There are several ways to fabricate metal and alloy products, and choosing one depends on the material type and specific fabrication requirements. Casting vs Machining is often a great debate among engineers, designers, and manufacturers when deciding on metal manufacturing technology.
The machining involves material removal to achieve the desired shape, whereas casting shapes the molten metal by solidifying it in a die or mold. These techniques have distinct capabilities, benefits, and limitations in production.
This article will discuss casting and machining in detail, including working principles, advantages, disadvantages, and key differences.
What is Casting?
Casting refers to the production of 3D metal parts by melting and shaping in a pre-determined shape of a die cavity containing a core and insert. It can produce various shapes and sizes of parts and products for multiple industries, using both ferrous and nonferrous metals.
The history of casting can be traced back to 4000 BC in old Europe, although scientists also believe in its use in ancient Mesopotamia and Chinese civilizations. Then, it is continuously developing and spreading throughout the world. Especially after the Industrial Revolution, this technology has evolved rapidly and has become an essential part of modern manufacturing.
How Does Casting Work?
The metal cast process starts with melting the raw metal into the furnace( which can or can not be directly connected to the casting machine). The liquid metal is then poured into a die cavity either annually or automatic systems. Inside the die, solidification occurs in multiple phases: nucleation, crystal growth, grain structure formation, and directional solidification.
Nucleation is the initial stage of solidification, where small solid particles (nuclei) form in the molten metal as it cools below its liquidus temperature. Once stable nuclei form, they grow into larger crystals (grains) as the metal continues to cool. The size, shape, and orientation of grains in the solidified metal depend on the cooling rate and alloying composition. After solidification, ejector pins or other mechanisms facilitate the safe ejection of parts. They might require post-processing and minor machining to achieve tight precision and finish.
Advantages of Casting
- Simple for Complex Parts: Using a crossposting die setup, you can cast complex metal parts with hollow sections and irregular contours.
- Material Diversity: It is possible to use scrap and low-grade metals/alloys to cast metal components as long as the melting point is witstandable by die and equipment.
- Cost-effective for Large Volumes: Once you invest in a die and tooling, it can produce up to millions of identical items, reducing the per-part cost of casting.
- Size Flexibility: Techniques like sand and investment casting can cast parts up to a few meters in size, whereas the die cast method is flexible up to 300 x 650 mm or higher.
- Incorporation of Inserts: To make parts with multiple materials, pre-fabricated inserts can be boned in casting, e.g., bimetallic bearings.
Disadvantages of Casting
- Prone to Defects: Casting components are prone to porosity, shrinkage, warping, and surface crack defects. Incorrect process variables and low quality of raw materials mainly cause these.
- Labor Intensive: Especially when comparing casting vs machining, the metal casting techniques are more labor intensive, from pouring to ejection.
- Harmful Emissions: The melting and handling of waste produce harmful emissions like CO, VOCs, and Metal Fumes.
What is Machining?
Machining involves the use of cutting tools to remove the material from work and shape it into the desired form. Rotating or stationary tools fed the material from the work bar in a determined path until the desired size, shape, accuracy, and finish are achieved. Consequently, computer programs control all of these processes in CNC machining.
CNC lathes, mills, drill presses, grinders, EDM machines, and many other equipment perform machining processes. Lathes are for turning, mills for milling, drill presses for accurate holes, and so on.
Advanced technologies are applicable for high-precision components and prototypes. In CNC, g and m codes dictate the tool and work movement through the computer. So, you can get CNC machining metal parts with the exact same specifications as its CAD model.
The development of machining technology started in the 17th century, with major developments in the 1950s, including the invention of CNC. Then, it is continuously becoming more precise and effective over time. AI-integrated CNC automation is what we have today.
Advantages of Machining
- High Precision: The muti-axis CNC machines can offer tolerances as tight as ± 0.001”, even for complex items like turbine blades and medical components.
- Quick for Small Batches: Unlike casting, it does not need extensive tooling, making it simpler and faster for prototypes and low volumes.
- Excellent Repeatability: A CNC setup can create identical parts in terms of dimensions, finish, and quality using a CAD model and CNC program.
- High Automation: CNC machining process automates production, reducing human error and labor costs. The machines operate 24/7 and boost efficiency.
- Material Range: Besides metals, it is suitable for plastics, composites, and ceramics.
Disadvantages of Machining
- Difficult for Internal Features: Machining is challenging for intricate internal geometry, like deep cavities and channels.
- Size Restriction: Large parts and products are challenging due to the limitation of spindle movement.
- Material Wastage: The process of removing material to achieve designed geometry waste material informs of chips and dust.
Machining Vs Casting: Types and Techniques
Casting and machining both have distinct types and techniques that cover enormous possibilities in manufacturing; let’s discuss.
Types of Machining
Milling, turning, drilling, and grinding are the main types of machining operations. These differ in terms of what cutter types are compatible, the movement of the cutter, and application preferences.
Milling
Milling involves rotating cutters attached to the spindle, which moves along multiple axes to shape the work material. This type of machining is diverse and works with complex designs. The 4, 5, or higher axes machines are popular for car chassis, orthopedic implants, and injection molding dies.
Turning
In turning, the workpiece rotates at a determined RPM, and the cutter feeds material to create axially symmetrical items like cylindrical and conical shapes. CNC lathes are suitable for this.
Drilling
Drilling is for circular hole creation where a twist drill bit rotates and feeds material with multiple cutting edges. The hole size (⌀) depends on the diameter and length of the drill bit. CNC drilling is compatible with aluminum, stainless steel, copper, plastics, and thermosets. Meanwhile, drilling on hard materials like titanium alloys requires bits of carbide or tungsten alloys.
Grinding
Grinding refines the surface of machined parts to meet the desired dimensional tolerance and finish quality. The grinder involves a rotating abrasive wheel for removing material in minor amounts. Moreover, CNC grinding can achieve roughness levels of 3.2 to 0.3 µm.
Types of Casting
You can choose from different types of casting, which are classified based on the structure of the mold/die used to cast the metal.
Sand Casting
This method involves a mold of silica, green sand, or other fine-gained sands that forms a cavity by placing the pattern, compacting sand around the pattern, and removing the pattern. Sand casting molds are useable for single or multiple cycles, and they work with high melting point metals.
Die Casting
Die casting is an advanced and popular method for large volume production of non-ferrous metals. The die is made from tough and thermally stable material like stainless steel. Meanwhile, Milling, Turning, EDM, and other precision CNC machining techniques make reusable casting dies.
Investment Casting
This metal casting process involves the use of a wax pattern with a coating of refractory material and binder. Then, immediately after pouring molten metal, it replaces wax and shapes the parts/products. Investment casting makes large and complex replicas of large and complex patterns, such as turbine parts. Subsequently, this is mainly beneficial for prototyping and small-batch productions.
Key Differences Between Machining and Casting
Let’s make a head-to-head comparison of casting vs machining based on various aspects;
Tooling
CNC processes use cutting tools like end mills, drill bits, face mills, fly cutters, and gear cutters based on what type of design you are machining. On the other hand, casting techniques require more extensive tooling arrangements, from pattern creation to mold or die-making.
Precision and Accuracy
CNC machines produced more accurate parts than cast, with tolerances as tight as ± 0.0025mm in some cases. Meanwhile, the standard tolerance for machined items is ± 0.13mm. Die casting and other precise techniques can achieve as low as ± 0.10 mm. In high volumes, it can provide higher repeatability.
Material Compatibility
Material options for casting depend on which technique you are using. For instance, the sand cast process is flexible with ferrous metals with high melting points, but die casting best works with non-ferrous metals. On the other hand, non-metallic materials are also machinable, including thermoplastics, thermosets, composites, and ceramics.
Design Complexity and Size
Complex geometries with sharp edges, fine details, and irregular contours are better to craft with a lathe or mill machine, but intricate parts with internal features are easier to cast. Consequently, large-sized items are suitable for machines, while there is a restriction on the size of machined parts based on the machine’s capability.
Production Volume and Speed
Production volume and speed are related in both machining and casting technology. If production volumes are high, casting offers more speed. On the other hand, CNC integration allows for quicker delivery of small volumes and prototypes.
Strength of Parts
Although CNC machined parts are more structurally intact and strong, casted parts can also retain almost the same original strength of metal through controlled processes and directional solidifications. However, heat treatment of cast parts significantly improves their strength and other mechanical properties.
Suitability for Prototypes
Machined prototypes are faster and simpler than cast them. A minimal investment in tooling is enough to make prototypes with CNC equipment if you have a CAD model of the desired item. However, specialized techniques like investment casting can match the speed of CNC prototyping in some scenarios.
Production Cost
Tooling and initial setup costs are higher for castings, although they are cost effective for mass production. On the other hand, machined parts are less costly for small and medium size production volumes.
Where Casting and Machining are Used?
Application Preferences in casting vs machining are different. It is best to cast metal parts if the size is large, the design is complex, and it costs more to use CNC or other processes. Similarly, complex and precise parts requiring tight tolerances are preferred to produce with machining.
| Industry | Applications of Casting | Applications of Machining |
| Automotive | Engine blocks, cylinder heads, wheel rims, and suspension parts. | Gears, pistons, crankshafts, brake discs, etc. |
| Aerospace | Engine housings, turbine blades, structural elements. | Turbine discs, fasteners, and hydraulic parts. |
| Medical | Foundation for medical equipment, frames, etc. | Surgical items, implants, and medical device parts. |
| Industrial Machinery | Valve bodies, gearboxes, and heavy equipment frames. | Precision gears, shafts, bearings, and tooling components. |
| Consumer Goods | Kitchen and cookware, decorative products, door handles, etc. | Small-sized components for different consumer devices. |
| Electronics | Heat sinks, enclosures, and connectors (die-cast). | Precision connectors, micro-components, and housings. |
| Construction | Pipes, fittings, beams, bars, and architectural items. | Fasteners, brackets, and precision construction tools |
Which One Should I choose? Machining or Casting
Deciding on casting vs machining for your manufacturing projects requires careful consideration of design characteristics and overall requirements.
The following are the key factors to consider;
Production Volumes
One of the key factors is the size of the volume you want to produce and possible scalability. CNCs are better suited for a small number of production batches, but it is possible to scale them with design adjustment at a minimal cost. On the other hand, the metal cast process is ideal for large volumes.
Complexity of Parts
Next, consider the size and complex features of the design to be manufactured, such as small internal features, profile curvatures, etc. Design with small-sized internal geometries is better with CNC as micro-tools can be used.
Desired Precision and Repeatability
In mass production, cast metal items are more repeatable as a single die setup is used for numerous cycles. On the other hand, computer-controlled tools make consistent parts, but not as repeatable as casting in large-scale manufacturing. However, the machining is more precise and provides tight tolerances. So, consider what level of precision and consistency your part needs before deciding.
Material Type
Which material you are using in production also matters. Fewer material options are available for casting, whereas the list of machinable materials is more diverse. It is difficult to die-cast metals with high melting temperatures.
Material Wastage
Wastage not only increases the cost of materials but also makes it difficult to recycle them. So, determine how much material will be wasted from the workpiece while machining. On the other hand, there is minimal material wastage if you cast them.
Speed of Production
Consider the project timeline and compare it with the speed of production to check whether it is feasible for you or not. For prototypes & small batches, machining techniques are faster. But, metal cast methods are speedy for medium to large volumes.
Conclusion
Overall, choosing the right fabrication method between casting vs machining is entirely influenced by the design you have and requirements for performance like strength, precision, finish quality, etc. It is recommended to choose machining techniques for high precision, low-to-medium volumes, and material flexibility. Similarly, casting is better if designs have intricate shapes, cavities, and large dimensions with production requirements.
Often, manufacturers also apply a combination of both techniques, first casting and precision CNC processes. It allows leveraging the cumulative benefit to make high-quality near-net shape parts.
RapidDirect is a leading supplier of CNC machined and metal cast items, offering multi-axis CNC machining and die-casting services worldwide. We can also make custom dies for you.
Even if you are worried about the high tooling cost of casting for prototyping, we can provide vacuum casting services at competitive prices.
FAQs
Although screws can be cast, it is preferred to machine them so that you can achieve accurate thread strength and durability. CNC thread cutting tools can make various sizes of threads. Even small screws are possible to make with Swiss machining.
The machined aluminum parts maintain the original mechanical and physical properties of the raw materials, whereas the strength of the cast aluminum depends on the grain structure.
Die cast method is suitable for large volumes and complex shapes, whereas machining is best for intricate parts with tight tolerances in small to medium volumes.
