CNC machining is known for its suitability for manufacturing complex structures and shapes with high precision. 3-axis CNC milling centers also benefit from this feature. However, when dealing with complex geometrical parts, the functionalities of 3-axis CNC machines may be compromised, as the cutters can only access the workpiece from three angles, limiting their ability to handle intricate designs.
This article introduces you to the operational principles of 3-axis milling centers and discusses why they may not be the best choice for complex designs. We also explore alternatives for manufacturing these complex structures.
How Does 3-Axis CNC Milling Center Work
As the name indicates, the 3-axis CNC milling center operates by moving the cutting tool along three linear axes—X, Y, and Z – to machine the workpiece. The movement of the cutters against the workpiece along the three axes in CNC milling is simultaneous, so the machinist does not need to adjust or reposition the workpiece. The X-axis involves moving the tool side-to-side, the Y-axis moves it front-to-back, and the Z-axis moves it up and down.
In CNC milling, the workpiece is usually fixed on the work table, allowing the cutting tools to work against the material in multiple directions. This enables precise cutting and shaping. The CNC system, controlled by computer software, directs these movements according to a programmed design.
CNC milling involves rotating the cutting tool at high speeds to remove material from the workpiece, creating the desired shape and features. This method is highly efficient for producing parts with simple to moderately complex geometries, such as flat surfaces, slots, and holes. However, the limitations of only three axes mean it struggles with more intricate, multi-dimensional shapes that require additional rotational movement of the cutters.
Why 3-Axis CNC Milling is not Suitable for Complex Geometrically Parts
Below, we will examine why 3-axis CNC milling centers are not the best fit for complex geometrical parts.
Cost
Compared to more sophisticated multi-axis CNC milling centers, the 3-axis machines are generally more cost-effective, particularly for more straightforward tasks. However, they can become inefficient and costly when producing complex geometries. The need for multiple machine setups and workpiece repositioning increases labor and machining time, driving up overall costs.
Efficiency
The limited range of motion in 3-axis CNC milling requires the operator to reposition the workpiece frequently to access different angles and features. This additional handling increases production time and reduces overall efficiency, making it less suitable for complex parts.
Precision
With 3-axis milling, achieving high precision for intricate features on complex geometrical parts is challenging. The need for multiple setups can compromise the technique’s dimensional accuracy, introducing errors and inconsistencies, thereby diminishing the overall precision and quality of the final product.
Limited Axis Movement
Complex geometrical parts often require simultaneous multi-axis movement to create intricate features, like undercuts. However, a 3-axis CNC milling center experiences restricted movement, making handling these advanced machining tasks more challenging.
Surface Finish
With 3-axis milling centers, high-quality surface finishes on complex geometries can be challenging. The need for multiple machine setups can lead to surface mismatches and imperfections, affecting the part’s overall finish.
Tool Access
In complex parts manufacturing, the limited cutting motion on 3-axis machines may make fabricating specific features more difficult, as the setup obstructs or prevents the device from reaching particular angles. This limitation requires creative workarounds or additional tooling, further complicating the process and potentially affecting the part’s integrity and accuracy.
Alternatives for Complex Geometrically Parts
When 3-axis CNC milling is insufficient for complex geometries, several alternative methods may provide better precision and efficiency for metal and plastic parts.
For Metal
Here, we will focus on alternative metal components and parts manufacturing methods.
Metal Casting
Metal casting is a broad manufacturing process that involves heating metal pellets, rods, or sheets into a molten state. The liquefied metals are then poured into a pre-made mold cavity, where, upon cooling, they take the shape and structure of the intended design, including the intricate details and features.
This process is highly effective for producing complex geometries and large parts, as taking the metal into liquid form allows it to be more flexible for the intended fabrication. In addition, it does not compromise the materials’ original mechanical and chemical properties while also giving the product a decent surface finish. However, the machinist or OEM manufacturer needs to engage in further post-processing operations to improve the overall quality of the product.
Metal casting comes in different forms, including die casting, sand casting, investment casting, vacuum casting, and permanent mold casting. Each method has its peculiarities. For example, sand casting uses a sand mold, vacuum casting occurs in a vacuum environment, limiting shortcomings like gas porosity, and permanent mold casting uses a reusable mold.
That said, die casting and sand casting are the most common metal casting processes. However, because of sand casting’s shortcomings, die casting is often used in industries with high-precision components. The technique requires injecting the mold cavity into a die (mold) cavity at high pressure, where it takes the intended shape of the part.
3D Printing
Unlike CNC milling, 3D printing is an additive process. It builds parts layer by layer using metal powders, filaments, wires, and a laser or electron beam. Compared to other manufacturing techniques, this technology creates highly intricate and complex geometries that would be challenging or impossible with traditional methods.
Metal 3D printing is ideal for prototyping, low-volume production, and custom parts, offering design flexibility and rapid turnaround times. Although the material and equipment costs can be high, the ability to produce lightweight and complex structures often justifies this investment.
Like CNC milling, it yields high-precision parts, regardless of design complexity. Also, the process does not compromise the mechanical and chemical properties of the metal, allowing the manufacturing of durable parts for various industries. However, an advantage over CNC milling is that it does not encourage material wastage. Since it’s an additive process, unused metal parts may be reused for subsequent part fabrication.
For Plastic
While metals are more common, particularly for complex structures, plastics also have vast applications, particularly for fabricating lightweight substances. Below are alternatives to 3-axis CNC milling centers for precision manufacturing complex plastic parts.
Injection Molding
Injection molding is comparable to metal casting, as it also involves heating its raw materials, this time plastics, into the molten state before injecting the molten plastic into a pre-designed mold (usually made of aluminum), where it solidifies. The liquefied plastic takes on the shape of the aluminum mold as it fills it, and upon cooling, it takes on intricate geometries and intended fine details.
This manufacturing method is highly efficient for mass-producing complex plastic parts with consistent quality and high precision. Plastic injection molding is cost-effective for large production runs due to its fast cycle times and low per-part cost. However, the initial cost for mold creation can be substantial, making it less suitable for small batches or prototyping.
Plastic injection molding is a versatile process capable of creating almost any plastic structure, making it suitable for producing plastic components for various industries. The manufacturer must use engineering-grade plastics to ensure enough strength to suit the intended application.
3D Printing
While 3D printing can create metal parts, this additive manufacturing process is more common for creating parts using plastics and polymers. Plastic 3D printing follows the same method as described under metal 3D printing. The technique involves building parts layer by layer using various plastic materials, such as acrylics, nylon, polyethylene, polycarbonate, etc.
This method is ideal for producing complex geometries, intricate designs, and custom parts without the need for molds. Plastic 3D printing is highly flexible, allowing rapid prototyping and low-volume production. While the per-part cost can be higher than injection molding for large runs, the technology’s ability to create detailed and unique parts makes it a valuable alternative for specific applications.
Conclusion
This article explores why CNC milling centers may not be the best fit for manufacturing complex structures and parts. However, they are still valuable for more straightforward machining tasks and may fit small-scale machining shops well. However, when dealing with components with intricate details, their precision standards may become compromised. In this case, machinists should consider opting for our discussed alternatives or more sophisticated CNC technologies.
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