The six patterns and what each means
Examine the edge under magnification rather than at arm's length, because the difference between chipping and attrition is not visible otherwise.
| Pattern | Appearance | Cause | Correction |
|---|---|---|---|
| Flank wear | Uniform band on the relief face | Normal abrasion at slightly high speed | Reduce speed about 10 percent, or move to a more wear-resistant grade |
| Crater wear | Crater on the rake face behind the edge | Coating lacks hot hardness for the temperature | Change to a coating with greater hot hardness, or reduce speed |
| Chipping | Irregular fracture of the edge | Edge too brittle, feed too high, or vibration | Tougher grade, stronger edge prep, check holder runout and rigidity |
| Built-up edge | Welded material on the edge | Speed too low, edge blunt, adhesion | Increase speed, sharper polished edge, different coating family |
| Thermal cracking | Cracks perpendicular to the edge | Thermal cycling, often from intermittent coolant | Reduce coolant variation, consider dry cutting with an AlTiN or AlCrN grade |
| Notch wear | Localised groove at the depth of cut line | Same depth of cut every pass | Vary the depth of cut between passes |
Where a worn holder masquerades as a worn tool
Chipping on one flute and not the others is a statement about runout rather than about the tool. If a single flute carries more load than the rest, it fails first while the others still have life, and replacing the tool changes nothing because the holder is the cause.
Before adjusting parameters for a chipping problem, measure runout at the cutting edge. If it is outside the tool's specification, the holder, collet or nut is the problem.
Normal wear against abnormal wear
Some wear is the process working correctly. A uniform flank wear land developing gradually across the whole edge is normal, and the tool is being used up as intended.
Abnormal wear is anything localised, anything sudden and anything that appears on one part of the edge rather than evenly. Those patterns point at a specific cause and are worth investigating before changing anything.
Recording wear to find the pattern
The value of examining worn tools comes from doing it consistently. Keeping the failed tools from a batch and comparing them shows whether the failure is consistent, which points to a process cause, or random, which points to a material or setup variation.
That record is also what makes a conversation with a tool supplier productive: a description of the wear pattern is worth more than a statement that the tool did not last.
Frequently asked questions
Why did my tool chip instead of wearing out?
Chipping means the edge fractured rather than abraded, which points to a brittle grade, too high a feed for the edge preparation, or vibration. Check holder runout first, because chipping on one flute only is a runout problem rather than a tool problem.
What causes crater wear?
The coating or grade lacks hot hardness for the temperature being generated on the rake face, so the material diffuses into the tool. Moving to a coating with greater hot hardness, or reducing speed, addresses it.
What is thermal cracking and how do I stop it?
Cracks running perpendicular to the cutting edge, caused by rapid temperature cycling, frequently from intermittent coolant or interrupted cuts. Reducing coolant variation, or moving to dry cutting with an AlTiN or AlCrN grade, is the usual remedy.
Why is my tool worn in one spot only?
Notch wear at the depth-of-cut line, caused by the tool meeting the same boundary on every pass. Varying the axial or radial depth between passes so the notch does not form at the same point is the most effective correction.
Is some tool wear normal?
Yes. A uniform flank wear land developing gradually across the whole edge is the tool being used up as intended. The patterns worth investigating are the localised, sudden or one-sided ones, because those indicate a specific correctable cause.