Let's Talk about "PRESSURE", and metalcutting Yin and Yang
During our first blog, we discussed what two things it takes to make a chip, HEAT and PRESSURE. Moving forward to our second blog we focused on the HEAT portion of that equation and explained how we regulate the temperature at the shear zone. Today let’s focus on the PRESSURE side of the equation.
The pressure in a metal cutting operation is created by the feed rate. Feed rate is defined as the axial movement of each cutting edge made along the workpiece during one revolution of the rotating component. For single-point lathe tools, feed rate is measured in inches or millimeters per revolution. For multiple cutting edge tools, feed rate is measured in inches or millimeters per tooth. As the workpiece material is feed past the cutting edge the material is deformed and displaced as it moves past the cutting edge and rakes across the top surface or the rake face of the cutting tool. This deformation compresses the workpiece material and the resulting chips begin to segment as they flow across the rake face of the tool.
When you look metal chips closely you will notice that the surface of the inside of the chip is very rough or undulated and the surface of the outside of the chip is smooth and shiny. Take a look at the chip pictured in our logo to above. After taking a closer look at the inside of the chip you will see lines that run perpendicular to the width of the chip. The distance between the lines is a chip segment. The photo the right illustrates how the segments are formed. You may wonder why the chip segments do not come apart as each segment. The answer is in seen on the outside or smooth surface of the chip. Remember it takes “HEAT and PRESSURE” to make a chip. It’s between 1200 to 1400 degrees Fahrenheit at the shear zone. The workpiece material is plasticized. As the hot chip flows across the rake face of the tool just behind the cutting edge the chip segments are friction welded or burnished together. It’s not until the chip flow is obstructed in some fashion that the chip breaks. That is where the rake angle of the tool and chip flow management comes into play, but we will save that for another blog.
For now, let’s summarize where we are at. What two things does it take to make a chip? HEAT and PRESSURE created as the cutting tool is feed into the workpiece plastically deform the workpiece material to form the chip. How do we regulate the temperature of the heat? The speed of the workpiece material passing the cutting edge for a given unit of time creates friction and heat. The faster you go the more heat at the shear zone you get, the slower you go the less heat you get. Where does the pressure come from? The feed rate of the tool into the workpiece for a given unit of measure and time generates the PRESSURE. It is important that these two elements in the equation are balanced correctly for the material being machined to maximize both tool life and productivity.
Heat and Pressure are the Yin and Yang of metal cutting. If the heat is too high at the shear zone, we will plasticize the tool and have rapid tool wear. So always use the given manufacturers speed and feed recommendations based on the workpiece material you are machining.
Stay tune for my next blog where we will begin discuss the differences in cutting tool geometry their importance when being applied to various workpiece materials. We will start with three angles; lead, rake, and relief. Until next week!