Corner wear and notch wear are predominant reasons for tool failure of end mills. Notch wear becomes larger because the boundary between the heat generating part of the cutting edge and air tends to oxidize. Corner wear increases as it is simultaneously subjected to two different cutting forces. These forces are directed to the peripheral and end cutting edges. End cutting edge wear decreases towards the centre due to the concave angle of the cutting edge. Crater wear makes the radial rake angle smaller, and decreases the sharpness, but this has less of an affect on the tool as notch wear and corner wear.
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The types of tool wear found on solid end mills includes corner wear, peripheral cutting edge wear, notch wear, end cutting edge and crater wear. The image shows the different types.
Solid ball nose end mills
Solid ball nose end mills suffer from a different form of tool wear. When machining low hardness workpiece materials the centre of the end cutting edge is prone to wear as shown in figure a. This is because cutting speed near the centre is very low and so the cutting edge but tends to crush the workpiece rather than cut it. Continued machining creates wear as shown in figure b. It is believed that this is due to welding created by the crushing action.