Since the technological advances gained momentum towards the automation of manufacturing processes (especially towards the design of machines capable of being programmed to carry out all manual tasks of an operator automatically, CNC has become a staple in the manufacturing world, looking to reduce machining errors and increase the effectiveness of different industrial processes.
CNC (computer numerical control) machining is a swift, efficient, precise, and adaptable process using CNC machines: tools such as mills and lathes guided by computer instructions control the precision of the instruments compatible with a variety of hard and soft engineering materials (including metals)
The outcome of a CNC machining project and the functionality of the produced parts closely depend on the material that has been utilized. What is more, CNC machining operation may be perplexing and costly therefore the material selection is an important step in CNC manufacturing that requires particular attention because of its intrinsic properties.
Let’s overview the main aspects and technical qualities before deciding whether to choose a hard or soft metal type for your metal parts’ production process.
Materials for CNC
When it comes to CNC, the technical properties of the material and its type determine the metal manufacturing process that will be assigned. Based on the mechanical properties, metals are classified into 2 different taxonomies: Hard Metals and Soft Metals.
Hard metals are widely used to make tools, mechanical parts, profiles, and abrasives. Considered composite materials, this group is characterized by its metallic nature and a particular combination of properties such as hardness and resistance. As a general rule, hard metals are composed of 90-94% tungsten carbide (reinforcement phase) or other carbides, hard but at the same time very brittle, and 6-10% cobalt, nickel, or iron, which act as binders, also called durable components. These relatively soft, ductile, low melting point binders take care of the stage known as the fluid bonding phase, in which the carbide particles become wet during sintering. The binder permeates the agglomerated particles, compacting the body by contraction of the hard materials down to the smallest possible size. The hard metal carbides are surrounded by the metal matrix of the binder, but the heterogeneous metallographic constituents remain separate. In recent years hard metals have improved considerably, with new applications and continuous improvements in quality and production technology, thus proving their high qualities. This naturally leads to an increase in the range of production processes and demand.
Soft Metals, also called alkaline, are recognized as having a single electron in their last energy level or outermost shell. In addition, in all metals of the given type, that electron is located in an s orbital, which, based on similar characteristics, assigns them to a group with very similar properties within the known chemical elements. These kind of metals are generally shiny, soft, and malleable, which is why they can be easily cut. What is more, such metals are quite light and have high reactivity and, on the other hand, they are good conductors of electricity and heat.
Characteristics of Metals for CNC
Only by having a deep understanding of the metals’ mechanical and physical properties the proper material for any given application might be chosen. Let’s discuss some of them in deeper detail.
Wear and Fatigue
The first thing we should always know is that when we are working on a prototype, there is no need to be concerned about the wear of the material. However, if we are making final parts that will be fabricated under conditions of high stress the selection of the material needs to be a high priority. The properties of the material fatigue we should take into consideration are the following:
- Fatigue and Resistance: It is a measure to evaluate the stress level below which a material can presumably endure an infinite number of stress cycles. This measure has been studied in great detail to aid manufacturers to pick the right material for their applications. Fatigue is considered responsible for 90% of failures when metallic elements are involved. The material selection process is complicated: limitations and errors tend to occur if the material is selected wrongly. For this reason, if the desired metal part needs to withstand high-stress levels, fatigue limit is the value that should be analyzed carefully.
- Environmental Cycling: This is a critical value to be tested because most of the time materials are tested within a controlled environment that reflects the natural conditions of the environment. They are tested in high temperatures, low temperatures, abnormal humidity conditions, rapid thermal changes, and many more. As a result, it is highly advised to select a proper metal that can work under the conditions that are expected.
- Creep: Creep is a particular term to define the tendency of a material to move slowly and deform over time under the influence of persistent mechanical stresses. It usually happens when the material is exposed to high levels of stress. Creep values are critical when the material is exposed to elevated temperatures. The resistance to creep can be improved or controlled using special alloys to fiddle with the melting temperatures of metals. Creep is a significant value in specific industries, for example, aerospace applications; therefore, materials such as aluminum are highly avoidable because of their low melting point and tendency to creep.
Corrosion in Metals
Corrosion resistance is one more property to consider in the material selection process for CNC manufacturing. The final selection will generally result from several compromises between corrosion resistance and economic factors. Oxidation is a result of the interaction that the material has with the environment. All metals can corrode. The myth that certain metals do not suffer corrosion comes from the fact that some metals corrode quickly while others do not and, therefore, to the untrained eye, it might seem that certain metals are immune to corrosion. Iron, for example, can rust extremely fast, but when you have stainless steel, which is iron in alloy with other materials, it makes the material resistant to corrosion. If you need corrosion resistance, stainless steel is usually the best option. Other alternatives could be anodizing aluminum, but this adds a new step in the process that is not necessarily good if you have a time-sensitive project.
When it comes to selecting a material, thermal properties are also a crucial factor to be considered. Metals will react differently when put under different temperature levels. Some metals will expand at any given point, others will melt or even suffer a high degree of creep.
Last but not least, different materials have different production requirements when it comes to manufacturing, that is why it is important to select a material suitable to that particular manufacturing process and the machinery. In other words, it is the ,,Ability of a material to be worked or shaped into the finished component” (Farag, 1979) and it is sometimes referred to as “workability”. Thus, terms such as “weldability”, “castability”, “formability”, “machinability” are used to describe how easily the material can be used for specific processes.
Finally, another essential factor you need to evaluate when selecting a material is its cost (economical factor). The expenses involved will impact many of the decisions to be made in designing and producing a product. As a result, the overall cost of the product must be taken into account as soon as possible. The amount required in terms of production volume, production rate, and economic batch size may be the basis for other economic considerations.
To sum up, selecting the material for your metal manufacturing process is a multi-criteria decision-making process. There are several factors to take into account, such as wear and fatigue values, resistance to corrosion, thermal properties, the manufacturing process itself, and the cost. For example, prototyping faces less risks and is a less cost-sensitive process, for this reason, the errors should be identified at prototyping stage where the complexity of the design, costs involved and familiarity with the selected materials are evaluated.
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