Knowing the Problem is Only Half the Battle! Part 2
Updated: Oct 22, 2019
Determining Tool Wear Mechanisms and Correcting for Them.
BUE or Built-up-Edge! Part 2 of 10
During our last blog, we introduced you to the three groups of failure mechanisms; abrasive wear, heat induced wear and mechanical wear. Cutting tools are going to wear out that is why they are considered perishable tooling. The key is to have the tool wear out on your terms not on its terms. The only acceptable form of tool wear is normal flank wear. As we stated last week “flank wear is your friend.” This week we will begin discussing heat wear mechanisms; how to identify them and better yet how to correct them.
There are three basic types of heat wear failure mechanisms; built-up-edge, cratering, and
thermal deformation. For this session, we will focus on built-up-edge or what is often called BUE for short. BUE or built-up-edge is just that; a buildup of material on the cutting edge. This build-up or adhesion of workpiece material onto the cutting edge is caused by cold welding. As you can see in the illustrations this buildup can be deposited on the top rake surface of the cutting tool or on the bottom clearance face of the tool. Let’s take a closer look at rake face built-up edge.
There are several cutting conditions and tool characteristics that can cause rake face buildup to occur. What two things does it take to make a chip; heat and pressure; however, when you do not have enough heat at the shear zone the workpiece can begin to cold weld to the cutting edge causing built-up-edge. I call it the “Goldilocks Effect,” you mustn’t be too hot, as to much heat will plastically deform the tool. You mustn’t be too cool, as the workpiece material will cold weld itself to the rake face of the tool. Therefore, the temperature must be “just right.” A word of caution; speed has a 2:1 impact on heat and tool wear so start with small incremental increases.
On soft or gummy materials, or materials with a high “modulus of elasticity” you may also see built-up- edge. These types of materials require sharp edged tools with minimal edge preparation, hones, or t-lands. The free cutting edge geometry allows for unobstructed chip flow and reduces compression at the cutting edge. Again, remember that when you reduce the edge preparation you make the cutting edge a little weaker and more susceptible to chipping so again, make small incremental changes.
Changing the rake angle to provide a more positive cutting action can also improve chip flow. Negative rake tools compress the chip more and increases the amount of surface contact between the chip and rake surface of the tool. Using a more positive rake face reduces pressure, and the surface contact between the chip and tool. Remember positive rake angles are weaker than negative so start with small incremental changes.
Last but not least, using PVD coated tools will improve chip flow as well. PVD coatings are thinner than CVD coated tools therefore shaper edges and freer cutting action. In addition, PVD coatings are smoother than CVD. Due to the high temperatures required in the CVD coating process CVD coatings have small micro cracks caused by differing thermal expansion rates of the substrate and coating during the coating process. The PVD coating process is conducted at a much lower temperature where thermal expansion is not an issue. That pretty much sums it up for rake face chipping.
Now let’s take a closer look at flank or clearance face buildup. Flank or clearance face buildup is caused by excessive rubbing between the workpiece and tool due to minimal clearance. This type of build-up is more often seen on Internal machining operations like ID boring, threading, grooving operations, and applications where the tool may deflect thus reducing the tool's clearance. In these cases, increasing the clearance angle of the tool is always helpful; however, care should be taken not to increase the clearance to much sacrificing tool strength. If tool deflection on ID operations is causing the build-up, using a more positive rake face geometry and or reducing depth of cut or the feed rate may reduce the deflection thus preventing the reduction in clearance. You can also adjust the tool above center. Given you know the material of your bar and workpiece, and the speed, feed, and depth of cut; you can calculate the amount of tool deflection. You can then set the tool above center and once it enters the cut if will deflect into the correct position and provide proper clearance. (click here for deflection calculation)
In summary, BUE or Built-up edge, is caused by the workpiece material cold welding itself to the rake or clearance face of the tool. This build-up or adhesion of the workpiece material onto the cutting edge negatively impacts tool life, part size, and surface quality. Once the build-up breaks off the tool it typically takes the cutting edge with it causing a chip. By using the methodology described you can eliminate BUE and once again find your friend “normal flank wear”.
Next week we will discuss the second heat failure mode “cratering”.