Tool wear is diagnosed from what you see at the edge, not from a single guess. This guide maps each wear type — flank, crater, built-up edge, chipping, thermal cracking, notch and plastic deformation — to its likely causes and the right correction order, with a drill-specific section and a printable inspection sheet.
Quick answer
CNC tool wear shows up as recognisable patterns at the cutting edge: flank wear (land on the relief face), crater wear (a dish on the rake face), built-up edge (welded material), chipping or edge fracture, thermal cracking (a network of cracks), notch wear at the cut depth, and plastic deformation (the edge rounds over from heat). Each pattern points at different causes — speed, feed, coolant, runout, overhang or the wrong grade — and the correction order matters: check runout, holding and coolant before you blindly change cutting parameters. No single wear mode has one cause, so always work a check list, not a hunch.
01Visual quick diagnosis
Even land on the side
Flank wear — planned consumption; watch the land width.
Dish on top face
Crater wear — heat under the chip; weakens the edge.
Ragged welded lump
Built-up edge — material welding on; ruins finish.
Chips off the edge
Chipping / fracture — overload or brittle grade.
Fine crack network
Thermal cracking — heating/cooling shock.
Groove at cut depth
Notch wear — hard scale or oxidation at the line.
02Normal wear vs premature failure
Normal, planned wear
- Gradual, even flank land that grows predictably
- Surface finish stays acceptable until near end of life
- You can schedule the change before failure
Premature failure
- Sudden chipping, cracking or built-up edge
- Wear that jumps between parts
- Size drift, rough finish or broken tool with no warning
03Flank wear
- What it looks like
- An even land on the relief (clearance) face; grows with parts cut
- Likely cause
- Abrasion from the workpiece; too high speed; worn grade; long run at the limit
- Check first
- Actual surface speed vs tool-maker window; grade suitability; are you at the planned limit?
- Then adjust
- Lower speed toward the recommended window; confirm grade; set a tool-life limit so it is changed before failure
Flank wear is the wear you plan around — it sets the tool-life number. The danger is only when it grows faster than expected or shows unevenly (which points at runout, not speed).
04Crater wear
- What it looks like
- A crater or dish on the rake face under the chip flow
- Likely cause
- High heat and chemical reaction with the chip; too high speed; weak coating
- Check first
- Cutting speed and coolant delivery; coating grade for the material
- Then adjust
- Reduce speed; improve coolant; move to a more heat-resistant grade/coating
Left alone, the crater undermines the edge until it fractures. It is heat-driven, so the fix is usually speed and coolant before grade.
05Built-up edge (BUE)
- What it looks like
- A ragged lump of workpiece material welded to the edge; holes grow, finish ropes
- Likely cause
- Low speed or poor lube letting material weld; gummy alloys; below-minimum feed
- Check first
- Coolant delivery and aim; whether feed is below the material minimum; edge sharpness
- Then adjust
- Raise speed/feed so the edge shears; improve lube (through-tool/MQL); sharper/polished geometry (DLC for aluminium)
06Chipping and edge fracture
- What it looks like
- Chunks missing from the edge; sudden size drift; scrap
- Likely cause
- Overload: too much feed, interrupted entry, long stick-out, runout, brittle grade
- Check first
- Holder and runout at the tip; stick-out length; whether the cut is interrupted
- Then adjust
- Shorten tool / better holder; reduce feed per tooth; add flutes; choose a tougher grade
07Thermal cracking
- What it looks like
- Fine intersecting cracks across the rake face
- Likely cause
- Repeated heating and cooling; coolant hitting a hot edge; varying temperatures
- Check first
- Whether coolant is applied intermittently; how much temperature varies through the cut
- Then adjust
- Stabilise parameters; steady, adequate coolant; more heat-resistant grade if needed
08Notch wear
- What it looks like
- A groove worn into the edge exactly at the cut depth
- Likely cause
- Hard surface scale, oxidation or abrasives concentrated at that depth
- Check first
- Material condition at the cut line; whether a tougher grade is needed
- Then adjust
- Different grade/geometry; sometimes a slight depth shift or pre-processing the surface
09Plastic deformation
- What it looks like
- The cutting edge rounds over or smears instead of staying sharp
- Likely cause
- Heat softening the substrate; too high speed/feed for the grade
- Check first
- Speed/feed vs grade limit; is the tool running hot?
- Then adjust
- Reduce heat (lower speed, better coolant); use a substrate that holds hardness at temperature
10Drill-specific wear
Drills show the same physics with their own signatures:
- Margin wear / oversize holes. The margin rubs and the hole grows; check runout and point grind first.
- Web thinning / splitting. The drill splits up the web from excessive thrust — usually too much feed or a dull point.
- Flank wear at the lips. The cutting lips dull; feed force climbs and breakage risk rises.
- Built-up edge on margins. Classic in aluminium; produces oversize, rough, out-of-round holes.
11How speed, feed and coolant affect wear
| Variable | If too high | If too low |
|---|---|---|
| Speed | Cratering, thermal cracks, rapid flank wear from heat | Rubbing, work-hardening, BUE — wears by friction |
| Feed | Chipping, edge fracture, deflection, vibration | Powdery chips, rubbing, BUE |
| Coolant | Usually fine; over-pressure can mist/aerate | Packed chips, heat, BUE, fast wear |
Table 1. There is a window for each. Stay inside the tool maker's recommended speed, then tune feed; fix coolant delivery before touching numbers.
12Runout, overhang and rigidity
Before changing any parameter, check the mechanical causes — they cause most premature wear:
- Runout at the tip. A 0.01 mm runout on a small tool means one edge does most of the work and fails early. Measure at the tip, not the holder.
- Stick-out. Longer overhang = more deflection = more edge load and chatter. Use the shortest tool that reaches.
- Holder and clamping. A poor collet or loose clamp lets the tool move in the cut. Invest in low-runout holding.
13Tool-life monitoring
- Set a life limitParts or time, based on planned flank wear — not run-to-failure.
- Schedule in-process checksAt an interval suited to the tolerance, not just at the end.
- Record wear at changeLand width, crater, BUE — so trends are visible across a run.
- Catch drift on first articleA changing edge shows up there, not at 500 parts.
14Troubleshooting matrix
| Wear type | Likely causes | Check order | Action |
|---|---|---|---|
| Flank | Speed high, grade weak | Speed → grade | Lower speed; tougher grade; set limit |
| Crater | Heat, weak coating | Coolant → speed → grade | Improve delivery; lower speed; heat-resistant grade |
| BUE | Low feed/lube, gummy material | Coolant → feed → geometry | Raise feed; better lube; polished/DLC edge |
| Chipping | Overload, runout, brittle | Runout → holder → feed | Fix runout; shorten; reduce feed; tougher grade |
| Thermal crack | Temp swing, coolant shock | Parameters → coolant | Stabilise heat; steady coolant; resistant grade |
| Notch | Hard surface at cut line | Material → grade | Tougher/right grade; surface pre-process |
Table 2. The "check order" column is the point: diagnose upstream causes before changing cutting numbers.
15Case studies
Both were fixed in steps 1–4 (runout, holder, coolant, grade) — not by slowing the spindle. Values are specific to those parts and setups, not general claims.
16Printable inspection sheet
| Check | Record | Pass / fail |
|---|---|---|
| Runout at tool tip | ___ µm | ☐ |
| Holder / clamp tight | yes / no | ☐ |
| Coolant delivery & aim | flood / through / MQL | ☐ |
| Stick-out length | ___ mm | ☐ |
| Flank land width | ___ mm | ☐ |
| Crater / BUE present | none / noted | ☐ |
| Speed vs tool-maker window | ___ m/min | ☐ |
| Parts since last change | ___ | ☐ |
Table 3. A one-page check run at every tool change on a critical feature. Trend the numbers across a run to catch drift before it becomes scrap.
FAQFrequently asked questions
How do I tell normal wear from a problem?
Normal flank wear is a gradual, even land on the relief face that grows predictably with parts cut — you plan tool changes against it. A problem is sudden or uneven: chipping, built-up edge, cratering, cracks, or wear that jumps between parts. If the edge looks different from the last good run, treat it as a fault, not consumption.
What is the difference between flank and crater wear?
Flank wear is on the relief (clearance) face — the land you see from the side; it is driven mainly by abrasion and sets a tool-life limit. Crater wear is a dish on the rake (top) face under the chip; it is driven by heat and chemical reaction with the chip, and it eventually weakens the cutting edge. They have different causes and different fixes.
What causes built-up edge and how do I fix it?
Built-up edge (BUE) is workpiece material welding onto the edge, common in aluminium, mild steel and stainless at low speed or poor lubrication. It makes the edge ragged, grows the hole and wrecks finish. Fix it by raising speed/feed so the edge shears, improving coolant delivery, using a sharper or polished geometry (DLC for aluminium), and confirming you are not below the material's minimum feed.
Why is my edge chipping instead of wearing gradually?
Chipping is usually impact or overload: too much feed, interrupted entry, long stick-out, runout, or a grade too brittle for the cut. Check holder and runout first, shorten the tool, reduce feed/tooth, and consider a tougher grade before chasing speed.
What are thermal cracks on a insert?
A network of fine cracks across the rake face from repeated heating and cooling — typical when coolant hits a hot edge, or with wildly varying cutting temperatures. Stabilise temperature (consistent parameters, steady coolant) and avoid thermal shock; switch to a more heat-resistant grade if needed.
Does higher speed always mean faster wear?
Mostly yes — heat is the enemy of edge life, and speed drives heat. But too low a speed also hurts: the edge rubs and work-hardens (especially stainless/titanium), and that wears the tool by friction instead. There is a window; stay inside the tool maker's recommended speed, then tune feed.
How does coolant affect tool wear?
Poor delivery is a leading wear cause. Insufficient flushing lets chips recut and loads the tool; heat that cannot escape accelerates wear; no lubrication promotes built-up edge. Fix delivery (aim, through-tool, pressure) before changing numbers — see the coolant guide.
What should I check first when wear is unstable?
In order: (1) runout at the tool tip, (2) holder and clamping, (3) coolant delivery, (4) stick-out/rigidity, (5) grade and coating, then (6) speed and feed. Most premature wear is fixed in steps 1–4, not by slowing the spindle.
Can I just use a harder, more wear-resistant grade?
Not always — the most wear-resistant grades are often the most brittle and chip under interruption or vibration. Match toughness to the cut: interrupted or vibrating cuts need a tougher grade even if it wears a little faster. The tool selection guide covers the trade.
How do I monitor tool life in production?
Set a tool-life limit (parts or time) instead of running to failure, schedule in-process checks at an interval suited to the tolerance, and record wear at change so trends are visible. Catch a drifting edge on the first article, not at 500 parts. Pair this with the process review we offer on incoming RFQs.
●The CNC drilling & parameters library
Tool wear is the symptom; the cause is usually upstream in speed, feed, coolant, rigidity or grade. Diagnose the pattern, then fix in the right order.
The complete hole-making framework: operations, drill types, accuracy, cycles and process selection.
How cutting speed, RPM, feed per revolution and chip load fit together, with starting points by material.
Built-up edge, long chips, burrs, deep holes and hole accuracy in 6061, 7075, 2024 and 5052.
Feed per tooth vs feed per revolution, chip thinning, and what chip shape tells you about your parameters.
What SFM and m/min measure, why diameter changes RPM, and how to convert between them correctly.
Convert surface speed and tool diameter into spindle RPM, in both imperial and metric units.
Feed rate from RPM and chip load (milling) or feed per revolution (drilling), plus reverse calculation.
Canned cycle selection, Q/R/P/K parameters, G98/G99 returns, commented examples and alarms.
A decision framework from operation and material to geometry, coating, holder, reach and volume.
Flood, through-tool, MQL, air blast and dry machining, plus concentration, filtration and troubleshooting.
Diameter, true position, cylindricity, finish, burrs, process capability and how each one is inspected.
Experiencing unstable tool life on a production part?
Send us the drawing and your process conditions — wear photos, material and hardness, tool type and grade, speeds, feeds, coolant and stick-out. We diagnose the wear pattern and return a correction plan that fixes runout, coolant and rigidity before touching your cutting parameters.
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