Knowing the Problem is only Half the Battle! Part 6
Updated: Nov 18, 2019
Determining Tool Wear Mechanisms and Correcting for Them!
Mechanical Failure, Flank or Clearance Face Chipping! Part 6 of 10
For the past, several weeks we have discussed the four modes of heat failure. The temperature at the shear zone must be just right, not too hot and not to cold. Now we will move into mechanical failure mechanisms. There are three primary modes of mechanical failure; they are chipping, notching, and mechanical fracture. In today’s discussion, we will tackle chipping as it is one of the most mis-understood.
I will never forget the man who taught me about insert chipping. His name was Nick Clashman. He was a very smart man with a passion for knowing how things work. He taught me how to read or as he called it "listen to the cutting tool". He used to state; “that if you pay close enough attention to your cutting tools, they will tell you exactly what is causing them to wear out,” or in this case chip. All you must do, is pay attention.
There are three different types of chips that can form on the cutting edge. Each one is created by a different mechanical action taking place on cutting edge. The three types of chipping are; flank or clearance face chipping, rake face chipping and edge chipping. Let’s start off with flank or clearance face chipping, but before we do; what two things does it take to make a chip, “Heat and Pressure.” As we have stated before pressure or cutting forces are always perpendicular to the cutting edge. If this force exceeds the transverse rupture strength of the cutting edge the cutting edge will chip.
Flank or clearance face chipping is just that; a large chip on the flank or clearance surface of the cutting edge. As Nick Clashman would say, “look at the cutting edge, what is it telling you?” When you look at the cutting edge and see a chip that is larger on the flank or clearance face than it is on the rake face it is flank or clearance chipping. You ask yourself, OK what now. If you know what type of chip it is you know where the force came from that created the chip. If you know where the force came from you can now fix the problem.
In flank or clearance face chipping the force came from a direction perpendicular to the rake face and parallel to the widest part of the chip. Another way of looking at it is the force hit the rake face and popped the flank or clearance face of. You can also look at it this way; the surface of the tool where the chip is narrow is where the force hit the tool, the surface of the cutting edge with the widest section of the chip is the direction of the force.
So how do we correct for Flank or clearance face chipping? What did Obi-Wan tell Luke to do with the force? Use the force. While you do your best to eliminate or reduce the force, by definition, plastic deformation requires force. Therefore, in this case, we want to start by “redirecting the force.” We can do that by changing the edge preparation. Using a larger hone, a wider T-land or greater T-land angle we can strengthen the cutting edge by “redirecting the cutting forces” into the bulk strength of the tool. Remember carbide has great compressive strength but very little transverse rupture strength. You always want the cutting forces directed into the bulk of the tool. Your next option to strengthen the cutting edge would be to change the rake angle of the tool. Using a more negative rake angle will also “redirect the cutting forces” take the cutting edge from a transverse state to a compressive state.
The last option would be to use a tougher grade of carbide. That would be a grade with more cobalt and larger tungsten carbide grains, which would typically have a larger number in the grade designation. Remember cobalt is like the glue or binder that holds everything together. The more glue the more difficult it is for the force to dislodge the larger grains of tungsten carbide. It is important to note: using a tougher grade of carbide will require you to slow down your cutting surface speed. Tougher grades of tungsten carbide cannot take as much heat as harder more ware resistant grades.
In summary, flank or clearance face chipping is caused by an impact on the rake face of the cutting edge that produces a larger chip on the flank or clearance face of the cutting edge than the rake face. There are three primary options when correcting for flank chipping. The first two address the redirection of the cutting forces into the bulk strength of the tool. This can be accomplished by increasing the edge preparation by adding or increasing the hone size and/or increasing the T-land size or angle. You may also strengthen the cutting edge by using a more negative rake angle. These actions will strengthen the cutting edge by redirecting the cutting forces and by doing so puts the cutting edge into a state of compression versus transverse rupture. The final method of reducing flank or clearance face chipping would be to use a tougher grade of carbide. But remember if you use a tougher grade of carbide you may need to reduce your speed.
Visit us next week when we address the second form of mechanical failure; rake face chipping.