Can Your Tool be too Hard or too Tough?
Heat and Pressure - Impact on Hard and Tough Cutting Tool Materials!
We have talked about Heat and Pressure, how they are both required to form chips and how we regulate both. Now let’s look at the impact of “Heat and Pressure” on the materials used in metalcutting. Before we go too far down the road we need to establish a baseline on the two most important characteristics of cutting tool materials their hardness and toughness. As you may recall in my last post, “The Yin and Yang of Metalcutting” speed and feed must be balanced to maximize both productivity and tool life. Too much heat and we plastically deform our cutting tool, too much pressure and we fracture the cutting tool. So, let’s look a little more closely at material “Hardness versus Toughness” and their relationship to “Heat and Pressure”.
When we talk about a carbide cutting tool hardness we are also talking about its wear resistance and/or its ability to take the heat generated in the metalcutting operation. As you recall the temperatures at the shear zone are between 1200-1400 degrees Fahrenheit. In most cases the higher the hardness of a carbide cutting tool the greater its ability to take the heat and achieve longer tool life. Given this fundamental truth of metalcutting you would are forced to believe you should always use the hardest carbide cutting tool grade as possible to get the best tool life. While this is true we need to remember "The Yin and Yang of Metalcutting”.
I like to compare the “Yin and Yang of Metalcutting” to one of Sir Isaac Newton’s laws; “for every action there is an equal and opposite reaction”. In other words, “for every good thing in metalcutting there is a bad thing”. We say the harder a carbide cutting tool grade is the more heat it can take and the more wear resistance it has. That’s a good thing, but for every good thing there must be a bad thing. When I think of hard materials I think of glass. If I took a cutting torch to a piece of glass how long would it take to melt? A pretty long time, right? Glass can take a lot of heat before it melts, somewhere between 2500 to 2900 degrees Fahrenheit to be exact. Now that’s a lot of heat, but how much pressure can it take?
That’s another story all together. What happens when you hit a piece of glass with a hammer? It shatters. That is the same situation you have with hard carbide cutting tool grades. They can take the heat generated at the shear zone but there is a limited amount of pressure they can take. High feed rates, heavy depths of cut, vibration and/or impacts and interrupted cuts can cause hard carbide cutting tool materials to chip or break.
On the other end of the spectrum we have tough carbide cutting tool materials. When I say tough, think rubber. What happens when you hit a piece of rubber with a hammer? It bounces back. Tough cutting tool materials can “take a licking and come back ticking”. They can take more pressure, vibration, impacts or interrupted cuts. That’s a good thing, right? Yes, it is, but remember for every good thing there must be a bad thing. While tough materials can take the pressure, they can’t take the heat. Rubber melts at 750 to 1000 degrees Fahrenheit. That is a big difference from 2500 to 2900 Fahrenheit for glass. Now you have the two ends of the spectrum; hard carbide that can take heat but not pressure, and tough carbide that can take pressure but not the heat. When selecting the right carbide grade, speed and feed, it’s kind of like Goldie Locks and the Three Bears; not too hot, not to cold, but just right.
Stay tuned for our next blog when we will compare the composition of carbide to rice crispy treats.