Tool Failure is an inevitable aspect of a machine tool. Whenever your machine tool is not giving a satisfactory metal cutting finish, then there is a high chance of Tool Failure.
Effects observed during Tool Failure.
During Tool Failure, certain adverse effects are observed during a metal cutting operation.
- Higher power consumption
- Overheating of cutting tool
- Development of the burnishing bands on the work surface
- Extremely poor surface finish developed during tool failure.
- Work dimensions do not meet specifications.
Causes of Tool Failure
Tool Failure can occur due to one or more of the following reason:
- Thermal cracking and softening
- Gradual wear
- Mechanical chipping
Thermal Cracking and Softening
During the metal cutting process lot of heat is generated. Due to the heat, the tooltip and the area closer to the cutting edges become very hot. Every cutting tool has a certain limit to withstand the elevated temperature; when it exceeds that limit, the tool material starts deforming plastically. The tooltip and the cutting edges are mostly affected due to cutting pressure and the high temperature. When this plastic deformation occurs, the tool loses its cutting ability due to softening.
Causes of Thermal cracking and Softening
- High Cutting Speed
- High Feed Rate
- Small Nose Radius
- Wrong Tool Material
- Excessive Depth of cut
Operating temperature of common tool material
- Carbon tool Steels 200 C – 250 C
- High-Speed Steels 560 C – 600 C
- Cemented Carbides 800 C – 1000 C
Due to fluctuation in temperature due to overheating, the tool material faces local expansion and contraction, which give rise to temperature stresses or thermal stresses. These stresses cause cracks to de known as Thermal cracks.
Mechanical Chipping
Mechanical chipping is observed in the area near the nose or the cutting edge of the tool.
Causes of Mechanical chipping
- High cutting pressure
- Excessive wear
- Mechanical Impact
- High vibration
- Weak tip
Mechanical chipping is generally seen in carbide-tipped and diamond tools due to their high brittleness nature.
Gradual Wear
When a tool is found to lose weight or mass after extended use for a period of time, it is suspected that the loss in some material is due to wear. There are two types of wear found to occur in cutting tools.
Crater wear
This type of wear generally occurs on the face of the cutting tool. Crater wear is observed in ductile materials like steel and steel alloys, where continuous chips are produced during a metal cutting operation. The formation of crater wear is due to the pressure of the hot chip sliding up the face of the tool.
Flank wear
This type of wear occurs in the flank region below the tool’s cutting edge. Flank wear occurs due to abrasion between the tool flank and the workpiece, due to which e excessive heat is generated, causing plastic deformation leading to flank wear. This type of wear is the most common cutting tool wear and is more noticeable while machining brittle materials. The workpiece loses its dimensional accuracy, power consumption, and poor surface finish due to flank wear.
Prevent Tool Wear
Tool wear is inevitable, and every tool will have to experience tool wear at some point in time. There are certain things you can do to prolong tool life are as follows:
- Adjusting the cutting speed and optimizing coolant usage
- Reduce vibration
- Reducing feed rates
- Adjusting Depth of cut
Tool Failure can affect your work performance and finish. So it is advisable to inspect your cutting tool after prolonged use to identify the various signs of tool wear. The beginning of most of the wear and cracks might be microscopic, and it would be hard to detect the defect visually. However, specific adverse indications, as mentioned above, can give you a clear indication that it is time to change your cutting tool.