Machinability is a term used to describe the ease with which a material can be machined to achieve desired specifications. This encompasses various processes such as cutting, drilling, milling, and turning. Machinability of materials is a fundamental concept in manufacturing that directly influences core engineering design decisions like material selection, cutting parameter selection, and budgeting.
In this post, we will explore what is machinability, how machinability is measured and the factors that influence it.
What Is Machinability?
Let’s start with the main question: What is machinability? In short, machinability refers to the ease of cutting (processing) materials to achieve the desired part quality. The quality of the parts here refers to characteristics such as dimensional accuracy, tolerances, and surface smoothness.
Materials with high machinability usually require less time and power to process, have less tool wear, and have better surface quality. It can be understood that from a production perspective, materials with high machinability are always more popular. However, this may not always align with the views of designers who seek high strength, high performance, and thermal stability, which is not always the case for easily machinable materials.
Measurements Of Machinability
Cutting Speed:
This is the speed at which the cutting tool moves relative to the workpiece. A higher cutting speed often indicates better machinability, as it can lead to increased productivity. However, it must be balanced with the tool’s durability and the material’s properties.
Tool Life:
The duration a cutting tool can be used before it needs replacement or resharpening is a critical factor. Materials that allow tools to last longer without excessive wear are considered to have good machinability.
Understandably, materials with high machinability do not cause high tool wear and thermal damage, so the tool life is long. On the other hand, hard-to-cut materials like steel quickly wear down the tool.
Surface Finish:
The quality of the surface produced after machining is an important measurement. A better surface finish indicates higher machinability, as it reflects efficient cutting action and minimal tool friction. For example, hard materials have low machinability and have a rough surface finish due to chipping and friction.
Power Consumption
Due to cutting force, machining consumes power. Difficult to cut materials require greater force to cut. Therefore, they consume more power. For materials that are easy to cut, the situation is the opposite.
Due to the very direct relationship between processability and power consumption, it is a commonly used measure of material processability.
Cutting Forces:
The amount of force required to perform the machining operation is another measurement. Lower cutting forces are indicative of better machinability, leading to reduced energy consumption and wear on tools.
Chip Formation:
The nature of the chips produced during machining can also reflect machinability. Short, manageable chips typically indicate good machinability, while long, stringy chips may suggest difficulties in the process.
Factors Affecting Machinability
Material Properties:
The most important set of characteristics that affect processability are material properties. Due to the unique characteristics of each material, engineers must understand the impact of each characteristic on machinability in order to make informed decisions.
1.1 Hardness
Hardness is a key factor in determining the machinability of materials, as it determines the difficulty of “cutting” surfaces. Due to the fact that machining tools mainly interact with the surface of the workpiece, hardness is an important characteristic of machinability.
Usually, harder materials such as Inconel require greater power to cut because the tool needs to apply more force. In addition, tools wear out faster when processing hard materials. In short, high hardness means low machinability.
1.2 Resilience
Resilience is another key parameter that determines machinability. High toughness materials such as high carbon steel are good at absorbing cutting forces and resisting deformation, requiring higher cutting forces and more durable tools.
In addition, due to its high toughness, the material will produce long and viscous chips. Although this is beneficial for maintaining smooth cutting action and effective heat transfer, long chips often wrap around the tool, leading to cutting delays and surface wear of the workpiece.
1.3 Thermal conductivity
The processing generates heat due to material shearing. Therefore, thermal management of the cutting interface is crucial for an effective cutting process. In terms of heat transfer, it largely depends on the thermal conductivity of the material.
Difficult to cut materials typically have lower thermal conductivity, which means that the thermal energy generated at the cutting interface does not dissipate quickly. This can lead to various negative effects, such as thermal softening of workpieces and tools, shortened tool life, and reduced dimensional accuracy. A typical example of this material is titanium, which has all these issues.
Low thermal conductivity also prevents the use of high cutting speeds and feed rates, as the generated heat is not effectively transferred.
Cutting Conditions:
Speed and Feed Rates: The optimal combination of cutting speed and feed rate can enhance machinability. Different materials respond better to specific rates.
Coolant Use:
Machinists often apply coolants and lubricants to the tool-workpiece interface to enhance the machinability of materials. These enhance heat removal and frictional properties of the material, leading to smoother cutting action, better surface finish, and a higher tool life. The application of coolant can reduce friction and heat, improving tool life and overall machinability.
Machining Method:
Different machining processes (e.g., turning, milling, drilling) may be more or less suitable for certain materials. The choice of method can greatly influence the efficiency and quality of machining. Most of the time, the equation is simple. Higher speeds, feeds, and depth of cuts decrease machinability, and vice versa.
For example, a negative rake angle reduces cutting loads and improves chip formation, which are signs of high machinability. However, it also makes the tool weaker.
Heat Treatment:
The heat treatment processes applied to a material can alter its hardness and machinability. For example, annealing can improve machinability by softening the material.
Workpiece Geometry:
The shape and complexity of the workpiece can also impact machinability. Intricate designs may require specialized tooling and setups, affecting efficiency.
Conclusion
Understanding machinability is essential for optimizing manufacturing processes. By measuring factors such as cutting speed, tool life, surface finish, cutting forces, and chip formation, manufacturers can evaluate and improve their machining operations. Additionally, recognizing the various factors influencing machinability enables better material selection and process planning, ultimately leading to more efficient production and higher-quality products. By focusing on these elements, manufacturers can drive innovation and maintain competitiveness in a rapidly evolving market.