by Jan Andersson
Tool wear patterns can be used to maximize productivity
Tool wear is a topic often discussed, but rarely in terms of how we utilize wear patterns to maximize our productivity, minimize the tool life impact and gain predictability in the process. The three most common wear patterns are also the three most predictable wear patterns we see in every process: flank wear, crater wear and notch wear.
We have been told many times that flank wear is an abrasive wear. In the contact zone between the work piece and the cutting tool material, the abrasive nature of the material being machined causes wear. Can we make a material less abrasive? No, of course not. We need to adjust the cutting tool material to accept the abrasive nature of the process. Or do we? The secondary cause of flank wear is a chemical wear pattern, which is directly related to thermal and thermal management. The more efficiently we remove the heat from the cutting zone, the less impact the chemical wear will have, hence reducing the flank wear development. If the flank wear develops rapidly and is more of a deep V or U shape, then we get a flank wear that will cause premature end of life. Increasing feed rates will result in a larger chip area (roughly depth of cut times feed rate), which will in turn increase the mass of the chip, increasing its ability to transfer the heat away from the cutting zone, hence reducing the flank wear. What we end up with is a flank wear that is wider and shallower. And if you look at the flank of the insert, it looks like a smiley face, which is what you want as this will increase functional tool life.
Crater wear occurs on top of the insert. A crater that is developing right behind the microgeometry and is deep but not wide, will weaken the insert, increasing the risk of catastrophic failure. Checking the insert before the end of life and doing so frequently will give us the data we need to react in the correct way. Crater wear is primarily a chemical wear. As the chip is formed, it gets its initial lift from the microgeometry and will curl towards the top of the insert, making impact and causing the heat from the chip to develop a chemical wear. The secondary cause of cratering is mechanical; the forces of the chip make contact, aggregating the crater development. By increasing feed rate, we get a larger chip area, causing the chip to travel further away from the microgeometry before contacting the top of the insert, and we will also have a greater mass to transport heat away from the cutting zone, resulting in a lessened impact of chemical wear. As the insert travels further, the crater will develop further away from the microgeometry and grow wider, but shallower and towards the centre of the insert, reducing the risk of catastrophic failure and providing a more predictable process. Also, due to the shape and growth direction of the crater, we can also allow a greater degree of crater wear to develop, resulting in longer functional tool life.
Notch wear is primarily a mechanical wear with a secondary chemical component. Notch wear, also called depth of cut notch, is caused by the surface structure from prior operations or material conditions. These include the material changes and stresses in a forged surface, material changes in a surface structure of a cast material, and stresses in surface from prior machine processes. All these will cause a microlevel structure that is difficult to machine and concentrates stresses in a small area right at the depth of cut. Secondary cause is a chemical wear. A notch that is developing deep and as a narrow V shape will weaken the insert and potentially cause catastrophic failure. Again, increasing feed rate will give a larger chip area and a more efficient thermal evacuation, reducing the chemical impact. This in return will cause the notch to develop as a wider, but less deep notch, greatly reducing, or eliminating the catastrophic failure risk.
In all three instances, we increase the feed rate resulting in a larger chip area, which increases our thermal evacuation and reduces the chemical wear. It is also related to how the chip forms and where it impacts the insert surface. All these will result in effectively longer tool life and a more predictable tool life. As we increase our feed rate, we also gain productivity. SMT
Jan Andersson is the North American director-indexable inserts for YG1.
