By manufacturing process of machining, materials can be shaped into the desired products. However, machining materials is not always an easy task, because the properties of the materials and the specific machining conditions play a vital role in determining the smoothness and efficiency of the entire process. All such considerations are related to a key word “machinability ”.
Machinability is an essential property that characterizes the ease of removing material from a workpiece during a machining process. Materials with good machinability are highly demanded in manufacturing as they enable faster and more efficient machining, ultimately resulting in cost reductions and enhanced product quality.
In this article, we will illustrate the concept of machinability, exploring the factors that influence it. Moreover, we will discuss various methods that can be adopted to improve machinability and how to measure it.
What Is Machinability?
Machinability refers to the ease of machining a material, specifically its ability to be cut, shaped, or modified by varied machining processes. In other words, it measures how readily a material can be machined into a desired form.
The machinability of a material is an important indicator of evaluating time and cost for producing a product from it. To ensure the production efficiency, tool life, and the quality of the final product, it’s very necessary to understand what determines the machinability and what measures can be taken to improve it.
What Are the Factors Affecting Machinability?
The machinability of a material depends on both its physical properties (what it's made of) and its condition (how it has been processed). The physical properties are fixed, but the condition can vary greatly.
Physical properties
Work hardening: It refers to the phenomenon that a metal becomes harder and stronger as it is plastically deformed. This hardening can make the workpiece tougher to cut, leading to increased tool wear and difficulty in maintaining precision.
Thermal expansion: Thermal expansion coefficient measures the degree of thermal expansion of solid materials. The higher of the coefficients, the materials are prone to greater expansion when heated, which can affect the precision of machining.
Thermal conductivity: It is the ability of a material to directly conduct heat. Materials with high thermal conductivity dissipate heat more quickly, reducing the thermal load on the cutting tool and improving tool life.
Modulus of elasticity: It measures materials’ resistance to elastic deformation. Materials with higher modulus of elasticity are stiffer and less prone to bending under cutting forces, which can improve dimensional accuracy during machining. However, excessively stiff materials may also be more brittle and prone to cracking.
Condition factors
Microstructure: It refers to the distribution and arrangement of grains and phases within a material. Uniform, fine-grained structures generally enhance the machinability of the material, while coarse or uneven grain structures can lead to unstable machining and increased tool wear.
Grain size: Smaller grain sizes often result in better machinability as they reduce the likelihood of crack formation and chipping.
Heat treatment: It can significantly impact the machinability of materials by altering their mechanical properties. We will elaborate on this later in the text.
Hardness: Harder materials are generally more difficult to machine as they resist cutting, leading to higher tool wear.
Tensile strength: It measures the maximum stress a material can withstand under tension before breaking. Materials with high tensile strength are harder to machine due to their resistance to cutting, which can increase tool wear and machining forces.
Besides the five factors above, the machinability of materials is also influenced by various other aspects such as machining methods, cutting tool material and geometry, cutting parameters, lubrication and cooling, equipment status, etc.
How Can Machinability Be Improved?
As above, the introduction of those factors allows us to understand clearly how they influence machinability. Metals' inherent properties, such as modulus of elasticity, thermal expansion, and thermal conductivity, are their unchangeable physical characteristics. Nevertheless, there are approaches to altering the conditions and the machining process to make the workpiece easier to machine. Specifically, we can improve machinability from the following two major categories.
Category One: Without Altering Material Properties
This approach focuses on optimizing conditions encountered during machining processes. Here are some specific methods as below.
Material Selection
Select materials with inherently better machinability. Some materials possess favorable machinability due to their chemical composition and microstructure. Prioritize materials with moderate hardness, good thermal conductivity, and low adhesiveness.
Select and Upgrade Cutting Tools
Select tools made from appropriate materials (e.g., carbide, high-speed steel) based on the material being machined. Use tools with optimal rake angles, clearance angles, and cutting edge designs to reduce cutting forces and improve chip evacuation.
Optimize Machining Parameters
By optimizing cutting parameters such as cutting speed, feed rate, and cutting depth, tool life can be extended and surface finish improved, ultimately enhancing machinability. For example, increasing cutting speed and feed rate within reasonable limits can enhance material removal rates, but excessive increases should be avoided to prevent overheating and tool wear. Additionally, by appropriately adjusting cutting depth—utilizing larger depths for rough machining and smaller depths for finish machining according to the task at hand—one can ensure both surface quality and machining efficiency.
Apply Lubricants and Coolants
Applying suitable lubricants and coolants to minimize friction, heat generation, and tool wear during machining operations, to ultimately improve the quality of the machined surface.
Improve Machine and Workpiece Setup
Using a machine tool with high rigidity and optimal maintenance, along with the adoption of suitable fixtures and supports to uniformly clamp the workpiece, can effectively prevent deformation and movement. This ensures consistent and precise results.
Adopt Suitable Machining Methods
Machining different materials to achieve desired shapes often requires specific types of equipment. By using a variety of equipment in a machine shop, such as wire electrical discharge machining (WEDM), it is possible to effectively handle hard materials or complex designs that other methods cannot manage efficiently.
Category Two: Altering Material Properties
Heat Treatments
Heat treatment is a highly effective method to improve machinability, yet it's crucial to recognize that applying heat treating and work hardening treatments to materials in the early stages of production can greatly increase their hardness, thereby rendering them more challenging to machine. Therefore, it's advisable to postpone heat treatments and other hardening processes till after machining. Quenching, often coupled with tempering, is the typical process used after machining to enhance the final mechanical properties of workpiece.
However, if postponement is impossible, you may consider annealing the workpiece prior to machining to soften the material and relieve internal stress. Here are the key points of these commonly used heat treatment methods.
Annealing: This process involves heating the material to a certain temperature, holding it for a period of time, and then slowly cooling it down. Annealing treatment makes the material softer, reducing resistance during cutting, decreasing tool wear, thus improving machinability.
Normalizing: This process heats the material above its critical temperature and then allows it to cool naturally in air. Normalizing refines the material’s grain structure, giving it a more uniform texture that improves workability. It also enhances consistency during the machining process and reduces tool wear.
Quenching and Tempering: Quenching is a process where materials are heated and then cooled rapidly, while tempering is a process that reheats the quenched materials to a lower temperature and then cools them. Materials that have undergone quenching are generally difficult to process, requiring tempering to optimize their hardness and toughness in order to make them easier to machine. Proper tempering temperature and time can greatly improve the processing performance.
Additives
The addition of specific elements can profoundly alter the machinability of a material. Some common ways are as below:
Lead addition: Adding a small amount of lead to a material can significantly enhance its lubricity. This reduces friction and wear on the cutting tool, making the cutting process smoother and the resulting chips easier to manage.
Other additives: Adding appropriate sulfur or phosphorus can improve chip fracture, reducing cutting forces and further enhancing machinability.
How Is Machinability Measured?
The assessment of a material's machinability is a multifaceted process that considers various factors. We often conduct qualitative evaluation from the following aspects.
Tool Life: Longer tool lifespan indicates easier machinability. Materials that allow tools to last longer under similar conditions are considered to have better machinability.
Tool Forces and Power Consumption: Lower cutting forces and reduced power consumption during machining indicate better machinability. These factors are often measured using specialized equipment that records the amount of force and energy required to machine the material.
Surface Finish: Materials that can be machined to a smooth finish without requiring extra processing have higher machinability.
Chip Form: Shorter, curly chips denote easier machining, while long, stringy chips indicate difficulty in machining.
Although these methods are commonly used, they primarily serve as qualitative references and may not be fully reliable due to the influence of various factors on power consumption, tool wear, and surface finish. To obtain a more quantitative perspective, let’s explore the AISI Turning Test Rating System.
AISI Turning Test Rating System
It’s the most widely adopted machinability rating system, conducted by the American Iron and Steel Institute (AISI). This system benchmarks the machinability of a material against B1112 steel, which serves as the reference standard with a Brinell hardness of 160. The machinability rating is expressed as a percentage, where B1112 steel is set as the baseline with a rating of 100%.
In this system, materials that are easier to machine than B1112 steel will have a machinability rating higher than 100%, while materials that are more difficult to machine will have a rating lower than 100%.
Materials
Machinability Rating
Aluminum 6061
480% – 320%
Aluminum 7075
480% – 320%
Low Carbon Steel 1010
64% – 40%
Low Carbon Steel 1018
80% – 44%
Medium Carbon Steel 1045
60% – 28%
Stainless steel 304
64% – 44%
Stainless steel 316
36%
Brass 260
105% – 100%
Brass 360
160% – 200%
Titanium Alloy Grade 2
30%
Titanium Alloy Grade 5
35% – 30%
Titanium Alloy Grade 23
28% – 25%
Magnesium Alloy ZK60A
65% – 60%
Magnesium Alloy AZ31
55% – 50%
Machinability Ratings Chart
Work With Chiggo for Hard-to-Machine Parts
Machinability is a key indicator of the time and cost required to manufacture parts. Materials with high machinability are easier to process, but this does not always equate to high performance. In some scenarios, materials with lower machinability are necessary. To optimize machining results, we can adjust machining methods and other variables.
At Chiggo, we have the expertise and advanced equipment to provide high-quality, cost-effective machining for various materials. Contact us for free design and manufacturing process optimization.
Common Machinable Materials
Aluminum
Aluminum is a soft, lightweight, and highly machinable metal. Among its variants, Aluminum 6061 is often recognized as one of the most machinable.
Steel
Although machining steel can be more complex than machining aluminum alloys, mild steel is generally easier to machine compared to high-carbon steels and provides a good surface finish. It produces short chips and does not cause excessive tool wear. Furthermore, certain stainless steel grades, such as 303, contain additives like lead to enhance machinability.
Plastics
Thermoplastics are difficult to machine because the heat generated by cutting tools can cause the plastics to melt and adhere to the tool. However, plastics such as ABS, nylon, PTFE, and Delrin offer excellent machinability.
Other Metals
Other machinable metals include brass, magnesium alloys, lead alloys, etc. Brass, a copper alloy, has soft nature with good tensile strength, exhibiting very good machinability. Magnesium alloys are lightweight metals with good machinability. Lead alloys, primarily composed of lead with various additives, offer low friction, good wear resistance and machinability, but their use is limited due to toxicity concerns.
Machinability vs. Workability
Workability refers to the ease with which a material can be shaped and formed into desired configurations through processes like bending, forging, drawing, and extrusion. It encompasses aspects such as ductility (ability to deform under tensile stress), malleability (ability to deform under compressive stress), and formability (ease of forming complex shapes without cracking). Workability includes both cold and hot working processes.
Machinability specifically relates to the ease with which a material can be cut, shaped, or finished using machine tools like lathes, milling machines, and CNC machines. It involves factors such as cutting speed (rate of material removal), tool wear (rate at which cutting tools wear out), surface finish (quality of the machined surface), and precision (ability to achieve tight tolerances and accurate dimensions). Machinability focuses on the material's behavior under cutting conditions and its interaction with cutting tools.
