Knowing the Problem is only Half the Battle! Part 10
Determining Tool Wear Mechanisms and Correcting for Them!
Mechanical Failure, It Broke! Part 10 of 10
We are finally here, our 10th and final post in our series on determining failure mechanisms and correcting for them. Today we will discuss “It Broke.” But first, I must digress. Now this is important! Cutting tools are just like your kids. If you listen to them, pay attention, and nurture them typically you will be able to control them and keep them out of too much trouble. But if you ignore your kids and let them go off to their own devises, bad stuff is going to happen. Yes, cutting tools are just like kids you must pay attention to them. That is where “broken tools” come in. This is one of my favorite failure mechanisms because it is the easiest to identify, yet the most difficult to troubleshoot for most folks.
In one of my past lives, I was the North American Quality Manager for a leading cutting tool manufacturer. One of my many duties was to conduct final review and approval of all quality claims and field all customer complaints related to tool failure and quality. I can’t tell you how many times I would receive calls from customers upset with the explanation of what happen to the tool. It seems that “it broke” was not the detailed scientific conclusion that most people were looking for. Every time I approved “it broke” as the final disposition of the tool, customers would get incensed. The typical reply from the customer would be; “any idiot can see that “it broke” but why. That’s where the problem lies, I have no flipping clue. Look at this tool, tell me what happened. Yes, it broke! Any other answer would be pure speculation. The problem with analyzing the failure mode is this; the part of the tool that could tell me what caused it to break is somewhere in the machine, chip conveyor, chip hopper or dumpster outside.
The issue with cutting tools that break is, the evidence of the crime is gone, nowhere to be found, gonzo’s archives in history. Without any evidence of the crime how am I supposed to solve it. Cutting tools don’t just break. Something else usually happens first. There is always a primary failure mode that goes on for far too long and the tool eventually breaks. To determine what that something else is, you need to see the part that broke off. Unless the tool broke right off the bat; which would usually indicate operator, or programmer error, one of the other nine failure modes went too far and caused the tool to break. Yes, I said it, someone screwed up, tools just don’t break right off the bat unless something really, really wrong happened. Usually when something really, really wrong happens a human is to blame. No offence intended here, but it’s the truth. Cutting tools are designed to take a lot of “Heat and Pressure” and are not going to break quickly unless you did something wrong. So why did the tool break, because some other failure mode like; flank wear, built-up-edge, thermal cracking, cratering, thermal deformation, chipping, or notching went too far.
The only way to be sure what really caused the tool to break is to start over. Put a new tool in and look at the cutting edge after a few minutes of cut. Then determine which of the nine failure modes start to happen first. That is what we call the primary failure mode. By the way you want it to be flank wear. Any one of the nine failure modes, if gone unchecked, could end up looking one of the other nine failure modes in a very short time. This is called secondary failure mode. Bottom[RD1] line pay attention.
So, what do I mean; with primary and secondary failure modes. Let’s start with flank wear because “flank wear is you friend.” That’s right we like flank wear because it is predictable; however, if flank wear is forming too fast or goes too far it can one turn into one of several other secondary failure modes. As flank wear grows the cutting edge, it begins to abrade away material increasing both the heat and pressure at the shear zone. If the resulting increase in tool pressure becomes too great the insert may break. If resulting increase in heat exceeds the hot hardness of the tool material, it could plastically deform. In both cases the primary tool failure mode was flank wear, the secondary failure modes were breakage and plastic deformation. You must always solve the primary failure mode or root cause of the problem
BUE (Built-up-edge) can lead to several secondary failure modes too. As the workpiece material builds up on the rake face of the tool it begins to obstruct the chip flow. Eventually the build-up
will become too great and the chip flow will break it off. In most cases when the material breaks off it takes a piece of the carbide along with it. The remaining surface of the cutting edge looks like it chipped. If you correct for the secondary failure mode of edge chipping by using a larger edge preparation, hone, or t-land, it would only make the primary failure mode of BUE worse. If the chipped edge, caused by the BUE pull-out of material goes unchecked, it will more than likely become an even larger notch and eventually will cause the insert to break.
Thermal cracking can lead to secondary failure modes as well. You noticing a pattern here yet? Right, pretty much all primary failure modes to lead to a secondary failure mode if gone unchecked. Back to thermal cracking, do cracks cut? Of course not. So, what goes into the crack? Watch it! Yes, your workpiece material starts to fill the crack. When enough material gets shoved into the crack, the cutting edge either looks like it chipped or notched. If gone unchecked it will break too. Again, we need to solve the primary failure mode not the secondary.
Cratering and chipping can both lead to a secondary failure mode of breakage or thermal deformation as well. I consider both catastrophic failure. If the crater gets too big the cutting edge will weaken as its thickness and compressive strength erode. Chipping will do the same. With the cutting edge chipped away the heat and pressure will increase and eventually it will either get to hot and melt or too weak and break. To sum it all up, tools do not just break. One of the other nine failure modes is always the primary failure mode, unless it is operator or programmer error.
Remember; “cutting tools are just like kids, if you ignore them bad stuff is going to happen.” You need to stop, look at them, listen to them, and it’s not a bad idea to write down what you see. You don’t want to have to learn things twice it you don’t have too. You must always search and correct for the primary failure mode.