How to determine whether a project is suitable for casting or machining?

There are many ways to manufacture metal and alloy products, and the choice of which method depends on the material type and specific manufacturing requirements. When choosing a metal manufacturing technology, casting or machining is often a point of contention between engineers, designers and manufacturers when communicating the project process.

Machining involves removing material to achieve the desired shape, while casting is a process that solidifies molten metal material in a mold and forms the desired product. Each of these technologies has its own unique capabilities, advantages and limitations in production.This article will discuss casting and machining in detail, including working principles, advantages, disadvantages and key differences, so that you can determine which process is better for your project.

What is Casting?

Casting is the process of melting metal and forming it into a predetermined shape mold cavity containing cores and inserts to produce the desired metal part. The casting process is applicable to ferrous and non-ferrous metals and can produce parts and products of various shapes and sizes for multiple industries.

The history of casting can be traced back to 4000 BC in Europe, although scientists also believe that it was used in ancient Mesopotamian and Chinese civilizations. Since then, casting technology has continued to develop and spread around the world. Especially after the Industrial Revolution, this technology has been rapidly developed and has become an important part of modern manufacturing.

What is machining?

Machining involves using cutting tools to remove material from a workpiece and machine it into the desired shape. A rotating or fixed tool feeds the material from the workpiece bar along a set path until the desired size, shape, precision, and finish are achieved. Therefore, in CNC machining, all 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 are used for milling, drilling machines are used for precise drilling, and so on.

The advanced technology is suitable for high-precision components and prototypes. In CNC machining, G and M codes control the movement of the tool and the workpiece through a computer. Therefore, you can get CNC machined metal parts that are exactly the same as the specifications of the CAD model.

The development of machining technology began in the 17th century and made significant progress in the 1950s, including the invention of numerical control machines (CNC). Since then, it has continued to become more precise and efficient over time. Today, we have CNC automation systems that integrate artificial intelligence.

How Casting vs. Machining Works

How does casting work?

The metal casting process begins by melting the raw metal into a furnace (which may or may not be directly connected to the casting machine). The liquid metal is then poured into the mold cavity periodically or through an automated system. Inside the mold, the solidification process 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 inside the molten metal as it cools below its melting point. Once a stable nucleus is formed, it grows 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 help to eject the part safely. These mechanisms may require post-processing and micro-machining to achieve exacting precision and finish.

How does machining work?

CNC machining is a computerized manufacturing process where software and code control the operation of the machine. The CNC machining process is subtractive machining, which is shaping an object by removing material from the workpiece until the desired parameters are achieved.

Design engineers input complex models into specialized software, which then outputs code that controls the CNC machine. The machine makes parts based on the constraints listed by the programmer in the software. The software automatically generates tool paths, cooling rates, feed rates, and various other parameters to make the part using the materials specified by the design engineer.

The coding language for CNC machines is called G-code. Creating a machined part involves rendering a computer-aided design (CAD) and converting it into computer-aided manufacturing (CAM) software. The CAM software then converts the tool paths for the CNC machine. After entering the program, the operator does a test run to find and correct any errors.

The process works with a variety of materials, including metals, plastics, wood, composites, glass, and foam. Each part requires a specific computer program to ensure tight tolerances, processing speeds, tool life, surface finish, and more.

Advantages and Disadvantages of Casting vs. Machining

Advantages of Casting

Simple for complex parts: Using a cross-die setup, you can cast complex metal parts with hollow sections and irregular contours.
Material diversity: Scrap and low-grade metals/alloys can be used to cast metal parts as long as the molds and equipment can withstand the melting point.
High-volume production is cost-effective: Once you invest in molds and tooling, it can produce up to millions of identical items, reducing the casting cost of each part.
Dimensional flexibility: Techniques such as sand casting and investment casting can cast parts up to several meters in size, while die casting methods are flexible and can cast parts up to 300 x 650 mm or larger.
Incorporation of inserts: To create parts with multiple materials, pre-made inserts can be incorporated into the casting, such as bimetallic bearings.

Advantages of machining

High precision: Multi-axis CNC machines can provide tolerances as low as ± 0.001”, even for complex items such as turbine blades and medical components.
Fast production of small batches: Unlike casting, it does not require a large number of tools, which makes prototypes and small batches simpler and faster.
Excellent repeatability: CNC setups can use CAD models and CNC The program creates parts that are identical in size, finish, and quality.
Highly automated: The CNC machining process enables automated production, reducing human errors and labor costs. The machine operates around the clock, increasing efficiency.
Range of materials: In addition to metals, it is also suitable for plastics, composites, and ceramics.

Disadvantages of casting

Prone to defects: Casting parts are prone to defects such as porosity, shrinkage, warping, and surface cracks. These defects are mainly caused by incorrect process parameters and poor quality of raw materials.
Labor-intensive: Especially when comparing casting and machining, metal casting technology requires more labor from pouring to ejection.
Harmful emissions: The melting and processing of waste produces harmful emissions such as CO, VOC and metal fumes.

Disadvantages of Machining

Internal Features Are Difficult to Machin: Machining is challenging for complex internal geometries such as deep cavities and channels.
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 includes debris and dust.

Types of Machining vs Casting Different Machining Technology Covered

Both casting and machining have different types and technologies covering a vast range of possibilities in manufacturing; the following is an explanation of the different machining technologies for the two processes

Types of Machining Technology

Milling, turning, drilling, and grinding are the main machining operations These operations differ in the types of tools they are compatible with, how the tools move, 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 often used for machining automotive chassis, orthopedic implants, and injection molds.
Turning: During turning, the workpiece rotates at a determined speed and the tool feeds the material, forming axisymmetric shapes such as cylindrical and conical shapes. CNC lathes are suitable for this type of machining.
Drilling: Drilling is used to machine circular holes, where a twist drill bit rotates and feeds the material using multiple cutting edges. Hole diameter (⌀) Depends on the diameter and length of the drill. CNC drilling is suitable for aluminum, stainless steel, copper, plastics, and thermosets. Meanwhile, drilling in hard materials such as titanium alloys requires carbide or tungsten alloy drills.
Grinding: Grinding refines the surface of a part to the desired dimensional tolerance and surface quality. A grinder uses a rotating grinding wheel to remove small amounts of material. Also, CNC grinding can achieve a roughness of 3.2 to 0.3 µm.

Types of Casting Techniques

You can choose from different types of casting, which are classified based on the structure of the mold or die used to cast the metal.

Sand casting: This method requires making a mold out of silica sand, green sand, or other fine sand, and forming a cavity by placing a pattern, compacting the sand around the pattern, and removing the pattern. Sand casting molds can be used for single or multiple cycles and are suitable for high-melting-point metals.
Die casting: Die casting is an advanced and popular method for mass production of non-ferrous metals. The mold is made of tough and thermally stable materials such as stainless steel. At the same time , milling, turning, EDM and other precision CNC machining technologies can make reusable die casting molds.
Investment casting: This metal casting process requires the use of a wax pattern coated with refractory materials and binders. Then, after pouring the molten metal, the wax pattern is immediately replaced with the wax pattern to shape the part/product. Investment casting can make replicas of large and complex models, such as turbine parts. Therefore, this is mainly suitable for prototyping and small batch production.

Main differences between machining and casting

Let’s compare casting and machining head-to-head from all aspects;

Tools: CNC Machining uses cutting tools such as end mills, drills, face mills, fly mills, and gear mills, depending on the type of machining design. Casting, on the other hand, requires a more comprehensive tooling configuration, covering everything from model creation to mold making.
Precision and accuracy: CNC machine tools produce parts that are more accurate than cast parts, with tolerances as precise as ±0.0025 mm in some cases. The standard tolerance for machined parts is ±0.13 mm. Die casting and other precision technologies can achieve accuracies as low as ±0.10 mm. In large-scale production, die casting can provide higher repeatability.
Material compatibility: The choice of casting material depends on the process you use. For example, the sand casting process is suitable for ferrous metals with higher melting points, but the die casting process is best for non-ferrous metals. On the other hand, non-metallic materials are also easy to process, including thermoplastics, thermosets, composites, and ceramics.
Design Design complexity and size: Complex geometries with sharp edges, fine details, and irregular contours are better suited to machining using lathes or milling machines, while complex parts with internal features are easier to cast. Therefore, large objects are suitable for machine processing, and the size of the machined parts will be limited according to the performance of the machine.
Production volume and speed: In machining and casting technology, both production volume and speed are closely related. If the production volume is high, the casting process can increase the speed. On the other hand, CNC integration can deliver small batches of products and prototypes faster.
Part strength: Although CNC machined parts have a more complete structure and higher strength, cast parts can also maintain almost the original strength of the metal through controlled processes and directional solidification. In addition, heat treatment of castings can significantly improve their strength and other mechanical properties.
Prototype suitability: Machined prototypes are faster and simpler than cast prototypes. If you have the product you need CAD models can be prototyped using CNC equipment with only a small investment in tooling. However, in some cases, specialized techniques like investment casting can rival the speed of CNC prototyping.
Production costs: Castings have higher tooling and initial setup costs, although they are more cost-effective for large-scale production. On the other hand, machined parts are less expensive for small to medium-volume production.

Is your project machined or cast?

When deciding whether to cast or machine a manufacturing project, you need to carefully consider the design features and overall requirements.

Here are the key factors to consider;

Production volume: One of the key factors is the volume you want to produce and the possible scalability. CNC machines are better suited for small-volume production, but the design can be adjusted to achieve scaled production at the lowest cost. Metal casting processes, on the other hand, are well suited for large-volume production.
Complexity of the part: Next, consider the size and complex features of the design to be manufactured, such as small internal features, contour curvature, etc. Since micro tools can be used, CNC It is better to design for small internal geometries.
Required accuracy and repeatability: In mass production, cast metal products have higher repeatability because one mold unit can be used for multiple cycles. On the other hand, computer-controlled tooling can produce consistent parts, but its repeatability is not as good as casting in mass production. However, computer machining is more accurate and has tighter tolerances. Therefore, before deciding, consider what level of accuracy and consistency your parts need to achieve.
Material type: The material used in production is also important. Casting has fewer material options, while there are a wide variety of machinable materials. 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 materials. Therefore, it is necessary to determine how much material will be wasted in the workpiece during machining. On the other hand, if the casting process is used, , there is little material waste.

Production speed: Consider the project schedule and compare it with the production speed to check if it is feasible. For prototypes and small batch production, machining technology is faster. But for medium to large batch production, metal casting methods are faster.

Conclusion

In general, choosing the right manufacturing method between casting and machining depends entirely on your design and the requirements for performance such as strength, precision, and finished product quality. It is recommended to choose machining technology with high precision, medium and low production, and high material flexibility. Similarly, if the design has complex shapes, cavities, and large sizes and there are production requirements, casting is a better choice.

Manufacturers often use a combination of two technologies, primary casting and precision CNC machining. This allows the cumulative benefits to be fully utilized to manufacture high-quality near-net-shape parts.

Xtproto is a leading CNC machining and metal casting product supplier, providing multi-axis CNC machining and die casting services worldwide. We can also customize molds for you.

Even if you are worried about the high tooling cost for prototype casting, we can also provide vacuum casting services at competitive prices.