole face. You need to check both zones separately.
Corner wear (the outer diameter edge) accelerates as the drill gets dull because the corner travels the longest path and sees the highest surface speed. I measure corner wear as the reduction in the outer corner radius. If the corner has gone from its sharp-ground radius to flat, the drill is running hot and the hole finish will be rough. The corrective action is to reduce feed until the next regrind interval. On critical holes, a worn corner will also drift the hole diameter undersize.
Center wear is harder to see because it is recessed in the notch area. I use a dental mirror or a borescope for this. Center wear shows as a rounding of the center-web edge. When the center goes dull, the drill starts to push rather than cut, which increases thrust load and risks pushing the drill off-center at the start of the hole.
A drill that shows heavy corner wear but light center wear ran too fast with adequate feed. Heavy center wear with light corner wear means feed was too high relative to speed. When both zones are worn evenly, the operating parameters were balanced and the drill simply reached its end of life.
Fire Crack / Heat Checking
Fire cracks look like a network of fine lines on the carbide surface, usually perpendicular to the cutting edge. They are the most dangerous wear pattern to miss because the drill looks intact but the carbide structure is compromised.
Heat checking happens when the cutting edge cycles between extreme hot and cold with every hole. The carbide expands and contracts until the surface develops a network of micro-cracks. I see this most often in dry machining or near-dry (MQL) applications, but it also happens in through-coolant drilling when the coolant flow is intermittent — a plugged coolant port that clears intermittently is a classic cause.
If you see fire cracks at 10x magnification, the drill is done. Do not send it back for another run. Those cracks propagate rapidly, and the next failure mode is a catastrophic tip fracture that leaves carbide fragments in the hole. Scrap that drill or, if the cracks are shallow enough, grind back past the heat-affected zone during regrind and inspect again.
Wear Pattern vs Root Cause vs Corrective Action
Here is the reference table I have pinned to the wall at the regrind station.
| Wear Pattern | What I See | Root Cause | Corrective Action |
|---|---|---|---|
| Uniform flank wear band | Even shiny land across cutting edge | Normal end of life | Continue regrind cycle; no change needed |
| Uneven flank wear (one side heavier) | Wear band wider on one edge or corner | Misalignment — bushing worn, spindle out, or drill deflection | Check bushing ID, align guide bushing to spindle, reduce peck increment |
| Heavy flank wear with galling | Smear or tear marks on clearance face | Insufficient coolant flow or pressure | Increase coolant pressure to spec (check pump wear), clean coolant passages |
| Micro-chipping under 0.3 mm | Small fractures, irregular edge | Interrupted cut — cross-hole, keyway | Reduce feed 20% at obstruction, or pre-drill cross-hole |
| Large corner break | Chunk missing from outer corner | Chip packing at exit before breakthrough | Add chip-cleaning dwell before exit, reduce final peck depth |
| Center-web chip | Center notch or web area broken out | Regrind geometry error — notch height or splitter wrong | Verify regrind setup: center notch height must match OEM spec |
| Crater wear (rake face) | Scooped-out divot behind cutting edge | Chemical/thermal wear from reactive material | Switch to higher-strength carbide grade or apply AlTiN coating |
| Built-up edge | Welded workpiece material on cutting edge | Edge too cold (low speed) or reground edge radius too large | Increase surface speed 10-15%; inspect reground edge under 20x magnification |
| Fire cracks | Fine crack network on carbide surface | Thermal cycling — intermittent coolant or dry/MQL cut | Scrap drill; if salvageable, grind past heat-affected zone and re-inspect |
| Corner flattened | Outer corner radius gone from sharp to flat | Excessive surface speed at outer diameter | Reduce feed; check if regrind restored corner radius correctly |
| Center edge rounded | Center-web looks dull, no defined cutting edge | Feed too high relative to speed, or drill wandering at entry | Reduce feed, check guide bushing fit and drill straightness |
Acceptable vs Unacceptable Wear Dimensions
Not all wear is created equal. Here are the limits I use before I will send a drill back for regrind or scrap it outright. These numbers vary by drill diameter — smaller drills have less carbide to work with.
| Drill Diameter | Feature | Acceptable (regrind) | Unacceptable (scrap or hesitation) |
|---|---|---|---|
| 3 - 6 mm | Max flank wear land | < 0.15 mm | > 0.25 mm |
| 3 - 6 mm | Corner wear (radius loss) | < 0.10 mm | > 0.15 mm |
| 3 - 6 mm | Edge chip size | < 0.10 mm | > 0.15 mm |
| 6 - 12 mm | Max flank wear land | < 0.25 mm | > 0.40 mm |
| 6 - 12 mm | Corner wear (radius loss) | < 0.15 mm | > 0.25 mm |
| 6 - 12 mm | Edge chip size | < 0.20 mm | > 0.30 mm |
| 12 - 25 mm | Max flank wear land | < 0.40 mm | > 0.60 mm |
| 12 - 25 mm | Corner wear (radius loss) | < 0.25 mm | > 0.40 mm |
| 12 - 25 mm | Edge chip size | < 0.30 mm | > 0.50 mm |
These numbers assume the drill is running within its recommended speed and feed range. If you are pushing parameters to the limit, the acceptable wear band shrinks — the drill has less margin before it fails catastrophically.
Any fire crack at any diameter means the drill should be scrapped or ground back a minimum of 0.5 mm past the deepest visible crack. Do not gamble on fire cracks.
What a Normal Worn Drill Looks Like vs a Problem
A normal worn drill that is ready for regrind has a uniform wear band across the flank, the corner radius is slightly reduced but still present, and the cutting edge shows no chips larger than 0.2 mm (on a 10 mm drill). The rake face is clean — no crater, no BUE. The tip color is even, usually a light tan or straw color from the heat of cutting. The margins show light polishing but no galling.
A problem drill has one or more of these: uneven wear band, a missing corner, fire cracks, a dull center web, or galling on the clearance face. The tip may be discolored — blue or purple indicates excessive heat. If the margins are galled or the drill body shows wear marks only on one side, you have an alignment problem that will not fix itself with a regrind.
Here is the rule I teach every new operator: if you look at a pulled drill and your first thought is “it looks okay, just a little worn,” it is probably fine. If your first thought is “huh, that looks weird,” something is wrong. Trust that instinct. Pull the job card, check the parameters, and do not put the next drill in until you understand what caused the weird pattern.
Key Takeaways
- Flank wear uniformity is the single most important visual check — uneven wear means misalignment, not tool life.
- Built-up edge means the edge is too cold, not too hot. Increase speed, not feed.
- Fire cracks are non-negotiable. Any visible heat checking means the drill structure is compromised.
- Micro-chipping under 0.3 mm on interrupted cuts is manageable with feed reduction; anything larger needs root-cause investigation.
- Corner wear and center wear tell opposite stories about your speed-feed balance.
- Keep the wear limit table at your regrind station and measure every drill before you decide to regrind or scrap.
- If the pattern looks wrong, stop and figure out why before the next drill goes in the hole. The drill is telling you something — listen to it.
For deeper dives, read the companion guides: Gun Drill Tool Wear Patterns for the full classification system, and Gun Drill Breakage Analysis for catastrophic failure modes.