Tool life can be long and productive, or short and disastrous. Understanding the basic forces in the metalcutting process that contribute to tool wear or failure will help you use today's tool technologies to ensure that your cutting tools will have a long and productive life.

In the violent world of metalcutting, cutting tools must resist extreme heat, high pressure, abrasion, and shock. Temperatures at the cutting edge can exceed 1000º C. Extreme heat degrades binders and other tool constituents, and can also trigger detrimental chemical reactions between tool and workpiece. Abrasion is always part of the cutting process. While in the cut, the tool is in constant contact with the workpiece, under pressures greater than 2000 psi (13 790 kPa).

Varying levels of thermal and mechanical shock also play a role in tool failure. Thermal shock--rapid heating and cooling of the tool--is most common in milling operations, in which the insert heats up while cutting and then cools while away from the cut. Mechanical shock is also a factor in milling, in machining interrupted surfaces, and even in turning, depending on the operation involved and the condition of the workpiece.

Basic failure mechanisms include crater wear, thermal deformation and cracking, nose wear, depth-of-cut notching, built-up edge, chipping, fracture, and flank wear. Tool manufacturers design tools to resist these mechanisms but must make compromises. For example, as cobalt content increases, a carbide insert's toughness generally improves while its hardness declines. Tradeoffs must be made among hardness (wear resistance), toughness (impact resistance), and chemical stability.

Hard coatings introduced in the late 1970s make it easier to balance hardness and toughness. Now toolmakers can fine-tune the elements of the substrate/coating system, combining the impact resistance of a high-cobalt substrate with the wear resistance of hard coatings such as titanium nitride or alumina. They now have ways to create zones of cobalt enrichment at the cutting edge, so that the edge is tough while the rest of the tool remains hard.

The first coatings, effective as they were, were applied by the chemical vapor deposition, or CVD, process. High temperatures used in CVD could degrade edge strength, however, so toolmakers developed alternatives--physical vapor deposition (PVD) and medium-temperature CVD. The lower temperatures used in PVD permit coating sharp insert edges; medium-temperature CVD coatings maintain edge strength better than higher-temperature processes. Coating materials such as CVD alumina ceramic and PVD TiAlN also broaden the ever-widening selection of substrate/coating systems that can be custom-tailored to resist specific wear mechanisms.

As complex as all this may sound, the list of reasons why tools fail is a short one--only eight basic possibilities. Any one of these factors, or a combination of two or more, can produce tool failure.

Crater Wear. Crater wear occurs on the rake face or top of the insert, typically when machining steels at elevated cutting speeds. Unlike abrasive wear, this kind of wear is caused by a chemical interaction between the hot chip and the workpiece material. When a tool is used to machine steels and other materials at high speeds, the tool material may dissolve into the chip or tiny particles of the tool may adhere to the chip and get carried away. In either case, a crater forms. Excessive cratering weakens the cutting edge, inhibits proper chip flow, and increases heat and pressure on the tool. Left unchecked, crater wear can lead to tool fracture.

This insert's cobalt-enriched substrate provides improved toughness, and its multilayer coating improves wear resistance.

To combat crater wear, tool manufacturers can increase the chemical stability of the tool material, as when they added titanium carbide (TiC) to tungsten carbide (WC) in the first successful steel-cutting carbide tool. Applying a hard coating to put a hard, inert barrier between tool and workpiece at high cutting speeds will also minimize crater wear. Tool geometry can also make a difference. A positive-rake tool will reduce tool pressure and decrease contact between the chip and the insert, and the reduction in pressure and contact can reduce crater wear.

Thermal Deformation. Heat and pressure generated by machining can cause the cutting tool's binder to soften, allowing the carbide grains to move. Little insert material is actually worn away, as it is in crater wear, but the nose of the insert becomes distorted. As thermal deformation progresses, heat and cutting pressure increase. Inconsistent part size and tool breakage can follow.

To minimize thermal deformation, users can pick inserts with lower cobalt content, finer carbide grain sizes, and a higher cubic carbide content (particularly TaC). Hard coatings also help minimize thermal deformation because they decrease generation of frictional heat.

Thermal Cracking. Large differences in temperature between the cutting edge and the bulk of the insert cause evenly spaced cracks perpendicular to the cutting edge. Interrupted cutting (as in milling) or machining materials like titanium that generate high heat when cut can cause these temperature fluctuations. Cracks will progress slowly, leading to chipping and eventually to tool fracture.

Choose a carbide tool grade with a high cobalt content for toughness and resistance to thermal shocks and cracking. If possible, avoid using coolant. If you must use it, be sure that the flow is strong, steady, and well-directed, so that it will moderate the temperature changes experienced by the tool.

Evenly spaced cracks perpendicular to the cutting edge show thermal cracking.

Nose Wear. When machining hard alloy steel, rubbing or abrasion and local deformation of the tool's nose into the workpiece can occur. As the tool nose wears, part size changes and surface finish deteriorates. Often, the workpiece material will smear.

Choose an insert grade with a higher cubic-carbide content and a larger nose radius to reduce nose wear.

Depth-of-Cut Notching. When machining stainless steels, high-temperature alloys, and work-hardening materials that generate high cutting temperatures, depth-of-cut notching can occur at the free end of the chip. A depth-of-cut notch can cause a burr to form, leading to tool fracture.

A popular way to minimize depth-of-cut notching is to use an insert grade with higher cobalt content. Alumina or TiN coatings can also help alleviate depth-of-cut notch. Because it extends the depth of cut over a greater length of the cutting edge, a lead angle tool is often helpful. A simple modification of the part program to vary cut depth may minimize the problem.

Tool fracture has a variety of causes.

Built-Up Edge. When soft materials like aluminum, brass, or soft steels are machined, the workpiece material can bond to the cutting tool chemically and mechanically. Built-up edge can increase tool pressure and cause poor surface finish, part size changes, and tool breakage. The buildup is often unstable, and is periodically washed away by the cutting action. Part size and finish will fluctuate. Tools can chip or break because of built-up edge, but users may not recognize it as the cause.

You can minimize edge buildup by using a smooth or polished cutting edge or a cutting tool with positive-rake geometry, or by increasing cutting speed. Picking a grade with lower cobalt and/or finer-sized carbide grains will raise the tool's abrasion resistance and can reduce surface roughening, and thereby help control edge buildup. A sharp cutting edge also can reduce the occurrence of built-up edge.

Chipping/Fracturing. Non-rigid setups, with vibration or inconsistent cutting pressures, can cause a tool to chip. Interrupted cuts can often cause chipping or fracturing. Tool fracture can occur when one or more failure mechanisms weaken the tool, or when cutting forces rise to such a level that the insert can no longer bear the load.

Pick a tool with high cobalt content and strong edge geometry, like a hone or chamfer. Don't overlook the possibility that part rigidity and the workholding system may be causing chipping.

Flank Wear. All tools wear, but some types of wear are more desirable than others. The motion of the tool's flank face against the surface of the workpiece causes flank wear. Most users think "uniform flank wear" is the most desirable type because it's predictable. When wear dulls the cutting edge, tool pressure and stress on the machine and the piece part increase. Excessive edge wear causes deflection of the part and a change in part size and forces more heat back into the part. The longer edge wear is left unchecked, the worse it gets. In the end, it can precipitate tool fracture.

Interrupted cuts often cause chipping.

Where edge wear is the predominant wear mechanism, use a more wear-resistant insert grade to extend tool life.

In a perfect world, a shop has the time and resources to optimize every metalcutting operation. Even in the real world, when the firm concentrates on high-volume, high revenue jobs, improvements in productivity are multiplied over a long production run, and it's worth researching, testing, and choosing the optimum grade for the cutting tool. In an environment of short production runs and rush jobs, however, shops often find that a lengthy tool evaluation process is not worth the investment, and that economical management of a large inventory of seldom-used cutting tools is too difficult.

Cutting tool manufacturers, responding to the problem, have begun to offer tools versatile enough to handle a wide range of workpiece materials and cutting parameters. Our company's KC8050 "universal" turning grade, introduced at IMTS 2000, is a case in point. It has a cobalt-enriched substrate and a multilayer coating that balances toughness and wear resistance.

Despite new materials, coatings, and substrates, the basic forces in the metalcutting environment haven't changed over the years. Tool designers must always juggle hardness, toughness, and chemical stability to get the combination that best protects users against tool failure. However, more than six decades of innovation in carbide tooling technology has increased the range of options available to match tools to the job and overcome the forces of failure and wear. Take advantage of them.