Tool wear describes the gradual failure of cutting tools due to regular operation. It is a term often associated with tools used for example on turning, milling, drilling and other types of machining operations where chips are made. We could also say “We started with a new cutting edge and at the beginning of the operation everything was working well. After a certain amount of time things started to change. Tolerances were out, surface finish was bad, vibrations occurred, more power was used and the many more things that can happen when the cutting edge has reached its end”.
What measures can we take to STOP this wearing out of our cutting edge?
Wear on carbide inserts is normal; don’t waste time and money trying to stop our cutting edge wearing out. This is an impossible task. Wear that is correct is safe, controllable and predictable. If after reading this article you still want stop the cutting edge wearing out then I can give you the answer now? Use a Cutting Speed of Vc=0m/min OR don’t use the tools. We can influence the wear behavior by changing the machining data. There is a relationship between a certain material and wear mechanisms. The objective is to have a predictable Flank Wear. A continuous wear and no wear peaks give us predictable behavior. Random wear is bad and gives us unpredictable productivity (volume). A great quote from a well-known American teacher of metal cutting :
"Knowing the problem is only half the battle!" -Mr. Ron D. Davies
Wear Failure Modes:
Normal Flank Wear
Normal Flank Wear, since it is predictable and consistent, is the most desirable wear condition.
Cause
Abrasive wear. Hard microscopic inclusions of carbide or work hardened material in the workpiece cut into the insert. Small pieces of coating break off and cut into the insert. The cobalt eventually wears out of the matrix, the carbide grains no longer have sufficient adhesion and they break out.
What to look for
- Relatively uniform abrasion along the cutting edge.
- Occasionally, metal from the workpiece that is smeared over the cutting edge can exaggerate the apparent size of the wear scar.
Corrective Actions
- Increase the feed.
- Reduce the cutting speed.
- Select a more wear resistant, harder carbide grade.
- Apply coolant correctly.
Crater Wear
Cause
A combination of diffusion, decomposition and abrasive wear causes crater wear. The heat from workpiece chips decomposes the tungsten carbide grains in the substrate and carbon leeches into the chips (diffusion), wearing a ‘crater ‘on the top of the insert. The crater will eventually grow large enough to cause the insert flank to chip, or may cause rapid flank wear.
What to look for
- Craters or pits on top of inserts.
- Chip breaking may improve after cratering starts.
Corrective Actions (Too rapid crater wear)
- Coatings containing thick layers of aluminium oxide are best.
- Apply coolant. High Pressure Coolant systems work well.
- Use a free cutting geometry to reduce heat.
- Reduce the cutting speed.
- Reduce the feed.
- Reducing the cutting edge angle will have a small, but positive effect.
Built up Edge (B.U.E.)
Cause
Material adhesion. BUE is a result of the workpiece material being pressure welded to the cutting edge. This occurs when there is chemical affinity, high pressure and sufficient temperature in the cutting zone. Eventually, the built-up edge breaks off and takes pieces of the insert with it, leading to chipping and rapid flank wear.
What to look for
- Shiny material on the top or flank of the insert edge.
- Erratic changes in part size or surface finish.
Corrective Actions
- Any coating will reduce built-up edge.
- Increase the cutting speed and or feed.
- Select an insert with a sharper cutting geometry and a smoother (polished) rake face.
- Apply coolant correctly, perhaps increase the concentration.
Chipping
Cause
A combination of thermal cycling (changing the temperature of the insert very rapidly), thermal load (Temperature differences between warm and cold zones) and mechanical shock causes thermal cracking. Stress cracks form along the insert edge, eventually causing sections of carbide to pull out and end up as chipping. Seen mostly in milling and interrupted cut turning.
What to look for
- Multiple cracks perpendicular to the cutting edge.
- Need to identify before chipping.
When to expect it
- Milling.
- Operations with intermittent coolant flow.
- Intermittent cutting operations in turning operations.
Corrective Actions
- Apply coolant correctly.
- Select a tougher carbide grade.
- Reduce the cutting speed and the feed rate.
- Use a free cutting geometry to reduce heat.
- Use a different machining method (ratio time in cut/time out of cut).
Plastic Deformation
Cause
Thermal overloading. Excessive heat causes the carbide binder (cobalt) to soften. Pressure on the insert makes the insert deform or sag at the tip, eventually breaking or wearing off or leading to rapid flank wear.
What to look for
- Deformation at the cutting edge.
- The dimensions of the workpiece may not be respected.
When to expect it
- High heat operations.
- High cutting speeds and feeds.
- Hard steels or work hardened surfaces and high temperature alloys.
Corrective Actions
- Apply coolant correctly. High Pressure Coolant systems work well.
- Reduce the cutting speed and feed.
- Select an insert with a larger nose radius. More Carbide Mass.
- Use a harder, more wear resistant carbide grade. Higher Hot Hardness
- Using free cutting insert geometry will have a small but positive effect.
Notching
Cause
Notching is caused when the surface of the workpiece is harder or more abrasive than the material further in, e.g. surface hardening from previous cuts, forged or cast surfaces with surface scale. This causes the insert to wear more rapidly in that part of the cutting zone. Local stress concentration can also lead to notching. As a result of the compressive stress along the cutting edge – and lack of the same behind the cutting edge – the insert is particularly stressed at the depth of cut line. Impact of any sort, such as hard micro inclusions in the workpiece material or slight interruptions, can cause a notch.
What to look for
- Notching or chipping at the depth of cut area on the insert.
When to expect it
- Materials with surface scale (cast or forged materials) or oxidation.
- Strain hardening materials.
Corrective Actions
- Reduce the feed and vary the depth of cut when using multiple passes.
- Increase cutting speed if machining a high temp alloy (this will give more flank wear).
- Select a tougher carbide grade.
- Use a chip breaker designed for high feeds.
- Prevent built-up edge, especially in stainless and high temp alloys.
- Select a smaller cutting edge angle.
- If possible use round inserts.
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