FundamentalsTool wear

Tool wear — types, mechanisms and criteria

How a cutting edge wears out. The wear curve, the five wear mechanisms, the eight wear types you can see on an insert with their causes and remedies, and the tool-life criteria of ISO 3685 and ISO 8688.

LiveUpdated 11 October 20267 min read

Wear on an insert corner: flank, crater and notch

ISO 3685 criterion for carbide: average flank wear VB: 0.3mm
Crater criterion KT at f = 0.25 mm/rev: 0.14mm
Wear mechanisms: 5
Wear types you can see on an insert: 8

Every cutting edge wears from the first second of its life. What matters is how fast, in which way, and when to stop. A worn edge raises forces and temperature, spoils dimensions and surface, and if it is left too long it breaks and can take the part with it. Reading the wear on a used insert is the quickest way to tell what is wrong with a cut.

The wear curve

Plot the width of the flank wear land VB against cutting time and almost every tool gives the same three-part curve.

I break-in
II steady wear
III accelerated wear

Schematic flank wear curve. The tool is changed when VB reaches the chosen criterion; ISO 3685 uses an average VB of 0.3 mm for carbide.

Break-in. A new edge wears quickly at first while the small unevenness of the newly sharpened edge is smoothed away.
Steady wear. Wear grows roughly linearly with time. The slope depends on the cutting speed, the tool geometry, the work material and the coolant. This is where the tool spends most of its life.
Accelerated wear. The worn edge runs hotter and is loaded harder, so wear accelerates; thermal cracks and plastic deformation follow, and finally the edge fails.

A sensible tool-change point sits in the steady region, before the curve turns upward.

Five mechanisms

Several physical mechanisms remove material from the tool. Which one dominates depends mainly on the cutting temperature, and therefore on the cutting speed.

Abrasion. Hard particles in the work material scratch the tool like an abrasive. It dominates at low speeds and temperatures.
Adhesion. Under high pressure and temperature, work material welds to the tool, and when the weld breaks it tears tool particles away. At low cutting temperatures the same mechanism builds up a built-up edge.
Diffusion. At high temperature, atoms move across the tool–chip interface and the tool loses its hardening elements. Diffusion is regarded as the main mechanism of crater wear. For carbide tools cutting steel at practical speeds, Arsecularatne and co-authors identify it as the dominant wear mechanism.
Oxidation. At high temperature an oxide layer forms on exposed tool surfaces. Being softer than the tool material, it is sheared away.
Thermal fatigue. Repeated heating and cooling, as in milling and interrupted cuts, cracks the edge and can eventually break it.

Plastic deformation of the edge is often listed alongside these: the binder softens at high temperature and the load pushes the edge out of shape.

Eight wear types on the insert

The table summarises the wear types that the troubleshooting guides of Sandvik Coromant, Seco, ISCAR, Mitsubishi and Kennametal describe. It lists only causes and remedies on which several of those makers agree.

Wear type	What it looks like	Typical causes	First remedies
Flank wear	Wear land on the flank, starting at the edge line and growing downward	Abrasion; speed too high; grade not wear resistant enough	Lower the cutting speed; use a more wear-resistant grade
Crater wear	A smooth depression on the rake face behind the edge	Diffusion at high temperature; speed or feed too high	Lower the speed and the feed; a wear-resistant coating (Seco names thick Al₂O₃); a more positive geometry
Notch wear	Local damage at the depth-of-cut line	Work-hardened, scaled or hard surface layer; burr formation; oxidation	Vary the depth of cut between passes; use a smaller entering angle
Plastic deformation	The edge sags or is pushed in; the shape changes permanently	High temperature together with high force; the binder softens	A grade with higher hot hardness; lower speed and feed
Chipping	Small pieces broken from the edge line; the insert can still cut	Vibration, interrupted cuts, hard inclusions, a weak edge, built-up edge	A tougher grade; a stronger edge (hone or chamfer); a more rigid setup; lower feed at entry and exit
Thermal cracks	Cracks roughly perpendicular to the edge, like the teeth of a comb	Rapid temperature swings in milling, interrupted cuts or intermittent coolant	A tougher grade; lower speed and feed; avoid intermittent coolant (see below)
Built-up edge	Work material welded to the rake face or edge	Cutting temperature or speed too low; sticky materials such as low-carbon steel, stainless steel, aluminium	Raise the cutting speed (all makers agree); a sharper, more positive edge; coolant
Fracture	A large part of the edge broken away; the insert is finished	Load beyond the strength of the edge, often after excessive wear or with wrong cutting data	A tougher grade or stronger geometry; lower feed and depth of cut; index earlier; check rigidity

Flank wearWear land growing down from the edge line
Crater wearDepression on the rake face behind the edge
Notch wearLocal damage at the depth-of-cut line
Plastic deformationThe edge sags; its shape changes for good
ChippingSmall pieces broken out of the edge line
Thermal cracksCracks roughly across the edge, like comb teeth
Built-up edgeWork material welded to the rake face or edge
FractureA large part of the edge broken away

The eight wear types of the table, drawn on the same insert corner with the main cutting edge on the right. Damaged areas are outlined in orange.

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The same three wear types in 3D on an 80° rhombic insert: the flank wear land under the main cutting edge, the crater on the rake face behind it, and the notch at the depth-of-cut line. Wear is drawn larger than life so that it can be seen at this scale.

When is a tool worn out?

A tool-life criterion is a predetermined value of a wear measure at which the tool is considered worn out. Standard tests use fixed values so that results from different laboratories can be compared.

ISO 3685:1993, single-point turning tools:

Tool material	Criterion
Sintered carbide	Average flank wear VB_B = 0.3 mm, if the flank is worn regularly
	Maximum flank wear VB_{B max} = 0.6 mm, if the flank is worn irregularly
	Crater depth KT = 0.06 + 0.3 f (mm, with f the feed in mm/rev)
High-speed steel	Catastrophic failure, or the VB_B limits above

For a feed of 0.25 mm/rev the crater criterion is KT = 0.06 + 0.3 · 0.25 ≈ 0.14 mm.

Flank, seen from the front

Section through the edge

ISO 3685 criteria for carbide

VB_B = 0.3 mm regular wear
VB_{B max} = 0.6 mm irregular wear
KT = 0.06 + 0.3 f = 0.14 mmFeed f0.25 mm

Where the measures of the table are taken, drawn schematically: the flank wear land seen from the front, divided into the zones C (corner), B (straight edge) and N (the end towards the depth-of-cut line) of ISO 3685, and a section through the edge with the crater depth KT. Move the slider to see the crater criterion for another feed.

ISO 8688-2:1989, end milling: a uniform flank wear of 0.3 mm averaged over all teeth is recommended.

From wear to tool life

The time to reach the criterion is the tool life T. It falls steeply as the cutting speed rises, which is what Taylor’s equation describes. See Taylor’s tool life equation for how to measure it, and how to choose the speed that gives the lowest cost.

Sources (14)

Every value and quotation on this page comes from the documents below.