2. Considerations for selecting the right CNC materials
2.1 Part requirement
Each design is unique and you know the part application better than anyone else. Put your requirements for the custom parts first. Start by listing the must-haves, and then move on to the nice-to-haves. This way, you can narrow down your material options and make a smarter choice.
2.2 Use environment
For many applications, the environment will be an absolute top-of-the-list factor. Heat resistance, cold resistance, flame retardance, UV resistance, and chemical tolerance are all environmental factors.
Components for outdoor applications shall be resistant to UV and rain, which need materials with durable properties and a robust design. By contrast, indoor application parts do not need to be exposed to extreme environmental conditions, more materials are available for choice.
Parts for medical equipment may have very tight tolerance and have to withstand high temperatures and harsh chemical sterilization. For other applications, the materials have to be food-grade or medical-grade.
2.3 Material mechanical properties
The mechanical properties of materials include strength, hardness, and toughness.
Strength
Strength is the materials’ ability to withstand an applied force without breaking or deforming. According to ISO (International Organization for Standardization), it is often measured in tensile strength, compressive strength or shear strength.
Low tensile yield strength: below 240MPa Plastics are always with low tensile yield strength, for example, PEEK, Nylon, and PC.
Medium tensile yield strength: 240MPa-550Mpa Aluminum is with medium tensile yield strength.
High tensile yield strength: above 550Mpa Stainless steel, alloy steel, and tool steel are with high tensile yield strength.
Hardness
Hardness is the material’s resistance to indentation or scratching. Hardness is measured in Rockwell, Brinell, and Vickers hardness tests.
Low hardness is below 50HRB.
Medium hardness is between 50 to 90HRB.
High hardness is between 90HRB to 50HRC.
Very high hardness is above 50HRC.
To convert different hardness, please visit steel hardness conversion.
Toughness
Toughness is the materials’ ability to absorb energy and fracture plastically before breaking. It is tested by impact tests or Charpy V-notch tests.
Strength VS Toughness
Strength and toughness are two related but distinct concepts.
Strength is the stability of the material under stress. The higher the strength, the less likely the material will deform when exposed to external loads. Toughness is the ability to absorb energy and deform without fracturing when exposed to external loads. The higher the toughness, the more likely the material will undergo a certain degree of deformation without losing the overall structure.
Strength and toughness are independent, and there is no inherent correlation between them. In other words, materials with high strength do not necessarily possess high toughness.
In component design, there is often a need to require a balance between strength and toughness.
2.4 Materials’ corrosion resistance
When CNC machined parts are used in environments with corrosive elements such as moisture, chemicals, or harsh atmospheric conditions, selecting corrosion-resistant materials can save the surface treatment for materials without corrosion resistance.
Stainless steels, aluminum alloys, and certain alloys like Inconel and titanium are known for their ability to resist corrosion in different environments.
2.5 Materials’ electrical conductivity
If the parts need good conductivity, select metals with good electrical conductivity, for example, copper and aluminum.
2.6 Materials’ thermal conductivity
Copper and aluminum parts have good thermal conductivity.
2.7 Materials’ thermal stability
If the parts are to be used in high-temperature environments, choose materials with good thermal stability, such as stainless steel, nickel alloy, and titanium alloys.
2.8 Materials’ magnetic properties
Understand whether the application has specific magnetic requirements. Some applications may need materials without non-magnetic materials to avoid interference with magnetic fields. If magnetic shielding is required, consider materials with high magnetic permeability.
Most stainless steel, aluminum, and copper alloys are typically non-magnetic, carbon steel and nickel are magnetic.
2.9 Materials’ machinability
It is very important to understand the material’s machinability because the machining cost is cheaper for easy-machining materials.
Plastics, mild steel, aluminum, and copper alloy are easy for CNC machining. The machining for stainless steel, tool steel, and titanium can be a challenge because of their hardness. Magnesium CNC machining is also not easy because it tends to fire during production.
Whether the material is easy to cut, milled, and turned, it is closely related to CNC machining precision and efficiency.
2.10 Materials’ weight and density
For the aerospace and automotive industries, the weight is critical. Choosing a material with a lower density is preferred. Lighter materials reduce the finished parts themselves.
2.11 Environmental factors
Consider the sustainability and environmental impact of the materials chosen.
2.12 Machining tolerance
Some materials are easier to machine with high precision and get tighter tolerance, for example, aluminum. While some are difficult to get tight tolerance, such as plastics.
2.13 Surface finishing
In most applications, cosmetic is not an important consideration. But for some for components where aesthetics are crucial, the surface finish becomes a significant consideration in CNC machining material selection.
Plastics can be supplied in different colors while metals can be colored using post-processing treatment. But keep in mind that post-processing costs money.
2.14 Material cost
Cost can be an important factor in choosing CNC machining materials, especially for projects with limited budgets. Plastics or cheap metals are good choices in this situation.
