Machinability of Materials in CNC Machining

03 November 2023
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Report by Danny Weller

Machinability is a crucial factor in machining processes, defining the ease of material removal with moderate force, typically quantified by the power needed for cutting and directly affecting the quality of the end product.  This not only leads to a smoother surface finish but also reduces equipment wear and tear. While achieving a superior surface quality is a common objective in machining, it can be particularly challenging when working with fine-grained metals.

In this article, we will delve into the strategies employed by engineers to improve machinability while ensuring product quality. We’ll also discuss the essential aspects of predicting machinability, the factors influencing machinability ratings, and how machinability varies in different materials. This knowledge is essential for optimizing CNC machining processes and achieving the best possible results.

 

Predicting Machinability

Predicting machinability can be challenging as it depends on the unique attributes of the workpiece, its environmental context, and its physical properties. Nevertheless, engineers place significant emphasis on two critical factors:

  • The characteristics of the work materials; including factors like grain size, microstructure, chemical composition, hardness, yield strength, tensile strength, and heat treatment.
  • The physical attributes encompass properties such as elasticity modulus, thermal expansion coefficient, thermal conductivity, and hardness.

 

Machining
Image: Simon Kadula

Factors that Influence the Machinability Rating

Hard materials have the disadvantage of low machinability, as they use more power heat and wear more tools. Here are some elements to consider when evaluating machinability:

Chemical Composition and Crystal Structure

The ease of material removal from a crystal is directly influenced by the crystal’s internal atomic arrangement and structure within its grains. Various factors, including heat treatment, work-hardening, and fabrication processes, can modify this crystal structure, leading to increased challenges in CNC machining. Furthermore, the machinability of a metal is linked to its chemical composition; for instance, the addition of carbon enhances its strength while simultaneously raising the difficulty level of the production process.

Tool Material

Selecting the right cutting tool with the appropriate hardness is vital for any CNC machining task, with a primary focus on factors like toughness and wear resistance. High-speed steel tools, available in M (Molybdenum) and T (Tungsten) varieties, surpass the performance of traditional carbon steel options. For even greater durability, tungsten carbide tool bits offer a longer lifespan, although they exhibit a degree of fragility.

Tool Geometry

The design of tools plays a pivotal role in the CNC machining process. To ensure a precise, smooth cut, it is crucial to maintain specific angles for the rake face, clearing faces, and chip breakers.

Cutting Depth

The depth of the cut is determined by the amount of material in contact with the tool. The introduction of carbon into metal enhances its strength, albeit at the cost of increased production complexity.

Drill bits
Image: Marcus Urbenz

Feed Rate

The feed rate refers to how quickly a drill bit moves into the job or how quickly the tool moves automatically during drilling.

Speeds

Optimizing cutting speed is the foremost factor for prolonging tool life, and need for adjusting it to align with the material’s machinability. While a high cutting speed may initially yield a flawless finish, it can lead to substantial tool wear, posing challenges in maintaining precise proportions.

Machine Rigidity

A machine that is not steady will deliver substandard results even if all the previously mentioned features are otherwise perfect. Among the design considerations for machines, this is what might impact accuracy:

  • Machine weight and toughness
  • Bearing quality
  • Drives using a ball screw
  • Clamping mechanism design 
  • A powerful spindle
Steel
Image: Yasin Hemmati

Machinability of Common Materials

Steels

The machinability of steel is significantly affected by its carbon concentration. High-carbon steels pose machining challenges due to their exceptional strength and the potential presence of abrasive carbides, which can accelerate tool wear. Conversely, low-carbon steels present their own difficulties as they are overly soft and can lead to adhesion to cutting tools, resulting in built-up edges and a shortened tool lifespan.

Stainless Steel

Stainless steel, known for its increased hardness, gumminess, and rapid hardening characteristics, presents a greater machining challenge compared to traditional carbon steels. To enhance the machinability of steel, a slight hardening process can be employed, reducing its gumminess due to sulfur and phosphorus.

Aluminum

“Aluminum is known for its ease of machining. However, with the material’s softer grades, there can be challenges, such as the development of a built-up edge that can result in poor surface quality. Good machinability in aluminum depends on utilizing fast cutting speeds, high rake angles, and high-relief angles.

Thermoplastics

Thermoplastics are difficult to manufacture due to their low heat conductivity. This produces heat in the cutting zone, which shortens the tool’s life and melts the plastic around the cutting zone.

Composites

Composites have the least machinability, because of the poor heat conductivity of the plastic resin, combined with its stiff or abrasive ceramic matrix.

AMFG DARK

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Find out how AMFG can supercharge your Additive Manufacturing and CNC Machining processes today.

 

 

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