Tool Wear Mechanisms and Tool Life: Diagnosis, Countermeasures and Economic Assessment

Tool wear is not a single event but a set of mechanisms with root causes that differ from one another. Being able to read which mechanism is dominant by looking at a worn edge is what makes the right countermeasure possible. This guide covers eight fundamental mechanisms, the ISO tool life criteria, the Taylor tool life equation and the economic assessment of tool life.

1. Two fundamental forms of deterioration

Tool deterioration falls into two groups. The first is gradual wear: the edge recedes slowly and predictably, the dimension drifts and the surface deteriorates. The second is sudden damage: the edge loses its function all at once through chipping, fracture or plastic collapse.

Production planning can only be built on gradual wear. For this reason the aim of parameter and tool selection is to suppress the sudden damage mechanisms and reduce deterioration to flank wear. The ISO 8688 series likewise builds its life criteria on measurable wear quantities; because sudden fracture does not give a measurable wear value, it is recorded as a separate form of deterioration.

2. Gradual wear mechanisms

Flank wear (VB)

This is the uniform wear band that forms on the flank face below the cutting edge. Its root cause is abrasion: the hard phases in the workpiece (carbides, oxide inclusions, cast skin) wear away the tool surface much like abrasive paper.

The visual diagnosis is a bright band of even width extending along the edge. The width of the band is measured as VB and forms the basis of the tool life criterion. Flank wear is the desired form of wear; because it progresses steadily, tool life can be predicted.

Should it progress too rapidly, the cutting speed has to be reduced and a more wear-resistant grade or coating selected. The effect of tool material selection on tool life is covered in detail in the tool materials guide.

Crater wear

This is the bowl-shaped cavity that forms on the rake face in the region where the chip flows. Its root cause is diffusion and chemical interaction: at high temperature the elements of the tool material pass into the chip. Because temperature rises exponentially with cutting speed, the crater is the clearest indicator of a speed error.

The visual diagnosis is a bright, hollowed region immediately behind the edge. As the crater deepens, the cutting edge is thinned from within and fractures without prior warning. ISO 3685 defines a feed-dependent criterion for the crater depth KT; in practice depths of the order of 0.05–0.10 mm are accepted as the end of tool life.

The countermeasure is to reduce the cutting speed, to use grades coated with aluminium oxide or AlTiN-type layers that act as a thermal barrier, and to improve cooling.

Notch wear

This is the localised wear concentrated at the depth-of-cut line, that is, at the boundary where the tool enters the workpiece. Its root cause is a surface layer work-hardened by the previous pass, cast skin or an oxide scale. Because this hard layer coincides with a single point on the edge, wear accumulates there.

The visual diagnosis is an abrupt deepening of the wear band at a particular height. It is most frequently seen in austenitic stainless steels and nickel-based heat-resistant alloys; the machining of these groups is examined separately in the milling stainless steel guide.

The most effective countermeasure is to vary the depth of cut between passes, so that the work-hardened layer does not fall on the same point of the edge every time. A smaller entering angle, round geometries and tougher grades slow down notch formation.

Built-up edge (BUE)

This forms when the chip cold-welds to the cutting edge under pressure. The welded material alters the geometry of the edge, breaks away after a time and tears particles from the tool surface as it goes. It therefore degrades both the surface quality and the integrity of the edge.

The visual diagnosis is an irregular deposit of workpiece material on the edge together with a bright, ragged surface. Low cutting speed, adhesive material and insufficient cooling are the principal causes.

The countermeasure is to raise the cutting speed, to select sharp geometries with a positive rake angle, to use coatings with a smooth rake face and to deliver the coolant directly to the cutting zone. The type, concentration and jet direction of the fluid also markedly influence the tendency towards built-up edge.

3. Sudden damage mechanisms

Plastic deformation

This is the change of shape of the cutting edge as it flows under temperature and pressure. When the cobalt binder phase of the carbide softens, the edge collapses or bulges sideways. The root cause is the simultaneous application of high cutting speed and high feed; in hardened materials the threshold is lower.

The visual diagnosis is a rounding of the edge as it loses its sharpness and a pronounced bulge on the flank face. Once plastic deformation has begun, flank wear increases rapidly.

The countermeasure is to reduce the cutting speed and the feed, to select a harder grade with a lower cobalt content, thick thermal barrier coatings and effective cooling.

Thermal cracking (comb cracks)

These are fine cracks perpendicular to the cutting edge and arranged parallel to one another. Because they resemble the teeth of a comb, they are known as "comb cracks". The root cause is thermal fatigue: in milling each tooth heats up as it enters the cut and cools rapidly with air or coolant as it leaves.

The visual diagnosis is a series of regularly spaced lines along the edge. When the cracks join up, the material between them breaks away and turns into chipping.

The countermeasure is to reduce the temperature fluctuation. In milling, either no coolant is used at all or it is applied uninterruptedly and in abundance; a half-pressure, irregular jet is the worst option. A tougher grade and a lower cutting speed delay crack formation.

Chipping and fracture

Chipping is the breaking away of small particles from the cutting edge, whereas fracture is damage to the edge or the tool as a whole. The root cause is mechanical overload: vibration, interrupted cutting, chip jamming, excessive feed or depth of cut, weak clamping and continuing to use a worn tool.

The visual diagnosis is irregular breakouts on the edge accompanied by a degraded surface. In the case of fracture the tool becomes unusable and the part is most often scrapped as well.

The countermeasure is to increase rigidity, to clamp the tool at the shortest possible length and with the shortest free overhang, to reduce the feed and depth of cut and to secure chip evacuation. For vibration-induced chipping, the chatter solutions guide can be followed.

4. Diagnosis and countermeasure table

On diagnosis: The same appearance can arise from more than one cause. The diagnosis has to be made by looking not only at the form of wear, but at the material group, the cutting speed and the cooling condition together. For distinguishing the material group, the ISO material groups guide is used.

5. ISO tool life criteria

Tool life can only be measured against a defined criterion. International standards quantify these criteria and make the comparison of different tools possible.

For measurement, ISO 3685 divides the cutting edge into zones; VB expresses the average width of the uniform wear band, while VBmax expresses the greatest localised width. The crater criterion KT is given by a feed-dependent relation. In milling, a limit of 0.3 mm for average VB and 0.5 mm for localised wear is widely used in practice.

In shop practice these values are not copied directly. While the criterion may be kept higher in roughing, in close-tolerance finishing operations tool life is deemed to have ended the moment dimensional deviation or surface roughness moves outside tolerance.

6. The Taylor tool life equation

The relationship between cutting speed and tool life is expressed by the Taylor relation:

vc × T^n = C

Here vc is the cutting speed (m/min), T the tool life (min), n an exponent depending on the tool material and C a constant for given conditions. In its extended form, feed and depth of cut also enter the equation as vc × T^n × fz^a × ap^b = C.

These ranges differ between sources and vary with the workpiece material. An exact value can only be determined by testing carried out for the relevant material–tool pair.

A numerical example: if a tool life of 30 min has been measured at n = 0.25 and vc = 200 m/min, then C = 200 × 30^0.25 ≈ 468 is obtained. When the speed is raised to 240 m/min, T = (468 / 240)^(1/0.25) ≈ 14.5 min results. A 20% increase in cutting speed roughly halves tool life. For parameter calculations, the cutting parameter tool can be used.

7. Economic assessment of tool life

Using a tool to the very end of its life is not always the cheapest option. In Sandvik Coromant's publications on manufacturing economics, cutting tool expenditure is given as approximately 3% of total production cost; the remainder is machine, labour and overhead. This ratio explains why savings made on the tool price are most often meaningless.

The tool-related cost per part is written as follows:

Ctool = (tc / T) × (Ck + Cm × td)

Here tc is the cutting time per part, T the tool life, Ck the cost of one cutting edge, Cm the per-minute cost of the machine and the operator, and td the tool change and resetting time.

The critical term of the equation is the product Cm × td. If a tool change takes five minutes, the cost of the machine time spent exceeds the price of the tool itself in most workshops. For this reason every measure that extends tool life quickly amortises the difference in tool price.

The economic tool life giving minimum cost is estimated through the Taylor exponent as follows:

Teconomic = ((1 / n) − 1) × (td + Ck / Cm)

At n = 0.25, td = 5 min and a Ck / Cm ratio equivalent to 10 min, the economic tool life comes out as 3 × 15 = 45 min. If the measured life is far below this value the cutting speed is too high; if it is far above, the machine is being run below its potential.

Planned replacement requires wear to be monitored. If the same job repeats, the life of a few tools is determined by VB measurement, after which this period is converted into a number of parts or cycles and a fixed replacement interval is defined. The correct choice of geometry also affects tool life directly; suitable types can be examined in the square end mills category.

Sources

• ISO 3685:1993 — Tool-life testing with single-point turning tools — iso.org
• ISO 8688-1:1989 — Tool life testing in milling — Part 1: Face milling — iso.org
• ISO 8688-2:1989 — Tool life testing in milling — Part 2: End milling — iso.org
• Machinery's Handbook — Cutting tool wear, tool life and the Taylor tool life equation, Industrial Press
• Sandvik Coromant — Wear on cutting edges — sandvik.coromant.com
• Sandvik Coromant — Manufacturing economics — sandvik.coromant.com
• Kennametal — Troubleshooting Indexable Milling — kennametal.com
• Harvey Performance Company — How to Avoid 4 Major Types of Tool Wear — harveyperformance.com