Knowing the Problem is only Half the Battle! Part 4
Determining Tool Wear Mechanisms and Correcting for Them!
Thermal Cracking! Part 4 of 10
What two things does it take to make a chip; heat and pressure.
We know that at the shear zone the temperature is 1200 to 1400 degrees Fahrenheit. Now, what happens when you pour cold water into a hot glass? You pick up the pieces of the shattered glass! So, what do you think happens when you put room temperature coolant on a 1200 to 1400 degree insert cutting edge? Exactly it’s not always a good thing.
What happens to most types of materials when they begin to heat up? They grow. What happens to most materials when they cool down, they shrink? The same thing is happening to the cutting edge of a tool. When it is in the cut it is 1200 to 1400 degrees Fahrenheit and the insert cutting, edge begins to grow. We call it thermal expansion. When the cutting edge of the insert is not in the cut it begins to cool down and when you place coolant on the cutting edge you get thermal contraction. This rapid succession of heating and cooling is called thermal mechanical cycling. If gone unchecked this thermal mechanical cycling will caus thermal fatigue in the form of cracks in the tool.
Thermal fatigue, happens when you have rapid temperature fluctuations. The rapid expansion and contraction of the cutting edge due to rapid heating and cooling causes the cutting edge to rip apart. Cracks form in the material eventually working their way to the cutting edge. Do cracks cut? No, of course not. What ends up in the cracks, your workpiece material. What happens to the crack after a lot of material is shoved into it? The cutting edge breaks and you have catastrophic failure.
This illustration provides a good representation of thermal cracks. The key to thermal cracking is to identify the failure mode before you reach the stage of catastrophic failure. To do that you need to understand a little more of how the crack are formed. They do not always start at the cutting edge. In many cases the cracks begin to form in the body of the tool just behind the cutting edge. While in the cut the cutting edge heats up and expands more rapidly than the material not directly in the cut. This unequal amount of expansion and contraction begins to rip the tool apart. It’s kind of like spreading your fingers apart on you hand. Your fingertips are the cutting edge when they get hot they spread apart, when the coolant hits, they collapse. The space between your fingers are the cracks that form. Just like the gaps in your fingers, the cracks that form on the rake and clearance surfaces of the cutting edge are straight and always perpendicular to the cutting edge.
Now that we know how to recognize thermal cracks how do we prevent them? The primary cause of thermal crack is thermal mechanical cycling or the rapid expansion and contraction of the cutting tool material. The best way to stop this thermal mechanical cycling is to turn off the coolant. Milling operations are always an interrupted cut therefore; when possible always run milling operations dry. This reduces the thermal shock and extreme fluctuations in the cutting tool temperature. If you are concerned about chip evacuation use compressed air to eject the chips. You may also want to turn the coolant off in lathe operations that include interrupted cutting. Any interrupted cutting operations where the cutting edge is entering and exiting the workpiece at a rapid rate will cause thermal mechanical cycling.
If you must use coolant, be sure you have a constant flow directed onto the cutting edge that is not obstructed by the chip flow. In grooving and some turning operations where the chip flow prevents coolant from reaching the cutting edge or it is only intermittent, it is better to bring the coolant flow under the cutting edge on the clearance side of the tool rather than the rake face of the tool. When using coolant, you will need to use a cutting tool material with a higher cobalt content. The higher cobalt/marshmallow content allows the tool to manage the rapid temperature changes and expansion and contraction without cracking. When cracks begin to form the added cobalt also slows down crack propagation. Be careful though, tools with greater amounts of cobalt typically must be run at slower surface speeds. This seems counter intuitive, you would think when using coolant, you could run higher surface speeds; however, the opposite is the case.
In summary; thermal cracks are caused by rapid temperature changes and the subsequent expansion and contraction of the cutting tool material called thermal mechanical cycling. This thermal mechanical cycling causes perpendicular cracks to form just behind the cutting edge which will grow. Once the cracks reach the cutting edge catastrophic failure will occur as the workpiece fills the cracks with material. In interrupted cutting applications, such as milling or interrupted turning, turn the coolant off to reduce the extreme temperature fluctuations. If coolant must be used, be sure it is applied directly to the cutting edge and is unobstructed. When using coolant; cutting tool materials with greater amounts of cobalt must be used. Cobalt, TaC and TiC cutting tool materials can withstand greater thermal fluctuations without cracking; however, they must be run at lower surface speeds.
Stay tuned to our next blog in this series where we will discuss Thermal Deformation.
IMPORTANT NOTE: There are many cases where coolant must be used; when machining superalloys or refractory metals, aluminum, some brasses and bronze materials. In cast iron, it's not required; however, it is sometimes used to keep the dust down. Use judgement; just remember coolant in interrupted cuts such as milling or interrupted turning requires cutting tool materials with higher cobalt content which requires slowing cutting speeds.