Knowing the Problem is Only Half the Battle! Part 1
Determining Tool Wear Mechanisms and Correcting for Them
Part 1 of 10, Flank Wear is Your Friend!
I was surfing the net and ran across a post form a large tooling manufacturer that illustrated the basic failure mechanisms of carbide cutting tools. The pictures were the basic one’s that all cutting tool manufacturers use, as were the descriptions and definitions. But that is where the post stopped. I was left there hanging, all dressed up with no place to go. They showed me how to identify them but left out how to fix them. That really bothered me, so with that said I decided to start this eight-part series on tool failure modes and HOW TO FIX THEM.
There are eight basic failure mechanisms which can be placed into three groups; abrasive wear, heat wear and mechanical wear. There is only one failure mode in the first group abrasive wear and that is what we call flank wear. In the second group; heat related wear we have four different types of failure mechanisms; built up edge, thermal cracking, crater wear and thermal deformation. In the final group, mechanical wear, we have three types of wear or failure mechanisms; chipping, notching, and fracture. In this eight-part series we are will describe and define each of the eight failure mechanisms and provide workable solutions to reduce and eliminate then to improve your tool life and drive productivity.
Before we get started there is one thing I must make perfectly clear, cutting tools are considered perishable tooling. Yes, they are supposed to wear out. That heat and pressure at the shear zone required to plastically deform your workpiece material to form chips and make your part is going to impact your cutting tool. It is going to wear it out; that is if you’re lucky and don’t break it first. The key is having it wear out on your terms and not its terms. That is where flank wear comes in. “Flank wear is your friend.”
What do I mean by “flank wear is your friend”? Of the eight types of failure mechanisms, flank wear is the one you always want to have. Remember, cutting tools are perishable and are going to wear out, you just want them to do it on your terms. Before we answer why it’s your friend we must first make sure you know how to spot it. As the photo illustrates flank wear is a “silver
or gray” area that’s width starts at the cutting edge and expands down the clearance or relief surface of the tool. The length starts at the depth of cut line and extends around the nose or radius of the tool. It looks like someone ran a file across the cutting edge; of course, carbide is harder than a file and it would not leave a mark, but use your imagination. Yes, it is “silver or gray” in color and not black. This is the most important and misunderstood misconception about flank wear. Most people think it is the “black” stuff on the clearance or relief surface of the tool. Let me make this perfectly clear; “Flank WEAR IS NOT BLACK it is SILVER”. The black stuff is carbon residue not unlike the stuff you see on your fireplace hearth. In fact, when cutting tool manufactures check tool life in their labs under a microscope they use acetone to remove the carbon residue to get an accurate measure. So, know that we know what it does and does not look like what causes flank wear.
As we know by now at the shear zone in a metal cutting operation there is a lot of heat and pressure. That high heat and pressure cause the cutting edge to break down over time resulting in wear. You want flank wear to be your primary failure mode because it is predictable. Flank wear is the only failure mechanism of the eight that is predictable. Normal flank wear can be plotted on a curve as you see here using the width of the wear over time on the axis.
There are three zones in the curve the A, B, and C zones. The A zone is call the break in zone when a small amount of wear can grow rapidly. Then you enter into the B zone or what is called the predictable wear zone. What we mean by that is; for a, given unit of time in cut you will have a given amount of flank wear growth. It is kind of the rise and run of a set of steps in your house, you go up the same amount for each step. As the cutting edge is being further abraded away and flank wear grows in the B zone, the heat and pressure at the cutting edge continue to rise and you end up in the C zone. Another name for the C zone is the “oh no zone” because “oh no you don’t know when it’s going to go.” Typically, you want to change the cutting edge just before you enter the C zone. This can only be done given the predictability of flank wear.
Once flank wear is achieved you can begin zeroing in on the grade of carbide, topography of the rake face, speed to product more tool life. When it comes to the carbide grade you typically want to use the hardest most wear resistant grade possible that still resists chipping. Harder grades of carbide have smaller grains of tungsten carbide that are tightly packed together. This allows them to dissipate the heat faster and last longer. That is the good thing but don’t go to hard as the harder the grade the less cobalt which make is more suitable to chipping. Using a tool with a rake face that contains bumps or divots will reduce the surface contact between the chip and tool reducing heat transfer and improving tool life. Using the most open chip breaker possible that will still product manageable is also a plus. Everyone believes you want the tightest chips possible, but the opposite is true. The tighter the chip the greater the heat and pressure exerted on the cutting edge and the faster the wear.
The next step after achieving flank wear would be to optimize your operation, but that is another topic all together. Stay tuned to next week’s blog on where we will begin to look at heat wear and failure mechanisms.