Casting and CNC machining: How to choose the right method?

The specific requirements of a part, such as its shape, size and geometry, affect the choice of manufacturing process. CNC machining and casting have advantages for different geometric shapes, so how do we decide which one to use? When determining the appropriate manufacturing method for a given part, several factors need to be considered:

 

Based on quantity and delivery time

In some cases, the choice of manufacturing process may be clear. If you need a large number of parts, tens of thousands or more, the casting method is usually more cost effective. Although it takes time to create the initial mold, each casting can be produced quickly and with relatively low add-on costs.

 

On the other hand, CNC machining can produce the first few parts faster and more economically, but as output increases beyond a certain point, the cost effectiveness decreases.

 

Therefore, the primary consideration when choosing a manufacturing method is the required yield. If you need dozens of parts for prototyping, or hundreds to thousands of parts for small batch production, you should consider CNC machining. For quantities of 10,000 pieces or more, sand casting or die casting would be more appropriate. However, for yields in between these extremes, various factors come into play and affect the best choice of manufacturing method.

 

Switching from machining to casting helps to save time and reduce waste. However, casting also has some disadvantages, including:

 

Rough edges: After casting, there may be excess material (called rough edges) that needs to be removed.

Porosity: Thick parts may sometimes have small holes (porosity) that may be revealed during machining. After processing.

Fluidity and brittleness: The flow of molten material during the casting process may cause visible ridges on the surface, and there may be cooler areas at the end of the fill that may be more prone to embrittlement.

In some cases, even if 10,000 pieces are cast, some parts may still require additional machining due to specific requirements. This can increase the overall cost of a part, but sometimes performance needs are more important than cost considerations.

 

When time is a key factor, CNC machining has significant advantages. Within hours of creating CAD models, machined parts can be produced. Casting, on the other hand, takes more time because it involves creating the mold and setting up the casting process. Even though the final production will be done by casting, machining offers a faster option. It allows parts to be modeled, machined and tested at a much lower cost than casting some prototypes.

 

By using machining, parts can be easily redesigned and fine-tuned according to test results. If needed, this can be done multiple times without the higher costs and longer delays associated with producing the molds and necessary fixtures. The production of prototypes can vary from a single part for laboratory testing to a large number of parts for testing. Beta testing or testing for marketing purposes.

 

Based on part geometry

When choosing the appropriate manufacturing technology, it is important to consider the geometry of the part. For example, designs that require a lot of material removal can be very time consuming and generate excessive waste. In this case, casting is usually a suitable choice. This is especially true for hollow parts with thin walls, large internal Spaces, and complex geometric shapes that require long machining times.

 

Manufacturing inner cavities through CNC machining, especially sealed inner cavities that require additional assembly or metal joining processes, is often more challenging. Casting, in contrast, simplifies the design and eliminates the need to assemble multiple small parts. Instead, the entire part can be cast as a whole.

 

However, using casting to create some features is challenging. For example, bottom cuts or internal shapes require complex dies and additional components such as cores and inserts. Sharp edges or corners are less suitable for casting, and all surfaces of the cast part need to be tilted. In addition, large or irregularly shaped thick-walled parts are best produced using CNC machining, as these shapes are difficult to fill and cool evenly during the casting process.

 

Based on the right tools

In metal casting, there are a variety of methods that can be used to produce end-use parts, such as die casting and sand casting:

 

Die casting is a similar to injection molding. It involves injecting molten metal, such as aluminum, copper, zinc, or their alloys, into a steel mold and then allowing it to cool and solidify. Sometimes, additional machining may be required to add threads, improve the sealing surface, or perfect specific details. Machining as a finishing step provides the flexibility to create multiple variants from a single casting.

Compared with die casting, sand casting is a less delicate process. The surface finish of sand castings is rough, and there may be slight differences between the two halves of the mold and the inserts used, which may require additional finishing. Sand casting is often used for parts that do not require a smooth and polished appearance, such as a car engine block. In more complex applications, such as creating engine coolant channels around the cylinder, additional machining may be required to achieve the desired final shape. Some design features specific to sand casting cannot be replicated by die casting or CNC machining.

CNC machining is usually used to produce a small number of parts, ranging from dozens to thousands. This is a fast and efficient process that does not require the same preparation tools as the casting method. In addition to small batch production, CNC machining can also be used to create prototypes of parts that will eventually be die-cast or sandcast in order to test and refine the design. It can also be used for bridge tools, which means using CNC machining to temporarily manufacture production parts when developing molds for mass production.

 

material-based

Casting produces less waste than CNC machining. During the casting process, only the material needed for the final part is used, with a small amount of additional metal flowing through channels such as sprue, runner and gate. It is simple to remove excess material and melt it down for reuse in other parts.

 

However, casting offers fewer material options than CNC machining. Almost all alloys can be CNC machined, but the range of metal alloys suitable for casting is more limited.

 

Rate of production based

The casting process usually takes several weeks from start to finish. Once the mold is ready, the casting process itself is relatively fast and suitable for producing high-volume parts. However, mold design and machining require some lead time. Although some casting methods use disposable molds, the molds also need to be inspected for quality and maintained. Die casting, on the other hand, uses permanent molds, and the time and cost of each part decreases as the volume of the part increases.

 

On the other hand, CNC machining can provide faster production speed, thus speeding up the turnover of small batch parts. No need to wait for mold manufacturing, just a CAD model, CAM program, a CNC machine and the expertise to start producing parts.

 

Based on repeatability and precision

Repeatability plays a crucial role in manufacturing, and the casting process excels at producing consistent parts. Once the mold and process are fine-tuned, each part can be made exactly the same. When die-casting aluminum, tolerances can be maintained in the range of +/- 0.25 mm (up to 25.4 mm), while for precision casting, tolerances for standard castings can be as small as 0.01 inch (up to 1 inch) and as small as 0.002 inch. More precise casting per 1 inch.

 

These tolerances may vary depending on the materials used. For example, aluminum has a higher melting point than zinc, so higher injection pressures and a more aggressive process are required. As a result, cast aluminum parts may have lower tolerances than machined aluminum parts. However, more stringent tolerances can be achieved with cast zinc. You can refer to our post to learn more about the difference between cast and machined aluminum.

 

CNC machining of metals is also a highly repeatable and precise process that enables tighter tolerances than casting. For standard metalworking, a reasonable expected tolerance is 0.025 mm (0.001 in). Precision machining can even produce parts with tolerances as small as +/ -0.0002 “through Runsom Precision’s specific process.

 

demand-based

The variability of demand is also an important consideration. If demand fluctuates or you only need to occasionally produce additional parts, CNC machining is usually a better choice. CNC machining enables faster production and on-demand manufacturing, especially with the advances brought about by the digital transformation of manufacturing. With CNC machining, you can order parts as needed, avoiding the risk of overproduction and overstock.

 

In contrast, if you expect high demand and are switching to production, casting becomes a good choice. Casting is ideal for large numbers of parts and provides a faster overall production process.

 

Casting is generally not recommended if you are currently in the prototyping phase of the design process. This is because every time you change the design, you need to pay for expensive mold re-creation or adjustment fees. This can significantly increase delivery times and costs. In fact, prototype die casting costs can be high. While sand casting is generally cheaper, prototyping is not much of an advantage.

 

On the other hand, CNC machining is a better choice for prototyping due to its flexibility to adapt to design changes. While your design is still evolving, all you need to do is modify the model and update the CAM program. No need to create a new mold or modify an existing mold. This saves time and resources during prototyping.

 

Feature casting CNC machining

Manufacturing process Molding The manufacturing process involves pouring molten material into a mold. The reduction manufacturing method involves the use of cutting tools to remove material from solid blocks or blocks.

Complex shapes Complex internal and external features of parts with complex geometries

Surface treatment often requires secondary processing to be smoother

The tolerances are not as tight as CNC machining, the dimensions are accurate, and the surface is smooth

Lead time Small batch Time Prototyping and the speed of small batch is significantly accelerated

Cost Low cost of mass production Saves the cost of small batches and prototyping

Volume ideal for high volume production Small batch production is more cost effective for prototyping

Materials usually suitable for alloys with lower melting points are suitable for most metals

Customization limited for more flexibility

Automation requires manual labor automatically

The speed is relatively slow and fast

The versatility is limited by the mold design and the shape, size and complexity of the parts are more diverse

conclusion

So, when to choose casting or CNC machining? The decision depends on several important factors: where you are in the development process, how many parts you need, how demand may change, and the specific shape of the part.

 

Ultimately, the best manufacturing process depends on the situation. You may have a unique combination of needs and need to solve problems creatively to find the best solution.

 

As we discuss in this article, each manufacturing method has its own advantages and disadvantages. By considering factors such as part shape, quantity required, quality requirements and project schedule, you can make an informed decision for your project. In fact, your project may even benefit from a combination of casting, CNC machining and finishing technologies

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