CNC milling
The general meaning of machinability refers to the easiness or difficulty of the material for machining. The highly machinable material requires less power to perform machining and does not get deformed while performing any CNC machining operation. On the other hand, the low machinability of the workpiece is challenging to manipulate into the desired shape. It requires more power and might affect the cutting tools.
This article will briefly discuss machinability, affecting factors, measurement, machinability improvement, and its relation with CNC machining.
What is Machinability?
In the subtractive manufacturing approach, material needs to remove from the workpiece to create the desired part. Machinability of the material is the measure of how easily it can remove the material during machining operations, such as milling, turning, drilling, and others.
The machinability of any material depends on its physical, mechanical, and chemical properties. Some important properties of the material that influence machinability are hardness, elasticity, mechanical strength, Work hardening, Thermal expansion, and thermal conductivity.
Whether a material is made of metal, an alloy, plastic, ceramics, or fiber, its machinability is determined by its characteristics and the machining conditions. However, the machining operators can feel the machinability of the workpiece from the machining results. The material with good machinability produces minimum wear and tear on cutting tools, smooth surface finishing, requires less time, and can be maintained tight tolerances.
Factors Affecting the Machinability of Martials?
There are different physical and mechanical properties that affect the machinability of the material. Let’s discuss them in detail.
- Hardness
The primary factor influencing a material’s machinability is its hardness. The work piece’s high hardness is generally considered difficult to process with machining operations.
The hardness of any material describes how difficult to deform the workpiece. In addition, hardness also suggests the compact internal structure of the material (crystal structure & grain boundaries). The compact internal structure of hard material challenges the machining tool to remove the material and requires high force.
- Thermal properties
Machining of every material creates heat. For example, metal & alloys generate temperatures as high as 800 to 900 degrees. The teeth of the cutting tool continuously contact and leave the workpiece. If the workpiece has low thermal conductivity, chips will not absorb much heat, and the tool will be subjected to a thermal load, which can affect machinability.
- Work Hardening
Hardening is the heat treatment process for metal and alloys, which involves heating the material at a specific level followed by a cooling process. It increases the hardness of the material. Basically, work hardening makes the machining process more challenging because of the increase in hardness. So, it is recommended to proceed with hardening after the completion of machining.
Machinability and CNC Machining
CNC machining of hard material
CNC machining is compatible with many materials, including metal and plastic, and common materials such as metals, alloys, plastics, composites, fiber, wood, and many more. The CNC machinability of each material is different because of its unique properties and machining conditions.
Related: Common Materials for CNC Machining
- Tool material & Geometry
The tool material also affects the machinability of the workpiece. The selection of tools should be based on the physical & mechanical properties of the workpiece material. Hardness and wear resistance are two important factors for cutting tool material that needs to be higher than the workpiece. For example, the carbide tool overcomes the hardness of carbon steel and offers the required machinability.
The geometry of the cutting tool also influences the machinability of the workpiece. For example, a specific angle is required for facing the operation.
- Feed rate
The feed rate also affects how easily the complex workpiece will be machined. The feed rate is directly proportional to the machinability. The highly machinable material can be processed with a high feed rate, while low-machinability material can be machined accurately with a low-feed rate.
For example, aluminum is more machinable than steel alloys, so a high feed rate is preferable for aluminum. The feed rate of aluminum in CNC machining ranges from 0.004 to 0.012 inch/tooth, while steel alloy (300-400 Brinell) is machined in a range of 0.0015 to 0.004 inch/tooth (SPEEDS AND FEEDS).
- Cutting Speed
The cutting speed and machinability of a material have a strong relationship. The cutting force is adjusted in the CNC machining operation based on the machinability of the workpiece. The higher cutting speed can cause damage to the tool for the workpiece with low machinability. The low-cutting speed is preferable for a low-machinability workpiece.
- Cutting depth
High cutting depth in a workpiece with low machinability is quite challenging in terms of accuracy, surface finish, and wear & tear in the tool. However, the highly machinable can be machined easily with a large cutting depth. Hardness is the prime reason behind this; the compact internal structure of hard material resists the cutting force of the machining tool.
- The rigidity of the CNC machine
The capacity and rigidity of the CNC machine also affect the machinability of the workpiece. How easily a workpiece can be processed depends on how rigid the machine is. The rigidity of the machine includes how tight the screws are, the clamping mechanism, bearing quality, weight, and hardness of the machine.
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How to Measure the Machinability of a Material?
Machinability is a very complicated material character that relies on different factors and is challenging to quantify into numbers. In addition, the machinability of the same workpiece can be different based on the type of machining, such as milling, turning, drilling, and so on.
A machinability index is a popular approach to quantify the machinability of the materials. It is used in manufacturing to compare the machinability of different materials.
Machinability index (I, %) = Vi/Vs × 100
Vi represents metal cutting speed for a specified time & Vs represents the cutting speed of standard steel for a limited time (What Is Machinability Index? Criteria Considered In Evaluating Machinability? 2018).
The relative quantification of material machinability is done by only varying the cutting speed. All other variables, such as tool material, geometry, and feed rate, remain unchanged. If you are interested to find out the machinability index of any material, there is an online machinability index calculator.
Machinability of some popular CNC materials (Zeng, 2007)
How to Increase the Machinability of Materials?
Improving machinability involves altering the fixed physical and mechanical properties to make it more machinable than the normal state.
There are three primary ways to increase machinability: adding additives, heat treatment, and controlling operational variables.
| Inclusion of Additives | Mixing the additives like lead and Sulphur in the material makes it more amenable for cutting. |
| Heat treatment | Heat treatment is one of the main ways to customize the mechanical properties of metal & alloys. For example, Annealing (heating followed by slow cooling) reduces the hardness and makes the material more malleable, which eases the machining process. |
| Controlling of operational variables | Various operational variables affect the CNC machinability of the workpiece. These factors need to be controlled in a favorable way to increase machinability. The operating factors include tool material, cutting speed, feed rate, and angle. |
Machinability of Some Popular CNC Materials
The common material in CNC machining projects includes steel, aluminum, copper, thermoplastic, composites, and plastics. Let’s discuss the machinability of these materials.
- Steel
The machinability of steel depends on the carbon content since it is directly associated with the hardness of steel. The high-carbon steel is challenging to machine and requires highly rigid carbide tools. Although, low carbon content steels are still challenging because they adhere to the cutting tools.
- Stainless steel
Stainless steel is not easy to work with CNC machines. The hardness and gumminess make it challenging. The best idea to make it highly machinable is slightly hardening with heat treatment.
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- Aluminum
Since aluminum and its alloys have a lower hardness than steel, they are highly machinable with CNC machining operations. High cutting speeds with a wide rake and relief angles are advised for a quality result.
- Copper
Copper shows high machinability than steel and lower than aluminum. It can be machined accurately with CNC with medium cutting speed.
- Plastics
Plastics don’t have much toughness, but the thermal effect makes it difficult to machine with CNC technology. The generated heat during machining can melt the plastics locally, lowering the tool life.
You might interested in CNC Machining: Plastic parts
- Composites
Composites have the least machinability because of their poor thermal conductivity and stiffness. The CNC machining of composites requires special tools and extra attention for operations.
Conclusion
Machinability is the ease or difficulty of manipulating material into the desired shape by subtractive manufacturing approach. It is one of the important factors to consider before jumping into any CNC machining operation. It is essential to fix the CNC machining variables, such as cutting speed, cutting depth, feed rate, and clamping mechanism, following the machinability level of the workpiece to get the best result.
Analyzing the part’s functionality according to the machinability of raw material is quite a complicated process. There are several factors to consider to optimize the machinability and get an accurate result- but don’t worry! Prolean is the leading on-demand manufacturing service provider for CNC machining projects. We consider machinability and other various factors in creating quality & accurate parts for you.
FAQ’s
What is meant by machinability?
The machinability of any material refers to how easily it can be shaped into the required shape by machining operations.
What properties of a material influence its machinability?
Several variables affect the machinability of a material, such as hardness, mechanical strength, elasticity, thermal conductivity, and work hardening. The other variables include operating conditions, tool material, and type of machining operation.
How can I increase the machinability of any material?
You can increase the machinability of a material by adding other compounds, such as lead & Sulphur. In addition, heat treatment and optimization of operating variables are the other two approaches.
How machinability of material and CNC machining are co-related?
While low-machinability materials are difficult to work with and require several considerations for CNC machining, high-machinability materials are easily compatible and create accurate parts.
Bibliography
(n.d.). SPEEDS AND FEEDS. from https://mae.ufl.edu/.
(2018). What Is Machinability Index? Criteria Considered In Evaluating Machinability? from https://www.mecholic.com/.
Zeng, J. (2007). DETERMINATION OF MACHINABILITY AND ABRASIVE CUTTING PROPERTIES IN AWJ CUTTING. SEMANTIC SCHOLAR.
Yes! you can easily manufactures the dies with rapid tooling approach.