What Is the Machinability of a Material?
Machinability refers to a material’s quality that makes it easy or difficult to cut, shape, or finish with any machining process. A material is more machinable when cut with normal cutting speed and minimal power without deforming the cutting zone. In contrast, a material has poor machinability when it requires more effort and high cutting forces, which results in more deformation. This property applies to all CNC machining materials, even though it is often used for metals.
Using material with improved machinability is more beneficial because it provides better parts with less deformation, tight tolerances, and excellent surface finish. These materials also facilitate faster lead times because they facilitate quick production of items. In the long run, using materials with good machinability reduces rapid tool wear and ensures extended cutting tool life, saving CNC machining costs for your project.
What Does Machinability Ratings Mean?
Machining ratings or machinability index is a standard method used to quantify the machinability of a material. This rating refers to a relative value determined from the material’s testing. Although physical factors such as a material’s tensile strength, hardness, and thermal properties can indicate its approximate machinability, machinability testing is the only way to be certain. According to a machinability rating system created by the American Iron and Steel Institute (AISI), materials machined by CNC turning and the surface finish, speed, and tool life are compared against the reference material, 160 Brinell B1112 steel.
Machinability ratings are similar to Fahrenheit and Celsius temperature scales in a way that randomly selected values reflect the reference point. The baseline is set around 0ºF and 0ºC in these temperature scales. The free-machining steel AISI B1112 is the reference point with a machinability rating of 100% in the AISI testing method. Hence, any material that machines easily and faster than B1112 steel attains a value above 100%. Conversely, materials that are harder on tooling and take longer to machine have a rating of less than 100%.
For instance, the SAE 6061 aluminum exhibits a 270% machinability rating, while the hard-to-machine ASTM Grade 5 titanium (Ti-6Al-4v) has a machinability of about 17%. Nevertheless, these ratings are a guideline for indicating a material’s relative machining difficulty or ease. However, product designers and machinists can make the machining of materials more efficient in some ways.
Common Ways of Improving a Material’s Machinability
Metals are often characterized by fixed physical properties such as modulus of elasticity, thermal expansion and thermal conductivity. However, you can make them more machinable by altering the condition of the workpiece and machining process. When a more machinable metal alloy is incompatible with the intended application, the only option is to find a way to improve the material’s machinability.
Heat treatment and work hardening treatment applied to a material in the initial stages of production can make it much harder to machine. Therefore, try as much as possible to postpone all fabrication processes and treatments that can result in material hardening till after machining. However, if delaying is impossible, consider annealing the workpiece before machining to soften the material and ease internal stresses.
You can effectively reduce production times by leveraging special equipment like wire electrical discharge machining in a CNC shop. Machine tooling made from different materials or tool geometry may accommodate higher cutting speeds and ensure extended tool life.
Moreover, you can best improve machining efficiency without changing the workpiece on a case-by-case basis. For instance, the mechanical properties of thermoplastics result in their machining difficulty. Since they are soft, they often melt or glue to the tooling. However, consider controlling the temperature with special coolants and modifying the machining parameters to effectively improve machinability if you cannot change to a material with higher machinability.
Factors Affecting Machinability
Several factors contribute to the machinability of a material, causing it to be relatively easy or difficult to machine. Here are some of these common factors that influence machinability:
Material Hardness
Although the chemical composition of a material often influences a material’s hardness, materials with high hardness are often problematic. They slow down cutting operations, require more cutting forces, and accelerate cutting tool material wear. Hence, the hardness levels of a material must be examined during selection.
Coolant and Lubricant
Cutting fluid plays a significant role in how machinable a material can be. Using appropriate lubricants and slower cutting speeds can reduce heat and friction between the workpiece and cutting tools, thereby improving the material removal rate and ensuring longer tool life.
Machining Method
CNC machining processes, including milling, drilling, grinding, turning, etc, affect a material’s machinability differently. Depending on a material’s property and preferred machining outcome, some materials are more suitable for specific machining processes than others. For instance, grinding is a more suitable machining process for working with brittle materials than turning due to their high hardness and low ductility.
Tensile Strength
The higher the material’s tensile strength, the more complex it is to machine. Materials with higher tensile strength have greater power consumption because they are highly resistant to deformation and require high cutting speed.
Thermal Properties
The thermal properties of a material are another critical factor in determining whether or not it is relatively easy to machine. The way materials dissipate heat differs. Some materials soften and gum up the machine tool because they cannot handle heat.
Typical Materials with Excellent Machinability
| MATERIAL | Material Grade | Machinability Rating |
|---|---|---|
| Low Carbon Steel | 1010 | 64% – 40% |
| Low Carbon Steel | 1018 | 80% – 44% |
| Medium Carbon Steel | 4140 | 36% – 16% |
| Medium Carbon Steel | 1045 | 60% – 28% |
| Stainless Steel | 304 | 64% – 44% |
| Stainless Steel | 316 | 36% |
| Stainless Steel | 17-4PH | 32% – 16% |
| Stainless Steel | 410 | 56% – 36% |
| Brass | 360 | 160% – 200% |
| Brass | 260 | 105% – 100% |
| Titanium Alloy | Grade 2 | 30% |
| Titanium Alloy | Grade 5 | 35% – 30% |
| Titanium Alloy | Grade 23 | 28% – 25% |
| Magnesium Alloy | AZ31 | 55% – 50% |
| Magnesium Alloy | ZK60A | 65% – 60% |
| Magnesium Alloy | WE43 | 45% – 40% |
| Aluminum Alloy | 6061 | 480% – 320% |
| Aluminum Alloy | 7075 | 480% – 320% |
Note: Materials from different countries may vary slightly in machinability ratings due to subtle differences in workmanship and composition.
Some materials are commonly used for different applications because they are easily shaped and formed using various processes. Here are other machinable materials:
Steel
Although machining steel is relatively more complex than aluminum alloys, steel grades with moderate carbon content, such as 303 stainless steel, are easy to machine. High-carbon steels are relatively hard materials that are difficult to cut, while low-carbon steels are gummy. However, using lead as an additive can make steel more machinable and improve chip formation. Sulfur is another element that helps to improve the steel’s machinability.
Aluminum
Aluminum is one of the few materials with exceptional machinability and high thermal conductivity. It is relatively easy to machine and cost-effective. 6061 aluminum is a typical aluminum grade for machining. 2011 and 8280 are other less common aluminum alloys with remarkable machining. They create very small chips and a superior surface finish.
Plastics
Thermoplastics are relatively difficult to machine because CNC tools generate heat that causes plastic deformation, causing it to stick to the tool. However, ABS, nylon, Delrin, and acrylic are typical plastics with the best machinability ratings. They are ideal for machined parts requiring tight tolerance and a good surface finish.
Other Metals
Brass alloys, magnesium alloys, and copper are typical examples of other machinable metals. Brass alloys are soft materials with good tensile strength. Copper shows great electrical conductivity aside from a good level of machinability. Magnesium alloys are lightweight materials. Although not as robust as aluminum, they exhibit good machinability and are suitable for low-density applications.
AT-Machining: Your Reliable Manufacturing Partner for Hard-to-Machined Parts
The machinability of a material is a significant indicator of the required time and cost of making a part from it. Machining works differently for every material since the process involves using a powerful machine tool to cut or shape the material to achieve desired surface finish, dimensional accuracy, and tool life. Understanding everything about how machinable a material is can ensure better machining outcomes.
AT-Machining is a top-tier CNC machine shop providing quality, cost-effective services. Our expert professionals can handle various materials irrespective of their machinability ratings to meet your product requirements. We leverage our expertise and cutting-edge facilities to optimize production and maintain quality. Contact us now, and we will optimize the fabrication process for your next project!
