Knowing the Problem is Only Half the Battle! Part 3
Determining Tool Wear Mechanisms and Correcting for Them
Cratering! Part 3 of 10
Last week we discussed built-up-edge one of the three heat failure mechanisms. This week we will teach you how to identify the second type of heat failure “cratering”. We will review the causes of “cratering” and detail the methodology used to eliminate it. But first we must remember; what two things it takes to make a chip; heat and pressure. This heat and pressure if not adequately managed will result in “crater” wear and possibly even catastrophic failure.
Cratering is rather easy to spot if you know what you are looking. The name of this failure mechanism truly says it all. Cratering is just that; a crater that forms on the rake face of the cutting to just behind the cutting edge. As you see in the illustration it looks just like a crater on the surface of the moon. As this crater grows it will eventually breakout to the cutting edge and catastrophic failure will follow.
Cratering is caused by high heat and pressure and the friction created by the constant chip flow across the rake face surface of the tool. This high heat and pressure causes diffusion or dissolution of the tool material into the chip. Yes, the tool material dissolves into the chip. You typically see this failure mode when machining high carbon steels at high surface speeds. There is carbon in both the carbide cutting tool material and in the high carbon steel. Carbon atoms have a lot of free electrons flying around the nucleus. When the carbon electrons in the tool get hot, they get excited begin bonding with the carbon electrons in the chip. As they diffuse from the tool the crater is formed.
The first thing that most people do to prevent cratering is to slow down the operation; however, that negatively impacts productivity and profitability. Before we slow down the speed to reduce the heat at the shear zone we would want to be sure we are using the hardest most wear resistant grade of carbide possible for the operation. Using carbide grades with higher concentration so titanium carbide is also helpful. Too many times; however, we only focus on the carbide grade or speed to reduce the heat. We must also take a close look at the tool geometry as well.
As you can see in this illustration the crater is forming right up against the wall of the chip forming geometry of the insert. When it comes to making chips, it is not always a good thing to have them as tight as possible. Yes, you want them to form in the “C” or “9” of”6” shape, but you want that to be as open a shape as possible. If we get to aggressive with the chip groove geometry as in the illustration, you will drive the chip right into the wall of the chip breaker causing a crater to form very rapidly. In these cases, you may want to move to a less aggressive chip breaking geometry i.e. move from a fine finishing to a medium machining, or from a medium machining to a roughing geometry. Having the tightest chip possible is not always a good thing. While you must have chip of a size and shape that can be easily handled by your chip evacuation system having tight chips creates more heat and pressure causing more rapid tool wear.
So, when cratering appears on the rake face of your tool, first be sure you are using the hardest most wear resistant carbide grade available for the material you are machining. In continuous cut applications ensure good coolant flow. Use a more positive rake face geometry. Finally, if all else fails you may reduce the speed and feed.
Stay tuned for next week’s blog when we will review the third and final heat failure mechanism “thermal deformation.”