Knowing the Problem is only Half the Battle! Part 5
Determining Tool Wear Mechanisms and Correcting for Them!
Thermal Deformation! Part 5 of 10
Today we will discuss the fifth and final heat related failure mechanism “thermal deformation.” So, want is thermal deformation? Thermal deformation is what happens when the heat and pressure at the shear zone are so great that they plastically deform the cutting tools cutting edge. Hence the term thermal deformation or heat deformity. This high heat and pressure being exerted on the rake face of the cutting edge plasticizes the tool. A depression forms on the rake face of the tool and a bulge forms on the clearance face. Basically, the tool gets so hot the cutting pressure squishes the cutting cutting edge. Think of a marshmallow or tube of toothpaste, you squeeze it in on direction and the material pushes out in another. The same thing happens to the cutting edge. Unfortunately; when the cutting tool material gets squeezed on the top or rake it bulges out on the side or clearance face underneath the cutting edge. The the plasticized bulge of material typically gets rubbed off.
In most thermal deformation failures you never get to see the clearance face bulge of material caused by the rake face depression. Usually the whole corner of the cutting edge is missing; scraped off I like to call it. When the whole nose of the cutting edge is missing a lot of times we think the tool broke or chipped when actually the primary failure mode is thermal deformation. It is extremely important to understand and recognize the difference between the two. If you correct for the wrong failure mode, you will only make the problem worse.
To determine the difference between "it broke" and "thermal deformation" you must look closely at the gray or silver surface remaining where the corner of the tool used to be. If the remaining material on the clearance surface looks like the material was scraped off and has straight vertical lines or striations; it failed due to thermal deformation. If the remaining material on the clearance surface looks like a piece of gravel or a rock with jagged rough edges it broke or chipped. As stated; it is important to understand and recognize the difference, thermal deformation is caused by the tools inability to withstand extreme heat and pressure whereas breakage is caused by the tools inability to withstand extreme mechanical impacts. The corrective actions for each of these is very different.
Thermal deformation of the cutting edge occurs when the extreme heat and pressure at the shear zone exceeds the hot hardness of the cutting tool material. The first corrective action to take would be to use a harder more heat and wear resistant grade of carbide. This would be a grade with a lower amount of cobalt and smaller more closely packed tungsten carbide grains. This type of carbide grade would have a smaller number in the ISO grade designation. You may also change the rake face geometry to one that that is more positive and/or one that contains bumps or divots which reduce heat transfer between the chip and rake face of the cutting tool. Using copious amounts of coolant targeted directly on the cutting edge in non-interrupted cuts can also reduce the temperature at the shear zone. These actions will not impact productivity; however, they will make the tool a little more brittle or fragile and susceptible to chipping or breakage.
The last two corrective actions you may take would be to slow down the cutting speed and/or reduce the feed rate. By reducing the speed and feed you reduce the heat being generated at the shear zone. Each of these will also have a negative impact on productivity but not by equal amounts. Speed has a ratio of 1:2 impact on tool life and “heat” versus feed rate which has a ratio of 1:1. Reducing your cutting speed by 10 percent will yield a 20 percent improvement on tool life; whereas, a 10 percent reduction in your feed rate will only yield a 10 percent improvement in tool life. Remember a key note when adjusting your running parameters, DOC (depth of cut) is free. DOC has a ratio of 1:0 impact on tool life. Once you have reach a depth of cut equal to 10 times your feed rate, further increases in DOC have virtually no impact on tool life. Therefore, if you are forced to slow down your speed or feed to reduce the heat you may be able to increase your DOC and reduce passes to regain your productivity.
In summary, thermal deformation is caused by extreme heat and pressure at the shear zone which exceeds the hot hardness of the cutting tool material resulting in the cutting edge being plastically deformed and scraped away. Your first choice to correct for thermal deformation would be to use a harder more wear resistant cutting tool material. Your second choice would be to reduce the heat at the cutting edge by using coolant in non-interrupted cutting applications or reducing the speed and feed rates.
Stay tuned for next week’s blog when we begin discussing mechanical tool failure modes.