There are many methods for manufacturing metal and alloy products, and the choice depends on the material type and specific manufacturing requirements. When choosing a metal manufacturing technology, casting versus machining is often a point of contention among engineers, designers, and manufacturers.
Machining involves removing material to achieve the desired shape, while casting creates a shape by solidifying molten metal in a mold. Each of these techniques has unique capabilities, advantages, and limitations in production. This article will discuss casting and machining in detail, including their working principles, advantages, disadvantages, and key differences.
The Difference Between Casting and CNC Machining
What is Casting?
Casting involves melting metal and forming it into a predetermined mold cavity containing cores and inserts to produce three-dimensional metal parts. The casting process can be applied to both ferrous and non-ferrous metals, producing parts and products of various shapes and sizes for a wide range of industries.
Casting dates back to 4000 BC in Europe, although scientists also believe it was practiced in ancient Mesopotamian and Chinese civilizations. Since then, casting technology has continued to evolve and spread throughout the world. Especially after the Industrial Revolution, this technology has rapidly developed and become an essential component of modern manufacturing.
The metal casting process begins by melting the raw metal in a furnace (which may or may not be directly connected to a casting machine). The liquid metal is then poured into the mold cavity periodically or via an automated system. Inside the mold, solidification occurs in several stages: nucleation, crystal growth, grain structure formation, and directional solidification.
Nucleation is the initial stage of solidification, when tiny solid particles (nuclei) form within the molten metal as it cools below its melting point. Once formed, stable nuclei grow into larger crystals (grains) as the metal continues to cool. The size, shape, and orientation of the grains in the solidifying metal depend on the cooling rate and alloy composition. After solidification, ejectors or other mechanisms facilitate safe ejection of the part. These mechanisms may require post-processing and micro-machining to achieve exacting precision and finish.
What is machining?
CNC machining involves using cutting tools to remove material from a workpiece and shape it into the desired shape. A rotating or fixed tool feeds material from a workpiece along a defined path until it achieves the desired size, shape, accuracy, and finish. Therefore, in CNC machining, all of these processes are controlled by computer programs. CNC lathes, milling machines, drilling machines, grinders, EDM machines, and many other devices can be used to perform machining processes. Lathes are used for turning, milling machines for milling, drilling machines for precise drilling, and so on. Advanced technology is used for high-precision components and prototypes. In CNC machining, G and M codes control the movement of the tool and workpiece via a computer. As a result, you can achieve CNC-machined metal parts that are identical to the specifications of the CAD model. The development of machining technology began in the 17th century and achieved significant advancements in the 1950s, including the invention of numerically controlled (CNC) machines. Since then, it has continued to become more precise and efficient over time. Today, we have CNC automation systems that integrate artificial intelligence.
Comparison of the Advantages and Disadvantages of Casting vs. CNC Machining
Advantages of Casting
- Ease of Use for Complex Parts: Using a cross-mold setup, you can cast complex metal parts with hollow sections and irregular contours.
- Material Diversity: As long as the molds and equipment can withstand the melting point, scrap and low-grade metals/alloys can be used to cast metal parts.
- Cost-Effective for High-Volume Production: Once you have invested in molds and tooling, you can produce up to millions of identical items, reducing the casting cost per part.
- Dimensional Flexibility: Techniques such as sand casting and investment casting can cast parts up to several meters in size, while die casting offers the flexibility to cast parts up to 300 x 650 mm or larger.
- Incorporation of Inserts: To create parts with multiple materials, prefabricated inserts, such as bimetallic bearings, can be incorporated into the casting.
Advantages of Machining
- High Precision: Multi-axis CNC machines can achieve tolerances as low as ± 0.001”, even for complex items like turbine blades and medical components.
- Faster Production for Small Batch Production: Unlike casting, extensive tooling is not required, making prototypes and small batch production simpler and faster.
- Excellent Repeatability: CNC setups can be customized using both CAD models and CNC The program creates parts that are identical in size, finish, and quality.
- Highly automated: The CNC machining process automates production, reducing human error and labor costs. Machines operate 24/7, improving efficiency.
- Material range: In addition to metals, it is also suitable for plastics, composites, and ceramics.
Disadvantages of Casting
- Defects: Cast parts are prone to defects such as porosity, shrinkage, warpage, and surface cracks. These defects are primarily caused by incorrect process parameters and poor quality raw materials.
- Labor-intensive: Especially when comparing casting to machining, metal casting technology requires more labor from pouring to ejection.
- Harmful emissions: The melting and processing of waste generates harmful emissions such as CO and VOCs. and metal fumes.
Disadvantages of Machining
- Difficult Internal Features: Machining complex internal geometries, such as deep cavities and channels, is challenging.
- Size Limitations: Large parts and products are challenging due to spindle motion limitations.
- Material Waste: The process of removing material to achieve the designed geometry produces waste, including chips and dust.
Types and techniques of machining and casting
Both casting and machining have different types and techniques, covering a wide range of manufacturing possibilities; let’s discuss them.
Types of CNC Machining
Milling, turning, drilling, and grinding are the primary types of machining operations. These operations Different operations differ in terms of compatible tool types, tool motion, and application preferences.
- Milling: Milling involves a rotating tool mounted on a spindle that moves along multiple axes to shape the workpiece material. This type of machining is versatile and suitable for complex designs. Machine tools with 4, 5, or more axes are commonly used for machining automotive chassis, orthopedic implants, and injection molds.
- Turning: In turning, the workpiece rotates at a defined speed while the tool feeds the material, creating axisymmetric shapes such as cylindrical and conical shapes. CNC lathes are suitable for this type of machining.
- Drilling: Drilling is used to create circular holes, where a twist drill bit rotates and feeds into the material using multiple cutting edges. Hole diameter (⌀) depends on the drill bit diameter and length. CNC drilling is suitable for aluminum, stainless steel, copper, plastics, and thermosets. Meanwhile, drilling in harder materials like titanium requires carbide or tungsten carbide drills.
- Grinding: Grinding refines the surface of a part to the required dimensional tolerances and surface quality. Grinding machines use a rotating grinding wheel to remove small amounts of material. Furthermore, CNC grinding can achieve a precision of 3.2 to 0.3 µm. Roughness.
Casting Types
You can choose from different types of casting, categorized by the structure of the mold/die used to cast the metal.
- Sand Casting: This method involves creating a mold from silica sand, green sand, or other fine sand. The cavity is formed by placing a pattern, compacting the sand around the pattern, and then removing the pattern. Sand casting molds can be used for single or multiple cycles and are suitable for metals with high melting points.
- Die Casting: Die casting is an advanced and popular method for mass-producing non-ferrous metals. Molds are made from tough and thermally stable materials such as stainless steel. Milling, turning, EDM, and other precision CNC machining techniques can also create reusable die casting molds.
- Investment Casting: This metal casting process uses a wax pattern coated with a refractory material and a binder. The wax pattern is then immediately replaced with the mold after the molten metal is poured, resulting in the part/product being formed. Investment casting can produce large, complex molds. Replicas of molds, such as turbine parts, are primarily suitable for prototyping and small-batch production.
Key Differences Between Machining and Casting
Molds
Casting: A mold (sand mold, metal mold, etc.) is required to form the part shape. The mold cost is the main initial investment.
Machining: No specialized molds corresponding to the part shape are required. General fixtures and cutting tools are sufficient. The main initial costs are in equipment and programming.
Precision and Accuracy
Casting: Conventional methods (such as sand molds) have low precision (tolerances in the millimeter range) and rough surfaces. Precision casting (die casting, investment casting) can achieve higher precision (tolerances in the 0.1mm range) and finish, but is still generally lower than machining.
Machining: The highest precision is achieved. Conventional machining can achieve IT7-IT8 accuracy (tolerances in the 0.01-0.05mm range), while fine grinding can achieve IT5 or even higher (micrometer level). Surface roughness can reach mirror-like finishes.
Material Compatibility
Casting: Able to handle flow Metal alloys with good mechanical properties. Some high-melting-point, refractory, and easily oxidized alloys are difficult to cast. Plastics can also be injection molded (similar to casting).
Machining: It has an extremely wide range of applications and can machine nearly any solid material (metal, plastic, wood, composites) as long as the tool is sufficiently hard. Machining ultra-hard materials (such as carbide and ceramics) is inefficient and costly.
Part Design Complexity and Size
Casting: It excels at producing extremely complex shapes, especially parts with complex cavities, curved surfaces, and thin-walled structures. It is the primary, or even sole, method for manufacturing large parts (such as machine tool bases and marine engine blocks).
Machining: Processing complex cavities and deep holes is extremely difficult (due to tool length, diameter, and accessibility limitations). Large, thin-walled parts are prone to deformation. It is more adept at machining external geometric features or relatively open internal features.
Production Volume and Speed
Casting: Large-volume production is extremely fast (especially die casting), with short cycle times. Small-volume production is slow and uneconomical ( High mold cost sharing).
Machining: Small-batch production is flexible and fast (no molds are required). Large-batch production is relatively slow, with longer processing times per piece. Multi-axis CNC and automated production lines can improve efficiency.
Part Strength
Casting: Castings may have defects such as pores and shrinkage, which serve as stress concentration points and reduce fatigue strength. The grain structure is not as dense as forgings, but the overall strength is good.
Machining: Rolled or forged billets are typically used. The material is dense and the fibers are continuous, resulting in mechanical properties (especially strength, toughness, and fatigue strength) that are generally better than castings. However, cutting may sever fiber flow lines.
Suitable for Prototyping
Casting: Prototyping is expensive and time-consuming (molds must be manufactured first), unless rapid casting technologies such as 3D printing sand molds/investment casting are used. Not suitable for single-piece prototype verification.
Machining: Ideal for prototyping. No molds are required, and designs can be quickly translated into physical objects through programming, facilitating design iteration and verification.
Production Cost
Casting: High initial mold cost + Lower marginal costs. Unit costs are lowest in large batches. Unit costs are extremely high in small batches.
Machining: Low initial cost (no tooling) + higher marginal costs (labor, tooling). Economical for small batches. Unit costs may be higher than casting for large batches.
Key Point: The cost break-even point. There is typically a critical “economic production lot” point. Below this point, machining is more economical; above this point, casting is more economical. This point varies greatly depending on part complexity, material, and precision requirements.
Design Considerations for Casting vs. CNC Machining
Before choosing a process, several design considerations will determine the best process for your needs:
- Machining Design: Machining design involves finding the right material, optimizing part geometry for the primary machining process, and minimizing the need for secondary operations. Designers should prioritize features that are easy to machine and can be quickly implemented with minimal or no effort. However, designers must ensure that material inventory is sufficient to accommodate machining allowances.
- Casting Design: Casting design involves precisely considering parameters such as block shape, draft angles, and material flow characteristics. Designers must develop creative molds that allow the material to flow efficiently and significantly reduce the chance of defects, such as porosity, shrinkage, or deformation. Considering draft angles and fillets at the part’s molding location facilitates easy disassembly and reduces the likelihood of casting defects. It is also recommended to consider the location of margins and heights to ensure uniform filling and solidification of the casting. By optimizing the casting design, manufacturers can produce highly functional parts with minimal defects and production costs.
Should Your Project Be Machined or Cast?
When deciding whether to use casting or machining for your manufacturing project, you need to carefully consider the design characteristics and overall requirements. The following are key factors to consider:
Production Volume
One of the key factors is the volume you want to produce and its potential scalability. CNC machines are better suited for small batches, but the design can be adjusted to achieve scaled production at minimal cost. Metal casting, on the other hand, is well-suited for large-scale production.
Part Complexity
Next, consider the size and complex features of the design to be manufactured, such as small internal features and contour curvature. CNC machines are often used because they can be used with micro-tools. Designing for small internal geometries is preferred.
Required Precision and Repeatability
In large-scale production, cast metal products offer greater repeatability because a single mold setup can be used for multiple cycles. Computer-controlled tooling, on the other hand, can produce consistent parts, but its repeatability is not as good as casting in large-scale production. However, computer machining is more precise and has tighter tolerances. Therefore, before deciding, consider the level of precision and consistency your parts require.
Material Type
The material used in production is also important. Casting has fewer material options, while a wider variety of machinable materials exists. Metals with high melting points are difficult to die-cast.
Material Waste
Waste not only increases material costs but also makes it difficult to recycle. Therefore, it is necessary to determine how much material will be wasted during machining. On the other hand, if casting is used, Material waste is minimal.
Production Speed
Consider the project timeline and compare it to the production speed to see if it’s feasible. Machining techniques are faster for prototypes and small-batch production. However, for medium to large-volume production, metal casting methods are faster.
Conclusion
In summary, choosing the right manufacturing method between casting and machining depends entirely on your design and the performance requirements for strength, precision, and finished product quality. Machining techniques with high precision, low to medium production volumes, and high material flexibility are recommended. Similarly, if the design has complex shapes, cavities, and large dimensions, and production requirements are in place, casting is a better option. Manufacturers often combine the two techniques—primary casting and precision CNC machining—to leverage the cumulative benefits and produce high-quality, near-net-shape parts.
Using Xtproto for CNC Machining or Casting Projects
If you have any uncertainty or difficulty making a decision, we recommend contacting Xtproto’s team of skilled engineers. We offer reliable and professional services in CNC machining, various casting processes such as die casting, sand casting, and investment casting, as well as secondary machining options. Our goal is always to help you achieve your goals with real results.Efficiently produce high-quality products at an affordable price. Discuss your project with us and we can provide effective guidance to help you make informed decisions or develop an integrated solution. Request an instant quote Experience our custom manufacturing services now and get your parts delivered faster, easier, and more efficiently.