Reading Tool Wear: Flank, Crater, Notch and Built-Up Edge

A used cutting edge is a record of what happened in the cut. The position and shape of the wear show whether the speed was too high or too low, whether the grade was too hard or too soft, and whether heat or mechanical load did the damage. Machinists who look at the edge before throwing the insert away can correct the cause. Those who only replace it repeat the same life on the next edge. This article describes six wear patterns, their causes and the correction for each.

Where to look

Use a loupe of 10–20× magnification and good light. Look at the edge before cleaning it: built-up material is part of the evidence.

The six patterns

1. Flank wear

An even, matt wear land on the flank face, parallel to the cutting edge.

This is the normal, predictable wear mode and the one you want. It is caused by abrasion from hard particles in the workpiece.

2. Crater wear

A hollow on the rake face, a short distance behind the cutting edge.

The chip heats the rake face to the point where tool material diffuses into the chip. It is typical of machining steel at high speed. A deep crater weakens the edge until it breaks.

3. Notch wear

A deep, local groove at the depth-of-cut line, where the edge leaves the workpiece surface.

At that point the edge cuts through the hardest part of the material: a work-hardened layer from the previous pass, forging or casting skin, or scale. It is common in stainless steels and heat-resistant alloys.

4. Built-up edge

Workpiece material welded to the cutting edge. It grows, breaks away and takes pieces of the coating and edge with it. The machined surface is rough and torn.

It forms when the temperature in the cut is too low for the chip to flow cleanly, and in sticky materials: aluminum, low-carbon steel, stainless steel.

5. Chipping

Small pieces broken out of the cutting edge, leaving a ragged line.

The cause is mechanical: the edge is too brittle or too sharp for the load it sees.

6. Thermal cracks

Fine cracks at right angles to the cutting edge, like the teeth of a comb. They appear mainly in milling.

Each tooth heats in the cut and cools out of it. With intermittent coolant the temperature swing is large, and the surface cracks from thermal fatigue. Pieces between the cracks eventually break out.

Two end states

If the patterns above are ignored, the edge ends in one of two ways:

• Plastic deformation: the edge is pressed down or bulges because heat has softened it under high pressure. Reduce speed and feed, or choose a harder, more heat-resistant grade.
• Breakage: the insert fractures. It is usually the final stage of excessive flank wear, a deep crater or notch, or severe chipping. The original cause is destroyed with the edge, which is why edges should be changed and examined before this point.

Quick reference

When to change the edge

Flank wear follows three stages: a quick initial break-in, a long period of slow steady wear, and a final stage where wear accelerates until the edge fails. The edge should be changed near the end of the second stage.

In practice the limit is often reached first by a symptom: surface finish out of tolerance, dimension drifting, burrs, noise or rising spindle load. Convert the limit into a number the operator can use: parts per edge or minutes in cut.

Speed has the strongest effect on tool life. As a rule for carbide tools, raising the cutting speed by 15 % cuts tool life roughly in half, and lowering it by 15 % roughly doubles it. Feed has a smaller effect and depth of cut the smallest. So to remove more metal per edge, increase depth first, then feed, and speed last.