Mastering Material Selection for CNC Machining: A Comprehensive Guide 

Material Selection for CNC Machining

The world of manufacturing offers thousands of materials, with new ones being developed daily. Each of these materials has characteristics that make it ideal for a particular function based on certain prevailing conditions. Narrowing the list of viable materials to a few can be daunting, especially without a clear starting point. This guide provides a comprehensive approach to the CNC machining material selection process.

We have delved into the material properties – such as the physical, machining, and mechanical properties, common materials used in CNC machining, the stages of the material selection process, and the basic and advanced factors to take into account when selecting materials. These key aspects will help you determine the optimal material for any specific process or function. Let’s get started.

Understanding Material Properties 

Material properties are fundamental considerations for engineers when designing products to function effectively. Material properties can be broadly grouped into physical, mechanical, and manufacturing/fabricating properties.

Physical Properties

Within the context of CNC machining material selection, thermal, chemical, optical, electrical, and magnetic properties, as well as density and resistance to corrosion and oxidation, are of particular interest. 

1. Density

The density of a material is calculated by dividing its mass by its volume. Engineers rely on this physical property as one of the many material selection factors during the design stage. For instance, they may select certain alloys, e.g., aluminum alloys, that have the same strength as, say, steel because the alloys are less dense and are, therefore, lighter.

2. Thermal Properties

Thermal properties include the following:

  1. Melting point: The melting point (MP) affects the machinability, weldability, and castability of materials. A decrease in MP improves all these factors. This is because materials with a high MP will require more energy and advanced tools to weld or machine.
  2. Thermal expansion: High thermal expansion increases the internal stresses and causes cracking. In ductile material, the differential expansion causes warping. Brittle materials, on the other hand, fracture when subjected to differential thermal expansion.
  3. Thermal conductivity: The best materials for CNC machining should be capable of conducting the heat generated during the cutting process. Materials with low thermal conductivity experience high temperature differences. This gradient causes inhomogeneous deformation of the part and thermal failure of the cutting tool. 
  4. Specific heat: Specific heat refers to the energy needed to increase the temperature of a unit mass of material by 1 degree Celsius or Kelvin. Machining material with a low specific heat will lead to a steep increase in their temperature, with the inverse holding true. The elevated temperature impacts the surface finish and accuracy of the machining process. It also increases tool wear and leads to negative metallurgical changes in the material due to alterations to its crystalline structure.

3. Electrical and Magnetic Properties

Electrical conductivity, which is the ability of metals to conduct electric current, is vital in machining processes like electrochemical machining (ECM) and electro-discharge machining (EDM). The workpiece must conduct an electric current for effective machining during ECM and EDM. Thus, alloys that are typically less conductive than pure metals may not be ideal materials in ECM and EDM.

When it comes to magnetic properties, some materials, such as pure nickel or certain iron-nickel alloys, experience magnetostriction. Materials that experience this phenomenon change their shape, by either expanding or contracting, when the magnetic field through them changes. Coincidentally, ultrasonic machining relies on the magnetostrictive effect, among other principles, to convert oscillating electric current to mechanical vibration. Thus, if you are machining material that will be used to make a transducer in an ultrasonically vibrating machine, consider using pure nickel or some iron-nickel alloys.

4. Optical Properties

Optical properties come into play during the surface finish stage. Very smooth finishes are extremely reflective, while rough surfaces reflect light randomly. Machinists control the machining and finishing processes to generate a desired optical property. However, some mechanical properties, like hardness, may make it harder to achieve certain finishes through polishing.

5. Chemical and Corrosion Properties

The chemical and corrosion resistance properties are important in material selection. This is especially so if the material and resultant parts are to be used in environments that require stable materials, e.g., in the petroleum, food, and chemical industries. Materials used in such environments should be resistant to chemical corrosion. Some corrosion-resistant materials include nickel, pure copper, tin, lead, titanium, plastics and composites, ceramic materials, metallic glasses, and tantalum. 

Mechanical Properties

The mechanical properties of materials affect their suitability for specific machining operations. This is because the deformation of the material is correlated to the applied load. This deformation may be low even when the load is exceptionally high and vice versa. The deformation may also be low even when the load applied is commensurately low or high when the applied load is high. This behavior depends on several mechanical properties: strength, stiffness, hardness, toughness, and ductility.

1. Strength

Strength refers to the ability of a material to resist externally exerted forces. A material with elevated strength will withstand a very high level of stress (force per unit area) before failure (i.e., fractures or permanent deformation). In contrast, a low-strength material requires very little force per unit area to fail. We, therefore, deduce that strength is a measure of a material’s resistance to stress. There are various types of strength:

  • Creep strength
  • Fatigue strength
  • Yield strength
  • Tensile and compressive strength    

2. Stiffness

Stiffness refers to a material’s ability to get back to its original shape or form after bending/deforming under load. This property mostly applies to cutting tools, which must be capable of resisting deformation during the cutting process. Thus, stiffness is a primary consideration when selecting materials to use when creating custom tools.

3. Hardness

Hardness refers to a material’s ability to resist localized plastic deformation. Coupled with tensile strength, hardness indicates that a metal is resistant to plastic deformation.

4. Toughness

Toughness refers to the energy needed to crack or break a material. It is an important mechanical property for parts that will suffer impact during day-to-day use.

5. Ductility and Brittleness

Ductility refers to the ability of a material to plastically deform or stretch thin when tensile forces are applied before failure. On the other hand, Brittleness is the opposite of ductility; it is the inability of a material to plastically deform when subjected to tensile stresses. A brittle material fails when subjected to tensile forces.

Machining Properties

General manufacturing properties include malleability, workability, weldability, formability, castability, ductility, machinability, heat-treatability, and grindability. However, within the context of CNC machining material selection, only three of these properties are needed: machinability, grindability, and heat-treatability.

1. Machinability

Machinability refers to the difficulty or ease of machining or fabricating a material. This machining property is affected by various factors, including the material’s thermal, physical, mechanical, and chemical properties, as well as the cutting speeds, feed rate, and properties of the cutting tool. For this reason, machinability is related to the entire machining system operating under a specific combination of conditions. 

In practice, machinists and engineers assess the machinability using various criteria, including: 

  1. Tool wear rates or tool life: machinability increases with a decrease in tool wear rates (or, relatedly, an increase in tool life) with other cutting conditions held constant.
  2. Chip form and burr behavior: This criterion is typically used to test the machinability of soft, ductile alloys. This is because such materials tend to form long, unbroken chips and, as a result, form burrs as the tool wears. Generally, materials that form long chips, which are harder to manage and flush out from the machining area, are said to be less machinable than those that form short chips. 
  3. Surface finish: The machinability of a material degrades when the achievable surface roughness under a given set of cutting conditions increases. This means that the machinability increases with an improvement in the surface finish achievable, all other factors held constant. 
  4. Tolerance: An increase in tolerance achievable under a specific set of cutting conditions is associated with a decrease in machinability, and vice versa. This criterion, like surface finish, is useful for assessing different classes of materials.
  5. Surface integrity: Materials that can be easily damaged due to the formation of residual stresses or galling of sliding surfaces are said to be less machinable.
  6. Cutting forces: Machinability increases as cutting forces decrease, and vice versa. And since cutting forces are directly correlated with power consumption, machinability similarly increases as power consumption decreases.
  7. Cutting temperature: Increased cutting temperatures mean the material has low machinability; high temperatures are associated with elevated friction and high cutting forces.
  8. Mechanical properties: Properties like hardness, ductility, and yield strength can also be linked to machinability. For instance, hard materials are less machinable, as are ductile materials that form long chips, as detailed earlier.

2. Grindability

As the name suggests, grindability is the general ease of grinding a material. It includes additional considerations such as the wear of the grinding wheel, surface integrity, surface finish/quality of the resulting surface, and more. Grindability determines the finishing process to be used. For instance, as stated below, grinding is not often the preferred finishing method when dealing with extremely hard materials. Machining processes like hard turning or hard boring are used in such cases. 

3. Heat treatability

Some materials, such as alloys, need to be heat-treated to achieve certain properties. Heat treatment processes are usually used alongside CNC machining processes to improve qualities such as machinability, hardness, or strength. Examples of heat treatment processes include quenching, case hardening, carburizing, precipitation hardening, annealing, tempering, and stress relieving. 

Common Materials Used in CNC Machining

The most common materials in CNC machining include:

  • Metals
  • Alloys
  • Plastics 
  • Composites
  • Wood

1. Metals 

Metals have high thermal conductivity and reflectivity. These materials are also malleable (meaning they can be thinned when hammered), have high tensile strength, and are tough and stiff. They are also ductile, meaning they deform plastically before fracturing or breaking. The list of metals includes iron, nickel, titanium, zinc, tin, lead, tungsten, silver, platinum, chromium, manganese, and gold.

2. Alloys

Alloys are created by mixing two or more elements, with at least one element being a metal. Examples of alloys include steels (carbon steels, alloy steels, stainless steels, and cast iron), aluminum alloys, magnesium alloys, tin alloys, zinc alloys, lead alloys, nickel alloys, copper alloys, nickel-based alloys, and cobalt-based alloys, just to mention a few.

3. Ceramics

Ceramics are inorganic compounds usually made up of one or more metallic elements and a nonmetallic element. The nonmetallic element can be oxygen (as in the case of aluminum oxide, also known as alumina, zirconia, magnesia, thoria, and beryllia), nitrogen (as in silicon nitride), or carbon (as is the case with silicon carbide, tungsten carbide, and boron carbide). Other examples of ceramic materials include magnesia, tungsten carbide, and boron carbide. Ceramics are generally hard and stiff and are excellent insulators of electricity. However, they are extremely brittle.

4. Plastics and polymers

Polymers are organic materials that are highly resistant to most chemicals, are good electrical and thermal insulators, and have low density. Plastics, on the other hand, are created by mixing polymeric materials with certain additives. Plastics are generally hard to machine, but that does not mean they cannot be machined. They are often used to make prototypes.

5. Composites

Composites are typically made by modifying the chemistry of two or more materials. For instance, thin fibers of glass and carbon can be added into a polymer matrix, creating a composite that is stronger and stiffer than the original polymer. Composites have a high strength-to-weight ratio. Thanks to the inexpensive polymer matrix, they are also less expensive than metals with the same properties.

6. Wood

CNC routers work on wood and there are different types of wood from which to choose: softwood, hardwood, and composite or engineered wood. Like other materials on this list, there are several factors to consider when determining the type of wood to use. These factors include density, hardness, strength, machinability, stability, grain size and direction, moisture content, tooling, and surface finish. 

CNC Machining Material Selection Process

There are tens of thousands of useful metallic and nonmetallic engineering materials. This sheer number makes material selection an extremely taxing task. What’s more, engineers must also consider the machining process available to them during the CNC machining material selection process. This is because some machining processes are more suited for certain materials than others. 

For instance, the hard turning process replaces grinding operations in hard materials. This is because hard turning can achieve excellent surface finish, roundness, and tolerance. Similarly, you will be more productive using hard boring to increase the internal diameter of an existing hole in a workpiece made of hard material, like hardened still, than if you opt for internal grinding.

The CNC machining material selection process, therefore, needs to be rigorous. Only then can you be assured that you have selected the suitable material that can be machined using the tools and CNC machines in your machine shop. The preferred practice involves considering the materials and machining process in the early stages of the design and defining them as the design rolls through the various stages. 

Stages of CNC Machining Material Selection

There are five main stages in the CNC machining material selection process:

  1. Assessment of the Requisite Material Performance
  2. Listing of Alternatives
  3. Initial Screening
  4. Comparison of Shortlisted Alternative Materials
  5. Selection of Optimum Materials 

1. Assessment of Material Performance Requirements

As detailed earlier, engineering materials have distinct properties that combine to influence their suitability during CNC machining material selection. But, in isolation, these properties serve little to no purpose if they do not align with the performance requirements of a part. For this reason, the first stage of the material selection process is the analysis of material performance requirements vis-à-vis the material properties and other parameters. 

In this stage, you should specify the material performance requirements, including:

  • Reliability requirements
  • Resistance to service conditions, e.g., corrosive environments and low or high temperatures
  • Functional requirements
  • Machinability requirements
  • Cost of material and how it impacts the overall quality of the machining process

2. Listing Alternative Materials

Once you have laid out your material requirements, the next stage of the CNC material selection process involves searching for materials that best meet those outlined requirements. To begin your search, looking at the entire range of engineering materials, including metallic and nonmetallic materials, is always advisable. This is because a number of materials can fulfill the basic functional requirement of a particular design. 

This second stage aims to create a list of possible alternatives without caring much about their feasibility. Organizations such as ASM International have comprehensive guides to the performance, structure, properties, processing, and analysis of metallic and nonmetallic engineering materials. Such guides can serve as a great starting point.

3. Initial Screening

The third stage involves eliminating unsuitable materials to create a more manageable list. This stage leans on the practicality of using materials. To help you with the screening, you can use quantitative methods like Ashby’s, Dargie’s, Esawi and Ashby’s, and cost per unit property method. You can use one or more of these quantitative screening methods. 

In addition, at this stage, you should also assess the material performance requirements based on rigid and soft requirements. Rigid requirements relate to the requirements that the material must meet, while the soft requirements are those you can compromise on.

4. Comparison and Ranking of Shortlisted Alternative Materials

While the screening process does narrow the list of possible materials, you still have to shrink this list further to a handful of promising materials. Like in the third stage of the CNC machining material selection process, you can use several quantitative ranking methods. These methods help you compare and rank the various options. The quantitative ranking methods include the weighted property method, digital logic method, performance index, limits on property method, and the analytic hierarchy process.

5. Selection of Optimum Materials

The final step in the CNC machining material selection process is selecting the optimum materials. It logically follows that materials that, based on the ranking methods, have the best performance scores are selected. And given that the selection is concurrently done during the design stage, then it goes without saying that the engineer will naturally capitalize on the material’s favorable properties when coming up with the final design. 

(For a more detailed discussion of the quantitative screening methods and quantitative ranking methods, refer to the book “Materials and Process Selection for Engineering Design.”)

Factors Influencing Material Selection

There are several factors influencing CNC machining material selection, including the following:

1. Material Properties

The properties of a material and its ability to meet performance requirements are perhaps the foundational factors influencing material selection. You cannot, for instance, select a brittle material for an application that requires ductility. Similarly, it would be illogical to select a material that is least resistant to chemical elements if the resultant part is meant to be used in a corrosive environment. The material properties also influence an additional consideration: durability.

2. Fulfillment of Material Performance Requirements

The first stage of the CNC machining material selection process involves listing the possible materials you can use and specifying the material performance requirements. The subsequent steps involve narrowing down the list of materials based on their ability to meet the specified requirements. Thus, one of the aspects to consider when selecting a material for a particular function is whether it fulfills the outlined requirements.

3. Cost

Cost is another fundamental factor in evaluating materials. Parts have a cost limit; exceeding this makes them impractical to machine as they won’t be cost-effective for buyers. If this cost limit is exceeded, engineers may be forced to change the design to enable the use of cheaper material.

When it comes to analyzing costs, engineers can conduct what is known as value analysis. This technique allows engineers to assess the value of a material by referencing it to another material that could serve the same function. To illustrate, consider material A, the reference material, that costs a given sum of money, say X. 

Suppose you are considering five possible alternatives in your CNC machining material selection process. In that case, value analysis calls for you to check the materials that exceed cost X and those below this price point, provided they can serve the same function. In such a case, you can choose the least expensive material or the more expensive material that is cheaper or simpler to machine. 

4. Product Design 

A part that serves a particular function may see the designer/engineer explore various alternatives and design concepts. In such a scenario, material A, say steel, may be perfect for design concept A, while material B, say plastic, may be ideal for design concept B. This is despite the fact that both design concepts serve the same function.

5. Machining Process

Some machining processes are better suited than others to create certain features. Similarly, some processes require fewer steps to machine a particular feature, perhaps because they support more axes. In a way, the choice of the machining process directly affects the time taken and, by extension, the cost of the part.

6. Product Scalability

Do you wish to create thousands or millions of parts for the mass market? If so, you should consider cost-effective and readily available materials. You should also select materials that have consistent properties regardless of where they are sourced. This combination of characteristics enables you to easily scale without compromising performance or quality. 

Advanced Considerations in CNC Machining Material Selection

1. Regulatory Compliance

The food, petroleum, medical device, and chemical industries have stringent standards and regulations governing material use. These regulations are usually enforced by government departments. For example, the UK Health and Safety Executive provides guidelines for accounting for corrosion when selecting materials for constructing plants and equipment.

2. Environmental Considerations

The increasing awareness of the public and companies on their impact on the environment has made environmental considerations an influential factor in the CNC machining material selection process. The choice of material and the machining process impact the power consumption. 

Manufacturing companies aiming to reduce energy consumption and environmental impact may opt for materials and processes that use less energy. After all, studies have shown that energy consumption is directly related to carbon emissions over the long term: an increase in consumption could lead to a rise in carbon emissions and vice versa.

Conclusion

To master CNC machining material selection, you must first be conversant with materials used in CNC machining and their properties of materials. Some of these properties include density, hardness, stiffness, machinability, heat-treatability, corrosion resistance, thermal properties, and grindability, just to mention a few, as they influence the ease with which you can machine a product. 

Next, you must specify the performance requirements of the material as they relate to the part you want to make. You should then use these requirements to narrow down the list of materials. This means that while you will start the CNC machining material selection process with tens – or even hundreds – of materials, you will end up with just a handful. You can also use quantitative methods to narrow down the list further. The selection process should also consider several key factors. These include the cost, environmental requirements, regulatory compliance, product design, scalability, etc. We contend that the CNC machining material selection is not always straightforward. This comprehensive guide, nonetheless, makes the process clearer and easier to navigate.

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