Introduction

Computer Numerical Control (CNC) is one of the most common manufacturing methods used nowadays. It allows us to increase production rates because of the automation of almost any process and offers high-quality surface finishes at relatively low costs. One of the main advantages of CNC manufacturing processes is the machining material availability, meaning that almost any kind of raw materials like wood, plastics, and metals can be re-shaped using CNC tools to achieve the desired product design.

What needs to be considered when selecting machining materials

Selection of the machining material will generally depend on the following factors:

• Design considerations regarding its final purpose.
• Manufacturing budget.
• Machinability of the material, which considers its mechanical properties.
• Thermal and chemical resistance.
• Cutting tools available for the process.
• CNC configuration (RPM, federate, coolant available for the process)

Using the correct cutting tools and the proper configuration of the machine are the keys behind the design of every CNC manufacturing process, and those are the first aspects to consider after selecting the machining material.

Machine configuration, such as the RPM of the raw material (in a lathe) or that of the tool (in a mill or drilling bench) also needs to be considered after selecting the machining material because of thermal and wear considerations. Using an improper configuration and tools will lead to poor surface finishes, damage to the cutting tool and it may alter the mechanical properties of the final product.

Regarding cutting tools, the most important characteristics are:

• Wear resistance
• Tenacity
• Rigidity
• Hardness
• Low friction coefficient

The difference between the hardness of the cutting tool and the machining material is what allows the CNC to re-shape the product, meaning that it is important to select the material first and then configure the CNC to use the correct tool for it.

Cutting tools are usually made of Carbon steel, alloy steels, high-Speed Steels, ceramics, diamond.

These tools are listed from lower to higher prices and it is understandable that as the hardness of the cutting tool is higher (like the case of diamonds, the material that has the highest hardness) its cost will also be considerably greater.

Different considerations Vs Different CNC machining material Selection

When comes to CNC machining manufacturing, there are three typical types of materials for this process, they are metals, plastics, woods.

Different considerations need to be taken into account when selecting the different machining material for the manufacturing process.

 #1- Considerations when machining metallic materials

CNC machining is characterized by its capability to turn hard metallic materials into machine components, achieving smooth surfaces and complex geometries. When working with metals, it’s important to consider the difference between the hardness of the raw material and the cutting tool.

If the raw material selected for the manufacturing process is cast iron, which has a Rockwell hardness of 86, it wouldn’t be feasible to use low carbon steel cutting tools because they have an average hardness of 60. Since the cast iron is harder than our tool, it is most likely that the latter wears or fractures faster.

Machining aluminum

When it comes to machining materials for CNC processes, aluminum is one of the preferred. Aluminum is characterized for its good mechanical properties while having a relatively low density, making it a strong and light material used in the naval, aerospace, and the automotive industry.

Aluminum is a versatile material, meaning that it can be used in most of the manufacturing processes. When the geometries are complex and molding, forming, or welding are not the best options, it can always be used as a raw material in a machining process. The properties and machinability of some of the most used aluminum alloys are listed below:

Aluminum AlloyDensity ( )Yield Strength (MPa)HardnessMachinabilityFormabilityWeldability
1100-AA271010525 to 28 (HB500)GoodExcellentExcellent
2024-T42780325110 to 120 (HB500)FairGoodPoor
6063-T5270021470 to 75 (HB500)FairGoodGood
7075-T62810503150 (HB500)FairPoorPoor

As can be depicted in the table shown above, harder and stronger materials are less likely to be used in other manufacturing processes rather than machining. For the strongest machining materials, better and more expensive cutting tools need to be used, meaning that prices will rise for the process.

Machining Steel

Another commonly used material in machining processes is steel. Steel is well-known for its considerable strength and since it has a considerable high weight, its mostly used in applications when strength is preferred over lightness.

It is important to keep in mind that most cutting tools are made of steel, a recommendation will always be to use tools with higher strength than the raw material to reduce heat generated by friction and wear of the tool.

The machinability of steels is given in comparison with the machinability of AISI 1212 carbon steel, which has a machinability rating of 100%. In the next table we mention the properties of some of the most used steels in machining processes:

Steel typeDensity ( )Yield Strength (MPa)HardnessMachinability
Carbon Steel AISI 12127870415165 to 170 (HB500)100%
Alloy Steel AISI 4142 annealed7850725187 to 230 (HB500)75%
Alloy Steel AISI 86207850385150 (HB500)77%
Stainless Steel Grade 416 annealed7750275260 (HB500)110%
ASTM Class 40 cast iron7150295 (Ultimate Strength, fragile materials do not yield)18048%

Ductility plays an important role in material machining. Ductile materials offer better machining conditions being less brittle and generating less friction, this is the main reason why cast iron has considerably lower machinability than, for example, grade 416 stainless steel.

For metals, RPM’s and feed rates must be low (300 to 1200) to avoid over-heating of the cutting tool. High RMP’s in the range of 4000 to 6000 is only used for clean surface finishing.

Proper coolant selection must be done to avoid corrosion of the machined surface or damages to the microstructure of the raw material, obtaining a poor final product.

When using other metals besides steel and aluminum, it is important to consider the costs of the process which is mostly affected by the costs of the cutting tools required. It’s important to keep in mind that harder thus stronger cutting tools cost significantly more than common steel tools.

When money is not the problem, with the proper configuration of the CNC machinery any kind of metallic material could be used in a machining process.

#2- Considerations when machining woods

Wood is extremely fragile, it brittles easily, and obtaining clean surface finishes it’s a hard task. They can be sorted into two main groups:

Softwood, used in structural timber.

Hardwood, mostly used in furniture, boat construction, tools, and barrels.

Wood is softer than metal, this means that higher RPM’s ranging from 12000 to 15000 are needed to obtain clean finishes. Working with wood is not easy, and using wood as machining material is specifically reserved for furniture and forging models making hardwood the most commonly used wooden machining material.

The preferred hardwoods to be machined are listed below:

• Maple
• Black Walnut
• Oak
• Cherry
• Cedar

High-Speed Steels (HSS) are ideal for machine woods because of their low friction coefficient. Having a low friction coefficient allows for higher RPM’s and feed rates, ensuring a better surface finish.

 #3- Considerations when machining plastics

When it comes to machining processes applied to plastic materials, there are some key factors to consider.

Primarily, cutting tools need to be sharp because machining plastic with highly worn tools causes brittleness of the raw material. Lower forces applied to the clamps and the use of supports to avoid deflections of the material during the machining process is part of the usual configuration required for the machining process of plastic materials.

RPM’s configuration will mostly depend on the machining process applied. For drilling of large diameter holes, low RPM’s ranging from 400 to 1000 are suitable. Other types of processes can be applied with high RPMs ranging from 15000 to 20000. The RPM range will also depend on the plastic material selected.

Coolant is not a big issue for plastic material machining, pure water, or compressed air are widely used.

Machining characteristics of some plastic materials are shown in the table below:

Plastic-typeMachinability
PETP PolyesterExcellent
PE PolyethyleneGood
NylonFair
PP PolypropyleneRegular
PTFE PolytetrafluoroethyleneRegular
PolycarbonatePoor

Wear-resistant HSS tools are preferred when machining plastics because they maintain their sharpness, allowing for better automation of the process, making it appropriate for mass production.

 Conclusion

Nowadays, material machining is one of the major manufacturing processes used in a wide variation of industries. Even though 3D metal printing is gaining recognition, the economy related to CNC machining maintains it at a peak position when it comes to achieving complex geometries for mass production processes.

At RapidDone, we have an outstanding CNC machining manufacturing capability, including 5-axis CNC machining capability, CNC turning capability, CNC milling capability.

We can work out a high-quality part for you with all kinds of material. To get a quote for your part or product, just get in touch with our team.